Archaeological involvements of physics

ARCHAEOLOGICAL INVOLVEMENTS OF PHYSICS

M.J. AITKEN Research Laboratoryfor Archaeology, Oxford University, 6 Keble Road, Oxford OX] 3QJ, England

NORTH-HOLLAND PUBLISHING COMPANY

-

AMSTERDAM

PHYSICS REPORTS (Section C of Physics Letters) 40, No. 5(1978) 277-351. NORTH-HOLLAND PUBLISHING COMPANY

ARCHAEOLOGICAL INVOLVEMENTS OF PHYSICS

M.J. AITKEN Research Laboratory for Archaeology, Oxford University, 6 Keble Road, Oxford OXI 3QJ, England Received August 1977

Contents: 1. Introduction 2. Thermoluminescence dating 2.1. Introduction 2.2. Application 2.3. Recent research and outstanding problems 2.4. Thermally stimulated exoelectron emissiop (TSEE) 3. Archaeomagnetism 3.1. Remanent magnetism 3.2. Instrumentation 3.3. The ancient geomagnetic direction 3.4. The ancient geomagnetic intensity 3.5. Lake sediments 3.6. Geomagnetic reversals

279 281 281 286 290 298 298 300 302 303 310 314 316

4. Radiocarbon dating 4.1. Introduction 4.2. Distortions in the radiocarbon timescale 4.3. Measurement 5. Location of buried remains 5.1. Introduction 5.2. Magnetic prospection 5.3. Electrical resistivity surveying 5.4. Electromagnetic techniques 5.5. Other possibilities References

318 318 321 330 ~33 333 334 338 339 344 345

Abstract: Accounts are given of the three main chronological applications of physics in archaeology: thermoluminescence dating, archaeomagnetism, and radiocarbon dating, with mention of direct detection using a Van de Graaff tandem accelerator. Following these, techniques for the location ofburied remains are discussed: magnetic prospection using proton free-precession magnetometry, electrical resistivity surveying, electromagnetic techniques, and other possibilities.

Single ordersfor this issue PHYSICS REPORT (Section C of PHYSICS LETTERS) 40, No. 5 (1978) 277—35 1. Copies of this issue may be obtained at the price given below. All orders should be sent directly to the Publisher. Orders must be accompanied by check. Single issue price Dfl. 34.00, postage included.

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1. Introduction Within some archaeological artefacts there is a record to which an archaeologist is blind but which a physicist can hope to read. As long ago as 1899 the Italian scientist Foigheraiter [1] measured the direction of the remanent magnetism carried by some Etruscan vases, and by assessing the most likely position in which the vase would have been placed for firing he sought to establish the ancient angle of inclination of the earth’s magnetic field. This study was the precursor of present day archaeomagnetism which Thellier [2] pioneered in the 1940’s using the remanent magnetization of the walls and floors of ancient pottery kilns. Essentially this uses dated archaeological material to obtain information which is of immediate geophysical interest but which eventually, when enough reference data for a region are available, allows a degree of“magnetic dating”. The remanent magnetism in baked clay, and in heated stones from prehistoric hearths, also records the ancient intensity of the geomagnetic field and when such data are available on a worldwide scale the ancient geomagnetic moment can be evaluated. The past variation of this is of more than academic geophysical interest because the dipole component of the geomagnetic field affects the flux of cosmic rays reaching the upper atmosphere and hence the production rate of ‘4C, with consequent distortion of the radiocarbon timescale. Radiocarbon dating of organic remains, based on the 5730-year decay of 14C activity below the starting level at “death” (defined as the cessation of carbon interchange with the atmosphere) has had the most dramatic impact of all involvements of physics in archaeology. The inception of this technique in the 1950’s, based on research by Libby [3] into the effects of cosmic rays on the earth’s atmosphere, brought about the “radiocarbon revolution” in prehistoric archaeology. Prior to this all dates earlier than 3000 B.C. the start of the astronomically-based Egyptian calendar had been a matter of conjecture and radiocarbon showed that civilization had developed substantially sooner than had been assumed. For instance the beginning of Jericho, formerly placed around 4000 B.C., was put back to 8000 B.C. Subsequently with improved measurement techniques, it became evident that in the period 3000—2000 B.C. radiocarbon dates were several centuries more recent than Egyptian calendar dates, and over the last decade calibration of the radiocarbon time scale by means of dendrochronology has shown radiocarbon dates to be too recent by about 750 years for the period 5000—3000 B.C. The details of this calibration are still being argued but the general effect is that civilisation is even older than radiocarbon had first indicated. A further implication is to call into question the precocity of the Eastern Mediterranean and the Near East in various technological developments and to undermine the diffusionist model of prehistory; the basis of the latter was that civilization spread outwards from the Near East and as this was also the basis of the chronology, it was not upset until the advent of external dating by means of physics. It is now accepted that independent invention took place although diffusion of ideas occurred too. Although radiocarbon is of predominant importance as the backbone of prehistoric chronology, even when all technical laboratory problems are solved and calibration into calendar years fully established there remains the difficulty that the “death” of the organic material being dated may not be contemporary with the archaeological event with which it is associated. In the case of wood or charcoal there are delays first between cellulose formation and felling of the tree and secondly between felling and archaeological utilization. Consequently, although not as precise, thermoluminescence dating has an important role because it is directly applicable to an artefact, namely pottery, which is of direct cultural significance successive cultural phases often being described —

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in terms of changing style and technique. An ancilliary role, and so far the one that has had the greatest impact, is in testing the authenticity of art ceramics; there has been a “thermoluminescent revolution” on the shelves of museums and art dealers. Of the three chronological techniques it is in thermoluminescence dating that there is greatest potential for physics development. On the other hand it is from archaeomagnetism that the physicist has most to gain in terms of advancing his own discipline in the context of the mechanism by which the earth’s magnetic field is generated. But as emphasised above it is with radiocarbon that the physicist has most to offer the archaeologist, though it must be stressed that radiocarbon is a joint physics and chemistry project in technique and that the understanding of its distribution in nature is a matter for earth scientists as well as botanists; however there are two new physics developments afoot in measurement technique utilization of lasers for isotopic enrichment and of particle accelerators for measurements. The final section of this review deals with the location of buried remains, in particular by means of proton magnetometer detection of the magnetic disturbance arising from the remanent magnetism of kilns, hearths, and ovens. Such techniques are small scale application of geophysical prospection for geological purposes and there is often no sharp demarcation line. Likewise dating by means of potassium—argon, uranium-series, and fission tracks are geological dating techniques which have limited, though sometimes important application in archaeology. Also excluded from the review are a whole range of physics techniques from which the resulting information is essentially chemistry. In this category there are the comprehensive neutron activation programmes of trace element analysis for determination of source of raw materials (and hence of trade routes), the studies for similar purposes of lead isotope ratios, and, the use of X-ray fluorescence in numismatics (to chart economic fluctuations by debasement of coinage). Recently particle accelerators have been used for surface diffusion studies [4—6]:for example the hydration layer in obsidian and glass grows with age and its depth in obsidian has been determined [4] by the gamma flux resulting from the resonance reaction ‘H (19F, c~y)’6Oas a function of the incident energy of the bombarding fluorine ion. Mössbauer spectroscopy is now [7—12] a well-established tool in characterizing ancient pottery as well as for determining firing temperature a parameter in the study of ancient technology. Finally, physical scientists should not overlook the already accomplished integration of biologists into the archaeological field (and of course an understanding of biological processes is an important necessity in radiocarbon dating quite apart from the use of dendrochronology for calibration). For instance tree and plant pollen preserved in acid soils and peat bogs leave a record of the dominant species during antiquity and hence of the climate. Recently, measurements made on the degree of conversion of amino acids [13] in bone (from isoleucine to alloisoleucine) have been used for dating on a long-term scale (perhaps as far back as a million years). Although there is the limitation that a constant temperature during burial has to be assumed, this method covers an important period of man’s early development which is beyond the present 50000 year range radiocarbon. A further treatment of various relevant topics will be found elsewhere [14, 15]. Current research is reported in Archaeometry as well as in more well-known journals; there is an annual Symposium on Archaeometry and Archaeological Prospection. —

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2. Thermoluminescence dating 2.]. Introduction Although thermoluminescence is a highly sensitive technique for measuring the trapped electron population in a semiconductor or insulator, being several orders of magnitude more sensitive than electron spin resonance, it is not a well understood phenomenon even in artificially grown crystals of controlled impurity content. In its use for dating ancient pottery we are dealing with minerals carrying an unknown suite of impurities, sometimes in a mixture of minerals of unknown identity. Therefore we have to discuss the process in terms of the generalized model of fig. 2.1 and develop a phenomenological approach in which as far as possible the characteristics of each sample are checked in respect of suitability for dating.

4

~~

~e~ron________

thermal release

E —T

L—

ionization

T

—6—L

L—

~ Light

Hoi~/~J

(i) Irradiation

~

(ii) Storage

___

(iii) Heating

Fig. 2.1. Energy-level representation of TL process. (i) Ionization by incident radiation results in trapping of electrons at T and holes at L. (ii) For a sufficiently deep trap (E I~eV) the lifetime of the trapped electrons is upwards of a million years. (iii) Laboratory heating causes eviction of the trapped electrons. A small proportion recombine with holes in luminescent centres with emission of light; alternatively they make non-radiative recombination at a killer centre.

The basic notion is that the firing of pottery by ancient man reset the “TL clock” to zero and that during the subsequent centuries the trapped electron population built up at a uniform rate due to the weak flux of ionizing radiation provided by radioactive impurities (a few parts per million of uranium, thorium and potassium-40) in the clay itself and in the soil in which the pottery was buried after the archaeological site fell into disuse. Suitable apparatus for detecting the consequent “natural” TL is shown in fig. 2.2. Because the minerals in pottery have a rather low TL sensitivity, and because the accumulated exposure during antiquity is only of the order of a thousand roentgen, the natural TL is usually rather dim. Not only is a fast heating rate required, together with a high sensitivity low noise photomultiplier (e.g. E.M.I. type 9635), but also it is important to have severe discrimination against thermal radiation by means of carefully-selected colour filters and to suppress any non-radiation-induced TL. This latter, usually referred to as “spurious” TL, is a surface effect induced during sample preparation; for most samples it is ade-

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X-Y Plotter Pulse amplifier (xl000)

If Ternperature(°C)

—

7

,,,,,,

7,,

7/1

~ Light guidc~

Blue filter

Photocathode

7

N,

~ Nichrorne heater plate

___________________

7

\\\\\\\ ///////

Nitrogen

________

1~rmceo;k

cl

,__..__fI:~T~~ San~ple

Heater current IOOA second to give heating rate of of 20 °Cper

Fig. 2.2. Apparatus for TL dating measurement.

quately suppressed by making the measurement in an atmosphere of dry oxygen-free nitrogen or argon. The “glow-curve” for natural TL a typical one is shown in the top right hand corner of fig. 2.2 has no TL below about 250°C.This is because the traps which are emptied below that temperature are too shallow to retain their electrons without serious loss during the centuries of burial. For dating purposes we are concerned with the TL emitted above 350°C;in practice the useful range does not usually extend beyond 450°Cbecause above that temperature thermal radiation becomes excessive despite colour discrimination and in any case the TL intensity usually falls off due to increased thermal quenching (dc-excitation of the luminescent centre by emission of a phonon instead of a photon). Of course for a single trapping level the TL glow-curve would be a single peak determined, in the simplest case, by the equation —

—dn/dt

=

nsexp{—E/kT}

—

(2.1)

as first discussed by Randall and Wilkins [16]. Here n is the number of filled traps at time t, s a frequency factor which expresses the number of times per second the electron presents itself in an attempt to escape from the trap, E the trap depth, k Boltzmann’s constant, and T the absolute temperature. The peak results from competing effects of the exponentially increasing probability of ejection of an electron and the rapidly decreasing number of electrons remaining in traps. For a single trapping level it would be about 50°Cwide; the glow curve of fig. 2.2 is evidently composed of a number of overlapping peaks and it is to be inferred that there are several types of trap present,

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either within the same mineral or in several different minerals. The presence of several types of trap has the advantage that it permits the “plateau test” (see fig. 2.3) to be used to check as to whether the traps associated with a given temperature range are deep enough for stable retention of electrons during antiquity.

Artificial TL

~:

Ntttlir~i~J

100

a 0 ~0 —

200

300

400

500

1000

500

0

a > I

100

_____

200 300 Temperature (°C)

_____-

400

500

Fig. 2.3. The plateau test. The upper part shows the glow-curves corresponding to the natural TL and to the artificial TL following 1000 rads of beta irradiation. The lower part shows the equivalent dose, which here is taken to be equal to (natural TL/artificial TL) x 1000 rads, as function of temperature. The onset ofthe plateau is indicative that a sufficiently high glow-curve temperature has been reached for the TL to be associated with traps that are deep enoughto retain their electrons with negligible leakage during archaeological times. The onset of the plateau is usually around 350°C.

2.1.]. The age relation In its simplest form this is age

— —

natural TL (TL per unit dose) x (dose per year)

(2 2)

TL per unit dose is the sensitivity of the sample for acquiring TL; it is measured by exposing the sample to a known dose of nuclear radiation from an artificial radioisotope source. The resulting glow-curve is referred to as “artificial TL”. Dose per year is evaluated from radioactive and chemical analysis of the sample and its surrounding soil. The unit of dose at present in use is the rad (Radiation Absorbed Dose) which is defined as the absorption of 100 erg per gram; the more familiar roentgen is the unit of radiation exposure. In air exposed to 1 roentgen of X-rays the absorbed dose is approximately 0.87 rad. The SI unit for dose is the gray (Gy), defined as the absorption of one joule per kilogram; it is equal to 100 rad.

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The natural dosage received by the pottery (see table 2.1) is a mixture of alpha, beta, and gamma radiation plus a contribution of a few percent from cosmic rays. On account of the high ionization density they produce alpha particles are substantially less efficient than beta particles and gamma rays. Hence it is necessary to rewrite eq. (2.2) in the form T

(2.3)

GN

=

X5D~+ x~(D~ + D~+ D~) where T is the age in years, GN

is the natural TL,

x5 and Xp are (TL per rad) for alpha and beta particles respectively, D5, D~,D~,D~are the respective annual dose-rates for alpha, beta, gamma and cosmic radiation. Experimentally it is convenient to define k

=

and denoting the equivalent dose by Q (= GN/xp) we have T—

(24)

kDa+Dp+Dy+Dc~

So far it has been assumed that the acquisition of TL is linear with dose and that, as is implicit in the caption to fig. 2.3, there is no change of TL sensitivity when the sample is heated to 500°C in the course of measuring the natural TL. For most samples these assumptions are upset by two Table 2.1 Components of dose.ratet for typical pottery and soil a

/3

Thorium-232 series: Radioisotopes before thoron Thoron and later products

309)> 738 429)

10.31 28.6 18.3)

20.81 51.4 30.6j

818

191

Uranium-238 series: Radioisotopes before radon Radon and later products

~ 779 426)

17.1’~40.94 23.8)

1.61 32.1 30.5j

852

190

177

Potassium-40

Total

Effective total”

136

41

177 1847

—

—

558

Totals

1517

206

124

Effective dose-rates

227 (41%)

206 (37%)

124 (22%)

Dose-rates are given in millirads per year and correspond to pottery and soil having 2.8 ppm of and a The counting efficiencyalpha of83%; the urauranium, 10 ppm ofthorium, and 2% potassium (measured as2K2O). corresponding count-rate wouldand nium be 10 thorium per kilosecond contributions for a thick to the sample count-rate ofareawould 13.8 cm be approximately equal. tt Assuming k = 0.15.

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effects, supralinearity and sensitization, which will be discussed in section 2.3. 2.1.2. Sample preparation For accurate dating the technique of sample preparation is intrinsic to the principles on which the age is calculated. This is because of the heterogeneous nature of pottery fabric it is a baked clay matrix in which there are mineral inclusions, sometimes ranging up to a millimetre in diameter; these inclusions, usually of quartz or feldspar, were either present in the raw clay or were added by the potter in order to improve refractory qualities. The TL sensitivity of these inclusions is much higher than that of the clay matrix but in general (potassium feldspars and zircons are notable exceptions) the radioactive impurities are carried in the matrix. The average range of the alpha particles from uranium and thorium, which provide a substantial part of the natural radiation dosage, is only about 25 microns and as a consequence the average dose received by an inclusion decreases with size once its diameter exceeds a few microns. Since the grain-size distribution varies between fragments, for accurate results it is necessary to separate out grains of a given size range and to use these for measurement of both natural and artificial TL. There are two basic approaches. In the fine-grain technique developed by Zimmerman [17] the material extracted consists of grains that are small enough for the attenuation of the alpha dosage to be negligible. The pottery fragment is crushed by slowly squeezing in a vice; grains in the size range 1 to 8 microns are separated by suspending in acetone and utilizing the fact that the settling time is determined by the diameter. After separation, the selected grains are re-suspended in acetone and deposited on aluminium discs (usually 10 mm diameter and 0.5 mm thick) in a thin layer of a few microns. About a dozen such discs are prepared from each sample and if all goes well the TL reproducibility is about ±5 %. In the quartz inclusion technique developed by Fleming [18] the grains selected are large enough for there to be severe attenuation of the alpha dosage, but not so large that there is serious attenuation of the beta dosage; by etching these grains in hydrofluric acid the outer skin that has~received alpha dosage is removed and in evaluating the age D5 is put equal to zero. The size-range of the grains is usually 90—150 micron, a small correction being made for the attenuation of the beta dosage. Weighed portions of these grains (typically 5 to 10 mg) form the samples on which the TL measurements are made. As just indicated the quartz technique is based on the assumption that the grains are devoid of radioactivity. However, recent measurements by Sutton and Zimmerman [93] indicate that in some grains there is a small but just significant content of uranium. Another recent quartz investigation, by Bell and Zimmerman [94] using an electron microscope, categories grains into “shiny” and “frosty”; with the latter the etching by hydrofluoric acid proceeds in an irregular way that does not correspond to the orderly removal of the outer skin assumed above. In passing one may note two other recent publications connected with quartz. In dating the sandy clay of the burnt floor and walls of a kiln, Valladas [95] found it possible to eliminate intrusive unheated grains by virtue of their lower magnetic susceptibility the heated grains were presumed to have acquired iron impurity from the clay matrix during firing. Courtois et al. [96] in dating Amazonian pottery noted a parasitic TL component that they ascribed to the deliberate addition of siliceous wood by the potter. —

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Thermoluminescent dosimetry (TLD) The most widespread practical utilization of TL is in measuring the accumulated exposure to

2.1.3.

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Mi. Aitken, Archaeological involvements of physics

radiation of patients undergoing radiotherapy and of research workers subject to nuclear hazard. For these purposes highly-sensitive artificial phosphors such as lithium fluoride, calcium sulphate, and calcium fluoride are used, with a variety of added impurities as activators. For dating, TLD is an attractive alternative to radioactive analysis for evaluation of beta and gamma dose-rates [19—23]. 2.2. Application 2.2.1. Archaeological dating Had radiocarbon dating not been well-established, thermoluminescence would have had the task of establishing the chronological framework of prehistoric archaeology. Leaving until later the question of TL potentiality prior to the 50000-year limit of radiocarbon’s range, let us consider what TL has to offer in a complimentary role [99]. First and foremost it gives a direct date for an object of archaeological significance. The changing technique and style of pottery often form the basis of archaeological chronology whereas the association of a radiocarbon sample with an archaeological phase may be open to question. Also, the event dated by TL is the actual firing of the pottery whereas, as mentioned in section 1, in the case of radiocarbon dating of wood or charcoal the event dated may be several centuries earlier than the archaeological association. Secondly there are some sites prolific in pottery on which suitable radiocarbon samples do not occur. The accuracy obtainable with TL in good circumstances is at present somewhat better than ±10 % of the age. It is unlikely that it will ever be improved beyond ±5 % because the stored information in the pottery is not recorded to any better accuracy due, for instance, to uncertainty about the average water content during burial (which affects the dose-rate). This is somewhat poorer than is usually quoted for radiocarbon though for some periods it is no worse than indicated by a realistic assessment of accuracy having regard to the short-term fluctuations discussed in section 4. The “good circumstances” required for high TL accuracy refer on the one hand to pottery having satisfactory TL characteristics and on the other to whether or not the actual burial situation permits a reliable estimate of the gamma dose-rate. The latter is determined by the soil or rock within 0.3 metre of the sample and unless the surround is homogeneous it is difficult to make a reliable estimate. These considerations mean that samples should be collected specifically with TL dating in mind and preferably by the TL specialist who in any case should visit the site either to bury TLD capsules or to make measurements with a scintillation spectrometer. Pottery may display unsatisfactory TL characteristics in a variety of ways. The TL may be too dim, the growth with dose may be unmanageably non-linear, it may exhibit anomalous fading (see section 2.3) despite passing the plateau test, or it may be largely composed of spurious TL. For one reason or another, a substantial proportion of archaeological sites cannot be dated by TL. Published accounts of some of the TL dating projects so far undertaken are listed in the refs. [24—44,97]. Two of these are now mentioned as illustration of the versatility of TL and the way in which it can be applied directly to objects of archaeological significance. The first [31] is the TL dating of the baked clay balls that abound on sites of the Poverty Point culture along the channels of the Mississippi and associated rivers. On one such site it is estimated that 24 million of these objects were made; it is presumed they were used for cooking the balls being heated in a fire and then put into pits along with pieces of food to be baked or roasted. Their dating is imp or—

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tant because it was through the Poverty Point culture that Mesoamerican influence reached the inhabitants of eastern North America; associated with this culture are impressive earthworks of monumental construction. Balls from five such sites gave dates which clustered around 1000 B.C. A sixth site, which was culturally distinct on account of there being associated pottery (which was also dated), gave a date almost 1500 years more recent, suggesting that the baked clay ball technology persisted for a long time. The alluvial nature of the Mississippi valley dictated the use of baked clay for cooking rather than heated stones as in some other cultures. In the Orkney Islands, lying to the north of Scotland, there are several hundred mounds of fire-shattered stones which are sometimes 20 metres across and one or two metres high. It is presumed that these had been used for boiling water in a stone trough in which the meat to be cooked was placed. (Indeed archaeologists excavating one of these mounds demonstrated that a sheep could be made edible in this way!) TL dating [33] of half-adozen stones from half-a-dozen such mounds gave ages which spanned the first half of the first millenium B.C., indicating that the mounds were mainly contemporary and that each continued in use for several hundred years. The technological interpretation suggested by the archaeologists concerned, Renfrew and Hedges, is that this type of cooking was common domestic practice in the period following deforestation of Orkney when the only fuel available was peat (which is not suitable for roasting meat directly) and before the introduction of the bronze cauldron. What are possible materials for TL dating prior to the 50000 year limit of radiocarbon? Baked clay in the form of pottery does not extend beyond about 10 000 years ago though occasionally earlier in the form of figurines: in Czechoslovakia for instance, there are baked clay fragments which have been TL dated by Zimmerman and Huxtable [43]. However fire was used by ancient man as a means of cracking flint for use in tools and weapons, and some such flints are sufficiently heated for use in dating [41, 42, 98]. Heated stones associated with fireplaces are another possibility [44]. All discussion so far has been concerned with samples that have been heated. There is also the possibility that deposited carbonate (e.g. stalagmites and travertine associated with cave dwelling) can be dated by TL. The vital question is whether or not the crystals start off with trapped electron population of zero on formation. Besides this, with semitransparent materials such as flint and calcite there is the possibility that ambient light has induced a significant TL signal in the sample; exposure to light can be controlled during excavation and in the laboratory but not during antiquity. Promising TL measurements have been made on fominerfera from ocean cores [45], and extension to shells associated with human habitation, although beset with problems, would be a very worthwhile development. 2.2.2. Recent geology The carbonate materials just mentioned are more directly relevant to quaternary geology than to archaeology. In this context the obvious material for TL dating would seem to be volcanic lava and the archaeomagnetic interest of dating the Laschamp lava flow in the French ChaInedes-Puys is discussed in section 3.6. Unfortunately it seems to be a fairly general rule that the feldspar minerals carried in volcanic lava suffer from anomalous fading. This phenomenon does not appear to afflict quartz (or calcite) but unfortunately the amount of quartz in lava flows of interest is rather meagre. However when clay or rock having satisfactory TL characteristics has been heated by the lava flow there is a possibility of obtaining a date indirectly.

