- Email: [email protected]
Selective filtering in PIXE spectrometry
Nuclear Instruments North-Holland
SELECTIVE
and Methods
FILTERING
in Physics
Research
B49 (1990) 65-69
65
IN PIXE SPECTROMETRY
C.P. SWANN Bortol Research Institute,
Unruersity of Delaware, Newark,
DE 19716, USA
S.J. FLEMING MASCA,
The Unioersrty Museum.
lJnicrersi@ of Pennsduaniu.
PA 19104, USA
The Bartol Research Institute’s PIXE facility has made extensive use of selective filters in the study of compositional patterns in archaeological artifacts. The purpose of such filtering is to reduce the intensity of the primary X-rays by up to two orders of magnitude. This allows for an equivalent increase in the beam current and a corresponding reduction in the detection limits for the higher Z trace elements. In this report we bring together all the filter combinations we currently use and discuss their purpose and effectiveness.
1. Introduction
The authors have now collaborated for more than eight years in the development and application of PIXE spectrometry. Over that time there have been a host of changes in equipment components, and several small shifts in our research protocol, but practical experience continues to confirm our initial belief that this technique is well suited to the study of the composition of a wide range of archaeological materials. By way of a summary of what we regard are the PIXE spectrometry’s primary strengths, we note that: i) It is a non-destructive technique. Therefore it works well within the constraint often imposed in museum work, that an artifact suffers the minimum aesthetic damage possible in curation [l], yet serves as a resource for knowledge about both the cultural and technological aspects of our past. (Some pretreatment of the artifact’s surface may be necessary, such as a light abrasion to remove corrosion products or accumulated grime, but not drilling or cutting.) X-ray fluorescence (XRF) spectrometry shares this strength: several other analytical tools - e.g., basic wet chemistry, neutron activation analysis (NAA), atomic absorption (AA) spectrometry, inductively coupled plasma (ICP) spectrometry, and scanning electron microscopy with an adjunct energy dispersive X-ray detector (SEM/EDAX) - do not. ii) It has high spatial resolution. When the Bartol PIXE facility is run in a “microbeam” mode, the area scanned on the artifact’s surface is typically 0.5 mm2 [2,3]. Therefore it works well in a number of interesting situations, such as the study of a smear of solder bonding a granulated decoration on an item of Classical jewelry [4], an individual line of ink on an medieval 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
Bible [4], or a single strand of color marvered into the surface of an Egyptian glass vessel [6]. Conventional XRF, with a minimum study area of 4 mm* or so [7.8] does not share this strength. More specialized forms of XRF. particularly the synchrotron-based “light source” [9], have very good spatial resolution, but such facilities are rarely accessible for the kind of archaeological research discussed below. Wet chemistry, NAA, AA, and ICP are simply not applicable in these circumstances. SEM/EDAX does, of course, have superior spatial resolution if the artifact is quite small (i.e., only a few centimeters across and deep) and can be conveniently and safely mounted in a vacuum during analysis. iii) It offers high detection sensitivity for many trace elements in a wide range of matrices. We have previously reported in some detail the PIXE detection limits for bronze, iron, glass, copper and iron ores, and metal-working slags [lo-121. We note here only that most of the elements which might characterize the raw materials of these matrices, or might significantly influence their working properties, are readily detected at the 100 ppm level. XRF is also a sensitive tool, provided the excitation source is energy-tunable in some way (as is the case with the synchrotron-based “light source” [9]). The broad energy bremsstrahlung sources routinely used in XRF facilities create Compton scattering tails in the X-ray spectrum that tend to limit detection sensitivities to be an order of magnitude poorer than those achievable by PIXE facilities run in a similar measurement configuration [13,14]. Wet chemistry, AA, and ICP are all capable of very good detection limits for a wide range of elements [15], though the preparatory chemistry is sometimes complex and not suited to small samples. NAA is an exceptionally sensitive tool for II. EXPERIMENTAL ARRANGEMENTS
C. P. Swam,
66
S.J. Fleming / Selective Jiltermg
certain elements - e.g.. most of the rare earths, hafnium, lanthanum, manganese, and scandium - that have proven crucial for clay source discrimination in the study of ancient ceramics (see refs. [16.17], among many others). But there are other elements that are hardly detectable by NAA - e.g., silicon, sulfur, lead, silver. and tin - all of which are recognized as important in the study of ancient metals and glassy materials. SEM/EDAX detection limits are invariably far poorer than PIXE. because the bremsstrahlung contribution to the background is inversely proportional to the mass of the particle used as the excitation source. iv) It is efficient in terms of the turn-around time for data production. The various X-ray spectra that we tie together to characterize an artifact’s composition (see below) usually takes less than an hour to gather (leadrich glass being the most problematic matrix; see ref. [18]). XRF shares this advantage of speedy operation: wet chemistry, AA, and ICP tend to be slow because of their preparatory chemistry. NAA data collection is slowed by the fact that it relies on detection of low levels of induced radioactivity of a range of elements with widely varying neutron cross-sections, and with decay half-lives that may differ by several orders of magnitude. Consequently, a series of activation exposures of different neutron fluxes, and a series of separate y-counting procedures (sometimes months apart), are needed to obtain the most reliable data.
