262 Nuclear Instruments and Methods in Physics Research B29 (1987) 262-265 North-Holland, Amsterdam

DEPTH DEPENDENCE OF “Be AND %A1 PRODUCTION RATES IN THE IRON METEORITE GRANT

Thomas GRAF ‘) Stephan VOGT *I, Georges BONANI 3), Ulrich HERPERS *), Peter SIGNER ‘), Martin SUTER 3),’ Rainer WIELER I) and Willy WOLFLI 3).

‘) Institut fti Kristallographie und Petrographie, ETH, NO C61, CH-8092 Ziirich, Switzerland

2J Abteilung Nuklearchemie, Universitiit zu Kiiln, Zii@icherstrasse 47, D-5000 Kiiln I, FRG

‘) Institut ftir Mittelenergiephysik, ETH, HPK G7, CH-8093 Ziirich, Switzerland

‘OBe and 26Al concentrations in samples from a cross section of the iron meteorite Grant were measured with a 6 MV tandem Van de Graaff accelerator. Noble gas data on samples adjacent to those used for radionuclide determinations had either been reported earlier or are presented here. A correlation between the production rates of the radionuclides and the shielding parameter 4He/2’Ne is observed. Both “Be and 26A1 activities decrease with increasing shielding. The 1oBe/21Ne ratio is constant and thus can be used to determine shielding corrected 21Ne exposure ages.

1. Introduction

Stable and radioactive nuclides produced by cosmic ray particles in meteorites are widely used to recon- struct the history of meteoritic bodies as well as the cosmic ray intensity in the past. The reliability and the resolution of such investigations depends on the accu- racy of the assumed production rates of the nuclides in question. These production rates are a function of the cosmic ray particle flux and energy distribution, the chemical composition of the target, and the size of the irradiated body as well as the position of the sample studied within this body. Various models on the pro- duction of cosmogenic nuclides in meteorites have been developed (e.g., refs. [l-4]). Validation of these models requires analysis of as many nuclides as possible on small samples taken from known positions in meteorites. The requirement of small sample size has long since been met for cosmogenic stable noble gas nuclides by static mass-spectrometry. Furthermore, 53Mn can be measured by radiochemical neutron activation analysis on samples of less than 100 mg too. Since a few years, accelerator mass spectrometry (AMS) allows the mea- surement of the activities of additional radionuclides such as “Be, 26A1, 36C1, and 41Ca in meteorite samples

around 0.1 g. Following this development, several com- bined investigations on the production of cosmogenic noble gases and radionuclides as a function of depth and size in stony meteorites have been reported (refs. [5-91). We now present such a study on a large iron meteorite. For our investigation, we chose the meteorite Grant, because it had been extensively studied for He, Ne, and Ar [l], such that its geometry during exposure to cosmic rays is reasonably well known. Additional

0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

noble gas measurements complement the earlier data

set. In this short report, we focus on the following:

1) Discussion of the depth profiles of “Be and 26A1 activities in Grant.

2) Correlations between radionuclide and noble gas profiles as a function of depth in the meteorite Grant. In particular, we verify the observation noted in a pilot study [lo], that the ratio of the production rates of “Be and *lNe in Grant is independent of shielding. The complete set of data will be presented elsewhere

[ll], together with an extended Signer-Nier type model [l], describing the production of radionuclides and no- ble gases as a function of depth in iron meteorites of any size.

2. Experimental procedure

A chemical procedure to separate “Be, 26Al, 41Ca, and 53Mn from a single sample will be given elsewhere [12]. To ensure bulk meteorite composition, we used samples of 300 to 400 mg weight, although the amounts of lo Be and 26Al required for an AMS determination are contained in less than 50 mg of the Grant iron meteorite.

“Be/‘Be and 26Al/27Al ratios were measured with the 6 MV tandem Van de Graaff accelerator at ETH Ziirich. The technique of the AMS determinations is described in [13]. With about 1 mg of 9Be tracer used for meteorite samples of 400 mg, the measured isotopic ratios ranged from 0.8 X lo-” to 2.5 X lo-“, yielding typically 9 Be3 + currents around 200 nA. For “Be/‘Be ratios of lo-“, about 170 l”Be3+ ions per minute were

T. Graf et al. / AMS studies on the iron meteorite Grant 263

recorded. Due to the lower yield of Al- ions in the ion source, 27A17+ currents were around 50 nA and 26A1/ 27Al ratios of lo-” yielded about twenty 26Al ions per minute. In 7 min, the “Be/‘Be ratio is determined to an accuracy of 5-6% whereas the 26Al/27Al ratio is measured within 20 min to an accuracy of 5-7%. Re- agent blanks yielded isotopic ratios of about 4 X 10-14, both for Be and Al, corresponding to a background level of about 0.07 dpm/kg for the samples investigated

here.

