41Ca: past, present and future

572

Nuclear Instruments and Methods in Physics Research B52 (1990) 572-582 North-Holland

41Ca: past, present and future * D. Fink, J. Klein and R. Middleton department of Physics, University of Pe~nsylvani4 Ph~iade~~~~ PA f9104, USA

Accelerator mass spectrometty measurements of “Ca have come of age. The ability to routinely obtain currents of CaH; of 5 pA and backgrounds of less than lo-“, and even 5 X lo-l6 at times, makes possible measurements with a precision of S% in an hour. Studies of 4’Ca in a wide variety of extraterrestrial materials addressing several very different problems, including the temporal constancy of galactic and solar cosmic rays, the determination of terrestrial ages of meteorites, and their pre-atmospheric size are now in progress. However, the goal of employing %a to date bones still remains elusive. The major experimental problem is the production of currents of sufficient intensity. But more fundamental, it seems likely that the radiocalcium dating model is seriously flawed. In this short review, we summarize the technical developments that have led to a successful technique to measure 41Ca, and discuss the more significant applications of 4’Ca both on the Earth and above.

1. Introduction

I. I. The past - a review The successful measurement of cosmogenic radionuclides by accelerator mass spectrometry can be attributed to nuclide-specific characteristics enabling nearly total suppression of corresponding isobars that are reP sent at far higher concentrations. For 14C, 26Al, and 2g1, this suppression takes place during negative-ion formation, whereby their isobars, 14N, 2bMg and r2’Xe, do not form negative ions. For “Be and 3bC1 other techniques are employed. As *OR, the isobar of “Be, has a higher Z than Be, it can be completely eliminated by ranging out the i”B. The isobar of “Cl, “S, is reduced by a combination of techniques: chemical purification and elemental discrimination at the detector. However, these methods would not be sufficient to measure %I at natural levels if ‘% were not a minor (0.017%) isotope of sulfur. For 4’Ca, an effective method to remove 4*K (natural abundance of 7%) did not arise so readily, nor to that matter, so easily. Here, the negative-ion situation is nearly reversed: Calcium only reluctantly forms negative ions, its electron affinity is only 43 meV, while potassium forms negative ions much more readily. Consequently, the search for high currents of Ca, and with it a means to reduce 4’K, was directed to the investigation of molecular negative ions. As early as 1981, experiments by Raisbeck and Yiou [I] demonstrated that the intensity of KH; is lo8 smaller than K-, and suggested that CaH; was probably the molecule of choice. Following this, Fink et al.

* Invited paper. 0168-583X/90/$03.50

[2] showed the injection of CaH; (using NH, and Ca metal) resulted in a 4’K/40Ca ratio of 2 x 1O-‘2 - a reduction of lo4 over other molecular beams (CaO-, CaF- and CaH-). But a problem remained: Independent of the choice of molecule, ion source out uts of Ca g ions were very low ( < 100 nA), restricting the ‘Ca/?a detection limit to - 5 X lo-“. At the time of the Niagara AMS conference in 1987, only five iron meteorites had been successfully analyzed for 4’Ca (3-Q Relatively large sample masses (0.3-1.049) and a minimal amount of carrier were used so that Ca/% a ratios could be kept above 1 X 10-12, but even then the measurements bordered on the limits of sensitivity and accuracy was poor because of low statistics. All in all, low ion source currents made measurement an arduous task and without CaH; beams at the microamp level, routine, high-precision measurements below the lo- l3 barrier were not possible. Spurred by the enterprising suggestion of Raisbeck and Yiou [7] to utilize 4’Ca to ra~omet~c~iy date late Pleistocene bones, Henning et al. [8] were able to make a set of measurements of 4’Ca in terrestrial samples by preceding the AMS measurement with an involved isotope pre-enrichment stage that increased their sensitivity by - 150-fold. Using a gas-filled magnet, they obtained a 4’Ca/cCa ratio of 2 x lo-l4 for a modem cow bone. The result, although higher than the predicted saturation value of 8 X lo-” (71 for 41Ca at the surface of the earth, provided the spark that rekindled ionsource development of CaH; beams for use on tandem accelerators. Soon after, Middleton demonstrated [9) that by choosing high-grade CaH, as a sputter target in a hip-intensity source [lo], CaH; currents of 5 to 10 pA were achievable. Without doubt, it is this single discovery that has made the measurement of 41Ca practical.

0 1990 - Elsevier Science Publishers B.V. (North-Holland)

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D. Fink et al. / “‘Co: past, present and future 1.2. The present Pennsylvania

- statuy of 4’Ca at the University

of

Capitalizing on this breakthrough, the AMS facility at the University of Pennsylvania has been engaged over the past two years in an in-depth program with the goal of a comprehensive survey of “Ca concentrations in a diverse list of terrestrial and extraterrestrial environments. Within this period, we have performed 4’Ca analyses on over 60 extraterrestrial samples ranging from iron, stone and Antarctic meteorites to lunar surface rocks and the Apollo 15 drill core. The 4’Ca,&a ratios in these samples range from 5 X lo-l4 to 5 x 10-12. Measurement reproducibility for this range is better than 5% - a precision comparable to that seen time, including now for 26Al and ‘“Be. Measurement that to reach stable ion-source output, tuning and counting, is about an hour for 5% statistics. Of no less importance, we have also analyzed about 15 terrestrial samples, including a suite of modern animal bones, calcareous rocks, and corals. Terrestrial 4’Ca/‘reCa ratios range from 5 X lo-l6 to 2 x lo- 14. The precision with which we can measure these samples is largely determined by the Poisson error: For reasonable measurement times, say two hours, it is better than 25% for 4’Ca/?a ratios in the low lo-l5 range. At present, our detection limit is 1 X 10-‘5. although at times we have recorded ratios for commercial CaH, blanks as low as 5 x 10-16. Typically, 30-35 extraterrestrial samples can be measured in a three day run on the accelerator. The aim of this paper is two-fold. The first is a presentation, in broad terms, of the technical developments that were required to bring us to our present level of measurement proficiency for 4’Ca. We shall highlight the essential features of sample preparation, accelerator operation and 4’Ca detection, that are of paramount importance to any laboratory embarking on a program of 4’Ca measurements. The second is to offer a perspective of research areas one can investigate with 4’Ca. Given the diversity of samples we have looked at, we are able to indicate where,the potential of 4’Ca can be maximized. We conclude by pointing out new domains which are worthy of pursuit and the necessary improvements in the detection capability of 4’Ca needed to realize this goal.