2. Selective
filtering
PIXE spectrometry uses as its excitation source a monoenergetic beam of protons (sometimes a-particles. sometimes deuterons). At first sight, therefore, the technique’s non-tunability would seem to work against its effectiveness as an analytical tool. But the fact that the excitation source is typically 1 MeV or above in energy means that cross-sections for X-ray production tend to be very high. A significant problem does arise, however, from the fact that those same cross-sections fall off sharply as a function of the atomic number of the element observed (19,201. At the same time, the efficiency of the Si(Li) detector also decreases with atomic number, setting up a detection bias strongly in favor of lower Z elements. As a result, PIXE spectrometry suffers from quite severe “pile-up” and count-rate limitations [19], particularly in the study of soda-lime glasses and other silicates such as glazed faience [21.22]. Here we should separate our work in PIXE spectrometry from that of most of our colleagues. The major research areas for the technique are soft tissue and blood cell medicine, plant physiology, and environmental pollution (see the review in ref. [lo]). The matrix (of substrate) for the trace impurities of interest is always a low Z organic compound. In these circumstances, pile-
up effects are near-negligible, and count rates are limited only by the very real risk of irrevocable damage of the matrix. At the same time, the bremsstrahlung contribution to background is minimal, allowing the PIXE detection limits to be more in the range of 10 ppm or less. For our part, the lowest Z matrix we contend with is silica; and more recently we have extended our interests into the study of the far higher Z matrices associated with leaded bronzes and gold alloys 1221. While a significant portion of the pile-up difficulties we face is routinely overcome by electronical rejection of count coincidence 1191, that will not remove the so-called “sum peak” which represents the true coincidence of two dominant X-rays within the time resolution of the detection and processing components of the system. “Sum peak” removal requires the filtering out of the high intensity X-ray contribution from the lower Z elements in some way. Use of such filtering is by no means a new idea. A decade or so ago it was common practice to include a thin low Z absorber (e.g., kapton, aluminum, or some kind of plastic) in the gap between an artifact’s surface and the face of the Si(Li) detector, so that the intensity of the lower Z X-ray contribution was simply degraded by attenuation. Thereafter these absorbers were modified to become pin-hole filters, to allow simultaneous measurement of a controlled fraction of the lower Z X-ray contribution [23,24]; and then stepped filters, so that the X-ray degradation was variably softened as a function of Z [25]. Rather than depend upon just X-ray attenuation effects, we have preferred to find ways to selectively suppress the X-rays from the dominant component of the matrix. In essence, that has meant taking advantage of the fact that the K, X-ray of an element is either completely or partially overlapped by a K-absorption edge of a lighter element. Thus, the Al Kabs (at 1.56 keV) will heavily suppress the Si K, (at 1.74 keV); the Co Kabs (at 7.71 keV) will heavily suppress the Cu K, (at 6.08 keV); and so on. In a similar way, the L, X-ray of some of the heavier elements is either completely or partially overlapped by a K-absorption edge of a light element. Thus, as Demortier has turned to his advantage so well in his study of ancient gold artifacts [26-281, the Zn Kabs (at 9.659 keV) will heavily suppress the Au L, (at 9.705 keV). Beyond this, we further manipulate the PIXE detection efficiencies by running the proton beam’s energy at about 2.0 MeV for the analysis of higher Z elements, but at 1.3 MeV for the analysis of lower Z elements. The reasoning here is that the X-ray yield will be increasingly attenuated the deeper the excitation occurs within the artifact. and that attenuation will affect the less energetic X-rays (i.e., those from lower Z elements) more severely. Thus, the use of a 1.3 MeV beam energy effectively enhances the observation of lower Z elements. All elements down to Si (Z = 14) are detected
C.P. Swam. S.J. Fleming /
Niql+ CoK,
CuK,
CuKp
I
I
I
t a
CoKp
t
t
1
1C
z
E
2 0
%
s 5
s 1c
l(
7.0
8.0 X-ray
energy,
9.0 keV
Fig. 1. A comparison of two PIXE spectra for a bronze matrix, each of which was gathered with the proton beam energy set at 2.0 MeV, but with a different array of filters included in front of the detection systems’s collimator: (a) selective filter (15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil); (b) simple absorption filter (123 mg/cm2 Si wafer). Note how both the filter arrays suppress the Cu K, relative to the Cu Ka to almost the same degree (their intensity ratios without filtering is close to 5 : 1). Quantitative analysis for nickel and cobalt was based in part upon the U.S. NBS reference materials #1275 (Ni. 9.76%; Co. 0.024%) and #Cl123 (a beryllium copper with Co. 2.3%).