The “Be standard “lla” used (‘“Be/9Be = 1.18 X 10m9) was kindly provided by Dr. G.F. Herzog, Rutgers University, New Brunswick. A comparison with other standards is given in ref. [14]. The 26A1 standard “A19” ( 26A1/27A1 = 1.19 X 10m9) was prepared at Cologne [15]. The half-lives used to convert the measured concentra- tions into the reported activities are 1.6 X lo6 a and 7.2 x 10’ a for “Be and 26A1, respectively. To test the reproducibility of the data, “Be determinations on four samples reported earlier [lo] were repeated. The data

agreed to better than 5%. The errors of the “Be and 26A1 determinations indi-

cated in the figures comprise the counting statistics as well as the reproducibility of the chemical procedure.

3. Results

Fig. 1 shows the position of the samples analyzed for this investigation in the cross-section of the Grant meteorite. Also shown are the locations of the samples measured in two earlier studies [l,lO]. The concentra- tions of “Ne as well as the activities of “Be and 26A1 are shown in fig. 2 in relation to the sample position along the bars.

The activities of the radionuclides show a trend similar to that of the “Ne concentrations. Near the postatmospheric surface of the meteorite, all nuclides are produced more efficiently than in more heavily shielded locations.

On close examination, some irregularities in the trends are observed. Note that both “Be and 26A1 are extracted from the same sample, whereas all the noble gases were determined on another sample from an im- mediately ajacent position. Therefore, possible chemical heterogeneities would affect either all radionuclides or all noble gases in a systematic fashion Unfortunately, the present data does not allow a rigorous examination

as to the cause of the irregularities, most clearly visible in two “Al outliers. The respective data points are indicated in a.ll figures by question marks. For samples of the size used here, variable Taenite/Kamacite ratios or microscopic troilite inclusions [16] are unlikely to affect cosmogenic nuclides as much as is observed for the outliers. The samples in question will be reanalyzed.

The new “Be data substantiate the conclusion drawn

Grant Reference Line

Normalisation Radius

BAR R

BARN

BARJ

BARF

BARB

-400 -200 0 200 tmml

Fig. 1. Cross section of Grant with the positions of the samples studied here (solid symbols). Open symbols show the locations

of samples studied for noble gases only (ref. [l] and new data).

The normalisation radius used in [l] is also indicated.

-400 -200 0 200

X 20 -

: X X

. ZX 0. : x

g -20 - BAR N

I X P 20. ;

a -

* X xA

9 O- : $ X X X %:” x ?

4 -20 - BAR J

Ll -400 -200 0 200

Distance from Reference Line [mm1

Fig. 2. Comparison of the concentrations of cosmogenic

nuclides ‘lNe (crosses), “Be (squares), and 26Al (triangles) in the four bars of the Grant meteorite. Duplicate analyses are

given by the average values only. Two doubtful 26A1 data

points are labeled by question marks (see text). For ease of

comparison, the data is plotted with the following normalisa-

tion: The mean values of the concentrations of all 21Ne‘

determinations and the mean of all “Be and 26Al activities are

set as unity. The data is then expressed as deviation from the

mean in percent. The mean values are: 21Ne = 5.84~10-~

cm3STP/g, lo Be = 3.43 dpm/kg and 26A1 = 2.41 dpm/kg. Er- ror bars indicated are typical for the respective nuclide.

III(c). COSMOCHEMISTRY/IN SITU PRODUCTION

264 T. Graf et al. / AMS studies on the iron meteorite Grant

I n n

n

3 n n

n A

?

1= 1

250 300 350

4He/21Ne

Fig. 3. Measured activities of “Be and 26A1 versus the depth sensitive 4He/21Ne ratio. The location of the individual sam- ples is indicated by the letter for the bar and the position with respect to the reference line as shown in fig. 2. The ordinate scales for the two nuclides are chosen such that the relative variations can be directly compared. The two doubtful 26A1 analyses indicated by question marks are not included in the

calculation of the respective linear best fit.

from the pilot study [lo], that the depth dependence of the production of the two nuclides *lNe and “Be in Grant are quite similar. On the other hand, fig. 2 reveals that the 26Al production varies less with depth than *lNe and “Be. This is indicative for a contribution by primary particles of lower energy and/or secondary particles to the production of 26A1. A more detailed examination of the depth dependence of the production rates of the three nuclides is given in the next section.