2. Advances in 4’Ca techniques and measurement methods An extensive review of our complete 41Ca system detailing techniques and methods has been published recently [ll]. Here, we present an overview of the advances, stressing those we deem most central to our method. We refer the interested reader to the above paper for an expanded description.

2.1. Somple preparation Three major steps are involved in transforming the raw material into a CaH, sputter target. Normally, 4’Ca is chemically extracted from the sample via oxalate precipitation. After conversion to CaO at lOOO”C, the oxide is mixed with excess Zr powder and reduced at high temperatures (15OO’C) to Ca metal by vacuum distillation. Finally, the metal is transferred to a second chamber filled with hydrogen and converted to CaH,. Efficiency for the entire conversion process is usually 80--w% and about 6-8 oxide samples can be converted to hydride per day. We prefer to start with 25-30 mg of CaO, which provides sufficient hydride to fill two cathodes. Once loaded, the cathodes are individually sealed in evacuated glass ampoules. Our preparation system, although time consuming, works well and on only two occasions from the 100 or more prepared, did we fail to get more than 1 PA of CaH.; from the ion source. Attempts with reduced carrier mass have not always been successful and about 10 mg of CaO seems to be the lower limit to guarantee a measurement. Reduction to l-2 mg will demand an alternative procedure. Meteorite sample masses ranged from lOC-300 mg and as they contain 4’Ca concentrations of about lo9 atoms/g, the use of 20 mg of Ca carrier results in a 41Ca/‘Ya ratio of - 5 X lo-“. This is - 200-fold higher than our present detection limit and thus reduction of sample mass by a factor of 10 would not severely impair measurement quality. For terrestrial bones and rocks, where carrier is not involved, sample masses are usually plentiful and one is not restricted to preparing only one or two cathodes. This is not an insignificant advantage considering their very low 4’Ca/‘Ya levels. 2.2. Accelerator techniques With ‘%aH; currents more often than not exceeding 2 uA within - 15 minutes, terminal loading made accelerator tuning difficult, resulting in unreliable and variable transmission measurements. We thus investigated two alternatives that indirectly measure the 40Ca transmission: (1) maintain mass-44 injection and analyze for ‘%a from ‘?aH,Dand (2) inject mass 45 and analyze for 42Ca from 42CaH;. The first approach proved unsuccessful (see appendix and ref. [l l] for details), but the latter turned out to be both reliable and convenient. Transmission for the selected (see below) 8 + charge state of Ca was deduced from the analyzed 42Ca*’ current (measured at the same terminal voltage for 41Ca) and the ‘?aH; current, after correcting by their natural abundance ratio. We cannot normalize to a negative-ion yield at mass 45 as this includes considerable contributions from both 44CaH- and 43CaH;. IV(d). NUCLEAR&ASTROPHYSICS

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We tried various charge states (at 8.50 MV) in order to select that one which minimized the detector count rate due to M/Q ambiguities without compromising beam energy (for isobaric identification) or transmission (see figs. 11, 12 in ref. [ll]). For 41Ca9’, an unacceptable 2 kHz rate from 32S7+ ions, originating as a fragment from 32S’2C- molecules, entered the detector. Tests with 8 + resulted in a total background count rate of < 200 Hz for CaH; currents of - 5 PA. Despite a small loss in beam energy, transmission using 8 + was slightly higher (8-11%). To identify 41Ca events from interfering peaks of 4’K and 40*42Ca (injected as beams of ‘%aH,Dand 42CaH;, respectively) we constructed a multi-anode gas (isobutane) ionization detector, similar to that in use at Rochester [12]. Its excellent resolving power for equal energy isobar pairs in providing an endi.e., 4’Ca-4’K, lies predominantly of-range, or residual energy signal. By fine tuning the gas pressure it is possible to optimize this parameter. In addition to a lo4 suppression of 41K at the ion-source (K does not form a stable tri-hydride but is injected as KHD-; see appendix of ref. [Ill), the detector provides an additional factor of at least 104. For exam le. with 5 4P uA from the source, we usually measure a K rate of no more than 5 Hz. For bone measurements with 4rCa/eCa ratios typically at 1 x lo-l5 and lasting one hour, we can ident:? unambiguously each 4’Ca event above a total peak K count of 1 X 104. Backgrounds from 40Ca and 42Ca with the same magnetic rigidity as 41Ca cannot be fully resolved by multiple AE signals alone and only marginally by total energy. They also cannot be eliminated by use of an electrostatic analyzer at the ion source since both are injected into the accelerator as negative hydride ions in their own right at mass 44, and not as tails of peaks from adjacent masses. A high-resolution velocity selector (Wien filter) positioned at the exit of the accelerator tank, rejects this portion of the white 40.42Ca spectrum. This stage of analysis is indispensable for all 41Ca measurements. At a CaH; source output of 0.5 uA and the filter off, the 42Ca and ‘%a rates are > 100 kHz and 20 kHz respectively, while with it active and set to transmit 4’Ca, both rates are cut to < 0.5 Hz. Despite the small 40*42Ca leakage rates, both impose limits to extending the detection limit to below 5 X lo-l6 - particularly those ?a events which by having correlated low-energy tails in both residual and total energy, enter the 4’Ca window. Fortunately, because ‘%a must down-charge in the accelerator tubes, which is much less favorable than up-charging, its rate is 5-10 times lower than that of 42Ca. We have positioned a second and identical velocity filter after our switching magnet, 3 m upstream from the detector. Preliminary tests show this filter to be of equal resolution to the first. Their combined action is expected to reduce even further these interfering peaks.