well in this mode. since their X-rays are able to penetrate the air gap between the artifact’s surface and the input point for the detection system (a fine collimator in front of the thin Be window of the detector’s face). To measure as low as Na (Z = 11) - as is crucial in the study of glass - that gap has to be bathed by a He gas stream during PIXE measurement. The impact of selective filtering in the analysis of a tin-bronze matrix (Sn, 3.5%) is illustrated in fig. 1. There the effectiveness of a selective filter configuration (15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil) is contrasted with that of a 123 mg/cm2 pure Si wafer acting as a simple absorber, over the X-ray energy range of 6.6 to 9.1 keV. With the selective filter, even though the Ni K,
Selectirre Jiltering
67
is only a low energy shoulder on the Co K, peak produced when the intense Cu X-rays from the bronze causes fluorescence in the Co foil, the Ni detection limit is still only about 95 ppm. With the simple absorber, the Ni K, X-rays are degraded to such an extent that it can scarcely be resolved against background until the artifact Ni content is more than 2%. A detection limit as poor as this is quite unacceptable in archaeological studies, since nickel is regarded as one of the primary elements for ore source discrimination in the study of ancient bronzes ]301. The various selective filters we routinely use at the Bartol PIXE facility are listed below. along with how they effect the X-ray spectrum. In each instance, the proton beam energy is at 2.0 MeV. unless otherwise stated. Bronze. Matrix: primarily Cu with additives of As and/or Sn. in the range of l-15%. Selective filters: i) for Z 2 27 < 29: 9.8 mg/cm2 Fe foil + 7.5 mg/cm2 V foil. This is a filter combination well-suited for Co determination [22]. ii) For Z > 29: 15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil. The Co foil suppresses the Cu K, and K, X-rays. The V foil cuts out the fluorescence which intense Cu X-rays excite in the Co foil. These filter combinations can also be used in the study of other copper-based materials such as smelting mattes, prills entrapped in slag [31], ingots and crucible spills; and for the study of silver alloys and artifacts rich in antimony (e.g., beads, and Sb-rich bronzes). Iron. Matrix: primarily metallic iron with some silicate-rich slag remnants occurring as stringers. Selective filters: i) For Z 2 26: 15.0 mg/cm2 V foil + 15.0 mg/cm2 muscovite. The V foil suppresses the sum peaks at 12.80 keV (Fe K, + Fe Ka), and 8.14 keV (Fe K, + Si Ks). The potassium-rich muscovite cuts out the fluorescence that intense Fe X-rays excite in the V foil. Copper smelting slag. Matrix: primarily an iron silicate (either fayalite or wustite), with copper either dissolved in the matrix, or occurring as discrete prills. Selective filters: i) For 20 < Z I 26: 3.8 mg/cm2 Al foil (*proton beam, 1.3 MeV). The Al foil suppresses the sum peaks at 3.46 keV (Si K, + Si Ka). ii) For Z 2 26: 15.0 mg/cm’ V foil + 15.0 mg/cm2 muscovite (as for iron: see above). These filter combinations are also used in the study of iron-rich silicate minerals such as vesuvianite, and iron oxides such as hematite and limonite [12]. Besides suppressing the sum peaks due to the Fe- and Si-contents of these materials, the V foil then also suppresses the sum peak at 10.09 keV (Fe K, + Ca K,). Glass. Matrix: primary silica with soda and other alkalis that sum to as much as 20% by weight; and various heavy elements that are responsible for coloration. Selective filters: i) For 20 I Z I 26: 3.8 mg/cm2 Al foil (*proton beam, 1.3 MeV). The Al foil suppresses the sum peaks at 3.48 keV (Si K, + Si K,), 5.43 II. EXPERIMENTAL ARRANGEMENTS
68
C.P. Swunn, S.J. Fleming / Selectrve filtering
keV (Si K, + Ca K,), and 7.38 keV (Ca K, + Ca K,). ii) For Z 2 26: 1.9 mg/cm’ Al foil + 7.5 mg/cm’ V foil + 15.0 mg/cm2 muscovite. The V foil suppresses the Si and Fe sum peaks as it does for iron-rich silicates (see above), but it is usually used in a lesser thickness, since the iron levels in glass are typically in the range of 0.5-3%. dependent upon whether it occurs as an accidental contaminant brought in with the sand used in glassmaking [32], or as a deliberate additive. Copper ores. Matrix: primary silicon- and calciumrich minerals (quartz, grossular garnet, wollastonite, etc.), with the copper-bearing minerals such as malachite or chrysocolla occurring intrusively [12]. Selective filters: i) For 20 2 Z I 26: 3.8 mg/cm2 Al foil ( *proton beam, 1.3 MeV) as for copper smelting slags: see above). ii) For Z > 29: 15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil. This filter combination, besides suppressing the Cu sum peak, as it does for bronzes (see above), now also suppresses the sum peaks at 9.79 keV (Cu K, + Si K,), and 11.74 keV (Cu K, + Ca K,). Gold. Matrix: primarily metallic gold, with additives of silver and/or copper, in the range of l-50%. Selective filters: i) For Z 2 20: 3.8 mg/cm2 Al foil + 15 mg/cm2 muscovite + 21 mg/cm’ brass foil. The Al foil suppresses the Au M X-rays. The copper and zinc in the brass foil suppress the Au L, and La X-ray peaks at 9.65 keV and 11.44 keV, respectively, and effectively remove all related sum peaks. As one might expect there are several areas of common ground between these various material categories, as far as selective filtering is concerned. The choice of filtering is often a judgement call based upon the kind of information sought from the PIXE study. For example, with lead-rich glasses (PbO > 4%). where either the glass has been deliberately colored yellow by addition of lead antimonate [6,21], or some lead oxide has been
added to bring down the melting point of the glass melt [18,33]. the PIXE data have been supplemented by a run with the brass-based filter used in the study of gold. Similarly, when a glass has a quite high Fe-content (Fe,O, > 5%). the filter combination used for Fe-rich slags, etc. (with a double thickness of V foil), becomes more appropriate (see table 1). That filter combination cannot be used for low Fe glasses, because the Fe K, line is the link between the lower Z and higher Z spectra, and too much filtering makes measurement of its intensity statistically problematic. At the same time, the use of a triple thickness of V foil (i.e.. at a thickness of 22.5 mg/cm2) only complicates such studies, because heavier elements that may be involved in the coloration process, such as Cu. Zn and Pb, will fluorescence the trace levels of iron in the detector’s Be window. This fluorescence will produce a false set of the Fe X-ray peaks that, in intensity, will rival or exceed the small amount of artifact-related Fe X-rays that manage to get through such a strong selective filter.