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2.2.3. Authenticity testing It is here that TL has had as revolutionary an impact as that of radiocarbon in archaeology. For testing authenticity it is usually a question of deciding between an age of less than a century and one of upwards of a thousand. Uncertainty about gamma dose-rate is then unimportant and it does not matter that the “burial circumstances” are unknown. The result of a TL test may alter the value of an object from an astronomical figure to a negligible amount; consequently although the answer does not need to be accurate it does need to be reliable and so tests need to be conducted with meticulous care. Reputable art dealers have doubtful pieces tested as a matter of routine, the usual fee for such a service being £50—~100.A sample of about 50 milligram is sufficient in most cases and this is obtained by drilling a small hole in an unobtrusive location. There are some styles of ceramic ware for which art historians have had doubts as a whole, and application of TL in some such cases has led to agonizing reappraisals. It has had academic impact in the sense that previously the art historian may have been studying the forger’s view of man’s cultural and artistic development rather than actuality. An example [24] of this is the so-called Hacilar ware anthropomorphic vessels and figurines said to be 7000 years old and to have come from the renowned site of that name in south-west Turkey. Because their supposed origin was from a cultural phase following soon after the first appearance of pottery in that part of the world, the fineness of technique and beauty of form attracted particular interest. Of the seven most important pieces tested the magnificent double-spouted vessels, each spout being in the form of a head with obsidian eyes only one was found to be genuine. Out of a total of 66 pieces tested there were 48 modern forgeries. In another application [46] to Chinese Hui Hsien ware there were no genuine examples among the pieces available for testing. Can a modern forgery be irradiated so that it will exhibit TL appropriate to a genuine object? An inexperienced operator could well be misled by even a clumsy attempt. To do the job properly the forger would need to employ a physicist having some training in TL and with appropriate experimental facilities available. Even then it is difficult to see how artificial irradiation could fool a technique such as zircon dating (see section 2.3). On account of their high uranium content zircons carry a very much higher equivalent dose than other constituent minerals in pottery and it is not possible to achieve this situation by external irradiation. Another possibility is by reconstitution, i.e. the fake object is made from ground-up ancient brick by means of some chemical cementing agent. If no heat is used in the process, then the TL age will be that corresponding to the ancient brick. Of course there is much more to it than this; it is likely that the chemical agent will reveal itself by excessive spurious TL and in any case there are other techniques that can be applied e.g. thermogravimetric analysis, and archaeomagnetism. —

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2.2.4. The Glozel controversy Discussion of “faked TL” has been brought to the fore recently by what is currently a cause célèbre in thermoluminescence the Glozel affair, an archaeological cause célèbre of the 1920’s. Beginning in 1924 a local farmer living in an isolated farmhouse south of Vichy, France, began to report the discovery of a wide range of objects most of which had no parallels in excavated material anywhere else in the world baked clay tablets with mysterious incised lettering in an unknown script, jars and other ceramics with and without inscriptions, phallic symbols in pottery, bones and pebbles with animal inscriptions. The latter led to the claim that the site was Neolithic and that the tablets represented the earliest writing in the world. This view was hotly contested mainly by non-French archaeologists who thought the whole thing was so unacceptable that it must —

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be a hoax, the objects being forgeries. An acrimonious controversy continued more or less continuously until the outbreak of war in 1939. During this time two international commissions of inquiry sat and pronounced exactly opposite decisions and some five law suits were fought with varying results. Subsequently the questionable nature of the finds became generally accepted and it was therefore a severe shock when TL measurements made on some of them by McKerrell et al. [47, 48] indicated that the ceramics were not recently fired but had probably been made about 2000 years ago. The majority ‘of archaeologists still vehementally rejected the notion that the site was anything but a hoax, partly on account of the history of the discoveries (though McKerrell et al. contest the accepted interpretation that this had been a shady business), and partly on grounds [49] that, even if the site was to be accepted as a unique manifestation of an otherwise unknown culture, the absence of run-of-the-mill pottery fragments typical of close-by sites of the period rules out its authenticity. This TL finding was the first that had received strong rejection either by archaeologists or art historians and the validity of TL in this particular application was questioned [50—5 3]. The proximity of a high grade uranium mine seemed a strange coincidence but TL measurements on zircons extracted from several of the objects ruled out artificial irradiation, at any rate for those objects. The notion that the objects had been made by cementation of ground-up Roman tile was encouraged by the generally poor quality of the pottery fabric and this possibility was investigated, for six objects, by Barbetti [54] using archaeomagnetic intensity analysis. Although the TL of an ancient tile might not be affected by reconstitution this would not be true of the thermoremanent magnetism (see section 3.1): the magnetic grains would be disorientated in the process and the resultant magnetic moment would be negligible by comparison with that acquired on re-heating a sample in the laboratory. The first object tested, a bisexual figurine, did in fact show this sort of behaviour, indicating either that the original firing temperature had been less than 300°Cor that reconstitution had taken place and that the temperature reached in this process had not exceeded 300°C.This finding was in accord with the poorly-fired appearance of the object. To explain why the TL measurements on it indicated that an age of about 2000 years rather than a geological age corresponding to unfired clay one must accept the first alternative and presume that the original firing had nevertheless been sufficient to reset the TL to zero. The results for the other five objects indicated firing temperatures in excess of 500°C.For two of them, which were from tablets carrying the strange script, reliable determinations of the intensity of the ancient magnetic field in which they cooled were possible. The values obtained were 46 and 47 microtesla, suspiciously close to the present-day value at Glozel of 46 microtesla. From comparison with the general trend indicated by published values for the ancient intensity in Europe (see fig. 3.7) Barbetti concluded that for these two tablets at any rate the date of last firing was unlikely to have been during the 3000 years between 1500 B.C. and A.D. 1500, noting that although this excluded the 2000 year-old ages it was not in strong conflict with the medieval TL age that had subsequently been found (51) for one of them. However, subsequent to Barbetti’s conclusions more detailed measurements have been made on well-dated pottery from central France over the period 50 B.C. to A.D. 200; these give indication that there was a rather rapid deviation from the general trend during the period, reaching close to the present-day value (J. Shaw, D. Walton, private communications). One problem in the Glozel investigation is the diverse nature of the objects and the tendency to forget that there may be a number of different origins, some ancient others modern. Also, in

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view of the conflicting accounts of the circumstances of excavation there is the possibility that the ancient ones came from a site remote from Glozel. Even so, the strange script remains a puzzle; according to Crawford [55] the script does not have the characteristics of a natural language but is a collection of signs, of which there are at least 133. Another possibility [56] is that the tablets themselves are ancient but the signs were added subsequently. Some tablets have small blobs of glass visible on and around the inscription surfaces and consequent the TL date that has been reported [57] specifically for one such tablet is of particular interest: it lies within the last few hundred years. However this does not conclusively prove a fairly recent origin for the inscription since there is the possibility that the tablet had been reheated through accidental association with glass-making activities that were going on at Glozel, perhaps as recently as the eighteenth century [57]. The Glozel affair has been discussed at some length for two reasons besides its intrinsic fascination. First because the validity of TL has been called into question on account of it, and secondly because as perusal of the cited references will illustrate, albeit with an extreme case, it shows the quagmire of embittered controversy and preconceived belief through which a physical scientist is liable to have to wade once he involves himself with material of doubtful origin. 2.3. Recent research and outstanding problems 2.3.]. Supralinearity and sensitization Evaluation of TL sensitivity as indicated in fig. 2.3 by measurements made on a sample from which the natural TL has been drained in the course of the first glow-curve is not, in general, a valid procedure. A substantial change in sensitivity may occur when a sample is heated; part may be due to changed transparency and reflectivity and part may be due td the radiation dose that the sample has received previous to heating, the latter being referred to as the pre-dose effect. Whatever the details of the mechanisms involved this sensitization between “first-glow” and “second-glow” presents a very serious complication in TL dating. The effect itself can be circumvented by using the additive method of equivalent dose evaluation (see fig. 2.4). This has the procedural disadvantage of requiring precise interrelation of the TL from different portions and the more serious objection that although the additive method checks for linearity in the acquisition of TL in the dose region above the level of the natural dose, it can say nothing about linearity at lower levels of dose. In fact a common characteristic of many TL minerals (and of some artificial phosphors, lithium fluoride for example) is a lower sensitivity during the first few hundred rads of dosage than subsequently; this supralinearity is illustrated in fig. 2.5. An empirical way of correcting for supralinearity is to use already-glowed portions to determine the second-glow growth characteristic, and to assume that despite a change in sensitivity on heating there is no change in the intercept A. The justification for this procedure is that it gives improved agreement in tests with known age samples. It must be admitted that it is a rather bland assumption and indeed with some samples a pre-dose effect shows itself in the supralinearity [58, 59]. Studies of the supralinearity characteristics of individual minerals are only of help as a general guide because the quantitative behaviour depends on the annealling conditions that the particular sample has experienced; also, in the fine grain technique one is dealing with a mixture of minerals of unknown identity. An advantage of the quartz inclusion technique is that if attention is restricted to the “benign” peak at 375°Cthe sensitivity change on heating is often less than 5 0/~giving grounds for belief that the supralinearity intercept will be free from pre-dose effects. too. Unfortunately

Mi. Altken, Archaeological involvements of physics

291

N+2fl

AD—

N+lfl

N

~ 0

500 10(X) Additive dose (rads)

1501)

Fig. 2.4. Additive method of evaluating equivalent dose (ED). Of three equal samples the first is used to measure N — the natural TL, the second to measure (N + 1$)— the natural-plus-artificial TL after a dose of /3 rads, and the third (N + 2$); in the above, /3 = 700. The supralinearity correction 4 is determined by means ofa grewth curve such as fig. 2.5 using samples from which N has been removed. The sum of ED and 4 equals AD, the archaeological dose.

the “malign” 325°Cpeak commonly dominates the TL from quartz. It is not difficult to suggest mechanisms that might give rise to supralinearity and pre-dose effects though far from easy to establish which is operative in any particular mineral. The simplest explanation is that additional traps are being created in the early stages of the irradiation, thereby increasing the sensitivity. An alternative is the “competition model” in which it is assumed that there is a second set of traps (which do not give rise to TL) competing for electrons. These are —

a I-

V V

E ~ Ei V

H 100

200

300

400

500

600

700

Radiation dose (rad) Fig. 2.5. Typical supralinear dependence of TL on radiation dose for pottery samples irradiated with beta or gamma radiation. The degree of supralinearity is measured by the intercept 4.

292

M.J. Altken, Archaeological involvements ofphysics

assumed to saturate earlier than the TL traps and so as the competing traps gradually approach saturation the competition is suppressed and more electrons are available for the TL traps. Another explanation is in terms of an enhancement in the probability that an electron freed from a trap will produce luminescence such enhancement could arise because of an increase with dose of the number of activated luminescent centres for instance. With increasing application to paleolithic samples low-dose nonlinearity becomes unimportant but allowance for the effect of approaching saturation becomes critical. Failure to make sufficient allowance can lead to substantial overestimate of age. There is not much work to hand on the question of whether in this dose region any reliance can be placed on the second-glow growth characteristic, and non-linearity in the additive first-glow growth characteristic is an insensitive indicator of approaching saturation. The situation is further complicated by the likelihood that at these high dose levels trap creation becomes important so that the TL growth characteristic continues to rise beyond the level corresponding to saturation of existing traps. In the case of the 100°Cpeak in quartz the pre-dose effect is so strong that it is the basis of an accurate dating technique particularly applicable to samples of the last thousand years [60, 61]. There is no sensitization until the sample has been heated to 400—500°Cand detailed investigation by Zimmerman [62] has established a model in which the essential mechanism is thermal transfer of holes from “reservoir” traps to luminescent centres (which are thereby activated). This is the only well-established model that we have in TL dating at present. —

2.3.2. Transfer dating In the context of thermoluminescent dosimetry, it is well known that with natural calcium fluoride electrons can be transferred from deep to shallow traps by illumination with ultraviolet light. By observing the TL from the shallow traps only, a measure of the deep trap population can be obtained without heating the sample to the high temperature that would be necessary for direct observation of its deep trap TL. Currently the effect in this and other phosphors is under development for ultraviolet dosimetry. The effect has been studied in quartz by Schlesinger [63]; for instance, by means of 250-3 50 nm light electrons can be transferred from a peak at about 500 K to peaks in the range 150—300 K, the sample being held at liquid nitrogen temperature. At that author’s suggestion preliminary studies have been made by Bowman and Bailiff [23, 64—66] with a view to utilization in dating. With the above discussion of nonlinearity and pre-dose difficulties in mind there is obvious advantage in being able both to measure the deep trap population (by transfer) and to empty the deep traps (by prolonged bleaching) without having to heat to 400°or 500°C,for heating is certainly responsible for some of the sensitivity changes that occur between first-glow and secondglow. Of course the bleaching with ultraviolet may give rise to sensitivity changes for other reasons but the work of Bowman [64] suggests that with careful selection of wavelength this is not necessarily so. There are also advantages simply because the TL is observed at a lower temperature. Interference by thermal radiation and spurious TL is avoided; this is particularly relevant when the TL is green or of longer wavelength, as in the case of calcite, and it is difficult to discriminate by means of colour filters. Also, thermal quenching of the luminescent centres is less. In principle the technique gives greater selectivity of the traps from which electrons are released and thus it may be possible to avoid traps having malign properties such as anomalous fading.

M.J. Aitken, Archaeological involvements of physics

293

2.3.3. Anomalousfading The expected lifetime of an electron in a trap of depth E and frequency factor s is given by =

~

exp(E/kT)

(2.5)

where k is Boltzmann’s constant and T is the storage temperature. Determinations of E and s by means of kinetic studies lead to lifetimes that are of the order of a million years or more for peaks occurring at glow-curve temperatures of 350°Cor higher. However grossly anomalous behaviour has been observed by Wintle [67] in the behaviour of feldspar minerals. For volcanic lavas of known age the observed TL was significantly too low sometimes by an order of magnitude, although the expected ages were no greater than 50000 years and the TL utilized was in the 400°—500°C region of the glow-curve. The results of short-term stability tests were more anomalous still: losses of 10%—40°/~ of the 350°—500°CTL in periods varying between a few hours and a few days were observed for some samples of sanidine, fluorapatite, labradorite, andesine, zircon, and bytownite. Not all samples of these minerals showed the effect, and for quartz and calcite the fading was insignificant over months of storage. The phenomenon has also been observed in lunar samples and it has been suggested by Garlick and Robinson [68] that it is due to the subsidiary escape of electrons from traps due to overlap of wavefunctions a wave mechanical “tunnelling” process. Apart from their discouraging implications for geological lava dating these observations call into question the basic validity of pottery dating using fine-grain samples or other forms of sample in which there is a substantial contribution from feldspar minerals (unfortunately the TL from feldspars is in general much brighter than from quartz). However, the satisfactory results obtained with known-age samples indicate that this behaviour is not common among the mineral constituents of pottery; also, the short-term stability tests that are now made routinely on fine-grain samples show that for most sites it is only the occasional sample which exhibits measurable loss (5 %—10 %) for a storage period of up to a month. However there are some sites for which the majority of samples are afflicted [36, 37]. The rate of percentage loss is initially rapid [92], getting progressively slower as the time elapsed since irradiation increases. To account for the initial rapid loss in terms of eq. (2.5) (and to explain the observation that there is substantial fading even for’ storage at 77 K) requires E ~ eV and s iO~s~and it is difficult to accept the thermal release model on which (2.2) is based as valid for such values. The tunnelling explanation [92] is more credible and in terms of this the progressive slowing dowii of the fading is seen as the using up of nearby centres, tunnelling to these being much more probable than to distant ones. From the dating point of view the crucial questions are first, whether for a sample which shows no anomalous fading over a period of months it can be reliably assumed that there has been none over thousands of years, and secondly whether study of short-term fading can be used to make a quantitative estimate of long-term fading. Comparison with radiocarbon can give some degree of answer to these two questions. But beyond the limit of radiocarbon, in the middle and lower paleolithic ages where there is most need for TL, the only empirical approach is through intercomparison of TL dates obtained on different materials if there is fading the amount is likely to be different. However, until satisfactory calcite dating is achieved the only generally available material is burnt flint (and that is rather sparse). Although short term tests and comparisons with radiocarbon may give no indication of fading in flint the utilisation [42] of this material in dating sites that are approaching an order of magnitude older —

‘~

—

294

M.J. Aitken, Archaeological involvements ofphysics

than the limit of radiocarbon raises the question as to whether the tunnelling probability can indeed be sufficiently low, and gives emphasis to the need for quantitative understanding of the process. An experimental study by Zimmerman [100], following the work of Wintle [92] suggests that there is a correlation between anomalous fading and the short-term delayed luminescence that follows irradiation; this may provide a much needed tool for investigating the affliction. Although the tunnelling process is only weakly dependent on temperature, the deeper the trap the lower should be the probability. Hence there should be advantage in utilizing traps beyond the usual 500°Climit of the TL glow curve; access to these is possible by means of the ultraviolet transfer technique and Bailiff [65] has obtained encouraging indications that anomalous fading can thereby be reduced and perhaps eliminated. 2.3.4. Trap depth determination One of the most widely used methods for determination of trap depths is by study of the temperature dependence of the TL during the initial rise of the glow-curve peak. The portion of glow-curve used needs to be sufficiently below the peak temperature for there to have been no significant emptying of the traps; the TL intensity is then proportional to exp { —E/kT}, and a plot of the logarithm of the intensity versus (kT)’ ‘ yields a straight line of slope E. Using this method the mean lifetime for the 325°Cpeak in quartz has been reported as 3000 years for geological alpha quartz [69] and as 200 years for quartz extracted from Romano-British pottery [70]. Both of these values conflict strongly with the observation that for this pottery sample the TL age based on the 325°Cpeak is within a few percent of the TL age based on the 375°Cpeak (for which the predicted lifetime is 40 x 106 years). There is even greater conflict with the observation that for burnt sand having a radiocarbon age of 30 000 years the TL age based on the lower peak is within 10 % of that based on the upper one [70]. Investigation by Wintle [70] indicates that the reason for this discordance is that as the glowcurve temperature increases, efficiency of the luminescent centres decreases because of increasing de-excitation by emission of phonons rather than photons (i.e. thermal quenching). Thus the TL intensity rises less rapidly than in the absence of the effect and an erroneously low value of E is obtained. Study of the prompt luminescence confirmed that the luminescence efficiency was in fact strongly temperature dependent. Use of alternative methods for determining trap depth (by isothermal decay and by peak shift with heating rate) predicted a lifetime of about 30 million years, a value that is consistent with the TL ages mentioned earlier. For the 230°Cpeak in the archaeological quartz the peak shift method predicted 40 thousand years instead of 2 years according to the initial rise method. For the 110°Cpeak in quartz and for the 275°Cpeak in calcite, thermal quenching effects are not significant; the lifetime of the latter is calculated to be 100 million years from trap depth measurements [71]. 2.3.5. Influence of temperature of irradiation For some phosphors the TL peak height response is slightly dependent on the temperature of irradiation [72]. For natural calcium fluorite and for CaSO4 : Tm the response is lower by 5—10% if the irradiation is carried out at 100°Cinstead of 20°C;for LiF (TLD-100) the response is about 10 % higher and for CaF2 : Mn it is the same to within ~ %. For fluorapatite and burnt limestone the response was the same to within 5 % but for the 325°Cpeak of an archaeological sample of quartz, the response was lower by 17 %. The possibility that the decreases were due to thermal untrapping of electrons was excluded. In the case of the quartz sample the effect was still