2.3. Measurement procedures Initial setup requirements involve: (1) optimization of all beam transport elements up to the detector Faraday cup using a 42Ca *+ beam at the same rigidity as 4’Ca, and then (2) setting the terminal voltage (8.50 MV) and GVM to the corresponding 41CaE’ energy by analyzing an equal energy 42Ca8+ beam (75.93 MeV) in the analyzing magnet cup. These two steps are done with all slits at f1.25 mm to accurately define the beam trajectory. For radioisotope counting the analyzing magnet field is returned to that in (1) above, the slits are opened to f5 mm and mass-44 injected C’CaH;). An offset Faraday cup following the injector magnet is positioned to intercept the mass-43 (‘?aH;) beam for normalization purposes while 4’Ca ions are being counted in the detector. For each sample (except blanks), we carry out at least two, and if necessary three, measurement blocks. A block consists of 3-6 consecutive 41Ca counting periods of 2-20 minutes duration depending on sample countrate. 42Ca transmission measurements open and close each block. Repeat measurements, even when measured in experiments separated by several months, have been consistent to within *lo%. All 4’Ca/‘%a ratios are normalized to our 4’Ca standard, prepared by thermal neutron irradiation of commercial CaH, to a nominal value of 5.41 X 10mr2. This irradiated material needs no further treatment and is pressed directly into a regular cathode. Typically, 8-12 new standard cathodes are measured per experiment. Reproducibility (or lo spread) in the standard measurements completed in each experiment is better than 5%. The normalization correction (the ratio of the mean of this set to the nominal standard value) is consistently 0.95, verifying our method for absolute determination of the sample’s 41Ca concentration. Comparisons of our procedural chemistry blanks (commercial CaO converted to CaH,) to nonirradiated commercial hydrides, show no indication of contamination in sample processing. Blanks inserted directly after running a standard gave no signs of increased 4’Ca, indicating that ion-source cross talk for Ca is < 10e4. Table 1 summarizes the results we have obtained with several standards prepared under different conditions. We prepared two standards by irradiating commercial CaH, in a low-intensity thermal (thermal/ epithermal > 50) neutron flux at the Penn State University Breazeale Reactor facility. The two standards were irradiated at separate times about a year apart and thin gold foils were used to determine the neutron fluence. A cross section of 0.41 f 0.02 b for the ‘?a(n,,, y) reaction was used to convert the neutron fluence to 41Ca atoms produced, and subsequently to a 4’Ca/%a ratio. The 41Ca/%a ratios measured for these two standards agree with their nominal values to within - 5%. In fact,

D. Fink et al. / “Ca: past, present and future Table 1 Sununary of 41Ca/40Ca measurements made on four different standards. The experimental 4’Ca/40Ca ratios represent the weighted mean of the number of independent determinations (column 2). The error is defined as the larger of the Poisson error and the estimated error of the weighted mean. Penn-I and Penn-II standards were prepared by thermal neutron activation of commercial CaH, at the Penn State University Breazeale Reactor Facility on two separate dates, March 1988 and February 1989. The Je~s~em/Argonne standard was obtained from successive dilution of enriched 41CaCOs purchased from ORNL. It has been mass spectrometrically determined to be 1.27% by weight in 41Ca. The UCSD/Tokyo standard was

obtained from Mabuchi [14]. Standard

Number of measurements

4’Ca counts

Measured 4iCa,/cCa (lo-=)

Nominal 4’Ca/cCa (to-‘21

Penn-I Penn-II

I9 11

56200 9500

5.25 +4% 1.11+4x

5.41 h 9% 1.0319%

Jet-us./ Argonne

3

2700

5.09 + 5%

5.26 2 10%

UCSD/ Tokyo

3

5800

2.41+ 4%

0.96

$

10%

we are uncertain as to which value is the more accurate. We have not made any correction for differences in stripping efficiency between 42Ca and 41Ca, but we expect the difference to be much less than 5%. The comparison with the Jerusalem/Argonne standard is interesting. This standard was made by successive dilution of highly enriched 41CaC03 material purchased by W. Kutschera from ORNL. It has been employed as a standard by the AMS groups at Jerusalem [4] and ANL [8]. The 41Ca/%a ratio of the original material has been independently determined by mass spectrometry at two laboratories (at ANL and at Cal Tech) to be 1.27% [13] by weight. Unlike the standard we prepared, its nominal value does not deend on the thermal capture cross sections for 40Ca and P 97Au, but rests on the reliability of the dilution procedure. Again there is very good agreement with the value we measure. Only for the standard prepared at UCSD do we encounter a discrepancy. We measure a “‘Cap”Ca ratio a factor of 2.5 higher than expected. The source material for this standard comes from the enriched 41Ca carbonate powder used by Mabuchi et al. [14] to determine the *iCa half-life. The reason for this difference is being investigated. However, the higher than expected value for this standard explains the discrepancy between the results obtained at Rochester (where this material was used for normalization) and those obtained at Penn for the small iron meteorite, Bogou.