3. Conclusions Archaeological materials are so diverse in their composition, we would hesitate ever to put foward the filters listed above as ideal for all applications. Only recently, we have initiated studies of various kinds of commercially available wrought aluminium sheet which, because of practical requirements such as bendability, etc., have minor amounts of medium Z impurities deliberately added to them. Here we have a range of materials which combine the properties of simple absorption and selective filtering in a way that may offer us further gains in the detectability of high Z elements in complex matrices. Overall, however, we
Table 1 The effect of vanadium foil as a selective filter in the study of Fe-rich
silicate matrices
Copper smelting slag from the Iranian Iron Age site of Tepe Hissar (SiOz. 39.0%: CaO. 11.5%; FeO, 16.2%; CuO. 0.15%; ZnO, 4.2%) Filter: 3.8 mg/cm* Al foil + 15 mg/cm’ muscovite + 7.5 mg/cm2 V foil Ag,O, 5 5190 ppm; Sb,O,, _< 11300 ppm; SnO. 5 8270 ppm: BaO, < 40520 ppm Filter: 15 mg/cm’ muscovite + 15 mg/cm2 V foil Ag,O, 5 260 ppm: Sb,O,. 5 805 ppm: SnO, 5 245 ppm; BaO,
2 1870 ppm
A dark blue frit bead from the Late Bronze Age Iranian site of Dinkha (SiO,, 51.3%; CaO, 3.8%; FeO. 5.2%; CuO, 20.3%; ZnO. 0.97%)
Tepe
Filter: 3.8 mg/cm2 Al foil + 15 mg/cm2 muscovite+7.5 mg/cm’ V foil Ag,O. 5 1750 ppm; Sb,O,, _< 5630 ppm: SnO, 5 3960 ppm; BaO. 5 16000 ppm F&a-: 15 mg/cm’ muscovite+ 15 mg/cm’ V foil AgzO. 5 88 ppm; Sb,O,. _i 130 ppm; SnO. I 130 ppm; BaO.
I 310 ppm
C.P. Swann, S.J. Fleming / Selectwe filtering
believe that the Bartol PIXE facility is now tailored to present and future archaeometric needs to an exceptional degree.
References
[II Ch. Lahanier,
F.D. Preusser and L. van Zelst, Nucl. Instr. and Meth. B14 (1986) 1. (21 C.P. Swarm and S.J. Fleming, Nucl. Instr. and Meth. B22 (1987) 407. [31 S.J. Fleming and C.P. Swarm, NucI. Instr. and Meth. B30 (1988) 444. [41 G. Demortier, Nucl. Instr. and Meth. B30 (1988) 434. [51 T.A. Cahill, B.H. Kusko, R.A. Eldred and R.N. Schwab, Archaeometry 26 (1984) 3. and S.J. Fleming. Nucl. [61 C.P. Swarm, P.E. McGovern Instr. and Meth. B45 (1990) 311. 15 171 E.T. Hall, F. Schweizer and P.A. Toiler, Archaeometty (1973) 53. PI K.L. Malmqvist, Nucl. Instr. and Meth. B14 (1986) 86. [91 K.W. Jones, B.M. Gordon, A.L. Hanson, J.B. Hastings, M.R. Howells and H.W. Kraner, NucI. Instr. and Meth. B3 (1984) 225. Microscopy 2 [lOI C.P. Swarm and S.J. Fleming, Scanning (1988) 197. [Ill H. Schenck and R. Knox, MASCA J. 3 (1985) 132. I121 C.P. Swarm and S.J. Fleming, Mater. Res. Sot. Symp. Proc. 123 (1988) 77. 17 (1975) 165. [I31 T. Florkowski and Z. Stos, Archaeometry 1141 A.J.J. Bos, R.D. Vis, H. Verheul, M. Prim, ST. Davies, D.K. Bowen, J. Makjanic and V. VaIkovic, Nucl. Instr. and Meth. B3 (1984) 232. Brochure L-655A, Atomic Spectroscopy P51 Perkin-Elmer (1981) 4. B. Gould. A. Killibrew and J. Yellin, in: (161 P. Goldberg,
[17] [18] [19] [20] [21] [22]
[23] [24] [25] [26] [27] [28] [29] [30]
[31] [32] [33]
69
Proc. 24th Int. Archaeometry Symp., eds. J.S. Olin and M.J. Blackman (Smithsonian Inst., Washington, DC, 1986) p. 341. R.G.V. Hancock, J.W. Hayes and E.M. Wightman. MASCA J. 3 (1984) 75. S.J. Fleming and C.P. Swarm. MASCA J. 4 (1986) 70. R. Woldseth, X-ray Energy Spectrometry (Kevex Corp.. Burlingame, 1973) pp. 2.20-2.30, and fig. 1.9. F.-W. Richter, Nucl. Instr. and Meth. B3 (1984) 105. S.J. Fleming and C.P. Swarm. Nucl. Instr. and Meth. B22 (1987) 411. S.J. Fleming, C.P. Swann, P.E. McGovern and L. Horne, these Proceedings (5th Int. Conf. on PIXE, Amsterdam, The Netherlands, 1989) NucI. Instr. and Meth. B49 (1990) 293. J.R. Bird, P. Duerden and D.J. Wilson. Nucl. Sci. Appl. Bl (1983) Table VII and 398. W.R. Ambrose, P. Duerden and J.R. Bird. Nucl. Sci. Appl. 191 (1981) 397. L.-E. Carlsson and K.R. Akselsson, NucI. Instr. and Meth. 181 (1981) 531. G. Demortier and T. Hackens, Nucl. Instr. and Meth. 197 (1982) 223. G. Demortier, Gold Bull. 17 (1984) 27. G. Demortier, Nucl. Instr. and Meth. B14 (1986) 152. E. Clayton, Nucl. Instr. and Meth. B30 (1988) 303. T. Berthoud. S. Bonnefous. M. Dechoux and J. Fran&x, in: Scientific Studies in Early Mining and Extractive Metallurgy, ed. P.T. Craddock (British Museum, London, 1980) pp. 87-102. S.J. Fleming and C.P. Swarm, Nucl. Instr. and Meth. A242 (1986) 626. W.A. Weyl, Coloured Glasses (Sot. Glass Technol., Sheffield, 1951) p. 6. A.F.G. Dingwall and H. Moore, J. Sot. Glass Technol. 37 (1953) 316.