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295

present when the dose-rate was reduced by a factor of 3000 but it was absent for a saturating dose of 40 kilorads. A possible explanation is that the effect is due to temperature dependence of the trapping cross section; however, on the basis that alpha particle tracks are microscopic regions of TL saturation this explanation is not consistent with the observation that the effect occurs as strongly for alpha irradiation as for beta. On the basis of the above results the effect is not significant for dating. Additionally, in one sample of archaeological quartz the effect was present in the 325°Cpeak but not in the 365°C peak and it is the latter on which reliance is usually placed for dating. On the other hand Khazal, Hwang and Durrani [73] have reported a strong temperature dependence for the 375°Cpeak in a sample of natural quartz, though using a heavy dose of gamma radiation. Relative to irradiation at room temperature they found a response of x 0.65 at 0°C,of x 0.018 at —25°C,of x 0.0054 at —78°C,and of x 0.0017 at —95.4°C. Strong temperature dependence has also been reported for the efficiency of ultraviolet transfer in natural calcium fluorite [65, 74]. The efficiency is the order of 50 % greater at 100°C than at room temperature, the effect getting stronger as the wavelength of illuminating light is increased. It has been suggested that the effect is associated with one or two excited levels of the donor trap, these levels lying a few tenths of an electronvolt below the conduction band. 2.3.6. Assessment of dose-rate The established system for fine-grain dating is to use thick-source alpha counting for evaluation of the uranium and thorium contributions and some form of chemical analysis (e.g. flame photometry, XRF, atomic absorption) for the potassium. For alpha counting the use of a disposable zinc sulphide screen remains supreme over other more sophisticated techniques on account both of cheapness and of low background. Using the a-value system [75] the conversion of the alpha countrate into effective alpha dose-rate can be made without knowledge of the stopping power of the sample or indeed of the decay chain details. However both of these are needed for evaluation of beta and gamma dose-rates and a reassessment of the energy release per parent nuclide disintegration has recently been made by Bell [76, 77]; this reassessment is based on published nuclear data tables and owing to the complexity of the decay schemes it is no mean task. The revised values [77] for the alpha, beta, and gamma contributions in millirads per year per ppm by weight of parent, are 73.8, 2.86, and 5.14 respectively for the thorium series, and 278, 14.6, and 11.5 for the uranium series. The beta and gamma dose-rates from 1 % of natural K2O are 68.2 and 20.5. For quartz inclusion dating the preferred method of dose-rate evaluation is by TLD and incidental to developing such a system for beta dose-rate evaluation Bailiff [65] has made an experimental check of the value derived by Bell from the nuclear data tables; there is agreement to within the 5 % limit of experimental error. For some pottery and soils the degree of escape of the radioactive gases Rn-220 and Rn-222 is sufficient to affect the dose-rates substantially. The earlier assumption that such escape does not occur on wet sites has been shown [78] to be invalid; it has also been shown [79] that the established method of alpha-counting in which the sample is sealed in a container can give rise to a gross overestimate of the activity if there is substantial radon emanation in the sample. A more satisfactory technique is to measure the alpha activity with the sample unsealed and to evaluate the gas escape in a separate experiment in which the sample is sealed in a gas cell such that the only particles which can reach the scintillator screen are from emanated radon. Confirmatory checks that laboratory assessment does give a reliable indication of the situation during burial can be

296

MI. Altken, Archaeological involvements of physics

made by radiochemical measurement of polonium-210; this is supported by 21-year lead-210 which is subsequent to radon in the uranium decay chain. 2.3.7. Environmental uncertainties However accurate the laboratory techniques and however well-behaved the TL minerals there remain several environmental causes of uncertainties. One of these is the change of radioactivity in a sample due to leaching or deposition by ground water; in geology uranium is well known for its mobility. Another is the attenuation of the dose-rate by the water content of sample and soil. Pottery commonly has a saturation water content of around 10 %, and soil 20 %; the average water contents during burial depends on the climatic history of the site and perhaps on water table variations, but even if known these are not easily translatable into water contents except when the site is known to have been excessively wet or excessively dry throughout. Even in the former case there remains doubt as to the extent to which water content affects the alpha dose-rate the pore structure of pottery is not necessarily fine enough for water to be interposed in the path of the majority of the alpha particle tracks [80, 81]. Specifically with regard to gamma dose-rate it has to be assumed that the sample has spent the major part of its burial time with the same surroundings as found by the excavator; quite apart from later erosion or other disturbance there is sometimes doubt as to how rapidly the sample became buried. There is also the question of how strictly the collector has kept to the requirement of a homogeneous surround of soil to a distance of 0.3 metre, a requirement that is often restrictive and irksome particularly if none of the samples on a site fulfill it. Hence there is strong reason to develop subtraction methods by which the gamma dose-rate is eliminated and at the same time to attempt to use impervious materials so as to avoid uncertainty due to water content. With any subtraction technique there is an inevitable increase in the size of the error limits, and with most samples, until evaluation of equivalent dose is more precise, what is gained on the swings is lost on the roundabouts. However for pottery samples in which the alpha particles contribute at least a third of the fine-grain TL Fleming and Stoneham [82] have demonstrated that dates accurate to ±12 % can be obtained by subtracting the equivalent dose obtained with quartz inclusions from the equivalent dose obtained with fine-grains. This removes dependence on beta and gamma dose-rates and besides allowing dating of pottery from burial contexts whichdo not meet the usual requirements it makes it possible to date objects from museum shelves, as demonstrated by application to four Renaissance terracottas [83]; however, besides the need for a strong alpha contribution there is also the requirement for a large enough sample (at least several cm3) to provide sufficient quartz. Several other subtraction methods have been proposed. Poupeau, Sutton, Walker and Zimmerman [44] suggest that for dating heated rocks from ancient fireplaces it is feasible to eliminate the gamma dose-rate by using two rocks of very different radioactivity, one high and one low; also, rocks have the advantage of low water content and low radon emanation. In application [97] to the paleolithic site of Pincevent in France the archaeological dose in quartz grains in a piece of granite was found to be 5600 rads whereas that in pieces of sandstone was only 1550 rads. Since the radioactive content of the sandstone was very low (< 0.2 ppm U, <0.6 ppm Th, <0.1 % K 20) its quartz was effectively acting as a monitor of the environmental dose (gamma plus cosmic) so that the net dose obtained by subtraction, (5600—1550), arose from only the internal radioactivity of the granite. In the “quartz attenuation” technique proposed [85], but not yet demonstrated, by Mejdahl and McKerrell the net quantity utilised is the difference in dose between inner and —

—

M.J. Aitken, Archaeological involvements of physics

297

outer parts of large quartz grains (about millimeter across) due to attenuation of beta radiation. The same authors also propose quartz-feldspar subtraction which utilises the difference in dose between large grains of potassium feldspar and large grains of quartz due to the beta contribution from internal potassium in the feldspar. Anomalous fading in the latter is likely to be a difficulty here. There is complete elimination of the need to make direct measurement of any radioactivity in the method DATE (Difference d’atténuation temporelle des emissions) proposed by Langouet et al. [86]. In this the dose-rate is evaluated by determining the equivalent dose indicated by a TL peak having too short a lifetime for linear accumulation. The ratio of this to the true equivalent dose (as indicated by a peak having a sufficiently long lifetime) can be used as a measure of the doserate as long as the too-short lifetime is known; effectively it is determined by measuring the equivalent dose ratio for a known-age series of similar samples. The basis of this method is most easily appreciated by recalling a simpler version of it that was used by Johnson [87] in dating contact-baked limestone. In this the lifetime of the lower peak was short compared to the age so that the peak was in thermal equilibrium at a level determined by ‘~

=

G1/r1

(2.6)

where x~is the (TL per rad) for this peak, G1 is the observed peak height, ~r1is its lifetime and D is the dose-rate. For a higher peak of long lifetime, X2DT

=

G2

(2.7)

where T is the age. Hence if x1 is known and the ratio Xl/X2 determined by artificial irradiation, T can be found. According to the preliminary results obtained by Langouet et al., the 325°Cpeak in quartz has a sufficiently short lifetime for their purpose. This conflicts with the earlier discussion under “Trap depth determination”; presumably different types of quartz are involved. 2.3.8. Zircon dating Zircon grains carry such a high concentration of uranium (typically several hundred parts per million) that the TL contribution from radioactivity in the pottery matrix in which they are embedded, and from external gamma rays, is barely significant. This powerful form of radioactive inclusion dating was first proposed by Zimmerman [88] and is under development at the Laboratory for Space Physics of Washington University, St. Louis, Missouri; its validity for archaeological dating has been tested by Sutton and Zimmerman [89] with zircons extracted from half-a-dozen pottery fragments of known age and its utility in authenticity testing has been demonstrated by application to the ceramic Qore of the famous Bronze Horse of the New York Metropolitan Museum of Art [90]. In its oiiginal form this technique involved measurement of TL sensitivity and alpha dose-rate as in conventional TL dating. Single grains were dated individually, alpha radiation being used for sensitivity measurement and induced fission tracks for the uranium and thorium content. However, although for some grains the age obtained was correct, for others it was substantially too low. On investigation this was found to be due to anti-correlation between TL sensitivity and radioactivity, the spatial distribution of the former being mapped by means of cathodoluminescence and of the latter by means of induced fission tracks. The TL sensitivity measured by artificial irradiation is dominated by the regions of high sensitivity and since these are remote from the uranium and thorium the value obtained is not relevant to age calculation. For obvious reasons the mapping cannot be done until all TL measurements on a grain are

298

Mi. Aitken, Archaeological involvements of physics

complete and hence the occurrence of zoning in a substantial proportion of grains means that there is a great deal of wasted effort. To circumvent this the so-called ‘natural method’ is being developed: after measurement of the archaeological TL, GN, for a grain, or a group of grains, the sample is stored for a time, t, of the order of six months after which the re-accumulated TL, GR, is measured again; the age is given by T

=

GNt/GR.

(2.8)

In concept the method could not be simpler but the experimental difficulties are severe. Apart from the need for high detection sensitivity in order to obtain a statistically meaningful number of counts, say a thousand, in the measurement of GR, it is vital to eliminate spurious TL which without extreme precaution is liable to mask GR. Quite apart from measurement problems there is the ever present difficulty that some grains exhibit anomalous fading. Nevertheless zircon dating is potentially of high importance just because it avoids uncertainties arising from water content and environmental radiation. Also, in the natural method it avoids the assumption that TL sensitivity is independent of dose-rate. In an attempt to reduce the storage time required for measurement of GR, Mobbs [91] investigated the possibility of storage at liquid nitrogen temperature so that lower temperature peaks, which are an order of magnitude more sensitive, could be utilized; the re-growth of the higher temperature “dating” TL would then be inferred by measuring the ratio of high temperature TL to low temperature TL following artificial irradiation. Unfortunately this ratio was found to be different from grain to grain and consequently it seems likely that it will be different between different sensitivity zones within a grain, in which case the approach is invalid. 2.4. Thermally stimulated exoelectron emission (TSEE) Many substrates that exhibit TL also show electron emission at about the same temperature. It seems that electrons are able to escape despite the fact that their kinetic energy is insufficient to overcome the surface potential barrier; it is presumed that these electrons are released into the conduction band from traps very close to the surface. The effect is measured by heating the sample inside a Geiger-Muller counter. A difficulty is that the sensitivity is highly dependent on surface condition and liable to change with heating. Rather surprisingly, in preliminary reports of investigation into dating application it is claimed that there is no effect analogous to spurious TL, i.e. there is no appreciable TSEE signal from unirradiated material.

3. Archaeomagnetism At the present time the declination (D) for London changes by roughly 1° every decade, becoming less westerly. The angle of dip or inclination (I) also changes and the secular variation of these two elements has been recorded for London, Paris, and Rome over the past four centuries from observations on suspended magnetized needles see fig. 3.1. This information is taken from Bauer [101], who collated early magnetic data from various parts of the world, and from later records. Besides the written record, this information is also stored in baked clay which has remained in position on cooling down from firing by means of thermoremanent magnetism (TRM) which —

M.J. Aitken, Archaeological involvements of physics 58

~

I’

60• 621800

f

64—

~.

66

— Rome

~‘

-

~

72 l800~\

~

1700 76 78

299

London

Paris

-

i

24

I 20

I 16

West I I 12 8 4 0 Declination (I)) I

East I

4

I

I

I

I

8

12

I I

16

Fig. 3.1. Secular variation from historical records — London, Paris, Rome and Boston. The time scale is indicated by dots at 20-year intervals. Prior to 1900 the curves shown are those obtained by Bauer [101]using recorded observations to determine an empirical formula; Bauer’s extrapolations into periods when only declination was measured have been omitted.

is induced by the geomagnetic field in iron oxide minerals in the clay as it cools from 700°C. This means that the baked clay acquires a weak but permanent magnetization in the same direction as the field at the time of cooling (and in the case of successive firings it is the last cooling that is effective). Clay has been used for thousands of years in the construction of hearths, ovens, and kilns. Hence whenever such a structure can be dated by reference to archaeological chronology, or by radiocarbon or thermoluminescent dating, there is the opportunity to obtain otherwise irretrievable geophysical information. Conversely, once the secular variation curve has been thus extended backwards into archaeologically interesting periods, the direction of remanent magnetization found in a structure can be used to establish its date. Unfortunately the secular variation curve is different for different regions of the earth’s surface so that calibration with known-age structures needs to be done for each region in which “magnetic dating” is to be used. Nor is the behaviour regular and although for Europe during the period covered by fig. 3.1 the data are approximated by incomplete ellipses, in earlier centuries (see fig. 3.3) the form is quite different. Although orientated samples that have remained undisturbed since firing are the primary source of data about the ancient geomagnetic direction, bricks and tiles can be used for determination of the ancient angle of dip since the requirements of stacking in a kiln usually ensures that one face or edge is horizontal (and irregularities in the stacking can be averaged out by measuring a large number of samples). The angle of dip can similarly be found from whole pots if the form is such as to necessitate that they should be baked standing upright. This is the case, for example with Chinese Yueh ware on account of the heavy glaze and ornamentation [102], but unfortunately

300

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the more usual technique in other cultures was to pack the pots in the kiln rather irregularly. It is also possible to determine the ancient intensity of the geomagnetic field by measurements on baked clay, whether in situ since firing, or as bricks, tiles, or fragments of pottery. This is because the strength of the magnetization acquired is proportional to the field intensity. If a sample is reheated and allowed to cool in the present day field, the ratio of the original magnetic moment to the new moment gives the ratio of the ancient intensity to the present day intensity. Since records of direct measurement of intensity are only available over the past two centuries this type of archaeomagnetic data is of particular interest geophysically as well as for radiocarbon dating (see section 4.2.1). Reliable results are not easy to obtain owing to the possibility of mineralogical changes occurring during the reheating. In such circumstances the mineral being re-magnetized may have different characteristics to the original mineral and this means that the constant of proportionality to field intensity will have altered. Another similar possibility is mineralogical change due to weathering in antiquity. The necessary techniques and criteria for reliable results were established by Thellier [103] and their first results showed that substantial variations had indeed occurred in the past they found that in Roman times the intensity in France was some 60 ~ higher than its present-day value. This result was accepted with some reluctance at the time, on the grounds that even if the geomagnetic dynamo could exhibit strong and rapid variations despite its inertia, the conductivity of the earth’s mantle would prevent corresponding variations being observed at the earth’s surface. However recent research suggests that variations of this magnitude sometimes occur within a few centuries. This is in line with the general indications from paleomagnetic investigation of geomagnetic reversals that the geomagnetic dynamo can display much greater agility than had been thought hitherto. Paleomagnetism is usually taken as referring to geophysical and geological applications whereas archaeomagnetism is reserved for applications in which there is some involvement with ancient man. The distinction between the two is becoming increasingly blurred both because of a growing appreciation on the part of geophysicists of the detailed magnetic data that archaeological remains can provide and because of the finding that the variations of D and I are discernible in cores taken from the sediments deposited in still water at the bottom of inland lakes. —

3.1. Remanent magnetism Thermoremanent magnetism results from the presence in the clay of fine grains of magnetite (Fe3O4), haematite (~—Fe2O3),and associated ferrimagnetic minerals; some grains, particularly those of haematite, consist of a single magnetic domain while others are multi-dominated. The detailed magnetic mechanism, which derives from the classic exposition of Néel [104], is complex and far from fully understood but phenomenologically the bulk behaviour can be described in terms of blocking temperature, TB, and coercive force, H~.Above its blocking temperature a grain is superparamagnetic, i.e. its thermal energy is large enough for its domain or domains to make transitions between different configurations with respect to the external field whereas when it cools below T~the configuration is fixed. The relaxation probability is a very sharply rising function of temperature so that a few degrees above TB realignment is possible within a fraction of a second wheras a few degrees below TB realignment only takes place within a very long time. The value of TB depends on the material of the grain and its size; in a typical baked clay there is a wide distribution from the Curie point downwards. Obviously those with the highest TB have the

M.J. Aitken, Archaeological involvements q[physics

301

“hardest” magnetization, being stable over millions of years, whereas those having a low TB, say below 150°C,are said to be -“viscous” in the sense that if the direction of the external field is altered the magnetization of these grains will gradually realign to the new direction. The grains are widely distributed in their coercive force values too but for grains having TB above 20°Cthe coercive force is usually large compared to the geomagnetic field. Some grains are so hard that for demagnetization an alternating field of the order of 0.1 tesla (10~oersted) is required, whereas for others 103_10_2 tesla is sufficient. Although an alternating field of the order of 10.2 tesla may remove a substantial part of the viscous component (VRM) of a sample, it is to be noted that some grains have high H~and low TB, and others have low H~and high TB. Detrital remanent magnetism (DRM) occurs in sediments deposited in calm water. The magnetic particles present inherit a remanence from the rocks from which they have been eroded and tend to be aligned by the geomagnetic field as they settle. In sediments rich in organic matter the dominant remanence is post-depositional (PDRM). Some of this is a chemical remanence (CRM), acquired for instance when haematite is formed from iron hydroxides precipitated in the sediment, alignment occurring as the grains are formed. However at any rate in some cases [262] the more important mechanism is probably grain rotation while the sediment is in the form of a slurry the grains subsequently being locked in position as compaction and drying-out occurs; sometimes gel-formation also plays a part. Whatever post-depositional mechanism is dominant, the direction recorded will correspond to a later time than the actual date of deposition. This delay may sometimes be as much as a century and it results in the smearing-out of the record due to lack of time resolution. The remanent magnetism of baked clay and lake sediments is a small part of the much wider field of rock magnetism; various reviews are available of which a recent one is by Stacey and Bannerjee [105]. Shear, or shock, remanent magnetism (SRM) is the name given to a remarkable form of magnetic remanence recently discovered in the course of archaeomagnetic studies of adobe sun-dried mud bricks that were proposed by R.L. Wilson and carried out by Games [106]. The remanence is acquired at the instant when the mud is thrown into the wooden mould that shapes the brick; the process has been simulated in laboratory experiments. The specific magnetization acquired is an order of magnitude weaker than for TRM but it is of comparable stability, an alternating field of about 0.1 tesla (1000 gauss) being needed for its demagnetization. Consequently when the brick is moved after being thrown, it retains its hard SRM although a VRM component may subsequently grow at the expense of the soft component of the SRM. For recently made adobe brick from Peru the VRM accounted for 25 % of the natural remanent magnetism (NRM) while for a 3000-year-old brick from Egypt it accounted for 60 %. However in both cases the VRM could be removed by alternating field demagnetization in 0.06 tesla so that a substantial part of the SRM could be isolated intact. SRM is proportional to the geomagnetic field in which the brick is being thrown, at any rate within the range 20 to 30 microtesla and its exploitation for ancient intensity determination holds high promise (see section 3.4). Remanent magnetization has also been reported as being acquired by brass coins when they are struck. Investigating a series of fourth century roman coins Tarling and Cope [107] found that the remanent magnetization vector made an angle around 80°with the plane of coin and that the polarity usually corresponded to the coin having been struck with the emperor’s face downwards, as was the numismatist expectation the striker considering it disloyal to hit the face directly. —

302

M.i. Aitken, Archaeological involvements of physics

3.2. Instrumentation The astatic magnetometer is the classic instrument of paleomagnetism; it consists essentially of two bar magnets of equal strength fixed rigidly at either end of a rod about 10 cm long and suspended by a fine fibre of phosphor-bronze or quartz. Since the two magnets are antiparallel the net torque due to the earth’s field is zero. On the other hand, the torque on the lower magnet due to a small sample positioned beneath it is substantially greater than that on the upper one and the resultant deflection is a measure of the horizontal component of the magnetization of the sample. A good quality instrument can detect samples having total moment of the order of 10-8 joule per tesla (10-s gauss cm3). Discussion of its fundamental sensitivity capability has been given by Blackett [108]. Until recent years it was unrivalled in this respect; an instrument of particularly high sensitivity, able to measure samples having a magnetization of only 5 x 10_b joule per tesla, has been described by Pozzi and Thellier [109]. The electromagnetic induction spinner magnetometer is more suitable for the large non-uniformly magnetized samples used for directional measurements in archaeomagnetism. The pick-up coils of such an instrument have to be complex in order to give uniformity of detection over the volume of the sample and rejection of interference by external fields due to passing cars etc. In the largest instrument of this type yet built [110] samples having dimensions up to 30 cm across are spun at 5 revs per sec; the minimum detectable moment is about 10—6 joule per tesla (10-s gauss cm3), which in the absence of interference rejection coils would correspond to an external 5 herz field of about 10—8 A/m (10-10 oersted). For small samples an ultra sensitive spinner magnetometer has been developed by Jelenek [111]. Fluxgate spinner magnetometers are now widely used in paleomagnetism; the instrument developed by Foster [112] has a minimum detectable moment of the order of 10~joule per tesla (10 6 gauss cm3) for a sample of volume 10 cm3. A particularly flexible and convenient development of the fluxgate spinner is the use of an on-line computer for signal analysis. This is the “Digico” magnetometer developed by Molyneaux [113]. In its archaeomagnetic version it can accommodate samples up to 5 cm across, it has a noise level equivalent to a total moment of 10—6 joule per tesla (10-s gauss cm3) for a measurement time of half a minute; in the standard version used for 10cm3 samples the noise level is lower by one or two orders of magnitude. Magnetometers based on nuclear magnetic resonance are not suitable for this type of measurement because they require the field due to the sample to be fairly uniform over the volume of the detector. This means that the sample would have to be at an appreciable distance from the detector and in consequence the sensitivity obtainable would be poor. At the other end of the scale of sophistication is the technique in which the sample is suspended on a fibre (or floated on a pool of mercury) and allowed to orientate itself in a uniform magnetic field; the procedure is simple but slow. The SQUID cryogenic magnetometer This Superconducting Quantum Interference Device utilizes the Josephson effect and operates at the temperature of liquid helium. Readers unfamiliar with the utilization of this device for magnetometry are referred to the introductory exposition by Swinthenby [114]; its specific use in paleomagnetism has been reviewed recently by Goree and Fuller [115] and utilization in archaeomagnetism for measurement of ancient intensity has been reported by Walton [116]. In sensitivity and response time SQUID magnetometers are superior to existing types of magneto-