515

3. Production modes and research applications of *‘Ca 3. I. Extraterrestrial

- meteorite and lunar samples

There are three reaction channels which dominate 4’Ca production in extraterrestrial environments. They are characterized by the energy range of the bombarding nucleon and by target species: (1) high-energy spallation by galactic cosmic-ray protons (and their secondaries) on iron and nickel, (2) solar-proton-induced reactions on titanium, and to a lesser extent, on neighboring elements such as 41K, the heavier isotopes of calcium, scandium etc., and (3) thermal neutron capture on %a. It is this wide variety in production modes and sensitivity to different bombarding fluxes and target matter that makes 4’Ca a powerful diagnostic tool. The size and composition of an extraterrestrial object, and the sample depth in the parent body determine the hierarchy of the production mechanisms outlined above. SCR effects are detectable only to a depth of a centimeter or two in objects with > 1% Ti. Neutron effects can readily be observed in the calcium-bearing stone phases of objects of sufficient size such that secondary neutrons have begun to thermalize. And in general, any object containing Fe and Ni, or for that matter elements with Z > 20, will produce 4’Ca from spallation reactions induced by GCRs. In what follows, we will describe in more detail these production modes with specific reference to our 4*Ca applications program. Let’s consider the simplest case first. 3.1. I, SpaIlation production in iron meteorites As iron meteorites contain negligible amounts of Ca, the only relevant production mechanism of 41Ca is GCR spallation on Fe and Ni. From our analysis of the 41Ca content in four small iron falls [15,16], which have pre-atmospheric radii estimated to be < 20 cm, we have determined a 4’Ca production rate of 24 &-1 dpm/kg. Our ‘%a analysis of Grant, a large iron (pre-atmospheric radius of 35-40 cm) of unknown terrestrial age, shows only a 15% decrease over 20 cm from post-atmospheric surface (17.7 j, 1.3 dpm/kg Fe) to center (16.4 f 1.2). The constancy of the 41Ca concentrations in the four falls and the absence of any significant decrease in 4’Ca across Grant, strongly suggest that GCR-induced production of 41Ca in iron is relatively insensitive to shielding depth [16]. Our preliminary profile of 4’Ca in the iron phase of the chondrite Jilin (radius - 85 cm) also shows little depth attenuation. These results indicate that 41Ca can be used reliably to determine terrestrial ages of Antarctic meteorites, particularly for young ages ( < 50 kyr) where 36C1is ineffective. Because the shielding behavior of 36C1(T~,~ = 301 kyr) is similar [17] to that we have observed for 4*Ca, the 4’Ca/36C1 ratio should provide shielding-independent terrestrial IV(d). NUCLEAR & ASTROPHYSICS

D. Fink et al. / 4’Ca: past, presenf and future

516 Terrestrial Age [kA]

9.5

c

600

400

200

I,‘,,,,,.,,

-

.

Antarctc

/

7.5

Average

0 71”

9

0

Baxter

,““,_I

Metemtes Of Small

IrOn

”

”

falls

(L6 fall)

9.5

”

1

10

log ( 36CI [atoms/g] ) Fig. 1. Log-log plot of 41Ca versus ‘kl concentrations measured in five Antarctic meteorites (solid points) used to determine the half-life of 41Ca. The upper horizontal scale is terrestrial age based on 36Cl and its saturation concentration of 5.28 x lo9 atoms/g. The rectangular data point is the average of the four small iron falls. The slope of the weighted linear regression line is equal to the ratio of the 4’Ca to 36C1 decay constants, resulting in a 41Ca half-life of (1.03+0.07) x 10’ years. Except where shown, error bars are contained within the symbol of the plotted points.

ages for cases where shielding may be partially responsible for the decrease in activity. However, at present the half-life of 41Ca is not known accurately - published measurements range from 100 to 200 kyr with errors as large as 30% [14,18]. An alternative method of determining the 41Ca half-life, which avoids the dependency on (n,,, v) capture cross sections common to previous measurements and which circumvents the requirement of an activity measurement, is to measure the decrease in 41Ca concentrations in the iron phase of Antarctic meteorites as a function of their terrestrial age, as established from their %l concentrations. We have carried out such an analysis on a carefully chosen selection of five Antarctic chondrites are [18]. When the results of these 41Ca measurements plotted against 36C1 concentrations using logarithmic axes (see fig. l), the five data points fall along a straight line with a slope of 2.91. From what was stated above concerning similar shielding behavior, the linear correlation in the 41Ca and 36C1 concentrations for these meteorites is a result of decay, and not of shielding differences or undersaturation. Thus the slope is equal to the ratio of their respective half-lives and we deduce a half-life for 41Ca relative to that of 36C1 of 103 + 7 kyr. For the future, a more detailed and systematic compilation of 41Ca and 36C1 concentrations in Antarctic

meteorites might also be used to examine the constancy of the GCR flux. Another promising application of 41Ca production in iron will be to determine exposure ages of meteorites. The most widely accepted means for determining the exposure ages of meteorites that have been exposed for hundreds of millions of years is the 41K-40K method developed by Voshage [19]. Ages determined using radioisotopes of shorter half life, such as 36C1-36Ar [20] and 26A1-21Ne [21], consistently provide exposure ages smaller by 30-50%, suggesting that the average GCR flux may have been larger during the past few million years than the average over the past billion years. By pairing absolute 41K concentrations with 41Ca activities, we were able to calculate 41Ca-41K exposure ages [16] for two of the meteorites we analyzed - these ages confirm the above discrepancy. 3.1.2. Production