II. EXPERIMENTAL
ARRANGEMENTS
SELECTIVE
and Methods
FILTERING
in Physics
Research
B49 (1990) 65-69
65
IN PIXE SPECTROMETRY
C.P. SWANN Bortol Research Institute,
Unruersity of Delaware, Newark,
DE 19716, USA
S.J. FLEMING MASCA,
The Unioersrty Museum.
lJnicrersi@ of Pennsduaniu.
PA 19104, USA
The Bartol Research Institute’s PIXE facility has made extensive use of selective filters in the study of compositional patterns in archaeological artifacts. The purpose of such filtering is to reduce the intensity of the primary X-rays by up to two orders of magnitude. This allows for an equivalent increase in the beam current and a corresponding reduction in the detection limits for the higher Z trace elements. In this report we bring together all the filter combinations we currently use and discuss their purpose and effectiveness.
1. Introduction
The authors have now collaborated for more than eight years in the development and application of PIXE spectrometry. Over that time there have been a host of changes in equipment components, and several small shifts in our research protocol, but practical experience continues to confirm our initial belief that this technique is well suited to the study of the composition of a wide range of archaeological materials. By way of a summary of what we regard are the PIXE spectrometry’s primary strengths, we note that: i) It is a non-destructive technique. Therefore it works well within the constraint often imposed in museum work, that an artifact suffers the minimum aesthetic damage possible in curation [l], yet serves as a resource for knowledge about both the cultural and technological aspects of our past. (Some pretreatment of the artifact’s surface may be necessary, such as a light abrasion to remove corrosion products or accumulated grime, but not drilling or cutting.) X-ray fluorescence (XRF) spectrometry shares this strength: several other analytical tools - e.g., basic wet chemistry, neutron activation analysis (NAA), atomic absorption (AA) spectrometry, inductively coupled plasma (ICP) spectrometry, and scanning electron microscopy with an adjunct energy dispersive X-ray detector (SEM/EDAX) - do not. ii) It has high spatial resolution. When the Bartol PIXE facility is run in a “microbeam” mode, the area scanned on the artifact’s surface is typically 0.5 mm2 [2,3]. Therefore it works well in a number of interesting situations, such as the study of a smear of solder bonding a granulated decoration on an item of Classical jewelry [4], an individual line of ink on an medieval 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
Bible [4], or a single strand of color marvered into the surface of an Egyptian glass vessel [6]. Conventional XRF, with a minimum study area of 4 mm* or so [7.8] does not share this strength. More specialized forms of XRF. particularly the synchrotron-based “light source” [9], have very good spatial resolution, but such facilities are rarely accessible for the kind of archaeological research discussed below. Wet chemistry, NAA, AA, and ICP are simply not applicable in these circumstances. SEM/EDAX does, of course, have superior spatial resolution if the artifact is quite small (i.e., only a few centimeters across and deep) and can be conveniently and safely mounted in a vacuum during analysis. iii) It offers high detection sensitivity for many trace elements in a wide range of matrices. We have previously reported in some detail the PIXE detection limits for bronze, iron, glass, copper and iron ores, and metal-working slags [lo-121. We note here only that most of the elements which might characterize the raw materials of these matrices, or might significantly influence their working properties, are readily detected at the 100 ppm level. XRF is also a sensitive tool, provided the excitation source is energy-tunable in some way (as is the case with the synchrotron-based “light source” [9]). The broad energy bremsstrahlung sources routinely used in XRF facilities create Compton scattering tails in the X-ray spectrum that tend to limit detection sensitivities to be an order of magnitude poorer than those achievable by PIXE facilities run in a similar measurement configuration [13,14]. Wet chemistry, AA, and ICP are all capable of very good detection limits for a wide range of elements [15], though the preparatory chemistry is sometimes complex and not suited to small samples. NAA is an exceptionally sensitive tool for II. EXPERIMENTAL ARRANGEMENTS
C. P. Swam,
66
S.J. Fleming / Selective Jiltermg
certain elements - e.g.. most of the rare earths, hafnium, lanthanum, manganese, and scandium - that have proven crucial for clay source discrimination in the study of ancient ceramics (see refs. [16.17], among many others). But there are other elements that are hardly detectable by NAA - e.g., silicon, sulfur, lead, silver. and tin - all of which are recognized as important in the study of ancient metals and glassy materials. SEM/EDAX detection limits are invariably far poorer than PIXE. because the bremsstrahlung contribution to the background is inversely proportional to the mass of the particle used as the excitation source. iv) It is efficient in terms of the turn-around time for data production. The various X-ray spectra that we tie together to characterize an artifact’s composition (see below) usually takes less than an hour to gather (leadrich glass being the most problematic matrix; see ref. [18]). XRF shares this advantage of speedy operation: wet chemistry, AA, and ICP tend to be slow because of their preparatory chemistry. NAA data collection is slowed by the fact that it relies on detection of low levels of induced radioactivity of a range of elements with widely varying neutron cross-sections, and with decay half-lives that may differ by several orders of magnitude. Consequently, a series of activation exposures of different neutron fluxes, and a series of separate y-counting procedures (sometimes months apart), are needed to obtain the most reliable data.