Mi. Aitken, Archaeological involvements of physics

303

meter by one or two orders of magnitude; further, in contrast to the spinner magnetometer it can measure a steady field and this allows a new approach in some applications. An incidental advantage of working at low temperatures is the availability of superconducting shielding and the high sensitivity would not otherwise be utilizable. In palaeomagnetic application the minimum detectable total moment for a standard 10 cm3 sample is the order of 3 x 10_li joule per tesla (3 x 10-8 gauss cm3). Figure 3.2 shows the magnetometer developed by Walton [116] for archaeomagnetic intensity measurements at the Oxford Research Laboratory for Archaeology. The sample, a squat core of pottery 2 or 3 mm across, is placed on top of a quartz rod. On raising the rod so that the sample is between the measuring coils the change of flux linking the set having a vertical axis is a measure of the vertical component of the sample’s magnetization. On rotating the rod the amplitude of the flux variation through the set of coils having a horizontal axis is a measure of the horizontal component. The horizontal and vertical coils are connected in series to the field coil of a single SQUID; this arrangement reduces the electronic complexity compared to a 3-SQUID arrangement, one for each component, and is ofcourse less costly. The instrumental noise level is equivalent to a minimum detectable moment of 10_il joule per tesla (10-8 gauss cm3) but in the laboratory environment in which the device is situated the present practical limitation arises from magnetic interference, despite the superconducting shield, at about a factor of four higher than this. The small sample size permits unobtrusive coring of museum exhibits as well as advantageous developments in the technique of intensity measurement (see section 3.4.1). 3.3. The ancient geomagnetic direction 3.3.1. Sampling procedure; magnetic distortion It is now well-established that in good circumstances, and with care and appropriate precautions, measurement of the remanent magnetization in a fired archaeological structure (such as a pottery helium

container liquid N2 CQolin9 P shijld room temp. wall liquid He coaxial line SQUID measuring _______

I

thermocouple

re-entrant cavity Fig. 3.2. Schematic diagram of SQUID magnetometer as used for archaeomagnetism by Walton [116].

304

Mi. Ait ken, Archaeological involvements of physics

kiln) can yield the ancient direction with a precision and accuracy of the order of 1°;the conditions that constitute good circumstances will become apparent in the course of discussion. The evidence that the remanent direction does indeed represent the geomagnetic field direction at the time of firing is from the basic work of Thellier [117], from measurements on experimentally-fired replica kilns [118, 119] and by measurements on ancient kilns lying within the period in which ancient scientists were recording magnetic data [120, 121]. Obviously, quite apart from instrumental capability, an essential requirement is that an accurate indication of orientation is attached to each sample before it is detached from the structure and taken to the laboratory for measurement. To do this a stump of baked clay is isolated (except at its base) using tools such as knives, chisels and hacksaw blades. This stump is then enclosed in a plaster mould of which the top surface is made horizontal and a line is sighted onto this surface from a theodolite of which the azimuthal scale has been related to geographic North by means of a sun-shoot. The use of a theodolite rather than a magnetic compass is preferred because of the risk that the reading of the latter will be distorted by the proximity of the magnetic material of the kiln in general and of the sample on which it is placed in particular [122]. The horizontal surface needs to be 3 or 4 inches across in order that a line scribed on it will yield the orientation to within a fraction of a degree. Another factor that hinders the use of small samples is the often “craggy” nature of the baked clay in a kiln; also, its general heterogeneity gives the possibility that small scale magnetic distortions will accompany the larger scale ones mentioned below. Thus an instrument for archaeomagnetic directional measurements needs to be able to accommodate larger samples than an instrument for paleomagnetic measurements; poorer angular precision is tolerable with the latter, and also, the material being sampled is usually homogeneous. Returning now to the question of “good circumstances” one obvious requirement is that the structure has not slumped since it last cooled down from firing. This consideration is particularly relevant to the walls of a pottery kiln. From the systematic deviation in ancient direction observed in rings of samples taken from around the walls of circular Romano—British pottery kilns, Harold [123] hypothesized that there had been outward tilting of the walls, typically by about 3°.The deviation was first harmonic with azimuth; I was too steep in northerly parts of the circumference and too shallow in southerly parts, D was too westerly in eastern parts and too easterly in western parts. The amplitude of the deviation for D was approximately twice that for I. However the explanation in terms of wall tilting was discounted when the same distortion pattern was found by Weaver [119] in an experimentally-fired replica kiln for which vat ious mechanical monitoring devices had indicated that no significant movement had occurred. This experiment does not rule out the possibility that in ancient kilns significant movement may have occurred but it does suggest that the first harmonic deviation pattern is due to magnetic distortion i.e. demagnetizing fields from the baked clay itself. The simple expectation for such distortion is that the magnetic lines of force will be “refracted”, the demagnetizing field being perpendicular to the surfaces of a parallel-sided sheet of magnetic material the tendency is for the lines of force to deviate so as to run more parallel to the surface. Such refraction gives rise to second harmonic distortion declination too westerly in the N.E. and S.W. quadrants, and too easterly in the N.W. and S.E. quadrants. In studying a series of circular pottery kilns Weaver [124] found first harmonic deviation to be dominant in some and second har-monic to be dominant in others; no model that explains the former has so far been proposed. Another expectation from consideration of magnetic refraction is that the remanent angle of dip will be a little too shallow in the floor and a little too steep in the wall. In a group of 6 kilns considered by Aitken and Hawley [125] —

Mi. Aitken, Archaeological involvements of physics

305

there was in fact a difference 3°or 4°between walls and floors (see tabie 3.1); quantitatively this is what would be expected if the specific magnetization was about i0~ A/rn2 per kilogram (10~gauss cm3 per gram). Whatever the cause, the deviations observed in practice indicate that comprehensive sampling of a structure is essential for reliable results; a dozen samples should be regarded as the minimum. To the extent that magnetic distortion is responsible there is advantage in choosing less strongly magnetized structures, but on the other hand poorly-fired remains such as prehistoric hearths are more likely to have suffered physical disturbance; to the author it has often seemed as though ancient man did a war dance on the hearth after its last use. Table 3.1 Comparison of remanent inclination in walls and floors of six pottery kilns (from ref. [125]) Walls Kiln

Mancetter/1 Mancetter/2 Mancetter/9 Gloucester Thetford/46 Thetford/115

Floors

—.~---.——_---——~

—~

No. of samples

Iw

12 12 7 15 22 29

66.3 62.2 68.1 67.0 66.6 67.7

±1.2 ±1.2 ± 1.5 ±0.9 ±1.0 ±1.0

—

No.of samples

‘F

5 10 7 13 11 12

62.7 60.9 63.2 62.7 63.6 63.2

±0.4 ±1.6 ±1.0 ±0.4 ±1.3 ±0.6

+3.6 + 1.4 +5.0 +4.3 +2.9 +4.5

± 1.3 ±2.0 ±1.8 ±1.0 ±1.7 ±1.2

3.3.2. Magnetic cleaning Removal of the viscous component (VRM) acquired by the grains having low blocking temperatures is most effectively achieved by thermal demagnetization i.e. heating the sample to 100°—150°C, letting it cool in zero magnetic field, and storing it there until measurement. In baked clay the VRM is typically around 5 % of the TRM; according to Weaver [124] the major part of the VRM of 2000-year-old samples is removed by heating to 100°C.The hardness of the VRM increases with age but because growth is proportional to the logarithm of the time the contribution of the hard VRM is relatively unimportant and the main part of the VRM is acquired within the last few hundred years. It may be noted incidentally that in principle this increase of hardness with age provides a dating technique. Practical realization has been demonstrated by Heller and Markert [126] in respect of building blocks from the 1800-year-old Hadrian’s Wall in the north of England; however because of the logarithmic dependence on time the accuracy obtainable is inevitably poor. Viscous magnetization can also be substantially eliminated by alternating-field demagnetization; typically a field of 0.01 tesla (100 gauss) would be used, though a more thorough approach is to use successively increasing fields until the remanent direction ceases to shift. However because a fairly wide range of blocking temperatures may correspond to a given coercive force there may be appreciable removal of TRM before a field sufficiently strong to remove all VRM has been reached. 3.3.3. “Magnetic dating” Directional results have been obtained in various parts of the world and a comprehensive list of references will be found elsewhere [14]. The secular variation curves obtained for England,

306

M.J. Aitken, Archaeological involvements of physics

the Ukraine, and Japan are shown in figs. 3.3, 3.4 and 3.5. From the point of view of magnetic dating the curves for England and Japan illustrate one difficulty that the same direction repeats itself from time to time. However there are restricted periods, such as AD 1300 to AD 1600 in England, when the change of direction was so rapid that magnetic dating can give a more precise date, say to within ±20 years, than any other scientific technique except dendrochronology (tree-ring dating), as long as there are other indications that the structure being dated should lie within those three centuries. Of course, the dating is not absolute since the time-scale of the reference curve is based on existing archaeological chronology. This reference curve needs to be established for each region of about 1000 kilometres across in which it is to be used for dating; the poorer the existing chronology the greater the need for magnetic dating, but greater too is the difficulty of establishing the reference curve. The curves shown represent work over a number of years. It is slow not only because of the need for comprehensive sampling and measurement but also because the number of structures being excavated in any one year is limited. Further, although for some structures the accuracy is good and the uncertainty in the average direction is less than 1°,for an appreciable proportion the uncertainty is 3°or 4°,or even worse; the change in direction over a century is sometimes only 5°or 10°so that for the latter category the value of the data obtained is marginal. —

3.3.4. Geophysical implications The archaeomagnetic results shown in figs. 3.3, 3.4 and 3.5 show that in short-term at any rate the secular variation does not follow any simple worldwide pattern. In particular the curve for England disposes of the nineteenth century notion that the direction repeatedly traces out an ellipse with a period of about 450 years, on the basis that the geomagnetic moment was precessing

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M.J. Altken, Archaeological involvements of physics WEST

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around the earth’s axis of rotation with an inclination of about 100. This notion is contrary to the now-established geophysical presumption that the secular variation is mainly due to transient irregularities, near the core-mantle boundary, in the dynamo current system in the core that generates the main geomagnetic field.

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308

Mi. Aitken, Archaeological involvements of physics

Compared to the precise and detailed knowledge that we have of present-day spatial variations the data available on time variation is inadequate and fragmentary. However over the last three centuries or so it is well established that the non-dipole components of the field, i.e. what is left after subtraction of the field due to a dipole at the centre of the earth, show a tendency to drift westward at about 0.2°of longitude per year. This westward drift was first discussed by Halley [127] who noted among other indications that the position of zero declination in the South Atla~tic had moved 23°to the West in 90 years. Detailed analysis by Bullard et al. [128] of the harmonic components of the field in 1907 and in 1945 give a value of 0.23°per year for the drift and various other analyses indicate that a drift-rate in the range 0.1°to 0.2°per year has been going on since A.D. 1600. The geophysical interpretation of this drift is that it is due to relative motion between the outer part of the core and the mantle the mantle rotates faster than the core and so magnetic features associated with irregularities in the current pattern in the core appear to an observer on the mantle to be drifting westward. Hence the past secular variation at a particular station should be given by the present-day spatial irregularities observed on looking westward along the line of latitude running through the station. Obviously if the form of the irregularity remained unchanged for long enough then the secular variation would repeat itself each time the mantle made one revolution relative to the core. However, the lifetime of the irregularities is expected to be only of the order of a few hundred years and so the correlation should not extend back for more than that length of time. For London there is in fact a credible correlation for the past few centuries, as was pointed out by Bauer [1] in 1899, but before that the archaeomagnetic data show as expected a complete breakdown of correlation [254]. Clockwise curvature of the secular variation curve accompanies the presently observed westward drift and detailed analysis [129, 130] shows this to be generally true for the harmonic components of significance in the present day geomagnetic field. However it has been suggested that there could be combinations of harmonic components such that the curvature is anticlockwise although the drift is westward; if so the portions of figs. 3.3 and 3.4 showing anticlockwise curvature do not necessarily imply a reversed direction of drift. Of course the ideal way to derive information about the direction of the drift is by comparison of archaeomagnetic data from different parts of the world. Comparative data is at present rather too sparse for a clear cut conclusion but the analyses of Yukutake [131] suggest that for the past thousand years the drift has always been westward (partially contradicting the inference drawn from fig. 3.4). This is also the viewpoint of Burlatskaya, Nechaeva and Petrova [132] who point to the observed retardation of the last maximum of the angle of dip as one moves from Japan (last maximum circa A.D. 1300) to Britain (last maximum A.D. 1700). The way in which irregularities in the current pattern might be generated has been considered by Hide and Malin [133] who suggest that they arise from the relative motion through the core of protruding bumps on the mantle at the interface, a kilometer or so high. Support for this is given by the observed correlation between features of the non-dipole field and irregularities in the gravity field of the earth. The best fit for correlation is obtained by rotating the present non-dipole field eastward by 160°of longitude; on account of the westward drift of the field it is conjectured that 500 or 600 years ago the best fit for correlation would have been obtained with zero longitude displacement and that the main features of the non-dipole field might have been generated at this time. It is possibly significant that the change from clockwise to anticlockwise curvature in fig. 3.3 occurred then too. —

—

—

M.J. Aitken, Archaeological involvements of physics

309

Although the archaeomagnetically observed secular variation curve observed for Japan bears little resemblance to that for England, when the data are expressed in terms of virtual pole positions there are some broad trends in common (see fig. 3.6). The virtual pole position for an observing station is the orientation of a dipole placed at the centre of the earth which would produce a magnetic field at the station having the observed values of D and I. Kawai and Hirooka [134] have calculated the virtual pole motion that best fits the archaeomagnetic data for England, Japan, and Arizona, and the volcanic lava data for Iceland. They propose as a working model that over the past 2000 years the representative dipole has executed a “quasi-hypotrochoidal” movement around the rotational axis of the earth; this movement is a basic anticlockwise precession of the dipole around the rotational axis, at an inclined angle of 11°and with a period of 1500 years, together with a superimposed smaller and more rapid clockwise precession, at an inclined angle of 7°and period 400 years. When both motions reinforce there is a net rapid anticlockwise motion, and when they are in opposition the motion is slow and clockwise. Because of the limited time-span of the data used the model is bound to be approximate, and indeed because of the sparseness of the observing stations the basic inference of worldwide correlation could be 180°

1300

250 1300,” 70° 1900

I

—-.--

/ / 1500

\

Il

80°

1600i

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1000

~6OO /

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00

00 Fig. 3.6. Virtual pole positions derived from archaeomagnetic data for England (solid line) and for Japan (dashed line). The virtual pole for an observing station is the orientation of a dipole at the centre of the earth such that the magnetic field would have the values of I and D observed at the station. Although the pole positions do not superimpose, it is to be noted that there is a broad similarity in the movement. Data used are from Aitken [251] and Hirooka [253].

310

Mi. Aitken, Archaeological involvements of physics

due to coincidence. Even if there are features in the secular variation that are due to dipole precession it would be surprising if there were not also features of comparable importance arising from non-dipole disturbances; the present day geomagnetic field contains features that imply a nondipole field vector of the order of 40 % of the dipole vector. 3.4. The ancient geomagnetic intensity

The primary interest in ancient intensity is the correlation of its variation with distortions of the radiocarbon time-scale, as will be discussed further in section 4.2.1. Figure 3.7 shows the summary of worldwide data on which this discussion is based. It is to be noted that the high values for circa 6000 B.C. are from only one region (Turkey) and much further work is necessary before we can be confident that the variation earlier than 2000 B.C. is indeed on a worldwide basis. The correlation with the distortion of the radiocarbon timescale gives support but there is danger here of circular argument. In view of the strength of the non-dipole field vector implied by the present-day spatial distribution mentioned above it is to be expected that in a given region there will be deviations of the order ±40 % from the worldwide average. In fact the intensity data for Central Europe shown in fig. 3.8 contains such deviations; these are unlikely to be due to a tilting of the main dipole towards or away from the measuring station since a tilt of 10° produces an intensity change of only 13 % at latitude 35°and less elsewhere. The sharp peak in the Central European data centred around 500 B.C. suggests the possibility that in certain periods intensity measurements could be used as a dating tool. As with the directional

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T 6000

T

7000 AD. B.C. Fig. 3.7. Variation of the geomagnetic dipole moment according to archaeomagnetic data — from Cox [255] with additional data. MA is the ancient value of the reduced dipole moment derived using the present-day geomagnetic colatitude of the site from which the sample was obtained; M0 is the present-day value of the dipole moment from spherical harmonic analysis. The data have been averaged over 500-year intervals and where shown the vertical bars represent the standard error of the mean value; the numerals mdicate the number ofdata contributing to each interval; the letters indicate the countries in which the sites were located: A — Central Asia, B — Bulgaria, Bo — Bolivia, C — Czechoslovakia, E — Egypt, F — France, I — India, J — Japan, M — Mexico, P — Peru, R — Russia, S — Switzerland, T — Turkey, Tu — Tunisia, U — United States.

M.J. Aitken, Archaeological involvements of physics

1000

A 1) B

(•

100(1

311

2000

Fig. 3.8. Comparison of geomagnetic intensity variation observed for Central Europe with variation of resultant intensity obtained by adding effect ofdipole movement to effect of variation ofdipole strength. (FA/Fo) is the ratio of ancient intensity to present day intensity at the sampling site. Open circles joined by straight lines are data for Central Europe (from Bucha [256]); dashed curve is 1 + 4 sin (2iv/8000)}, an approximate fit to the worldwide average data of fig. 3.7; the full curve is the resultant after addition of intensity variations due to dipole movement predicted by the model of Kawai et al. [134].

dating a pre-requisite would be the establishment of a reference curve for the region concerned but there is the advantage that intensity measurements can be made on pottery fragments and bricks, thereby increasing the availability of suitable material by several orders of magnitude. Inevitably the difficulty of finding material reliably dated with sufficient precision will remain. Of course the method would only be useful when used in conjunction with another technique, such as radiocarbon or thermoluminescence, by which the unknown sample could be dated to within a couple of centuries. The intensity measurement would then give greater precision as long as the period was one of rapid change. —

3.4.1. The Thellier technique

In the classic double-heating method of Thellier and Thellier [103] the sample is successively heated twice to each of a set of increasing temperatures (e.g. 100°C,200°C,300°C,400°C,etc.) and its moment measured, at room temperature, after each heating. If the first heating (and cooling) is carried out in zero field and the second one in the known field of the laboratory then the decrease of magnetic moment after the first heating to 300°C(say) represents the ancient magnetization carried by grains having blocking temperatures in the interval 200°to 300°Cand the increase of moment resulting from the second one represents the laboratory magnetization acquired by that same set of grains. The ratio of the decrease to the increase gives the ratio of the intensity of the ancient field to that of the laboratory field. A value for this ratio is obtained for each temperature interval and as long as the ratio remains the same as successively higher temperatures are reached it is presumed that no undesirable mineralogical changes have taken place. In practice the method is tedious and time consuming but, if carried out with meticulous rejection of samples for which the ratio changes, reliable results are obtained. There are three main causal categories to which a change in ratio can be attributed. First, the ancient firing may not have reached the Curie point or there may have been a second heating to a lower temperature than the Curie point. Secondly there may be mineralogical change because the atmosphere of the laboratory heating is not

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the same as that of the ancient firing; for instance, if the latter were in a reducing atmosphere then heating in air is liable to cause oxidation of magnetite to haematite. Thirdly, there may have been mineralogical change due to weathering in antiquity; for instance, hydrating conditions may have led to the formation, after firing, of goethite or lepidocrocite with very weak chemical remanent magnetization and when the laboratory heating takes place there is conversion back to haematite or maghaemite [157]. 3.4.2. The SQUID null change technique The high sensitivity of the SQUID cryogenic magnetometer permits the use of samples only two or three millimetres across and Walton [116] has developed a modification of the standard Thellier technique. Besides the advantages of minimizing damage to museum specimens and of being able to obtain sufficiently heated samples from partially heated stones from paleolithic hearths, the small sample size means that each heating and cooling can be done in less than a minute thereby minimizing the likelihood of mineralogical change. Quick heating also makes it practical to incorporate the oven in the magnetometer facility, so that the sample does not have to be remounted during the whole sequence of heatings and coolings; this sequence can be accomplished in less than an hour. The essential basis is that the field during cooling is adjusted until the magnetization gained on cooling from a given temperature to room temperature is equal to the ancient magnetization lost in that same temperature interval. Cooling in zero field is not utilized, but several heatings to the same temperature may be necessary in order to get the remagnetizing field adjusted so that there is no change in moment. Having got this field correct for, say, 250°C,at the next heating the temperature is increased to 300°C,and if the field is correct for this temperature too this is indication that mineralogical change has not intervened between 250°Cand 300°C;at the next heating a temperature of 350°Cis used, and so on until mineralogical change is manifested by the need to alter the remagnetizing field. The value of the remagnetizing field below the temperature at which change began gives the value of the ancient field and because of the high signal-to-noise ratio attainable with the SQUID an accurate (better than ±5 %) value can be obtained even though less than 20 % of the ancient magnetization has been removed. It has been assumed for the sake of simplicity that the remagnetizing field is in the same direction as the ancient magnetization; in practice it is not convenient to arrange for this to be so and the actual calculation is more complicated than the foregoing suggests. 3.4.3. Thermal remagnetization with coercivity spectrum monitoring Various intensity techniques have been tried in paleomagnetism in which alternating field demagnetization (and remagnetization) replaces heating. In the technique developed at Liverpool by Shaw [136] heating is retained but alternating field measurements are used to ascertain to what extent the magnetic characteristics of the sample have been changed by the heating. The steps are as follows: A. Measure the natural remanent magnetization (NRM) and repeat this measurement after alternating field (AF) demagnetization using successively increasing maximum fields. The plot of residual NRM versus maximum field utilized is effectively a coercive force spectrum. (In AF demagnetization the DC field is zero and the AF is smoothly reduced to zero from its maximum value; this randomizes all those domains having a coercive force less than the maximum field.)