by thermal

neutrons

in chondrites

and

the moon

Perhaps the ideal location in which to study the production of 41Ca by thermal neutrons is the moon. We have measured the 41Ca concentration as a function of depth in the Apollo 15 long core to a depth of almost 400 g/cm2 [22]. The average Ca and Fe contents of this material are - 7% and - 12X, respectively. The results are plotted in fig. 2. At the peak, which occurs at a depth of - 170 g/cm2, the production rate of 41Ca is nearly 1000 atoms/(kg-Ca)min. This is - 50 times larger than the production rate one would expect from the spallation of one kilogram of iron. Our measured 41Ca profile is in very good agreement with the model of lunar neutron production and thermalization presented by Lingenfelter et al. [23], and also consistent with the results of the Apollo 17 Lunar Neutron Probe Experiment reported by Woolum et al. [24]. The strong sensitivity of 41Ca production with depth evident in fig. 2 provides an excellent means by which the pre-atmospheric size of meteorites, and sample depth within a meteorite, can be determined. 41Ca is nearly an ideal monitor of thermal neutrons whereas most other neutron-produced nuclides possess major disadvantages. For example, the very short half-life of 6oCo restricts its utility to recent falls, and stable neutroncapture products (the Gd and Sm isotopes) lie under a noncosmogenic component and provide no temporal information. 36C1, although having an appropriately long half-life and large thermal neutron yield is produced from a trace element, whereas Ca occurs typically at the few percent level. In simple terms, only large bodies (radius > 50 cm) are capable of generating large thermal neutron fluxes. Near the surface, this flux is small, climbs to a maximum at depths in the range of 150-200 g/cm2, and thereafter decreases exponentially because the source of thermal neutrons (i.e energetic secondary particles) are

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- 41Ca in lunar surface

+ Lunar Basalt 74275 p Lunar Core 15001 1 0.01

n ‘nrrd

’ ’ 11111*’ s ’ 111111 ’ ‘11--*’ s n“U 1

100

Depth [g/cm 2] Results of 41Ca measurements in the X-rich lunar surtace rock 74275 (solid points) and that of the Apollo 15 core (open points). The results from 74275 were used to calculate the relevant parameters which determine the shape and magnitude of the solar cosmic-ray flux after appropriate corrections for spallation and neutron production were carried out. The data from the lunar core are consistent with models of lunar neutron production and thermalization, and highlight the strong sensitivity of neutron produced 41Ca lo sample depth within a large body.

diminishing. In smaller objects, fewer neutrons are produced and most of these escape before they thermalize - as a result the flux increases monotonically from surface lo center. Spergel et al. [25] have modelled the development of thermal neutrons in chondrites (and in the moon), and used it to predict the production profiles of 3”CI, 59Ni and @%o as a function of radius. Since “Ni production, like 4’Ca, is dominated by thermal neutron capture, we present in fig. 3 Spergel’s “Ni calculations (see fig. 10 of ref. [25]) normalized to that of 4’Ca by the ratio of their thermal neutron cross sections and target molar masses. Also included in the figure are our experimental data points for the Apollo 15 core, which compare extremely well with the calculated distribution based on a Zn-irradiation. The variation in 4’Ca with meteorite radius is clearly evident, with a maximum production rate of 2800 dpm/kg-Ca occurring at the center of a meteorite of radius 300 g/cm2 (- 85 cm).

The approach we have taken in the analysis of Jilin, a large chondrite, is as follows. Iron and stone phases were separated and 4’Ca measured in both. Measurement of the calcium and iron concentrations in the latter phase allowed the contribution from spallation of iron not removed from the stone during the separation process to be subtracted. This correction was crucial for samples near the surface where spallation production is significant but negligible at larger depths where neutron-produced 4’Ca begins to dominate. Our preliminary measurements on Jilin range from 300 to 2000 dpm/kg-Ca across its radius. a result consistent with fig. 3 and its estimated radius of 300 g/cm2. The interpretation of fig. 3 becomes more complex for 4’Ca contents lower than - 1500 dpm/kg-Ca. In this case. the model is less predictive and generates only a large range of possible depths and radii. Recently it has been suggested [26] that differences in bulk chemical composition may affect production rates by altering the development of the secondary flux. Such changes will also be reflected by a modification in the magnitude of the thermal neutron flux. Comparing the 4’Ca content in meteorites with large differences in elemental composition may help to shed light on this interesting idea. 3.1.3. Solar proton production

on the lunar surface

In the surface

of lunar rocks is recorded a history of the sun. Reedy [27] has recently summarized what has been learned from studies of cosmogenic radioisotopes. Measuring more than one nuclide allows an exploration of the SCR spectrum on different time scales and over different proton energies. To a large extent, interpretation has been hampered by the lack of relevant cross sections. The results of our 4’Ca measurements in a high-Ti basalt, 74275, [28] are presented in fig. 2 (together with those of the lunar core described earlier). In order to more accurately model this profile, we have also determined the excitation function for 4’Ca production on natural titanium by protons with energies in the range 35 to 150 MeV [see Fink et al. [29], these proceedings]. Converting these measurements to an SCR flux requires removing the contributions from other production modes, principally spallation and neutron capture. Also shown in the figure are their contributions. At the very surface, SCRs are responsible for more than half of the total 4’Ca, but at a depth of only 10 g/cm2 (- 3 cm), the SCR contribution is lost beneath the contributions of thermal neutrons (accounting for about 50%) and spallation. From these measurements, we conclude that the flux of protons from the sun was probably larger and softer [28] than the accepted value. However, a precise parameterization of the past SCR flux must await the results of our measurement of the excitation function for 4’Ca from 42Ca by low-energy protons. IV(d). NUCLEAR