2. Selective
filtering
PIXE spectrometry uses as its excitation source a monoenergetic beam of protons (sometimes a-particles. sometimes deuterons). At first sight, therefore, the technique’s non-tunability would seem to work against its effectiveness as an analytical tool. But the fact that the excitation source is typically 1 MeV or above in energy means that cross-sections for X-ray production tend to be very high. A significant problem does arise, however, from the fact that those same cross-sections fall off sharply as a function of the atomic number of the element observed (19,201. At the same time, the efficiency of the Si(Li) detector also decreases with atomic number, setting up a detection bias strongly in favor of lower Z elements. As a result, PIXE spectrometry suffers from quite severe “pile-up” and count-rate limitations [19], particularly in the study of soda-lime glasses and other silicates such as glazed faience [21.22]. Here we should separate our work in PIXE spectrometry from that of most of our colleagues. The major research areas for the technique are soft tissue and blood cell medicine, plant physiology, and environmental pollution (see the review in ref. [lo]). The matrix (of substrate) for the trace impurities of interest is always a low Z organic compound. In these circumstances, pile-
up effects are near-negligible, and count rates are limited only by the very real risk of irrevocable damage of the matrix. At the same time, the bremsstrahlung contribution to background is minimal, allowing the PIXE detection limits to be more in the range of 10 ppm or less. For our part, the lowest Z matrix we contend with is silica; and more recently we have extended our interests into the study of the far higher Z matrices associated with leaded bronzes and gold alloys 1221. While a significant portion of the pile-up difficulties we face is routinely overcome by electronical rejection of count coincidence 1191, that will not remove the so-called “sum peak” which represents the true coincidence of two dominant X-rays within the time resolution of the detection and processing components of the system. “Sum peak” removal requires the filtering out of the high intensity X-ray contribution from the lower Z elements in some way. Use of such filtering is by no means a new idea. A decade or so ago it was common practice to include a thin low Z absorber (e.g., kapton, aluminum, or some kind of plastic) in the gap between an artifact’s surface and the face of the Si(Li) detector, so that the intensity of the lower Z X-ray contribution was simply degraded by attenuation. Thereafter these absorbers were modified to become pin-hole filters, to allow simultaneous measurement of a controlled fraction of the lower Z X-ray contribution [23,24]; and then stepped filters, so that the X-ray degradation was variably softened as a function of Z [25]. Rather than depend upon just X-ray attenuation effects, we have preferred to find ways to selectively suppress the X-rays from the dominant component of the matrix. In essence, that has meant taking advantage of the fact that the K, X-ray of an element is either completely or partially overlapped by a K-absorption edge of a lighter element. Thus, the Al Kabs (at 1.56 keV) will heavily suppress the Si K, (at 1.74 keV); the Co Kabs (at 7.71 keV) will heavily suppress the Cu K, (at 6.08 keV); and so on. In a similar way, the L, X-ray of some of the heavier elements is either completely or partially overlapped by a K-absorption edge of a light element. Thus, as Demortier has turned to his advantage so well in his study of ancient gold artifacts [26-281, the Zn Kabs (at 9.659 keV) will heavily suppress the Au L, (at 9.705 keV). Beyond this, we further manipulate the PIXE detection efficiencies by running the proton beam’s energy at about 2.0 MeV for the analysis of higher Z elements, but at 1.3 MeV for the analysis of lower Z elements. The reasoning here is that the X-ray yield will be increasingly attenuated the deeper the excitation occurs within the artifact. and that attenuation will affect the less energetic X-rays (i.e., those from lower Z elements) more severely. Thus, the use of a 1.3 MeV beam energy effectively enhances the observation of lower Z elements. All elements down to Si (Z = 14) are detected
C.P. Swam. S.J. Fleming /
Niql+ CoK,
CuK,
CuKp
I
I
I
t a
CoKp
t
t
1
1C
z
E
2 0
%
s 5
s 1c
l(
7.0
8.0 X-ray
energy,
9.0 keV
Fig. 1. A comparison of two PIXE spectra for a bronze matrix, each of which was gathered with the proton beam energy set at 2.0 MeV, but with a different array of filters included in front of the detection systems’s collimator: (a) selective filter (15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil); (b) simple absorption filter (123 mg/cm2 Si wafer). Note how both the filter arrays suppress the Cu K, relative to the Cu Ka to almost the same degree (their intensity ratios without filtering is close to 5 : 1). Quantitative analysis for nickel and cobalt was based in part upon the U.S. NBS reference materials #1275 (Ni. 9.76%; Co. 0.024%) and #Cl123 (a beryllium copper with Co. 2.3%).
well in this mode. since their X-rays are able to penetrate the air gap between the artifact’s surface and the input point for the detection system (a fine collimator in front of the thin Be window of the detector’s face). To measure as low as Na (Z = 11) - as is crucial in the study of glass - that gap has to be bathed by a He gas stream during PIXE measurement. The impact of selective filtering in the analysis of a tin-bronze matrix (Sn, 3.5%) is illustrated in fig. 1. There the effectiveness of a selective filter configuration (15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil) is contrasted with that of a 123 mg/cm2 pure Si wafer acting as a simple absorber, over the X-ray energy range of 6.6 to 9.1 keV. With the selective filter, even though the Ni K,
Selectirre Jiltering
67