M.J. Aitken, Archaeological involvements of physics

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B. Give the sample an anhysteritic remanent magnetization (ARM) by placing it in an alternating field which is smoothly reduced to zero while a steady DC field of about the same strength as the earth’s field is applied. The alternating field is much stronger. C. Measure the ARM and repeat after AF demagnetization using successively increasing maximum fields. The plot of residual ARM versus maximum field utilized gives the coercive force spectrum of the ARM. D. Now give the sample a thermoremanent magnetism (TRM) by heating it to above the Curie point (say 700°C)and letting it cool in a known field. E. Measure the coercivity spectrum of the NRM. F. Give the sample another ARM. G. Measure the coercivity spectrum of this second ARM. Comparison of G and C indicates which parts of the coercivity spectrum have been altered by the heating. These parts are then discarded from A and E and the ratio of the remaining NRM to the remaining TRM is used to evaluate the ancient intensity. In results reported so far it is the lower part of the coercivity spectrum that is most liable to change i.e. the grains most vulnerable to the effect of heat are those with low coercivity. No reason has yet been proposed as to why there should be this correlation. Be that as it may the technique is highly effective and the monitoring provided by the coercivity spectrum at last provides a way of separating good grains from bad grains from the point of view of intensity determination. On the other hand in its basic form the technique can only deal with samples for which the ancient firing was to a temperature above the Curie point and for which there has been no secondary firing to a lower temperature. The latter may reveal itself by a change in angle during part A above but inadequate primary firing will go undetected giving an erroneously low value for the ancient field unless variants of the procedure, in which different temperatures are used in part D, are introduced. 3.4.4. Ancient intensity usiiig sun-dried bricks It is surprising to find that a sun-dried brick carries a stable magnetization acquired at the instant the mud is thrown into the mould and even more surprising that this SRM (see section 3.1) can be used for determination of ancient intensity. Yet this seems to be well-established by the work being carried out at Liverpool by Games [106] who has expectation that the method will yield ancient intensities with better than 5 per cent accuracy. The steps in the determination are similar to those in the one just described except that instead of thermal remagnetization the sample is given a laboratory SRM. For this the sample is crushed and mixed with water in order to re-form it into mud. A piston and cylinder arrangement is then used to artificially throw this mud into a cylindrical mould. As in the Shaw thermal method, alternating field demagnetization is used to check whether there has been any change in the coercivity spectrum due to the remagnetization procedure and it appears that the resetting of SRM can be done, with far less risk of change of magnetic properties than is the case when thermal treatment is involved. It appears too that the laboratory procedure for remagnetization satisfactorily replicates the original throwing into a mould by the brickmaker. In the preliminary results so far reported by Games four samples from a Peruvian adobe brick made in AD 1975 give an average value for the field of 29 ±2 microtesla, the error being the RMS deviation from the mean; this is in excellent agreement with the expected value of 28 microtesla. Other results reported are for Peruvian bricks made in AD 1450 and AD 700 and for an 18th

314

M.J. Aitken, Archaeological involvements of physics

dynasty (1440 B.C.) brick from Egypt. The ancient intensities obtained are in agreement with the worldwide average values (fig. 3.8). 3.5. Lake sediments The magnetic record carried by varved clays has been studied for some decades [137, 138] and deep-sea cores have played a major role in the detection of geomagnetic reversals [140]. However with marine sediments the time-resolution obtainable is usually poor because of the slow rate of deposition. Recently it has been found that the more rapidly deposited sediment at the bottom of inland lakes carries a reliable record and data on the long-term features of the secular variation of direction are being obtained. The data are not as precise or as detailed as that obtainable from archaeological structures, but because the data can span a dozen millennia periodicities are discernible which cannot be seen in shorter term studies. The record is presumed to be postdepositional in mechanism though as mentioned earlier the extent to which it is due to chemical change and the extent to which it arises from grain rotation is not definitively established. Figure 3.9 shows the declination and inclination values found [141] from long cores (of diameter 6 cm) taken from the bed of Lake Windermere, England. The upper meter spans the last 2000 years and it will be seen in the figure that the declination is broadly consistent with the historical and

Declination 100 Intervals West East

Inclination 400

800

~

:~‘

.

—

2600 ~9o0 ~ -<

.j

...

.

—

690t) ~l20(}

‘—10100 ..•. •.,

4

.

—11000

Fig. 3.9. Declination and inclination values (.) obtained for sediment cores from Lake Windermere, together with secular variation curves from historical recording and archaeological structures. The sediment data are from Thompson [141] plus later additions and are composite points from several cores; the declination scale is relative rather than absolute and the age scale for the curves has been adjusted so as to match the first westward and eastward maxima of the sediment declination values.

M.J. Aitken, Archaeological involvements of physics

315

archaeomagnetic values taken from fig. 3.3; the failure to record the low inclination of circa AD 1300 has not been fully explained though compaction of the sediment as the core tube is driven into the lake bottom is a possible reason. Indication of the declination variation of the last four centuries has also been found [142] in some, though not all, of a large number of cores taken from the upper meter of the sediment from Lake Geneva. Besides the rather limited comparison with historical and archaeomagnetic data, evidence that lake sediment does give a valid recording of the ancient declination is provided by the common period of about 2800 years that is found in sediments from various lakes of N.W. Europe spanning the last 11000 years as well as the approximate simultaneity of the radiocarbon ages assigned to the easterly and westerly swings; the evidence has been reviewed by Thompson [143]. Besides direction, the remanent intensity of magnetization is also measured. The more conservative interpretation of the latter is in terms of variation of the magnetic characteristics of the detrital minerals. Indeed this can be correlated [144] with land-use changes in the surrounding districts and other limnological factors; the swings of declination provide a valuable dating tool in this context. On the other hand for sediments that show no lithological changes the remanent intensity should reflect the intensity of the geomagnetic field at the time of deposition. The 2800-year periodicity has also been found by Creer and Kopper [145, 146] in the inclination records of a core from the Black Sea spanning 25 000 b.p. to 7000 b.p. and of samples from sediment in Jeita Cave, Lebanon, spanning the last fifteen millennia.* No declination measurements were made for the Black Sea case; in the declination record of the Jeita Cave sediment there is indication of a 1700-year periodicity. Cores from two of the Great Lakes of North America (Michigan and Erie) spanning the last 11500 years show [147] declination swings having a periodicity of around 2000 years (or perhaps less in the case of Lake Michigan) but no distinctive pattern in inclination. In discussing interpretation of all these data, Creer [148] points out that explanation in terms of wobble or precession of the central dipole field is ruled out by (i) the difference in periodicity between North America and Europe and (ii) the different motions of the central dipole that would be required to produce the observed variations at different lakes. Explanation in terms of westward or eastward drift of localized disturbances in the outer part of the core is not favoured because for sites occupying approximately the same latitude this model would produce similar secular variation patterns with a time lag due to longitude difference. The explanation put forward by Creer [148, 149] is in terms of the eight radial dipoles, together with a central axial dipole, that were proposed by Alldredge and Hurwitz [150] as a model giving a description of the observed 1945/1955 geomagnetic field. For the best fit these dipoles were located deep in the liquid core at 0.28 Earth radii from the geocentre. In the development of the model proposed by Creer the dipoles are allowed to oscillate, each with its own particular frequency. Depending on geographical position the secular variation at a given observational point may be dominated by one particular dipole and depending on position relative to that dipole the swings of declination may be strongly evident and swings of inclination barely discernible, or vice-versa. The declination variation at Lake Windermere is dominated by the dipole located under the region of the Persian Gulf; the nearer dipole in northern Greenland has less effect on declination because the horizontal lines of force from it are aligned almost north-south. The strongest effect on inclination at Lake Windermere is from the Persian Gulf dipole; however the absence of marked inclination swings could be because the combined effect of three other dipoles is of comparable importance. The Persian Gulf * For a definition of b.p. see p. 320.

316

M.J. Aitken, Archaeological involvements of physics

dipole would also dominate inclination variation in the Lebanon and the Black Sea and so observation of the same 2700—2800 year periodicity as for N.W. Europe is to be expected. On the other hand the declination variation at the Great Lakes would be dominated by two dipoles in the Pacific thereby explaining why a different periodicity is observed. These lake sediment studies are at an early stage; whether or not the simple radial dipole model continues to be satisfactory as more data are accumulated these studies show the inadequacy of the models adduced to describe the historic and archaeomagnetic data. The radial dipole model is preferred by Creer to that of Cox [151] for other reasons. Archaeological dating on the basis of the swings in declination (or inclination) found in paleolithic cave sediments is under investigation by Kopper and Creer [152—154].For these studies the samples are taken by pressing a small plastic cylinder into the archaeological section. On the basis that the declination or inclination swings have the same periodicity as for other sediment records in the Mediteranean region for which radiocarbon or other chronological controls are available, the time span represented can be determined. Absolute dating can be obtained if withth the record there is one horizon for which the date is known; this sometimes on the basis of paleoclimatic evidence through pollen or faunal remains or directly from the magnetic record if it spans an already-dated magnetic excursion in which the recorded direction is grossly abnormal. This type of magnetic dating is to be distinguished from the proposal [155] that the magnetic direction found in silt accumulated at the bottom of archaeological ditches might be used to determine the date of abandonment of the ditch by reference to archaeomagnetic curves such as those of figs. 3.3— 3.5. Quite apart from any other difficulties, such as whether the silt acquires its remanence rapidly enough when considered in the context of the application concerned, it is apparent from fig. 3.9 that there is likely to be a wide scatter in individual points and that the only reliable information that can be obtained from sediments is on the basis of averaging and curve fitting. 3.6 Geomagnetic reversals It is now well-established from paleomagnetic studies that during certain periods of the geologic past the earth’s field direction was reversed by 180°with respect to its present direction. When reversely magnetized rocks were first found one explanation put forward was that the minerals involved were abnormal in that they acquired remanent magnetism in the opposite direction to the applied field and indeed a few such minerals do exist. However this explanation cannot account for reversed magnetization in deep-sea sediments nor for the worldwide synchroneity that is in general observed for the ages of reversely magnetized rocks; this synchroneity implies also that it is a reversal of the dipole field (i.e. of the main current system) rather than an abnormally strong irregularity in the non-dipole field. Reversal of magnetic polarity on the part of the earth should not be regarded as too remarkable since this is known to happen with many stars, sometimes within a matter of days. The present polarity epoch is termed the Brunhes and this was preceded by the Matuyama reversed epoch, the transition occurring around 0.7 million years ago. The lengths of the last four polarity epochs (Brunhes, Matuyama, Gauss, and Gilbert) have been of the order of a million years but within an epoch there are the occasional occurrences of abnormal polarity lasting one or two hundred thousand years and sometimes very much less. These are termed events or excursions depending on the degree of reversal achieved. Within the last two million years there is at present evidence for seven or eight of these. One of them, the Lake Mungo excursion, was discovered by —

M.J. Aitken, Archaeological involvements of physics

317

Barbetti and McElhinney [156, 157] in the course of archaeomagnetic study of aboriginal fireplaces in Australia; so far no other events or excursions have been detected in archaeological remains though the first evidence for the Olduvai event at 1.8 million years was obtained from lava underlying the volcanic ash in which the hominid remains at Olduvai were found. Of the nine fireplaces examined at Lake Mungo the remanent direction in one implied a shift in the vrrtual pole by about 120°and the directions in five others were grossly abnormal. Radiocarbon dating of charcoal from the fireplaces indicate that the excursion was in progress from abotit 2700 b.p. to 3100 b.p.*; the average thermoluminescent age of 33 500 ±4300 years obtained for the first phase of the excursion by Huxtable and Aitken [159] on the baked clay itself does not differ significantly from the average radiocarbon date for that phase at the 68 % level of confidence. That there is no significant disagreement between the two techniques is of considerable interest because the archaeomagnetic measurements indicate that during the first phase of the excursion the geomagnetic intensity was higher than the present-day field intensity for the site by a factor of 3 and that during the second phase it was lower by a factor of 3. As discussed in section 4.2.1 the former would cause radiocarbon dates to be less recent than calendar dates and the latter more recent; however the global carbon-14 production would not have been significantly affected if the excursion was not worldwide but localized due, for instance, to the fields from the two radial dipoles located near Australia being reversed and stronger than the main dipole field. Whether or not there was manifestation of a contemporary excursion elsewhere is of considerable geophysical interest apart from relevance to radiocarbon dating. On the other side of the world in this time range the Laschamp excursion has been found in several lava flows of the French Massif Central by Bonhommet and Babkine [161]; originally it was placed between 9 000 and 20000 B.P. [162] but subsequent TL age determinations by Valladas et al. [163] give an age of 33000 ±4000 years and a recent potassium-argon dating by Gillot and Cassignol [164] does not exclude this. Thus it appears that the Laschamp and the Lake Mungo excursions could well be manifestations of the same worldwide occurrence; magnetic intensity determinations for the former are awaited. Also relevant is the low value of ancient intensity, one third of the present-day value found by Barbetti and Flude (private communication 1976) in clay that had been baked by another lava flow also in the French Massif Central, at Royat; the remanent direction in this baked clay was of normal polarity, though substantially disturbed from the axial dipole direction as was also the case for the overlying lava. Thermoluminescence dating of the clay by Huxtable and Aitken (private communication 1976) to the range 25 000 to 30000 B.P. suggest it could be the tail end of the Laschamp/Lake Mungo excursion. The revised dating for the Laschamp excursion does not support the notion that it correlates with the warming up of the earth from the last Ice Age, as pointed out by Labeyrie and Gillot [165]. The detection of polarity abnormalities in baked clay or lava flows is rather hit or miss because the record is not continuous. However, in respect of the interpretation of the sedimentary record a current controversy over the reality ofthe so-called Gothenburg “flip” is to be noted. The existence of this brief period of abnormal direction was first inferred from measurements on the lowest layer of a 14-metre core from a Swedish glacial deposit; subsequently it was identified in other coarsegrained Swedish deposits [135, 139], being dated to 12 400 B.P. by radiocarbon. However in examining two Swedish cores of fine-grain organic sediment spanning 11 000 to 13 000 B.P. Thompson and Berglund [158] find no evidence of any reversal or excursion of the geomagnetic —

* For a definition of b.p. see p. 320.

318

A’I.J. Aitken, Archaeological involvements of physics

field; they conclude that the previously observed abnormal directions are due to distortion by mechanical sedimentation processes, slumping, or weathering, and suggest that “the proliferation of unusual paleomagnetic directions in Scandinavia around 12000 yr b.p. is a reflection of changing climatic conditions”. The interpretation of scattered results in sections of core from sediment elsewhere in the world as manifestation of the Gothenburg flip is therefore called into question and likewise the time correlation stemming from such interpretation.

4. Radiocarbon dating 4.1. Introduction Radiocarbon dating is possible because of the presence of a minute concentration of the radioactive isotope 14C in all living plants and animals, and in the dissolved carbonates of the ocean. The concentration is about 1012 relative to natural carbon (12C) and is effectively uniform; the uniformity obtains because of comparatively rapid mixing throughout the carbon exchange reservoir which consists of the atmosphere, the biosphere and the oceans. The concentration stays approximately constant, because it represents the equilibrium level between the radioactive decay of ‘4C and its production from ‘4N in the upper atmosphere by cosmic ray neutrons. On the other hand, in animals and plants that are preserved after removal from the exchange reservoir (by “death”) the concentration decays exponentially with the 5730 year half life of ‘4C, and by comparing the concentration found in such material with the concentration for living material the time that has elapsed since death can be determined. The established method of determining the concentration is by measurement of beta activity; for living material this is approximately 15 disintegrations per minute (i.e. about 10_li curie) per gram of natural carbon. The idea of age determination by ‘4C arose in the course of a study of the effects of cosmic rays on the earth’s atmosphere by W.F. Libby and his group at the University of Chicago, in the late 1940’s. Libby [3] has described the method and its inception and has reviewed subsequent developments from time to time elsewhere [166—169]. 4.1.1. The production ofradiocarbon The nuclear reaction by which thermal neutrons produce radiocarbon in the atmosphere, is 14N + n

—~

14C + ‘H.

(4.1)

The cross section for this reaction is about 1.7 x 10—24 cm2. It is the dominant way in which neutrons interact with nitrogen and for oxygen the neutron cross section is lower by a factor of a thousand. The neutrons concerned are secondary particles produced by the incidence of the primary cosmic ray flux on the atmosphere. Although they may have high energy on formation, the neutrons rapidly become thermalized by collision and because of the dominance of reaction (4.1) the effective fate of each neutron is to produce a ‘4C atom. It is known from high altitude balloon measurements that the average neutron production rate is about 2/s cm2 of the earth’s surface; this yields a global production rate of 7.5 k~,of ‘4C per year. The neutron intensity builds up from zero at the outer limit of the stratosphere to a maximum at a height of around 15 km. and then falls off again; for the latitude range 50°—90° it reaches 3 ~ ofthe maximum value at about 3 km and about 0.3 °/~at sea-level. Being charged the primary cosmic .

M.J. Altken, Archaeological involvements of physics

319

ray particles are deflected by the magnetic field of the earth unless they are travelling parallel to the lines of force. Consequently the cosmic ray intensity is a maximum at the poles and a minimum around the geomagnetic equator; the corresponding neutron intensities in the stratosphere are in the ratio of about 5 to 1. However, the mixing of ‘4C in the atmosphere is sufficiently rapid and thorough for these inhomogeneities to be unimportant when considering the uniformity of the i4C concentration. It is presumed that the 14C atoms combine with oxygen to form heavy carbon dioxide, which, except in respect of radioactive decay and isotopic fractionation effects, is indistinguishable from the ordinary carbon dioxide of the atmosphere and so circulates in the same pattern through the carbon exchange reservoir. Although the latitudinal differences in 14C production may not matter, the dependence of the overall global production on the earth’s magnetic field intensity is important in view of the archaeomagnetic evidence that in the past substantial changes in the latter have occurred. A stronger magnetic field means a greater degree of shielding of the earth from the primary cosmic ray flux and therefore a lower production of 14C. 4.1.2. Half life and equilibrium level Radioactive decay is accompanied by the emission of a weak 160 keV) according to ‘4C

‘4N + /3~.