& ASTROPHYSICS

D. Fink et al. / 4’Ca: past, present andfuture

578 I

1

41 Ca in L-Chondrites

I

I

0

100

200

300

400

500

Depth [ g/cm’ ] Fig. 3. Estimated 4’Ca production rates as a function of depth in chondrites with different radii. The curves are based on the calculations carried out by Spergel et al. [25] for “Ni (see text). The magnitude of the maximum possible 41Ca activity for a given meteorite is strongly dependent on its radius. Only for bodies with radii < 300 g/cm’ does this maximum occur at its

center. The data points (filled squares) are the results of the Apollo 15 core (converted to dpm/kg-Ca) and are in good agreement with the calculations.

The app~cation of 41Ca to study SCRs looks promising, if limited. It is certainly desirable to measure 4’Ca (and other radionuclides) in at least one or two more rocks; no rock is perfect, and all suffer from a restricted view of the horizon, uncertain surface erosion, or even complicated exposure history (74275 was ejected from depth onto the lunar surface only 2.8 Myr ago) Comparing the results from several rocks provides a means of removing these influences.

Gd, Sm, Au, H and Cl) can effectively deplete the thermal neutron flux, thus reducing 41Ca production. 3.2.2. Exposure dating of rock surfaces The calculated 4’Ca/40Ca saturation value in calcareous rock (assuming all thermal neutrons are captured by Ca and the absence of erosion), has been estimated to be - 8 X lo-l5 [7]. The results that we have obtained for several surface rocks [30] seem to support this value. The highest ratio we have measured is 7 X lO_” in a tufa veneer from Egypt, a region characterized by low erosion and long exposure. However, our major observation, based on a painfully small number of samples, is that on the time scale relevant to 4’Ca (a few 100 kyr) erosion of Ca-bearing rocks is not negligible. Consequently, for most terrestrial environments the equilibrium surface 41Ca$aCa ratio will be considerably less than this theoretical saturation value and, importantly, will be strongly influenced by the local erosion rate. Perhaps this variability in surface concentrations of 41Ca might be turned to some advantage. Measuring erosion rates and surface exposure ages using ‘*Be and 26Al produced in situ in quartz is now fairly well established [31,32]. If a similar method could be developed using 41Ca in feldspar or limestone, it would find utility in a number of environments. Rock samples from regions of low erosion and high altitudes should necessarily be enhanced in 41Ca. The Dry Valley regions of Antarctica generally fit this description, and we have begun to carry out 41Ca measurements in rock samples from this area where we have also made “Be and 6A1 analysis. At present, a comparison of the results is not ossible but will be essential in validating the method. YlCa offers yet another advantage, in that its shorter half-life would make it easier to measure short exposures or high rates of erosion (> 10v3 cm/yr).

3.2. Terrestrial - bone and surface rock samples 3.2. I. Pr~~~tio~ surface

by thermal neutrons on the Earth’s

On the Earth, 41Ca isroduced primarily by capture of thermal neutrons by Ca at the surface. Other production modes, such as cosmic-ray interactions on 42*44Ca,Ti and Fe, and atmospheric production, are unlikely to account for more than a few percent of the total. Pr~uction is limited to the top few meters of the Earth’s surface (at a depth of 3 m the production is nearly 10 times less than at the surface) and in Cabearing rock materials, such as limestones, corals, feldspars, granites, etc. Production of 41Ca is dependent on target composition and is proportional to the ratio of the thermal cross section of Ca to the total thermal cross section of all the elements in the rock weighted by compositional abundance. High levels of trace elements possessing very large neutron capture cross sections (i.e.

3.2.3. Radiocalcium dating of bones Because Ca is a major constituent of bone, and since 41Ca’s half-life is ideal for dating the development of man, the use of 41Ca as a dating tool has attracted considerable attention - even before any practical method of measuring it at terrestrial concentrations existed. Even now, measuring 41Ca in terrestrial samples with 41CapCa ratios between lo-r6 and lo-l4 is difficult; it is this factor more than any other that has revented a comprehensive and systematic survey of g, Ca concentrations in bones. Unfortunately, the situation is not likely to improve in the foreseeable future there is no obvious way of significantly improving negative-ion currents, and alternatives, such as pre-enrichment are arduous, expensive and time consuming. Depte these difficulties several measurements of 41Ca/ Ca ratios in bones have been made. In general, the results from Jerusalem [33] and Argonne National

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D. Fink et al. / 4’Ca: past, present and future