is only a low energy shoulder on the Co K, peak produced when the intense Cu X-rays from the bronze causes fluorescence in the Co foil, the Ni detection limit is still only about 95 ppm. With the simple absorber, the Ni K, X-rays are degraded to such an extent that it can scarcely be resolved against background until the artifact Ni content is more than 2%. A detection limit as poor as this is quite unacceptable in archaeological studies, since nickel is regarded as one of the primary elements for ore source discrimination in the study of ancient bronzes ]301. The various selective filters we routinely use at the Bartol PIXE facility are listed below. along with how they effect the X-ray spectrum. In each instance, the proton beam energy is at 2.0 MeV. unless otherwise stated. Bronze. Matrix: primarily Cu with additives of As and/or Sn. in the range of l-15%. Selective filters: i) for Z 2 27 < 29: 9.8 mg/cm2 Fe foil + 7.5 mg/cm2 V foil. This is a filter combination well-suited for Co determination [22]. ii) For Z > 29: 15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil. The Co foil suppresses the Cu K, and K, X-rays. The V foil cuts out the fluorescence which intense Cu X-rays excite in the Co foil. These filter combinations can also be used in the study of other copper-based materials such as smelting mattes, prills entrapped in slag [31], ingots and crucible spills; and for the study of silver alloys and artifacts rich in antimony (e.g., beads, and Sb-rich bronzes). Iron. Matrix: primarily metallic iron with some silicate-rich slag remnants occurring as stringers. Selective filters: i) For Z 2 26: 15.0 mg/cm2 V foil + 15.0 mg/cm2 muscovite. The V foil suppresses the sum peaks at 12.80 keV (Fe K, + Fe Ka), and 8.14 keV (Fe K, + Si Ks). The potassium-rich muscovite cuts out the fluorescence that intense Fe X-rays excite in the V foil. Copper smelting slag. Matrix: primarily an iron silicate (either fayalite or wustite), with copper either dissolved in the matrix, or occurring as discrete prills. Selective filters: i) For 20 < Z I 26: 3.8 mg/cm2 Al foil (*proton beam, 1.3 MeV). The Al foil suppresses the sum peaks at 3.46 keV (Si K, + Si Ka). ii) For Z 2 26: 15.0 mg/cm’ V foil + 15.0 mg/cm2 muscovite (as for iron: see above). These filter combinations are also used in the study of iron-rich silicate minerals such as vesuvianite, and iron oxides such as hematite and limonite [12]. Besides suppressing the sum peaks due to the Fe- and Si-contents of these materials, the V foil then also suppresses the sum peak at 10.09 keV (Fe K, + Ca K,). Glass. Matrix: primary silica with soda and other alkalis that sum to as much as 20% by weight; and various heavy elements that are responsible for coloration. Selective filters: i) For 20 I Z I 26: 3.8 mg/cm2 Al foil (*proton beam, 1.3 MeV). The Al foil suppresses the sum peaks at 3.48 keV (Si K, + Si K,), 5.43 II. EXPERIMENTAL ARRANGEMENTS
68
C.P. Swunn, S.J. Fleming / Selectrve filtering
keV (Si K, + Ca K,), and 7.38 keV (Ca K, + Ca K,). ii) For Z 2 26: 1.9 mg/cm’ Al foil + 7.5 mg/cm’ V foil + 15.0 mg/cm2 muscovite. The V foil suppresses the Si and Fe sum peaks as it does for iron-rich silicates (see above), but it is usually used in a lesser thickness, since the iron levels in glass are typically in the range of 0.5-3%. dependent upon whether it occurs as an accidental contaminant brought in with the sand used in glassmaking [32], or as a deliberate additive. Copper ores. Matrix: primary silicon- and calciumrich minerals (quartz, grossular garnet, wollastonite, etc.), with the copper-bearing minerals such as malachite or chrysocolla occurring intrusively [12]. Selective filters: i) For 20 2 Z I 26: 3.8 mg/cm2 Al foil ( *proton beam, 1.3 MeV) as for copper smelting slags: see above). ii) For Z > 29: 15.6 mg/cm2 Co foil + 7.5 mg/cm2 V foil. This filter combination, besides suppressing the Cu sum peak, as it does for bronzes (see above), now also suppresses the sum peaks at 9.79 keV (Cu K, + Si K,), and 11.74 keV (Cu K, + Ca K,). Gold. Matrix: primarily metallic gold, with additives of silver and/or copper, in the range of l-50%. Selective filters: i) For Z 2 20: 3.8 mg/cm2 Al foil + 15 mg/cm2 muscovite + 21 mg/cm’ brass foil. The Al foil suppresses the Au M X-rays. The copper and zinc in the brass foil suppress the Au L, and La X-ray peaks at 9.65 keV and 11.44 keV, respectively, and effectively remove all related sum peaks. As one might expect there are several areas of common ground between these various material categories, as far as selective filtering is concerned. The choice of filtering is often a judgement call based upon the kind of information sought from the PIXE study. For example, with lead-rich glasses (PbO > 4%). where either the glass has been deliberately colored yellow by addition of lead antimonate [6,21], or some lead oxide has been
added to bring down the melting point of the glass melt [18,33]. the PIXE data have been supplemented by a run with the brass-based filter used in the study of gold. Similarly, when a glass has a quite high Fe-content (Fe,O, > 5%). the filter combination used for Fe-rich slags, etc. (with a double thickness of V foil), becomes more appropriate (see table 1). That filter combination cannot be used for low Fe glasses, because the Fe K, line is the link between the lower Z and higher Z spectra, and too much filtering makes measurement of its intensity statistically problematic. At the same time, the use of a triple thickness of V foil (i.e.. at a thickness of 22.5 mg/cm2) only complicates such studies, because heavier elements that may be involved in the coloration process, such as Cu. Zn and Pb, will fluorescence the trace levels of iron in the detector’s Be window. This fluorescence will produce a false set of the Fe X-ray peaks that, in intensity, will rival or exceed the small amount of artifact-related Fe X-rays that manage to get through such a strong selective filter.