=

/3

particle (maximum energy, (4.2)

The presently accepted value for the half life is 5730 years. This corresponds to a mean lifetime of 8290 years and it is convenient to remember that in approximate terms the disintegration rate is 1 % per 83 years. The predicted value for the equilibrium amount of ‘4C on earth, obtained by equating the decay rate to the production rate of 7.5 kg per year, is 62 tons; this is distributed throughout the carbon exchange reservoir. When death occurs (for example, when a cellulose molecule is formed in the wood of a tree) and the material happens to be preserved, the atoms in it are no longer “in exchange” with the reservoir and the concentration of ‘4C decays according to N

=

N 0exp(—0.693t/5730)

(4.3)

where N is the concentration at time t years, and N0 is the value of N when death occurred (at = 0). Experimentally the concentration is determined by measuring the f3 activity per gram of natural carbon and the date calculated from t

t

=

(5730/0.301) lg(C0/C)

(4.4)

where C is the observed /3 activity and C0 is value of C at death. Obviously the value of C0 cannot be measured directly and it has to4Cbeconcentration assumed that it theexchange same as for recently in isthe reservoir hasgrown been material. constant This basic assumption that the ‘ over past time was a reasonable one to make as a first approximation and from check measurements on known-age samples it appears that to within a few per cent it is valid back to 1500 B.C. Before that, however, there is evidence of a significant long term excess of up to 10 % lasting for several millenia. The magnitude of this long term excess and the extent to which there are short term fluctuations superimposed on it have been a prime concern of radiocarbon research over the past decade, together with discussion of the geophysical and geochemical causal mechanisms. The source of

320

M.J. Aitken, Archaeological involvements of physics

known-age samples is ancient trees, notably bristlecone pines from California, of which the annual rings are dated by dendrochronological techniques; on the basis of such measurements “straight” radiocarbon ages are calibrated into calendar years (see section 4.2). The half life of 5730 years is the mean of three determinations: 5695 ±40,5715 ±50, 5790 ±65 [170]. This value is 3% higher than the value 5568 which was used by Libby at the inception of the method on the basis of the best determinations then available. However, for the time being, laboratories continue to calculate and publish dates on the basis of the “Libby half life” so as to avoid confusion and risk of multiple correction; it is usual to quote such dates in “years b.p.” (literally meaning Thefore present” but for convenience defined as Thefore A.D. 1950”). In this article the convention suggested [171] by the editor of Antiquity will be followed: a.d., b.c., and b.p. are used for straight radiocarbon dates on the old half-life but A.D., B.C., and B.P. for radiocarbon dates that have been corrected to calendar dates either by the half-life correction only or by calibration from dendrochronological data (and also for dates from other techniques for which no systematic error is as yet known to exist). Needless to say there is confusion and controversy among archaeologists in this context and the situation has not been helped by the continued failure on the part of radiocarbon laboratories to agree on a recommended calibration curve. 4.1.3. The carbon exchange reservoir Plant life grows by photosynthesis of atmospheric carbon dioxide and, in turn, animals live off plants; consequently ‘4C spreads throughout the biosphere. Also, atmospheric carbon dioxide enters the oceans as dissolved carbonate, so this too contains ‘4C. The carbon dioxide withdrawn from the atmosphere by plant and animal life is eventually returned to it by the decomposition that follows death except for the very small proportion that is locked up in well preserved organic remains. The carbonate in the ocean is formed by an exchange reaction so that carbon dioxide leaves as well as enters the ocean again except for a small proportion which is locked up for long periods, this time in shells and other carbonate deposits. Thus the atmosphere, biosphere and ocean form an exchange reservoir throughout which the carbon atoms circulate; for the atmosphere and biosphere the residence time is short compared to the mean lifetime of a ‘4C atom but in the deep ocean the residence time is of the order of a thousand years so that its carbonate is depleted in ‘4C due to radioactive decay. The amount of natural carbon in the reservoir is 40 x 1012 tons; 93 ~ of this is in the deep ocean, 2 °i~in the surface (mixed) ocean, 1.6 in the atmosphere, 0.8 °/ in the terrestial biosphere and 2.6 in humus. The mixing into the reservoir of the equilibrium 62 tons of 14C gives an average weight fraction of about 1.5 x ~ 12 This concentration determines the value of C 0 in eq. (4.4) and the assumption 14C (as that C0 has always been the same could be upset by a change in the production rate of already mentioned), by a change in the overall size of the reservoir or, more subtly, by a change in the mixing rates which determine small non-uniformities in the ‘4C concentration in different parts of the reservoir. In considering possible non-uniformities the most obvious would be a greater concentration at high latitudes than at low on account of the deflection of the primary cosmic ray flux by the earth’s magnetic field. However, as mentioned in section 4.1.1, the mixing of carbon dioxide within the atmosphere is sufficient to remove any latitude dependence of 14C concentration; this was shown by an early test on recently grown wood samples from various latitudes [3] and has been confirmed subsequently. Since the atmospheric circulation systems of the northern and southern hemispheres are separate there is also the possibility of a difference between the concentrations in the two hemispheres, but measurements indicate that the concentration in the —

—

~

~

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321

southern hemisphere is only about 0.5 % lower than in the northern hemisphere [160]; this is attributed to the greater (by 40 %) area of ocean in the former compared to the latter. Besides non-uniformities due to large scale effects there are also non-uniformities due to isotopic fractionation during uptake and exchange of carbon. Thus the ‘4C concentration in plants is 3 or 4% lower than in the atmosphere from which they obtain their carbon dioxide; this corresponds to an apparent age difference of 240—320 years. There are small variations from species to species in the degree of this isotopic fractionation, and it is now common practice to evaluate the effect for each sample that is dated. This is done by measurement of the 13C concentration in the sample; this stable isotope is present to about 1 % and so it can be measured with a mass spectrometer. On the basis that the percentage dLepletion for an isotope is proportional to the difference in atomic mass, the depletion for ‘4C is taken to be exactly twice the depletion measured for ‘3C; whether or not the factor is exactly 2 is open to question but unless it is grossly different the resultant error in age will not usually be significant. Isotopic fractionation also occurs in the exchange reaction between carbon dioxide and ocean carbonate, there being an enrichment of 1.5 % when carbon goes from atmosphere to ocean. Hence relative to plant life there is an isotopic enrichment of ocean carbonate by about 5 %. In surface ocean this more or less compensates for the depletion that results from the longer residence time in deep ocean; after correction for isotopic enrichment surface ocean shows an apparent age relative to atmosphere and biosphere of around 400 years intermediate between deep ocean and atmosphere. as might be expected. —

4.2. Distortions in the radiocarbon timescale As measurement techniques improved it gradually became well-established that radiocarbon dates did not always correspond exactly with calendar dates. Wood from royal tombs in the pyramids gave dates that in the period 1500—3000 B.C. were several hundred years too recent according to the Egyptian calendar (based Ofl king lists, with an astronomical fix in 1800 B.C. by means of the recorded heliacal rising of Sirius). This led to much discussion of the accuracy of the Egyptian chronology, but eventually radiocarbon measurements on fossil trees dated by dendrochronology confirmed that radiocarbon dates did indeed diverge significantly from calendar dates. The interpretation of this divergence is that the ‘4C concentration in the atmosphere at the time when the sample fixed its carbon was different to the value assumed for C 0 in eq. (4.4). The existence of shortterm fluctuations with a time of one or two centuries was first noted by de Vries [172] at Groningen, using dated tree rings of the past 500 years; it has since been established that these are present in varying degree throughout the pe:riod for which dated tree-rings are available (so far the last eight millennia) and superimposed on a longer term variation which appears to be sinusoidal with a period of about 8000 years. The peak-to-peak amplitude of the short term fluctuations is 1 or 2 % and that of the long term variation about 10%; the latter corresponds to an age error of 800 years. Dated tree rings of such antiquity are available by means of the dendrochronological technique of cross matching the pattern of the inner rings of one tree with the outer rings of an older tree whichjust overlaps it. This is possible for species (such as pine, oak and sequoia) for which the ring width is determined by climate. The most important tree in the present context is the bristlecone pine which grows at an altitude of 3 kilometers in the White Mountains of California, sometimes attaining an age of several thousand years. By successive crossmatches, and many years of comprehensive study, Ferguson [173—175]of the Arizona Tree Ring Research Laboratory has esta-

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I I

I I

I

Calendar vears(millennua)

Fig. 4.1. Age error over the past 12 000 years: varve results superimposed on the tree-ring data of Suess [177]. The large full circles are the Scandinavian measurements of Tauber [250] and the dots with vertical lines are the American measurements ofStuiver [251]. There is as yet no explanation of the discordance, before 5000 B.C., between the two sets of varve measurements. There are American measurements more recent than 4500 B.C. (which are not shown in order to avoid confusion) and these are in good agreement with the tree-ring data. The meaning of the ages X and X’ are explained in the caption to fig. 4.2.

blished the calendar date for wood back to 6000 B.C. with expectation of eventual extension as older fossil trees are gradually located. The formidable task of radiocarbon measurement of 10-year bundles of these dated tree-rings was undertaken first by Suess [176, 177] at the University of California La Jolla Laboratory and later by Ralph [178] at the University of Pennsylvania (MASCA) Laboratory and Damon [179] at the University of Arizona. For millennia earlier than the limit of the tree-ring chronology, an indirect source of calibration going back to 10000 B.C. is the annual layers of sediment on lake bottoms, notably the glacial varves in Sweden. Before this it is a matter of intercomparison with thermoluminescent dating, or, of prediction from the calculated effect on cosmic-ray intensity of geomagnetic field variation as determined by archaeomagnetic studies. However, although these can give useful information in evaluating long-term trends, it is only tree-ring dating that is precise enough to calibrate shortterm errors of the order of a century. The archaeological implications of the tree-ring calibration “the second radiocarbon revolution” have been more serious than a further lengthening of prehistoric time-scales. The revision of dates has accentuated the difficulty, already indicated by uncorrected dates, of accepting the traditional interpretation of archaeological evidence in terms of outward diffusion from the Near East. Back to 3100 B.C. the chronology of the Near East is based on links with Egypt and its astronomically-based calendar and, compared to the corpus of dates that has been built up for regions remote from it, the available radiocarbon dates for the Near East are sparse. Hence any revision of the radiocarbon time-scale creates a chronological “fault-line” geographically located where dependence on radiocarbons begins [180]. According to the tree-ring calibration the first temples in Malta predate the pyramids by several centuries and the trilithons of Stonehenge (phase III) are earlier than Mycenae. The beginning of settled farming communities in central Europe, the Neolithic age, already put 1500 years earlier by conventional radio-carbon dating than had been assumed, is pushed further back a further 700 years to circa 5000 B.C. or earlier. These implications are by no means fully accepted and being accustomed to regional chronologies archaeologists have questioned whether the indications of trees grown at an altitude of 3 kilo—

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,I,l#.~/.•

1000

A/V

1~

___________

Ill::_______ ~—A.D.

B~C~_~~Calendar years (millennia)

Fig. 4.2. Lower: Age error according to 9-sample averaging of tree-ring measurements (derived from Ralph et a! [178]). ~pp~: Suess curve from fig. 4.1 for comparison. The value obtained by measuring upwards from the conventional horizontal axis X indicates the error when the radiocarbon age is calculated on the basis of the Libby half-life of 5568 years. Measuring from the inclined axis X’ gives the error on the basis of the revised half-life of 5730 years. The figure also indicates the excess concentration of carbon-14 in the atmosphere above the nineteenth century level: an age error of + 83 years on the revised half-life corresponds to an excess of 1 per cent.

1000 0—~. ——————S

__ ~—AD

BC—.-

Calendar years (millennia)

Fig. 4.3. Age error trends indicated by tree-ring measurements, on the basis of the 5730-year half-life. Ralph et al. [178] full curve; Damon et al. [179] dotted curve; wendland and Donley [261] dashed curve. The horizontal line at 310 years indicates the average over the period and corresponds to an excess atmospheric concentration o approximately 4 per cent above the nineteenth-century level.

meters in California are relevant to low altitude samples on the other side of the world. Currently effort is being devoted to the establishment of tree ring chronologies in Europe from Irish bog oaks, from submerged forests on the West coast of Britain, from oak trunks accumulated in gravel —

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deposits of the rivers Rhine, Main and Danube; recently published results [244] from the Irish material call into question the reality of the short term fluctuations, at any rate for lowland Europe. However the results of renewed measurements made on Egyptian samples, with careful exclusion of dubious samples, give affirmation of the general validity of the tree-ring calibration for Old World archaeology, through there are still some inconsistencies with regard to radiocarbon dates in the Aegean; also, with respect to the Egyptian chronology it is possible to argue that in some periods the radiocarbon dates are about two centuries too ancient. Having regard to the various factors that complicate radiocarbon dating and to the fragmentary nature of some of the evidence on which the Egyptian chronology is based, even if the reality of this small discrepancy is accepted the agreement is a remarkable tribute to scientific technique on the one hand and historical scholarship on the other. 4.2.1. Influence ofgeomagnetic field strength The variation in ‘4C production rate, Q, resulting from a change in the earth’s magnetic moment, M, through modulation of the cosmic ray flux was first considered by Elsasser, Ney and Winkler [181], who concluded that

Q

cc

1/M°~52.

(4.5)

The value of M determines, for a given latitude, the minimum momentum of cosmic ray particle that can reach the atmosphere, and consequently (4.5) is based on the observed spectrum of cosmic rays. The effect has been considered subsequently by others, including Ramaty and Lingenfelter [186, 187], who are in agreement with (4.5) as a good approximation. If one tentatively accepts that the archaeomagnetic data of fig. 3.7 indicate a sinusoidal variation of the geomagnetic moment there is reasonable agreement between the predicted concentration variation and the long-term concentration variation implied by the bristlecone-pine calibration. The basis of comparison is along the following lines. By plotting the magnetic data in the form shown in fig. 4.4 the best sinusoidal representation for the predicted production rate, Q, is obtained. This is close to Q(t)

=

Q 0(1

—

~ sin 2ivt/8000)

(4.6)

where Q0 is the average production rate and t is the time before present. It is assumed that the variation has persisted for several cycles. The resultant concentration variations are attenuated because the magnetic period is of4C. the Calculations same order as[182] the response of the global reservoir as indicate time an attenuation factor of onedefined and by the meanlag lifetime of ‘700 years; hence the prediction for the concentration variation seventh a phase of about is approximately C(t)

=

C 0 {1

—

~ sin 2ir(t + 700)/8000}.

(4.7)

Remembering that a 1 % change in concentration causes an age error of 83 years we see that this equation predicts an age error amplitude of about 400 years. In view of the paucity of the magnetic data this is in remarkable agreement with the age error variation actually observed see fig. 4.3. However, it would be rash to assume that there are no other important influences, or, that because of the agreement the early part of the archaeomagnetic data, which is from a rather restricted part of the earth’s surface, is definitely representative of the geomagnetic field as a whole. A much stronger influence on radiocarbon production than the ±50% variation of the last —

M.J. Aitken, Archaeological involvements of physics I I

I

I

I

I

I

I

I

2I

3I

4I

~I

6I

325

8

06 —II

0I~ \.D.

I3.(.

Calender

7I

ears (millennia)

Fig. 4.4. (Redrawn from Suess [260].) Production rate variation implied by the geomagnetic moment values evaluated by Bucha [256]; the minimum value ofM occurs circa 4500 B.C. in this figure instead of 3500 B.C. as in fig. 3.7 because the radiocarbon dates have been 2where MA/Mo is the ratio of the ancient moment to the present day corrected. The production rate is proportional to (MA/MQY°” moment.

seven millennia will have been the low intensity that usually accompanies a geomagnetic polarity episode whether it is a permanent reversal, a short event, or only an excursion; evidence from geological research indicates that the intensity may fall to less than 20 % of the average value. During the period of applicability of radiocarbon dating the last 50000 years manifestations of reversed polarity have been found in volcanic lava flows near Laschamp in the French Massif Central and in aboriginal fireplaces at Lake Mungo, Australia. Whether these are both part of a world wide excursion or whether they represent two separate regional excursions has been discussed in section 3.6. If the former then on the basis of the Lake Mungo results there is likely to have been marked effect on radiocarbon production the ancient intensity was three times the present day value at the site during the first phase of the excursion and one-third of the present day value during the second phase. Although the thermoluminescence age of 33500 ±4300 years obtained [159] for fireplaces of the first phase is not significantly different from the average radiocarbon age of 30 400 years for that phase the wide error limits allow the possibility that the atmospheric radiocarbon concentration was in excess of the present-day value by 45 %. It should be noted that the high intensity associated with the first phase would have caused a deficit in concentration so that ifone accepts from the preliminary evidence adduced in section 3.6 that the excursion was worldwide then one has to interpret the thermoluminescent date not being more recent than the radiocarbon date as due to the duration of the first phase being too short to have affected the radiocarbon concentration or that it was preceded by an earlier, so far undetected, low-intensity phase that had already caused the concentration to rise above its present-day value. “Present-day value” is here taken to mean the value indicated by wood grown around AD 1850 before the atmospheric concentration was altered by man. His first influence was through the burning of fossil fuel in which the 14C had long since decayed [183]; it is estimated that by 1950 the atmospheric concentration of 14C had been decreased by about 3 % due to this cause. Subsequently the testing of nuclear weapons has increased the atmospheric concentration by about a factor of 2; as this “bomb carbon” mixes into the ocean the excess level will gradually fall to about —

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—

—

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3 O/~ assuming hopefully that there are no further substantial explosions. The sharp injection of —

carbon-14 into the atmosphere in this way allows useful study of mixing rates between different parts of the exchange reservoir. In particular the 1961—2 explosions at high altitudes introduced carbon-14 into the same regions as are most important in natural carbon-14 production and so the route revealed as the artificial excess moves through the various components of the reservoir is also that for natural carbon-14. Another use of these sharp rises in carbon-14 concentration is in confirming the integrity of sample materials. Measurements [184] on a bristlecone pine and an oak showed that wood formed immediately before the sharp rise in atmospheric concentration did not contain any excess carbon-14; this was also true [185] for a red cedar as long as the cellulose structure was used, but the resin extract showed a sharp rise in carbon-14 activity for rings formed up to a dozen years earlier than the occurrence of the atmospheric excess. 4.2.2. Influence ofsunspot activity Cosmic ray data taken over the past few eleven-year solar-cycles indicate that when sunspot activity is high the cosmic ray intensity is depressed. It is presumed that with high sunspot activity there is an associated intensification of the weak interplanetary magnetic field carried by the solar wind, and that this deflects cosmic rays away from the earth’s vicinity, particularly the lower energy component responsible for carbon-14 production. Corresponding diminutions in high altitude neutron flux have been observed. From an analysis of these data Lingenfelter [186] has put forward the following empirical relation for the effect on production of carbon-14:

Q 2.64 S/337 (4.8) 2 of the earth’s surface) where Q is the carbon-14 production (expressed as atoms per second per cm =

—

and S is the annual sunspot number. Over the last six solar cycles S has varied between near zero at sunspot minimum to around 100 at sunspot maximum and so, according to Lingenfelter’s relation, an overall variation of 10 % in carbon-14 production rate is to be expected. More detailed calculations [187] for a specific fairly strong cycle give a variation of 20%. If the atmospheric carbon-14 concentration varied by the same percentage then radiocarbon dating would hardly be a practical proposition the percentages quoted correspond to age shifts of 800 and 1600 years respectively. However, because of the slow response time of the reservoir a production rate variation with a period of around ten years will be severely attenuated and the resultant concentration variations should be less than 0.1 % corresponding to an age shift less than 8 years. Records of sun-spot activity have been kept at Zurich since AD 1700 and estimates based on such phenomena as aurora borealis and solar flares have been made back to AD 300, extending to 600 B.C. with lessening reliability. These data indicate that the intensity of the maximum of the 11-year cycle has varied between 50 and 150 and that the activity has been greater in even centuries than in odd ones suggesting a 200 year periodicity in the amplitude of the cycle. For such longer periodicities the attenuation due to the response time of the reservoir is less severe and variations in atmospheric variation of the order of 1 % are to be expected. The observed deviations of radiocarbon age from calendar age indicated by measurements on dated tree-rings give evidence that such an effect does occur [188]; for instance the sixteenth century was a period of high sunspot activity and this agrees well with the gradual decrease in atmospheric concentration occurring during that century. Detailed studies [189] over the past 1000 years have confirmed correlation on a statistical basis. —

—

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In view of this it is reasonable to presume that heliomagnetic modulation is the cause of similar short-term variations revealed by tree-ring measurements for earlier millennia. Support for this is given by the observation (see figs. 4.1 and 3.7) that the short-term variations are less pronounced during periods when the geomagnetic moment is high; the heliomagnetic fields affect the lower energy cosmic rays and when the geomagnetic moment is high the~eare deflected away from the earth anyway. Another effect associated with high solar activity is the occurrence of solar flares; the associated particles will produce carbon- 14 and hence there could be a tendency to obscure the heliomagnetic modulation effect. Lingenfelter and Ramaty [187] estimate that the increase in atmospheric carbon- 14 concentration due to a single event could be of the order of 1 % and that solar flares may be responsible for some of the sudden short-term increases implied in fig. 4.1. These authors also consider possible increases in carbon-14 production that might arise from a supernova explosion. Although the amplitude predicted for concentration variations associated with the 11-year cycle is below detectable limits, a detailed study back to A.D. 1890 by Baxter and Walton [190] gives evidence for concentration variations whichdo show correlation with this cycle. The materials used were samples of whisky, wine, seeds, cerial, and wool for which the year of plant growth was known. The overall magnitude of the variation sometimes reaches 3 % corresponding to an age shift of 240 years; the authors propose an explanation of this anomalously large amplitude in terms of hold up of carbon-14 in the stratosphere before release into the lower atmosphere from which plants draw their carbon dioxide. It is suggested that transfer from the stratosphere is affected by solar activity through the influence which incident ultraviolet and corpuscular radiation have on relevant temperature gradients. A further study by Farmer and Baxter [191], of single rings for the years A.D. 1829 to 1865 from an oak grown in Britain also shows variations; the overall amplitude is between 1 and 2% and there is significant correlation with the 11-year cycle. On the other hand the same authors found no such correlation for the rings of a New Zealand pine spanning A.D. 1910 to 1950. Also, Damon, Long and Wallick [192, 193] have examined the A.D. 1940 to 1954 rings of a Douglas fir grown at high altitude in Arizona and although the variations show some correlation with the eleven year cycle, the overall amplitude is fairly small. These authors review other evidence from single-ring measurements and find no support for the existence of larger variations. Recent measurements on single year tree-rings of a Douglas fir from the State of Washington indicate that the 11-year cycle i4C amplitude is less than 0.2% for the period A.D. 1820 to 1900 (M. Stuiver, personal communication). It is to be hoped that the latter viewpoint is the correct one as otherwise the implication is that samples spanning only a year’s growth are liable to an error of ±120 years (assuming the effect to have had a similar magnitude during earlier millennia). Thus if short growth period materials are used there is risk of this error, while if a decade’s worth of wood growth is used in order to average it out there is the uncertainty as to how many years elapsed between formation and the archaeological event being dated. Support for existence of significant short term variations in antiquity has recently been provided by Burleigh and Hewson [194] from careful measurements made on fifteen antler picks of around 2000 b.p. from Grimes Graves in East Anglia. The archaeological evidence is that the time span represented by these antlers did not exceed a decade, yet to explain the spread of the radiocarbon dates obtained, which is in excess of experimental error, a time span of the order of 120 years has to be assumed. The preceding paragraphs were written before the presentation by Pearson [244] in August 1977 —