4. Discussion

Radionuclide determinations allow essential exten- sions of studies of cosmogenic nuclide production. On the one hand, noble gas data alone do not permit to unambiguously deduce the size of the meteoroid and the position of the sample investigated within the meteoroid [17]. On the other hand, if the size and depth depen- dence of production rate ratios of pairs of stable and radioactive nuclides are known, shielding corrected ex- posure ages can be deduced. Thus, it is essential to

understand the production of the radionuclides in terms of meteoroid sizes and sample position. For iron meteorites, the availability of shielding corrected pro- duction rates is even more important than for stony meteorites, because the former cover a much larger size spectrum, which impedes the use of “average” produc- tion rates. Moreover, iron meteorites have generally been exposed to the cosmic radiation for much longer times than their stony counterparts. Thus, iron meteorites can be used to monitor the cosmic ray inten- sity over times of up to more than a billion years, provided that radionuclides of appropriate half-lives can be determined. @K (T,,, = 1.25 x lo9 a) in iron meteorites has been extensively studied (e.g. ref. [17]), while two nuclides with much shorter half-lives are presented here. Also, iron meteorites may have ter- restrial ages which are not negligible compared to the half-lives of 26A1 or even “Be. Thus, the knowledge of the size and depth dependence of the ratio of the saturation activities helps us to recognize such speci- mens.

For the following discussion, we assume the ter- restrial age of Grant to be sufficiently short to not affect the 26A1 activity. This assumption is justified, since the radiochemically determined 36C1 activity in Grant is comparable to that measured in iron meteorites with known date of fall (cf. ref. [3]).

The 4He/21Ne ratio in iron meteorites is one of several size and depth indicators [1,17]. Therefore, we now examine the relation between the activities of the radionuclides and the 4He/21Ne ratios in correspond- ing samples. Fig. 3 shows the correlation of the “Be and 26A1 activities with the 4He/2’Ne ratio. Excluding the two doubtful 26A1 data points, the following linear best fit lines to the data are deduced:

“Be[dpm/kg] = ( 7.85 f 0.90) - (0.015 f 0.003)

x4He/*‘Ne (1)

and

26Al[dpm/kg] = (3.47 _t 0.60) - (0.0036 f 0.002)

x4He/*‘Ne. (2)

Whether a linear approximation for the two correla- tions is justified has to await additional model calcula- tions. Furthermore, note that these relations hold for Grant only, because they depend on the size of a meteoroid.

Fig. 4 shows three correlations between two ratios of cosmogenic nuclides. The figure emphasizes the similar- ity of the “Be and *lNe profiles noted in fig. 2. Also, the depth dependence of the 26A1/21Ne and ‘“Be/26A1 ratios is obvious. Within the framework of the Signer-Nier model, data points from samples of iron meteorites of any size and depth should, as shown by Voshage [17], plot on a single straight line in each of the

T. Graf et al. / AMS studies on the iron meteorite Grant 265

250 300 350

250 300 350

4He/21Ne

Fig. 4. Upper parts: production rate ratios P(“Be)/p( 21Ne) and P( 26Al)/P( 2’Ne) in [atoms per atom] as a function of the

shielding parameter 4He/2’Ne. The exposure age of Grant is

thereby assumed to be 695 Ma [17]. Lower part: activity ratio

26A1/‘0Be. The doubtful 26A1 data points are not used to calculate the respective linear best fits.

two upper panels in fig. 4. The shielding independent P(*‘Be)/P(*‘Ne) ratio

(fig. 4), where P(X) is the production rate of nuclide X, allows the deduction of shielding corrected exposure ages t of iron meteorites from the following equation:

t =21Ne,,,,,/P(‘oBe) X (0.772 * O.OSO), (3)

where *lN%,, is the *‘Ne concentration in [atoms/unit of mass] and P(“Be) the production rate of this nuclide in [(atoms/unit of mass)/unit of time]. The error stated does not include the uncertainty of the exposure age of Grant reported in [17].

The production rate ratio P(“Be)/P( *‘Ne) of 0.772 f 0.05 given here is within error limits identical to the value of 0.81 deduced from the pilot study [lo].

A linear fit to the data shown in the middle panels of fig. 2 yields:

26A1/ *’ Ne[ atoms/atom]

= (0.13 f 0.12) + (0.0014 + 0.0004) X4 He/*‘Ne.

(4)

Combining eqs. (3) and (4) yields the relationship between ‘“Be/26A1 and 4He/ *‘Ne indicated in the bottom panel of fig. 4.

5. Summary

AMS permits to analyse “Be and 26A1 in samples sufficiently small to investigate the production of these nuclides in iron meteorites as a function of shielding. In the meteorite Grant, the ratio “Be/**Ne is constant over the whole range of shielding, whereas 26Al/21Ne

versus 4He/21Ne can be approximated by a straight line. These relationships lead to a refined shielding correction for production rates of cosmogenic nuclides in iron meteorites and are expected to give a new impetus to studies of the exposure history if iron meteorites and their use as monitors for the cosmic radiation in past times, especially if the presently achieved analytical accuracy can be further improved.

This work is supported by the Deutsches Bundesmi- nisterium fur Forschung und Technologie and The Swiss National Science Foundation.

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