3. Conclusions Archaeological materials are so diverse in their composition, we would hesitate ever to put foward the filters listed above as ideal for all applications. Only recently, we have initiated studies of various kinds of commercially available wrought aluminium sheet which, because of practical requirements such as bendability, etc., have minor amounts of medium Z impurities deliberately added to them. Here we have a range of materials which combine the properties of simple absorption and selective filtering in a way that may offer us further gains in the detectability of high Z elements in complex matrices. Overall, however, we
Table 1 The effect of vanadium foil as a selective filter in the study of Fe-rich
silicate matrices
Copper smelting slag from the Iranian Iron Age site of Tepe Hissar (SiOz. 39.0%: CaO. 11.5%; FeO, 16.2%; CuO. 0.15%; ZnO, 4.2%) Filter: 3.8 mg/cm* Al foil + 15 mg/cm’ muscovite + 7.5 mg/cm2 V foil Ag,O, 5 5190 ppm; Sb,O,, _< 11300 ppm; SnO. 5 8270 ppm: BaO, < 40520 ppm Filter: 15 mg/cm’ muscovite + 15 mg/cm2 V foil Ag,O, 5 260 ppm: Sb,O,. 5 805 ppm: SnO, 5 245 ppm; BaO,
2 1870 ppm
A dark blue frit bead from the Late Bronze Age Iranian site of Dinkha (SiO,, 51.3%; CaO, 3.8%; FeO. 5.2%; CuO, 20.3%; ZnO. 0.97%)
Tepe
Filter: 3.8 mg/cm2 Al foil + 15 mg/cm2 muscovite+7.5 mg/cm’ V foil Ag,O. 5 1750 ppm; Sb,O,, _< 5630 ppm: SnO, 5 3960 ppm; BaO. 5 16000 ppm F&a-: 15 mg/cm’ muscovite+ 15 mg/cm’ V foil AgzO. 5 88 ppm; Sb,O,. _i 130 ppm; SnO. I 130 ppm; BaO.
I 310 ppm
C.P. Swann, S.J. Fleming / Selectwe filtering
believe that the Bartol PIXE facility is now tailored to present and future archaeometric needs to an exceptional degree.
References
[II Ch. Lahanier,
F.D. Preusser and L. van Zelst, Nucl. Instr. and Meth. B14 (1986) 1. (21 C.P. Swarm and S.J. Fleming, Nucl. Instr. and Meth. B22 (1987) 407. [31 S.J. Fleming and C.P. Swarm, NucI. Instr. and Meth. B30 (1988) 444. [41 G. Demortier, Nucl. Instr. and Meth. B30 (1988) 434. [51 T.A. Cahill, B.H. Kusko, R.A. Eldred and R.N. Schwab, Archaeometry 26 (1984) 3. and S.J. Fleming. Nucl. [61 C.P. Swarm, P.E. McGovern Instr. and Meth. B45 (1990) 311. 15 171 E.T. Hall, F. Schweizer and P.A. Toiler, Archaeometty (1973) 53. PI K.L. Malmqvist, Nucl. Instr. and Meth. B14 (1986) 86. [91 K.W. Jones, B.M. Gordon, A.L. Hanson, J.B. Hastings, M.R. Howells and H.W. Kraner, NucI. Instr. and Meth. B3 (1984) 225. Microscopy 2 [lOI C.P. Swarm and S.J. Fleming, Scanning (1988) 197. [Ill H. Schenck and R. Knox, MASCA J. 3 (1985) 132. I121 C.P. Swarm and S.J. Fleming, Mater. Res. Sot. Symp. Proc. 123 (1988) 77. 17 (1975) 165. [I31 T. Florkowski and Z. Stos, Archaeometry 1141 A.J.J. Bos, R.D. Vis, H. Verheul, M. Prim, ST. Davies, D.K. Bowen, J. Makjanic and V. VaIkovic, Nucl. Instr. and Meth. B3 (1984) 232. Brochure L-655A, Atomic Spectroscopy P51 Perkin-Elmer (1981) 4. B. Gould. A. Killibrew and J. Yellin, in: (161 P. Goldberg,
[17] [18] [19] [20] [21] [22]
[23] [24] [25] [26] [27] [28] [29] [30]
[31] [32] [33]
69
Proc. 24th Int. Archaeometry Symp., eds. J.S. Olin and M.J. Blackman (Smithsonian Inst., Washington, DC, 1986) p. 341. R.G.V. Hancock, J.W. Hayes and E.M. Wightman. MASCA J. 3 (1984) 75. S.J. Fleming and C.P. Swarm. MASCA J. 4 (1986) 70. R. Woldseth, X-ray Energy Spectrometry (Kevex Corp.. Burlingame, 1973) pp. 2.20-2.30, and fig. 1.9. F.-W. Richter, Nucl. Instr. and Meth. B3 (1984) 105. S.J. Fleming and C.P. Swarm. Nucl. Instr. and Meth. B22 (1987) 411. S.J. Fleming, C.P. Swann, P.E. McGovern and L. Horne, these Proceedings (5th Int. Conf. on PIXE, Amsterdam, The Netherlands, 1989) NucI. Instr. and Meth. B49 (1990) 293. J.R. Bird, P. Duerden and D.J. Wilson. Nucl. Sci. Appl. Bl (1983) Table VII and 398. W.R. Ambrose, P. Duerden and J.R. Bird. Nucl. Sci. Appl. 191 (1981) 397. L.-E. Carlsson and K.R. Akselsson, NucI. Instr. and Meth. 181 (1981) 531. G. Demortier and T. Hackens, Nucl. Instr. and Meth. 197 (1982) 223. G. Demortier, Gold Bull. 17 (1984) 27. G. Demortier, Nucl. Instr. and Meth. B14 (1986) 152. E. Clayton, Nucl. Instr. and Meth. B30 (1988) 303. T. Berthoud. S. Bonnefous. M. Dechoux and J. Fran&x, in: Scientific Studies in Early Mining and Extractive Metallurgy, ed. P.T. Craddock (British Museum, London, 1980) pp. 87-102. S.J. Fleming and C.P. Swarm, Nucl. Instr. and Meth. A242 (1986) 626. W.A. Weyl, Coloured Glasses (Sot. Glass Technol., Sheffield, 1951) p. 6. A.F.G. Dingwall and H. Moore, J. Sot. Glass Technol. 37 (1953) 316.
II. EXPERIMENTAL
ARRANGEMENTS