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at the INQUA meeting in Birmingham of painstaking measurements on a 1200-year section of a tree-ring chronology based on sub-fossil oaks from the North of Ireland. This chronology is “floating” in the sense that its calendar date is not known; on conventional radiocarbon dating it spans 4550 b.p. to 3600 b.p. The radiocarbon determinations were made with meticulous care on the cellulose extracted from 200 g samples each containing 20 rings, and, after taking account of all sources of measurement error, the estimated precision was ±25 years at the 68 % level of confidence. The important finding was that to within this degree of precision, which corresponds to ±0.3 %, the relationship between radiocarbon age and tree-ring age is not significantly different to a straight line, i.e. there are no short-term variations. Because 20-year bundles of tree-rings were used this has relevance to the earlier discussion in this sub-section rather than to the controversy over a possible effect due to the 11-year sunspot cycle. The implication is either that the ±1 % “wriggles” in fig. 4.1 during the period concerned are an unsuspected artefact of measurement, or, that there was a radiocarbon variability in the atmosphere at the 3 kilometre altitude at which the bristlecone pines grew in California that was not present in the atmosphere of low altitude Ireland. Either way the elimination of the wiggles would mean the elimination of ambiguous radiocarbon ages which can correspond to more than one calendar age and, in the words of Pearson et al. [244], “restore radiocarbon dating to the position of the absolute dating method as was foreseen at its inception”. 4.2.3. Effect of climate The atmospheric ‘4C concentration is dependent both on the total amount of ‘2C in the exchange reservoir and the rate of exchange between its different compartments; thus if the atmospheric residence time before entry into the ocean (a few decades) increases there will be an increase in the atmospheric ‘4C concentration. It would be remarkable if climatic regime did not influence these parameters and the extent of this has been considered by a number of authors. It was early noted out by de Vries [172] that the peak concentration occurring around A.D. 1700 followed a general worsening of the climate as evidenced by glacier advance. This peak also followed a period of low sunspot activity, in agreement with other evidence that there is correlation between sunspot activity and climate. The question then arises as to whether solar activity affects the carbon-14 concentration directly, through heliomagnetic modulation, or indirectly via its effect on the climate. It has been suggested that climate is affected also by geomagnetic intensity; measurements on deep-sea sediment cores spanning the past half-million years show a tendency for colder climate during periods of higher intensity. Thus it is possible that the geomagnetic effect on carbon-14 concentration is via the climate also. However, in this case the approximate quantitative agreement of prediction with observation (section 4.2.1) suggests that the effect is due to direct modulation of cosmic ray intensity. It is of interest to note here that measurement of stable isotope ratios in tree-rings has been proposed as means of determining past climate. Wilson and Grinsted [195] find that within a single ring of a pine grown in New Zealand there is a 3 % increase in the deuterium/hydrogen ratio between early wood and late wood and they interpret this as the influence of the 10°Cvariation in monthly mean maximum/daily temperature between summer and winter on isotopic fractionation in the biochemical reactions leading to the formation of cellulose. On the other hand Libby et al. [196, 197] in examining the D/H and 180/160 ratios of whole rings regard the isotopic composition of the rain and atmospheric CO 2 as the predominant influence; it is well known that this is dependent on temperature and 180/160 ratios in the polar ice caps and else-

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where have been studied extensively the heavier isotope is depleted in ice laid down in cold periods. Libby et al. find good correlation with annual temperatures for European oak and fir during the centuries for which temperature records have been available; measurements on a Japanese cedar indicate a fall of about 1.5°Cover the last two millennia. Another stable isotope study of tree-rings is that by Stuiver [198] in respect of ‘3C/’2C. In this he estimates the net release of CO 2 from biosphere to atmosphere between A.D. 1850 and 1950 as about twice the amount released from fossil fuel. Such4C; CO2 can distinguished both arebedeficient in ‘3C.from that released from fossil fuel because the latter is deficient in i 4.2.4. Effect of lightning and volcanic activity Libby and Lukens [199] have suggested that the ‘4C production from lightning-generated neutrons could amount to 1 % of the production by cosmic-ray neutrons. Variation in the frequency of lightning storms over the centuries might be partly responsible for the observed short term fluctuations in ‘4C concentration. Another possible effect is that due to volcanic activity; CO 2 14C but the question is whether released from volcanoes would be expected to be deficient in the amount is quantitatively significant. —

4.2.5. The validity of the bristlecone pine record As mentioned in section 4.1 it is reasonable to question whether the 14C record carried by high altitude trees in California can justifiably be used to calibrate radiocarbon dates from materials grown from lowland Europe and elsewhere. One suggestion made is that since the cosmic-ray neutron flux at 3 km is an order of magnitude greater than at sea level, and about three per cent of the maximum flux reached in the stratosphere at 15 km, there may be significant in situ production of ‘4C from ‘4N in the wood. However, experiments [200, 201] in which wood has been irradiated in a nuclear reactor indicate the effect to be unimportant, due to the low content of nitrogen the order of 0.1 %. Another less easily quantifiable possibility [199] is that there is in situ production from lightning bolts, particularly on account of the vulnerability of high altitude trees on exposed ridges. However, even if such production was significant it does not necessarily upset the validity of the record since ‘4C produced in this way need not be incorporated into the molecules forming the cellulose structure and it may be removed in the course of the chemical pretreatment. Another question raised is whether there is any significant incorporation in the cellulose ofinner rings of carbon acquired by the tree in a later year; that this does not happen in recent wood has been checked very sensitively by testing pre-1954 rings for the presence of excess ‘4C due to nuclear weapons tests as mentioned in section 4.2.1. The recent publication [244] of results from a floating tree-ring sequence of Irish oaks calls into question the reality of the short term fluctuations in the bristlecone pine record, as discussed in section 4.2.2, but gives no reason to doubt the reality of the long term variation; however, the sequence is floating and the long-term variation cannot be confirmed until the sequence has been fixed in calender years by dendrochronological linkage to a living tree. Apart from the question of the validity of the raw data there has been considerable discussion of its correct statistical treatment. Several different calibration curves have been published [178, 179, 202]; although there is strong disagreement over the amount by which the error limits for calibrated dates should be increased above those for dates quoted in radiocarbon years, disagreement over the corrections themselves is not too serious, though all of this will need reassessment in the light of the Irish results. —

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4.3. Measurement Because the specific radioactivity is low (less than 7 picocurie per gram of natural carbon) and the energy of the emitted beta particle weak (Emax = 160 keV), accurate measurement is difficult and tedious particularly for old samples. In general counting times are upwards of 24 hours. Even with stringent precautions the background count-rate is of the same order as that due to an old sample. The counting chamber must be constructed from materials that are free from radioactivity and it must be shielded both from environmental gamma radiation and from the small percentage of cosmic rays that do reach the earth’s surface; about ten or twenty tons of steel are used and additionally the chamber is surrounded by anti-coincidence counters. In the original technique used by Libby the sample was converted to carbon dioxide by combustion and then reduced to carbon black by heating with magnesium. The carbon black was then painted onto the walls of a modified Geiger counter. One disadvantage was that the painting process entailed exposure to the atmosphere of the highly absorptive carbon and risk of contamination became particularly serious subsequent to the beginning of hydrogen-bomb testing in 1954. The solid-carbon technique was superceded by measuring the sample as the gas of a proportional counter and this remains the preferred technique in many laboratories. The carbon is converted into carbon dioxide, methane or acetylene. The volume of the counter is typically several litres and it is filled to a pressure of one or two atmospheres, so that the amount of carbon measured is one or two grams; the count-rate for modern carbon is of the order of 10—20 counts per minute and with good shielding the background can be as low as a tenth of this. More recently liquid scintillation counters have been introduced. The carbon is converted into an organic liquid, such as benzene, which is used as solvent for a liquid scintillator. This is contained in a 20-ml vial viewed by two photomultipliers in coincidence. Because a given amount of carbon is contained in a much smaller volume than in the case of gas counting shielding against cosmic rays is easier. Another advantage is the ease with which measurement of sample, standard, and background can be alternated automatically thereby eliminating the effects of drifts in detection sensitivity or background. The technique can utilize larger samples than can the gas counter; with ten grams of carbon the count-rate for modern material is somewhat over a hundred per minute. On the other hand the chemistry is more complex, the background is less predictable and is not such a convenient system for handling small samples. Continued development of proportional gas counting has led to impressive results notably at Groningen by Mook and Vogel, by Oeschger [211] and by Stuiver, Robinson and Yang [203]. Using a 4.5 litre counter, heavily shielded in a counting room that is 11 meters underground. Stuiver Ct al. report a limiting age of 61 000 years for a 6 gram sample. This has been extended to 74000 years by prior isotopic enrichment in a thermal diffusion column, a fivefold enrichment of ‘4C relative to ‘2C being obtained in two months processing of a 120 gram sample. Obviously such treatment has to be restricted to samples of particular importance but is is of general interest to note that wood samples do exist with ‘4C contamination levels which are more than a factor of ten thousand below the level of 14C level in modern wood. 4.3.1. Solid state techniques Jeifreys, Larsen and French [204] have reported the detection of beta particles from natural carbon by means of the tracks they produce in photographic emulsion. The technique is proposed as a cheap way of obtaining approximate dates on not-too-old samples.

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Another suggestion has been the use of thermoluminescence dosimetry [205]. The dose-rate within a sample of modern carbon is about 6 millirad per year. Although there are thermoluminescence dosimeter materials (e.g. rare-earth activated calcium sulphate) that are sufficiently sensitive to measure such rates with an exposure of a few months, the problems of background suppression appear to be insuperable because there is no equivalent to an electronically-operated anti-coincidence shield. Likewise there is no discrimination against contaminating alpha activity. 4.3.2. Laser enrichment As mentioned above isotopic enrichment by means of a thermal diffusion column is slow and requires large samples. Use of a laser is an attractive alternative and enrichment of ‘3C with respect to 12C has been achieved by Moore et al. [206, 212] using a laser beam to secure preferential photo-dissocation of formaldehyde. The enrichment factor that can be achieved depends on the amount of overlap between the rovibrational lines of the molecular electronic spectra of the different isotopes and in a feasibility study using a tunable dye laser in the region of 330 nanometers Hedges [207] has obtained enrichment factors for ‘4C with respect to 12C of the order of 100; the enriched sample is in the form of carbon monoxide which is collected in a liquid helium cold trap. An enrichment factor of 100 corresponds to an extension of the radiocarbon range by about 30 000 years but because the amount of sample that can be processed in a realistic time is limited to a few tens of milligrams, at any rate with currently available laser powers, it is only practical to use the process in conjunction with some form of mass spectrometer for measurement rather than conventional counting of radioactive decay. Following on from other mass spectrometric investigations [212, 213] Hall and Hedges [207] have proposed the use of a high transmission mass spectrometer (specially designed at the Rutherford Laboratory by J.R.J. Bennett) in which discrimination against 14N~ions and molecular contaminants is achieved through conversion of the ion beam from C~to C by means of a thin foil; this exploits the finding that the production in the foil of N from N~is very much lower than for C. The advantage of mass spectrometer detection for small samples, which has been pointed out by a number of authors from time to time, is readily appreciated by recalling that for ‘4C with its 5730 year half life during a day of conventional radioactivity measurement only about one per three million of the ‘4C nuclei present are registered. Hall and Hedges consider that an overall laser plus mass spectrometer efficiency of 10 4is feasible implying that 10000 counts could be obtained from only 2 milligram of initial sample. Of course the capability to utilize such small samples has tremendous advantages in archaeological application and its successful realization would open up new vistas: the dating of single seeds becomes a possibility and likewise the dating of collagen from small pieces of old bone. Prior enrichment is dictated by limitations in discrimination that are inherent in a low energy mass spectrometer. However, as mentioned below, successful detection of ‘4C using a Van de Graaff accelerator as a high energy mass spectrometer has been achieved without prior enrichment; nevertheless the laser enrichment is of considerable interest since it would allow extension beyond whatever age limit is reached with the Van de Graaff alone. 4.3.3. Cyclotron detection The utilization of a cyclotron as a highly sensitive mass spectrometer has been discussed recently by Muller [208] and using the 88-inch machine at Berkeley he has demonstrated its performance in respect of tritium dating, the 12.5-year half-life of 3H making it important in hydrology. The

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M.J. Aitken. Archaeological involvements of physics

cyclotron has a low background rate and a high mass resolution (i~\m/m< 3 x 10~).The acceleration of particles to several MeV, rather than several keV as with an ordinary mass spectrometer, makes it possible to distinguish nuclei of different charge which have the same mass number (such as 14C and ‘4N); this is by utilizing a detector that measures the particle’s rate of energy loss, dE/dx. Muller also points out that as application extends to older and older samples accuracy ~f 14~ measurement becomes less and less important; for instance an accuracy of ±10% means an age uncertainty of ±10% of the mean life, i.e. ±830 years, which is insignificant for a sample that is several tens of millennia old. He also discusses the detection of ‘°Bewhich is produced in the atmosphere from oxygen and nitrogen by cosmic rays and has a half-life of 1.5 million years. In utilizing a cyclotron for 14C the major sources of background foreseen by Muller is 14N from residual nitrogen in the sample and the positive ion source of the cyclotron; he estimates that it would be necessary to reduce this by a factor of a million in order to be able to utilize the dE/dx counter without risk of saturation and radiation damage effects and he discusses several ways in which this might be attempted. 4.3.4. Van de Graaffdetection Subsequent to Muller’s publication two successful experiments to detect ‘4C from natural samples have been reported using tandem Van de Graaff accelerators; at McMaster University by Nelson et al. [209], and at Rochester University by Bennett et al. [210]. These accelerators utilize negative ion sources and whereas there are stable negative carbon ions this was not expected to be the case for nitrogen; consequently the problem of ‘4N contamination should be absent and this was confirmed by Purser et al. [84]. Another advantage of the tandem Van de Graaff is that its ion source is external and easily adapted to sample changing. Also it can accelerate ‘2C, 13C and ‘4C simultaneously so that, using the 13C/’2C ratio to correct for isotopic fractionation in the sample, the date can be obtained from the 14C/12C ratio. Nelson et al. used a terminal voltage of 7 MV in which the negative carbon ions were converted to C4~by a helium gas stripper. Thus the particles had a final energy of 35 MeV and they were identified in a AE E detector telescope consisting of two Si surface barrier transmission detectors operated in coincidence; a third detector vetoed events in which ions passed through the first two. The sample used was a quarter of a gramme of charcoal obtained from a nineteenth century tree which was placed directly in the caesium sputter source of the acceleratOr. The particles entering the telescope have a finite range of magnetic rigidities but the total energy (E) measurement separates those having different mass numbers. Energy loss-rate (AE) measurement separates those which have different charge and the output from the detector is displayed as a two dimensional spectrum (see fig. 4.5). The peak due to 14C contains about 800 counts and the spectrum in the vicinity of the 14C peak indicated that less than 1 count in the 14C peak was due to background. This is equivalent to what would be expected from a sample having an age of 50000— 60000 years, an extremely impressive result for a first attempt in which the operating conditions of the accelerator were for from optimum for this specific application. Bennett et al. used a terminal voltage of 8 MV so that after stripping in a carbon foil the emergent C4~ions had an energy of 40 MeV. After passing through a second stripper and further bending magnets, the particles were detected using a sophisticated AE E ion chamber. Using less than a gram of contemporary charcoal as sample the ‘4C count-rate was about 300 per minute for a ‘2C current of 1.2 ~A. A background measurement was made using a sample derived from fossil fuel; for a 12C current of 10 j~Athe 14C countrate was only 05. per minute and even this —

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amount may 2C have been due to contamination with carbon. Thus normalized current the background is down bycontemporary a factor of at least 5000 andwhen this corresponds to the same ‘ to an age limit of more than 70 000 years, an extremely impressive first-shot result. Subsequently (private communication) these authors have measured several samples already dated conventionally and obtained agreement to within error limits. A seven or eight MV tandem Van de Graaff is hardly within the range of radiocarbon finance and one thinks that for routine use with the goal of obtaining the ultimate in terms of maximum age or minimum sample size it would be desirable to have an accelerator dedicated to radiocarbon measurement. A tandem Van de Graaff in the energy range 1 to 2 MV costs very much less and this may be adequate; however, selective detection becomes more difficult at lower energy and elimination of molecular contaminants by the stripper is less effective.

5. Location of buried remains 5.1. Introduction As with an iceberg, only a small fraction of a country’s archaeology is visible above the surface; the rest has been buried by gradual accretion of soil to depths varying between a few inches and

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some tens of feet. Often there is no surface indication until deep ploughing or building activity throws up fragments and there may be little time between the first discovery of an archaeological site and its total obliteration. This puts a premium, first on techniques for site discovery, and secondly, on techniques for rapid exploration of a site so as to reduce to a minimum the amount of fruitless trial trenching. Experience and intuition enable an archaeologist to see relationships between surface features and reliefs which are often insignificant to the layman and new sites continue to be discovered in this way. There are many sites too which are known by local tradition or legend, or which show themselves by obviously man-made disturbances, e.g. the circular banks and ditches of an Iron Age hill-fort. However, by far the most powerful weapon of site discovery is aerial photography, and because a site may only show up in this way at a particular time of day in a particular season in a particular climatic sequence, each year’s aerial surveying brings its new revelations. The role of geophysical techniques is in site exploration, by detecting abnormalities in the physical properties of the ground. These techniques should not be overestimated; they do not usually relieve the archaeologist of the need to use his spade but they do suggest the most fruitful spots in which to insert it. Of course there are occasions when a geophysical survey produces such a complete plan that valid archaeological interpretation is possible without excavation, but such occasions are rare and interpretations made without confirmatory excavation have to be used cautiously. Aerial photography sometimes produces such a complete and clear-cut plan for a site that a geophysical survey is redundant; however the situation is more often that the two are complementary the geophysical survey gives a more detailed plan, or gives precise location of features seen from the air only vaguely. Sometimes a site which records strongly on geophysical survey may be completely unseen from the air and vice versa. The geophysical techniques routinely used in archaeology are magnetic and electrical resistivity surveys. There are other techniques too, some of which are applicable in special circumstances, and others that have not yet been fully exploited. A general point is that the pattern of geophysical disturbance will not be less complex than that of the archaeological remains except when the sought-for feature has some physical property that is absent from the surroundings. Metal objects are one such type of feature; another is kilns and ovens, by virtue of the thermoremanent magnetism (TRM) in the baked clay of their structure. —

—

—

5.2. Magnetic prospection In a substantial pottery kiln there is the order of 500 kilograms of baked clay. Taking the specific TRM magnetization to lie in the range iO~to 10_i A/rn2 per kilogram (10~to 10’ emu per gram) the total magnetic moment should lie in the range 0.05 to 50 A m2 (50 to 50000 ernu). What remains of a kiln is usually irregular in shape but on the basis that the bulk of it is spherical, for a total moment of 5 A m2 (5000 emu) the resultant field at a distance of 2 meters is about 100 nanotesla (10~oersted); the usual unit in geomagnetic prospection is the gamma, equal to 1 nanotesla. Thus for the detection of a buried kiln we need an instrument that can easily resolve an “anomaly” of 1 part in 500 in the normal geomagnetic field of around 50 microtesla (0.5 oersted) in mid latitudes; for less substantial features such as oven, hearths and funeral pyres, the anomaly is at least an order of magnitude less. Besides the requirement of high sensitivity, rapidity and convenience are necessary for an instrument to be a practical proposition; a spacing of not more than a metre between readings is usually

M.J. Altken, Archaeological involvements of physics

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necessary so that to cover a hectare (100 m x 100 m) requires 10000 readings. The first instrument to fulfill these requirements was the proton free-precession magnetometer. Following the laboratory observation of free precession in the Earth’s field of protons in water in 1953 by Packard and Varian [214] this phenomenon was utilized for geophysical measurements, and in 1958 it was first used [215] by the Oxford Research Laboratory for Archaeology for archaeological prospection, specifically to locate an important group of Romano—British pottery in the vicinity of Durobrivae that were thought to lie under the new route for the Great North Road near Peterborough, England. The detecting head of the instrument consists of a 200-mi sample of proton-rich liquid such as water or alcohol contained in a polythene bottle, around which is wound a coil of about 1000 turns. This coil has two functions which it fulfills in sequence. First by means of a current of about 1 A it provides a d.c. polarizing field of the order of 10 millitesla (100 oersted) which aligns a fraction (about 1 in 10~)of the protons along the axis of the coil, which is positioned roughly East—West. Secondly, on cutting off this current after about one or two seconds, it is used to detect the rotating magnetic moment of the protons as they precess en masse in the process of lining up in the earth’s field. The Larmor precession frequency is about 2000 Hz for the field of about 50 microtesla (0.5 oersted) that pertains in mid latitudes, and the precision to which this frequency can be measured is the precision to which the intensity of the earth’s field is determined. For water and alcohol the relaxation time of the precessing protons is a few secohds and various techniques are available, e.g. a phase-locked amplifier, for measurement of the frequency of the induced signal to an accuracy of 1 part in iO~or better. This is amply sufficient to detect the size of anomaly mentioned above and on the majority of archaeological sites there is no point in measuring to better than 0.5 gamma since there are liable to be random variations of a few gamma in the field due to irregularities of the ground surface. Besides this spatial irregularity there is also temporal irregularity due ionization currents in the upper atmosphere. This irregularity is typically of the order ofa fraction of a gamma on a short term (minute to minute) basis but it is superimposed on a stronger diurnal variation usually of the order of 100 gamma. The strength of these fluctuations is linked to sunspot activity and during the active years .of the sunspot cycle it is advantageous to use an ancilliary reference detector in a fixed position; during magnetic storm days, when changes of the order of 100 gamma may occur within half an hour, such an arrangement is essential. The usual mode of operation is to divide the site into 10-meter squares and to survey each of these as a unit, positioning the detector at 1-meter intervals by means of a string mesh stretched over the square. One operator moves the detector and the other records the reading indicated by the electronic package, which is kept in a fixed position; in this way a 10-meter square can be surveyed in about 10 minutes. The height of the detector above the ground surface is typically 0.3 meter; this is a compromise between increased “soil noise” if the detector is nearer the ground and reduced sensitivity if it is further away. Figure 5.1 shows a plot of the readings obtained on a Romano-British site over a region in which there was a pottery kiln and a filled-in ditch, as well as a horse shoe. Needless to say, if iron litter is frequent the weaker anomalies due to archaeological features are masked and prospection is impossible. Nor is it possible to operate in the vicinity of iron fences, electricity pylons or steel framed buildings. Besides masking any archaeological features, the irregular fields may contain such strong gradients that the instrument cannot operate. This happens when the intensity at one end of the volume of the detector liquid is so different from that at the other end that the precessing protons are out of phase; the limiting gradient is about 20 gamma per meter. The linear anomaly shown in fig. 5.1 is due to the magnetic susceptibility of the filling of the

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ancient ditch being greater than that of the subsoil into which the ditch was dug. The importance of soil susceptibility was manifested during the first prospection for kilns in 1958; a number of 50-gamma anomalies turned out an excavation to be filled-in rubbish pits. Although this complicated the finding of kilns it substantially extended the archaeological scope of magnetic prospection. Ancient rubbish pits are of great interest to archaeologists, being a capsule in time of artefact types and pottery styles; the tracing of defensive ditches around military camps and hill-forts was another application that became possible as well as the general mapping of sites. Walls and buildings produce small anomalies due to the intrusion of non-magnetic stone into weakly magnetic soil, and using pattern recognition techniques meaningful interpretations can be obtained [2 16— 221]. The enhanced susceptibility of topsoil relative to the subsoil or rock from which it has been derived is due to the conversion of weakly magnetic haematite (x—Fe2O3) into maghemite (y— Fe2O3), the susceptibility of the latter being greater than that of the former by two orders of magnitude. The conversion, often referred to as the Le Borgne effect [222—226],proceeds by reduction to magnetite (Fe304) and subsequent re-oxidation. Two possible mechanisms have been proposed: (i) Fermentation at ordinary temperatures; this is favoured by alternating periods of humidity which give anaerobic conditions suitable for oxidation.