Laboratory [13] are higher than we have measured at the University of Pennsylvania 1301, and the overall spread of results for modem bones is two orders of magnitude. At a single site, we have measured a range of a factor of seven among different species, and looking at a single species in several locations, the range of values was more than a factor of three [30]. What causes these variations? Perhaps it should not be too surprising that coeval bones from different locations should have different 4*Ca concentrations. We have already noted the important role played by erosion in determining the steady-state value of 41Ca in surface rocks. At each location, local environmental factors will be different and these will affect the “zero” age of the 41Ca clock. The surprising result was the variability in the 41Ca/40Ca ratio of bones from different species, all from the same location. The necessary follow-up experiments have not been done. We have not repeated these measurements, nor measured the v~iabi~ty among indi~du~s of the same species from a single location, nor looked at a stratigraphic sequence in which there was some independent age control or even measured paired samples of bone and source rock. These are all experiments for the future. Kavana et al. [34] have put forth an interesting possibility for another source of the variability in these samples. It is well documented that nuclear weapons testing in the 50s and 60s resulted in several radionuclide “bomb spikes”: The concentration of 14C in the atmosphere nearly doubled, and the 36Cl concentration in ice samples, deposited at the peak of the marine nuclear-weapons program, is two orders of magnitude higher than the natural pre-bomb level. They suggest a similar effect may have occurred for 4’Ca. Clearly, the requirement for producing anthropogenic 41Ca was present - an intense neutron flux with exposure to large amounts of ‘?Za in the corals of the atoll testing sites. The difficulties lie in estimating how much bomb-induced 41Ca entered the atmosphere, and its chemical and kinetic behavior once in the atmosphere. Measurement of 41Ca in Greenland ice, if at all possible, is needed in order to answer these questions. If a bomb spike of 41Ca is recorded in the ice and enough of it was distributed globally, it could be another reason for the large variability of the 4’Ca concentration observed in modem bone. 3.3. Medical applications A final, and as yet largely unexplored area of research, involves the utilization of AMS in biological tracer experiments. With its unparalled sensitivity, AMS offers the potential to expand the concept of tracer experiments to include the administration of long-lived radionuclides with doses up to lo6 lower than would be

needed if detection were done by decay counting. For the case of Ca, the only radioisotopes of biological interest are 45Ca and 47Ca with half-lives of 165 and 4.5 days, respectively. Their half-lives are too short to conduct a multi-year investigation into the loss of bone calcium responsible for osteoporosis - a disease that most frequently inflicts post-menopause women, but in general, humans of advanced age. In some studies, %Sr has been used as a surrogate for Ca. Its half-life of 28.1 years is perfect, but the doses required for many experiments are above the acceptable limits for human experiments. In contrast, both the half-life and decay mode of 41Ca effectively renders it biologically harmless making it an ideal candidate for long-term tracer studies of bone resorption and loss in humans. At an AMS detection limit of about lo6 41Ca atoms, it is sufficient to administer tens of nanograms of ‘“Ca in an average weight human to arrive at a 41Ca/40Ca ratio of - lo-l2 in bone. This dose is a factor of lo6 below the annual limit for oral intake in man 1351. Elmore et al. [36] have commenced a feasibility study by prelabeling dogs with 41Ca and 45Ca, in order to determine their concentration in serum and bone over an extended period. By initially following both radioisotopes to the level where 45Ca falls below detection due to decay (a few months), it will be possible to ascertain the applicability of 41Ca vis-a-vis AMS measurement and role as a tracer. Preliminary measurements have been carried out at the University of Pennsylvania (see Elmore et al. [36]). The second phase would entail 4’Ca measurements over the period of a year or more. If successful, 41Ca will present an attractive means, free of exposure to any radiation, with which one can identify women at a high risk of developing post-menopausal osteoporosis prior to the onset of damaging bone loss.

4. Conclusion aud expectations

It should be apparent from the foregoing that the field of AMS-based measurements of 41Ca is really just beginning. Because Ca is an important element in many materials, and because of the varied ways in which 41Ca is produced, me~urements of 41Ca are useful in a number of studies. A summary of measured and expected 41Ca concentrations in the diverse extra-terrestrial and terrestrial samples described in section 3 is presented in table 2. The problem of measurement has been solved satisfactorily for extraterrestrial samples where concentrations are typically in the 10-l’ to lo-l3 range. Even though the overall detection efficiency is relatively smalI, about 0.02% (0.2% in ionization efficiency and 10% in accelerator transmission), a measurement with 5% precision requires about an hour. IV(d). NUCLEAR

& ASTROPHYSICS

D. Fink et al. / 4’Ca: past, present andfuture

580

Table 2 Summary of expected and in some cases, measured, 41Ca concentrations in samples from different enviroments discussed in the text. The table shows the large variation in %a/%a ratios as a function of sample origin and production mode. Object

Principal production mechanism

Target

Iron meteorite (r < 300 g/cm2)