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337

(ii) The cumulative effect offire over the centuries ground clearance by burning being postulated as an integral part of ancient methods of cultivation. During the burning of vegetable matter, air is excluded from the underlying soil so that reduction occurs; during cooling, air reaches the soil and allows re-oxidation. Consequently, a thin layer of soil underlying the fire acquires an enhanced susceptibility and subsequent disturbance of the soil through cultivation, followed by further fires, ultimately produces enhancement throughout the topsoil. Although it has not been possible to demonstrate the fermentation mechanism experimentally, laboratory measurements indicate that the susceptibility of soil from rubbish pits is substantially higher than that of nearby topsoil or that of the silt from a ditch, suggesting that the fermentation mechanism makes an important contribution. The feature responsible for one of the strongest pit anomalies detected by the author was, on excavation, declared by the archaeologist to have been an Iron Age latrine. Similarly, on one or two occasions a magnetic survey has duly located last year’s Elsan pit. Further evidence for the importance of the fermentation mechanism is provided by Tite and Linington [227] in a study of topsoil, not associated with archaeological sites, from regions with different climatic regimes. They find, for instance, that in tropical soils for which thereis a pronounced dry period (that is, from the wet and dry tropics) the conversion to maghemite is in excess of 5 % whereas in soils from the rainy tropics the conversion is less than 2 %; they suggest that similar studies on paleosoils might yield information about ancient climate. Earlier work by Tite and Mullins [228] had demonstrated that in England at any rate the susceptibility of soils on archaeological sites was significantly higher than that of nearby soil not associated with archaeological occupation. Thus there is the possibility of using susceptibility as a means of locating the general whereabouts of sites. This could be done by laboratory measurement of collected samples or by field survey using an electromagnetic instrument of the type primarily designed as a metal detector (see section 5.4.3). The proton magnetometer being a passive device only responds to sharp changes in susceptibility; there is no anomaly, except at the boundaries, over a region of topsoil having uniformly enhanced susceptibility. —

5.2.1. The proton gradiometer Although the use of a fixed reference detector is adequate to deal with the diurnal variation due to ionspheric currents (except on magnetic storm days), in the vicinity of d.c. train lines where the changes are irregular and rapid, it is essential to make measurement at the fixed point simultaneously with measurement in the roving detector. A dual detector instrument for this purpose has been developed at the Rheinisches Landesmuseum, Bonn, by Scollar [221]; this has the additional facility of punched-tape output ready for computer analysis. A simpler though less precise method is to add together the proton signals from the two detectors before amplification and to deduce the difference in field from the observed beat frequency. Since the beat frequency for a difference of 10 gamma is only 0.4 Hz, and bearing in mind that the duration of the precession is limited to a few seconds by the relaxation time of the liquid, for sensitive operation it is a matter of measuring the time to reach “first zero”. Although this can be done electronically field experience indicates that the complexity is not worthwhile and that visual or audio observation of the composite signal is adequate. A simple version of such an instrument is shown in figure 5.2 and various circuits have been published [e.g. 229—23 1].

338

M.J. Aitken, Archaeological involvements of physics

MuItivibrator~

Meter Fig. 5.2. Simple version of proton gradiometer: block diagram.

5.2.2. Other types ofmagnetometer On sites where the anomalies are weak and the soil noise is low there is advantage in an instrument having higher sensitivity than the 1 gamma obtainable with the standard proton magnetometer. Various ingenious developments have been incorporated that give almost an order of magnitude improvement while still using the basic free precession phenomenon but there are two other types of nuclear magnetometer that have been extensively used in archaeological prospection. One of these, the caesium magnetometer based on optical pumping, has been extensively used [232, 233] by the University Museum Philadelphia (MASCA) since 1964. Its sensitivity is 0.005 gamma; another advantage is that its indication is continuous so that it is well-suited to rapid-scan surveys. The other is the double resonance proton magnetometer in which alignment is achieved by means of the Oberhauser-Abragam effect; as used by the Centre d’Etudes Nucléaires de Grenoble [234] this has a sensitivity of 0.01 gamma, and like the caesium magnetometer its indication is continuous; both types are differential as is essential if use is to be made of their high sensitivity. The fluxgate gradiometer also gives continuous indication; its sensitivity is the order of 1 or 2 gamma. This type of magnetometer initially developed [235] for archaeological application at the Oxford Research Laboratory for Archaeology and now, after further development and automation, it is in routine use in Britain by the Ancient Monuments Branch of the Department of the Environment. The two fluxgate elements are mounted, with axes vertical, at either end of a specially constructed rigid rod, about 1—1~meter long, which is carried approximately vertical; rigidity is essential because a fluxgate element measures the component of the field along its axis and in order to keep false signals due to bending below 2 gamma the alignment of the two elements must be maintained to within 0.06 degrees. With the nuclear magnetometers it is the total intensity that is measured and the detector need only be orientated roughly. 5.3. Electrical resistivity surveying Like the magnetic technique, resistivity surveying in archaeology is a scaled-down version of a geological technique. Its first archaeological use was in England in 1946 by R.J.C. Atkinson. Resistivity is largely dependent on water content and one intuitively expects wide differences between stone, clay, wet soil, dug soil, sand, etc. Walls, building foundations, roads, and ditches show up clearly with this technique; tombs, pits, underground cavities have also been detected.

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The instruments available for resistivity measurements are more tedious in operation than those for magnetic prospecting. In particular, probes have to be temporarily inserted a few ifiches into the ground in order to make electrical contact. However, the drawback of interference from iron and igneous rocks is absent, though on the other hand resistivity measurements are seriously affected by rainfall. In general with this technique and also to some extent with the magnetic one, while some archaeological features show up sharply whatever the terrain, it is not always possible to distinguish others from small-scale geological effects. To avoid interference by contact resistance it is usual to employ the four probes in line; the outer pair carry the current and measurement is made of the voltage developed in the ground between the inner pair. It is usual to have the probes equally spaced and the detection depth is roughly equal to the interprobe separation. 5.4. Electromagnetic techniques For large scale geological resistivity surveys the need to insert probes is not a serious drawback but for the closely-spaced readings that are required in archaeological application the consequent reduction in the speed at which the ground is covered is substantial. Various attempts have been made to circumvent this handicap but so far without practical success. One of these was the socalled Soil Conductivity Meter (SCM) an instrument of the type indicated in fig. 5.3; it consists of a transmitter coil and a receiver coil mounted on the same shaft about a meter apart. The transmitter coil is fed with a continuous sinusoidal current, at a frequency of 4 kHz in the standard SCM. The plane of the transmitter coil is vertical whereas that of the receiver coil is horizontal; the receiver coil is orientated relative to the transmitter coil so as to avoid any pick-up of the primary transmitter field, and the only signals received are due to the secondary fields from eddy currents and magnetizations, induced in the ground by the primary field. Field trials with the SCM at South Cadbury Castle have shown that although it can detect pits and ditches this is due to the induced magnetization signal rather than the eddy-current signal. Consequently it is not a substitute for conventional resistivity measuring techniques; as substitute —

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for the proton magnetometer it has the advantages of simplicity, cheapness and convenience of operation, but the disadvantage that its depth of detection is less and so it is only effective for shallow, near-surface features. This is because with the SCM there is both a go and return signal and the sensitivity falls off as the sixth power of the depth whereas with the proton magnetometer there is no transmitted signal and the sensitivity falls off only as the third power. Theoretical considerations and model experiments confirm [236] that at the standard frequency of 4 kHz the SCM response is dominated by the magnetic signal. Since the conductivity signal is 90°out of phase with the magnetic signal use of a phase sensitive detector enables the two to be separated; however although this can be done (and is done in a number of commercially available instruments) the depth at which a conductivity feature can be detected remains pitifully small in comparison with the capability of a probe-type instrument. The model experiments indicate, for instance, that for a pit of depth 0.65 meter and diameter 0.45 meter filled with topsoil and dug into non-conducting subsoil the conductivity signal from the pit becomes less than 5 % of that from the layer of topsoil under which it is buried as soon as the topsoil layer exceeds 0.3 meters; for the magnetic response the corresponding thickness of topsoil layer is 0.6 meters.* 5.4.1. Use of radio transmissions The depth of detection with an instrument of the SCM type would be increased by positioning the transmitter coil at a greater distance from the receiver coil, given that the transmitter power could be appropriately raised. However, any significant increase beyond the usual separation of about a meter would make the instrument unwieldy, particularly since the rod carrying the two coils must be rigid enough to keep the mutual orientation of the coils sufficiently precise to avoid linkage of the primary field. The need to have the receiver rigidly linked to the transmitter is avoided if the transmitter is at a sufficient distance from the site for the primary field to be uniform in strength and direction over the whole area of the site. This has been achieved in a novel way by using the routine transmissions of radio broadcasting stations. In the field trials carried out in France at the Centre de Recherches Geophysiques, Garchy, by Tabbagh [237], the sites were at distances of the order of 500 kilometers from the transmitters, which were on frequencies of 164 kHz and 180 kHz and had power of about a megawatt. The transmitter primary field has its magnetic vector horizontal, so by having the axis of the receiver coil vertical, pick-up of the primary field is avoided. The receiver coil is made compact by using a ferrox core and the axis kept vertical simply by allowing it to hang freely under gravity. Magnetizations and eddy currents induced in the ground by the primary field (strength about 1 gamma) produce a localized secondary field having a vertical magnetic component, and this gives rise to a signal in the receiver coil. If there is variation in the magnetic or electrical characteristics of the ground, the strength of the signal varies from point to point. (Fig. 5.4 shows the profiles obtained in traversing across a tumulus.) Strong signals are to be expected from long linear features (such as walls, ditches, and roads) but the signal becomes zero if the feature is perpendicular to the direction of the transmitter; it is therefore desirable to utilize two transmitter stations having bearings from the site that differ by about 90°.Calculations show that for a circular cylinder of diameter 1 m buried so that its axis is 1 m deep, the vertical secondary field strength is 4 % of the primary field strength for a resistivity contrast of 100 ohm-meter. * The foregoing was written before my attention had been drawn to the EM31 (Geonics, Ontario) in which the coil spacing is 3.7 meter.

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Fig. 5.4. (a) Secondary magnetic fields (verticalcomponent) due to radio transmissions observed in traversing through centre oftumulus surrounded by stone-faced bank and ditch (from Tabbagh [237]). The traverse made an angle of84°with the direction of one transmitter (1) and 63°with the direction ofthe other (2); the frequencies were 164 kHz (Allouis) and 180 kHz (Europe 1) respectively. (b) Resistivity measurements along the same traverse using Wenner configuration with an interelectrode spacing of 1 m.

5.4.2. Radar Various commercial radar systems are available for subsurface profiling and one of these has been utilized by Bevan and Kenyon [238] of the University Museum Philadelphia (MASCA) to survey an historic site containing buried walls and cellars etc. The system used had a peak power of 35 watts, an average power of 5 milliwatts, a peak frequency of about 100 megahertz, a pulse length of 15—30 nanoseconds and a pulse repetition rate of 50 kilohertz. Radar has also been used by Michael [239] in the search for fossil bristlecone pine logs, required for extension of the radiocarbon calibration, in alluvial deposits, as well as by others at Chaco Canyon, New Mexico [240], and at the pyramids at Giza. 5.4.3. Metal detectors In archaeology, metal detection is only of use in special cases, e.g. when large metal objects are among the primary objectives of the excavation such as in an Iron Age chariot burial or in obtaining prior warning of small metal objects during actual excavation. The limited utility stems partly from the short range of detection, and partly from the frequent occurrence of unwanted metal objects in the surface. Nevertheless in the hands of over-enthusiastic amateurs metal detectors constitute a serious threat to the intact preservation of archaeological sites; this is not because large quantities of archaeological objects are actually detected, but because random holes are dug into sites in following up false indications. Besides iron litter on the surface, such indications may arise because of the magnetic response from the soil. The conventional metal detector, which has been the subject of a great deal of electronic sophistication by thilitary and civil authorities and by treasure hunters both on land and under water, is a continuous wave device employing either separate transmitter and receiver coils as in the SCM or a single coil forming part of a sharply-tuned oscillator circuit. In either case a rigid coil system is essential and since greater —

—

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depth of detection requires a proportionally larger system this is a cumbrous requirement to meet in archaeological application and even more so in underwater use by a diver. In the Pulsed Induction Metal Detector, developed for archaeological application by Colani [257], pick-up of the primary field is avoided by keeping the receiver circuitry inactive for the duration of the transmitter pulse and no reliance is placed on the rigidity of the coil system; it can also be of whatever shape best suits the application. The operation of the instrument is as follows (see figs. 5.5 and 5.6). The primary field transmitted into the ground consists of a d.c. magnetic pulse lasting for about ~ millisecond. Although this induces a voltage in the receiver coil both at switch-on and at switch-off, by keeping the receiver electronics inactive until 50 microseconds after switch-off there is no indicated signal from the receiver. (The coil inductances are low so switch-off can be achieved in a few tens of microseconds.) However if a metal object is linked by the transmitter field, eddy currents will be induced in the object at switch-off (as well as at switch-on). These eddy currents persist after switch-off, decaying away slowly for a good conductor and rapidly for a bad one. Thus in this case the receiver coil is linked by a secondary d.c. magnetic field, and a voltage is generated due to the changing magnetic linkage as this field decays. Ablock diagram of the instrument developed by Colani [244] is shown in fig. 5.7. The transmitter is pulsed at a repetition rate of 11 times per second (it is important to avoid sub-multiples of 50 Hz); during each pulse a current of 15 Ais passed through the transmitter coil, which consists of 6 square turns of side about 1 meter and has an overall inductance of about 200 microhenry. The receiver ioop is of 14 turns, of side about 0.6 meters and having an overall inductance of about 400 microhenry. The receiver is controlled by means of a sampling pulse which activates it 50 microseconds after each transmitter switch-off for a duration of 25 microseconds. The receiver pulses are integrated over a second, and the output displayed on a meter. Various dimensions of coil can be used and considerable development of the circuitry has taken place since the prototype model just outlined. A later design has been described by Foster [242]. Besides the advantage of flexibility in coil design, the pulsed device is more sensitive than

Receiver Transmitter

~ /

/

/,

Transmitter held

/ ,/,I

/

~I

~

1—teld from eddy currents in metal object

Fig. 5.5. Pulsed induction metal detector: coil system.

M.J. Aitken, Archaeological involvements of physics

343

(a)J —~

(b)

millisec—

v

Fig. 5.6. Pulsed induction metal detector: waveforms (after Colani [244]). (a) Primary magnetic field pulse produced by the transmitter coil. (b) Receiver response in absence of metal object. (c) Receiver response with metal object present. The shaded area indicates the time during which the voltage is sampled.

Transmitter loop

Tr:nsmitt:r

Receiver loop

Amplifier Sapler

Integrator Meter

Sampling pulse generator Fig. 5.7. Pulsed induction metal detector: block diagram (from Colani [244]).

commercially available metal detectors of the conventional type; for instance a brass disc of diameter 0.3 m and thickness of 3 mm can be detected at a distance of 1.5 m. Like the conventional detectors its usable sensitivity is ultimately limited by magnetic response in the soil [243] though by a more subtle phenomenon (magnetic “viscosity” see section 3.2) and less seriously. In underwater use it is less affected by the conductivity signal from sea water. In early years of use it went by the name DECCO Decay of Eddy Currents in Conducting Objects. —

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5.5. Other possibilities 5.5.1. Radioactivity Most soils and rocks contain significant amounts of uranium, thorium and potassium and there are substantial variations in content in the various constructional materials used by ancient man. Portable gamma spectrometers are conveniently available from mineral exploration but the practical problem is one of depth of detection. If all energies of gamma ray are accepted the response is determined predominantly by the radioactive content of the upper layers and the contribution from soil or rock deeper than 0.3 meter is small. On the other hand if, in order to obtain greater detection depth, only high energy gamma rays are accepted, the count-rate becomes impractically low. Alvarez et al. [241] have utilized cosmic ray muons in an attempt to detect a possible second burial chamber in the pyramid of Chephren near Cairo. A directional detector was set up in the small unfinished chamber located centrally in the pyramid at about ground level and the flux of particles coming from different directions measured. It is estimated that an abnormality of one meter in thickness would just be detectable and consequently the presence of a burial chamber would cause the flux from that direction to be abnormally high. The experiment was very slow because the overall count-rate inside the 450-foot-high pyramid is only about one-sixth of the outside count-rate of about 1 per cm2 per second. Another nuclear technique that has been tried [245] is neutron scattering for the detection of buried walls and cavities. The probe, which was laid on the ground, consisted of a source of fast neutrons and a detector sensitive to slow neutrons. The notion was that the count-rate in the detector would be a measure of the moisture content, much higher in soil than stone, within a hemisphere of 0.2 meter. Although some significant spatial variations were noted in the trial it was concluded that the effect would not be of practical value except on sites with very thin soil cover. 5.5.2. Miscellaneous The use of gravity surveys has been considered by Linington [246], particularly in respect of locating large empty chambers; th’ese latter are one of the most likely types of feature to have sufficient density contrast to make detection possible. Even so the estimated anomaly is only 0.1 milligals, i.e. a change in the force of gravity by 1 part in 10~.Field measurements failed to locate any significant anomalies over known chambers though this may have been due to instrumental difficulties (the overall spread in repeated measurements being 0.3 milligals). Another limiting factor is the extreme sensitivity to distance from the centre of the Earth, a change of about 0.25 milligals occurring for a change in height of 1 meter. Seismic techniques are powerful in geophysical exploration but they do not lend themselves to the smaller and more detailed scale required for archaeology. A more promising development [247] is the “sonic spectroscope”, for the detection of cavities in walls and possibly also in the ground. The resonant frequency of a wall of solid brickwork is very low, in the range 5 to 20 Hz, but where there is a cavity the resonant frequency is higher. The wall is caused to vibrate by means of a loudspeaker emitting sound-waves of gradually increasing frequency (e.g. from 20 to 3000 Hz). The response of the wall is detected with an accelerometer or geophone, and recorded on a magnetic tape as a function of frequency; in order to improve the signal-to-noise ratio the first recording at any given spot is then used to control the relative amplitudes of the frequencies emitted by the

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loudspeaker on a second run, and by repeating the whole cycle several times the occurrence of any abnormal resonant frequencies can be detected. In underwater prospecting, side-looking sonar is the most promising of any of the prospecting techniques. A Roman wreck at a depth of 100 meters has been located [248] with this technique by the Scripps Institute of Oceanography. The equipment, which was towed at a depth of about 50 meters, consists of two transmitting hydrophones fed every ~ second with 200-watt pulses of 100 kHz carrier frequency, together with two associated receivers linked to recorders in the surface craft, the recorder trace being triggered by the transmitter pulses. The transmitting hydrophones provide a vertical fan of sound energy (of horizontal width ~°) on either side of the towed vehicle at right angles to the direction of motion. The maximum range of the device is about 400 meters. In a smooth-bottomed region, a more or less steady response is seen on the recorder trace, mainly from the sea-bottom fairly close-in. A wreck above the bottom gives rise to an additional re3ponse and its range can be determined by the distance of the response along the time-axis of the recorder. Although instrumental seismic techniques have not proved fruitful on land, it should be mentioned that some archaeologists have good success with the simple technique of bosing in which the ground is thumped with a pickaxe handle; over pits, tombs, and ditches there is a distinctive resonant note, the intensified wavelength being governed by the size of the feature beneath. Bosing is successful where the topsoil is thin and firm, with an unstratified rock such as chalk beneath. Another simple technique employed by some archaeologists is to observe their sites when under snow and to note the pattern of melting. The possibility of utilizing the differing thermal conductivity of different features is being studied by Tabbagh [249] based on temperature gradients associated with diurnal and annual heat flow; detection is by means of an airborne scanning radiometer operated in the thermal infra red, wavelength range 8—14 micron.

Acknowledgments I am 3.1, 3.3, Physics fig. 3.2,

grateful to Oxford University Press for permission to reproduce figs. 2.1, 2.2, 2.3, 2.4, 2.5, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.1, 4.2, 4.3, 4.4, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, and 5.7 from my book and Archaeology (2nd ed.), to the editors of Archaeometry for permission to reproduce and to the editor of Science for permission to reproduce fig. 4.5.

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