Cosmic-ray induced spallation

Fe

Chondrite

~~~-neutron capture Surface Maximum (size dependent) Total for small meteorites from thermal-neutron capture and spallation

Concentration

WVkl

Moon Surface

Subsurface

Cosmic spherules Earth Surface Ca Ca plagioclase

Solar-proton induced reactions Solar-proton induced reactions Thermal-neutron capture Total including solar-proton, cosmic-ray-induced spallation, n-capture Thermal-neutron capture (peak concentration at - 180 g/cm’ )

23.8

Ca Ca 20%Fe+ l-S% Ca

Ti

Ca Ca 14% Fe f 7.6% Ti + 7.4% Ca Ca

286 71 28 32

loo0

Spallation

20% Fe+0.5% Ca

Thermal-neutron capture beak-neutron capture Thermal-neutron capture

Ca 14% Ca 30% Ca

Unfortunately, the picture is not so bright for terrestrial measurements, where concentrations are usually in the lo-l4 to lo-l5 range (or even less). At a current of 5 PA and 10% transmission, the count rate of a sample with a 41Ca/40Ca ratio of lo-l4 is 100 counts/h. At lo-l6 it is only 1 count/h. Unfortunately there is no clear path to higher currents or a significant improvement in accelerator transmission - the only plus is that there is usually no limit in sample size. The alternative of pre-enrichment, while possible, is unattractive because of its expense, the time involved and introduction of a source of new error. Particularly so, considering the unpro~sing results that have resulted from such preliminary studies of bones. On the other hand, if a radiocalcium method of dating bones were shown to be effective, then the investment in isotope separation would be justified. The new field of biomedical applications of AMS would seem to benefit from the inclusion of 41Ca as a tracer. One looks eagerly forward to the first results from these studies. 41Ca measurements by AMS are now at the stage where results from the fields of applications outlined in

O-60 O-2500 5-130

4.8 1.9 0.3 c 0.7

[atoms/g x 109]

Ratio [4’Ca/CaX10-‘5]

1.9

o-4.7 O-200 0.39-10

O-300 O-13000 500-13ooo

22 5.6 2.2 2.5

370 150 2300

78

5200

0.37 0.15 0.02 i 0.05

4900 10

10 (10

this work are just beginning to mature for the harvest to come. The next few years will bear the fruit.

We would like to emphasize the importance of collaborators in fields of interdisciplinary research such as AMS. Without them, all would be in vain. In particular, we would like to acknowledge the contributions of Gregory Herzog and Stefan Vogt, who have prepared most of the extraterrestrial samples we have measured, but more importantly, have been responsible for focussing our attention on important problems. K. Nishiizumi and J. Arnold have also been a source of both samples and ideas. Thanks go to H. White as well who has contributed much to the technical aspects of these meas~ements. Finally, we thank the National Science Foundation for the support of our tandem accelerator without which this work would not have been possible.

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D. Fink et al. / “‘Ca: past, present and future

Appendix As mentioned earlier, we had hoped to determine 4*Ca/eCa ratio by measuring the @Cast current arising from ?ZaH,Dwhich is injected into the accelerator at the same time as 4’CaH;. In principle the 40Ca8+ current corresponding to ‘?ZaH; injection can be calculated from the former current by making a correction for the isotopic abundance of deute~um (0.45 nA measured should correspond to 1 p A of %Ca8+ from ?aH,). Unfortunately, the currents measured this way proved variable and in general about 50% less than those measured directly. After several attempts we were forced to abandon this simple and elegant method of measu~ng 4’Ca/@Ca ratios. Later, off the accelerator, we decided to study this apparent isotope effect further with cathodes containing calcium hydride, calcium deuteride and a SO/50 mixture of hydrogen and deuterium. The calcium hydride, deuteride and hydride/deuteride mixtures were pre-

3

-_ ii = iL, 2

z

I CaH-

1

Cali;

0

-i

pared in the apparatus shown in fig. 2b of ref. [ll], following the procedure described therein. All measurements were made on our ion-source test facility with an ion source not dissimilar to that used on the accelerator, except that it had a spherical ionizer. To our intense surprise the spectra recorded with the calcium hydride and deuteride were quite different and in particular the CaH;/CaHratio was about 3 compared with a CaDq/CaDratio of about 1.5. We were so surprised by this difference that we repeated the experiment several times using freshly prepared calcium hydride and deuteride, but with essentially the same results. Simulated spectra showing the average relative intensities of the molecular ions are shown in fig. 4 where CaH- and CaD- have both been no~alized to unit intensity. Also shown is a spectrum from a cathode containing calcium hydride made from a SO/SO mixture of hydrogen and deuterium. Given the large difference in spectra from the hydride and deuteride cathodes, the latter comes as no surprise. We are at a complete loss to understand why the ratio of CaX-/CaXy is so different for hydrogen and deuterium, and are forced to conclude that it is a very large isotope effect resulting from the factor of two difference in mass. In light of this it is perhaps not surprising that @CaH,Dcould not be used to determine the @CaH; current and that this method had to be abandoned. However, the measurements with negative ions were quite reproducible and we do not understand the occasional variability in the ‘*CaH,D-/?ZaH; ratio that we observed on the accelerator.

-

References [I] G.M. Raisbeck, F. Yiou, A. Peghaire, J. Guillot and J. Uzureau, Symp. on Accelerator Mass Spectrometry, eds.

[Z]

(31 [4]

MASS

( a.m.u.)

Fig. 4. The upper two spectra were measured with cathodes prepared from IOOSgcalcium hydride and 100% calcium deutride. The intensities of the CaH- and CaD- peaks have been normalized to unity. Surprisingly, whereas the intensity ratio of CaH; to CaH- is about 3, the ratio of t&D< to CaD- is only - 1.5. The lower spectrum was measured with a 50/50 mixture of calcium hydride and deuteride. The relative intensities, although surprising, are roughly in accord with expectations based on the upper two spectra.

[S] [6]

[7] [8]

W. Henning et al., Argonne National Laboratory, ANL/PHY-81-l (1981) p. 426. D. Fink, M. Paul and G. Hollos, Workshop on Techniques in Accelerator Mass Spectrometty, eds. R. Hedges and E. Hall, Oxford, UK, 1986, p. 23. M. Paul, D. Fink, P. Englert and S. Theis, Meteoritics 20 (1985) 726. D. Fink, M. Paul, G. HOBOS, S. Theis, S. Vogt, R. Stueck, P. Englert and R. Michel, Nucl. lnstr. and Meth. B29 (1987) 275. P.W. Kubik, D. Elmore, N.J. Conard, K. Nishiizumi and J.R. Arnold, Nature 318 (1986) 568. G. Korschinek, H. Morinaga, E. Nolte, E. Preisenebrg, U. Ratzinger, A. Urban, P. Dragovitsch and S. Vogt, Nucl. Instr. and Meth. B29 (1987) 67. G. Raisbeck and F. Yiou, Nature 277 (1979) 42. W. Henning, W. Bell, P. Billquist, B. Glagola, W. Kutschera, Z. Liu, H. Lucas, J. Yntema and M. Paul, Science 236 (1987) 725. IV(d). NUCLEAR

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