10Be and36Cl depth profiles in an Apollo 15 drill core

Earth and Planetary Science Letters, 70 (1984) 157-163 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

157

[6]

1°Be and

36C1depth

profiles in an A p o l l o 15 drill core

K. Nishiizumi x D. Elmore 2 X.Z. Ma z and J.R. Arnold t Department of Chemistry, B-017, University of California at San Diego, La Jolla, CA 92037 (U.S.A.) 2 Nuclear Structure Research Laboratory, University of Rochester, Rochester, N Y 14627 (U.S.A.)

Received April 30, 1984 Revised version received July 2, 1984

Cosmic-ray-produced 1°Be (tl/2 = 1.6 × 106 years) and 36C1 (/1/2 = 3.0 × 105 years) have been measured in the Apollo 15 long core for study of galactic cosmic ray production profiles using tandem accelerator mass spectrometry. From these experiments, the half-attenuation length for 1°Be production and 36C1 production were calculated to be 120 g/cm 2 and 132 g / c m 2 (150-400 g / c m 2 region). The measured half-attenuation length for 1°Be is slightly longer than that predicted by the Reedy-Arnold theoretical model. The flatter and somewhat deeper maximum seen in the 36C1 profile compared to the 1°Be, 26A1and 53Mn profiles can be explained by production from secondary thermal neutrons on 35C1.

1. Introduction The study of cosmogenic nuclides in extraterrestrial materials has two related and interacting purposes: to learn about the history of the sun and of the galactic cosmic rays (GCR), and to study the history of the moon, meteorites and other solid bodies in the solar system. Reedy and Arnold have developed a model for calculating the production rates of various cosmogenic radionuclides as a function of depth in the m o o n [1]. This model can be usefully extended to the production profiles in meteorites of various sizes. To check and improve the theoretical model, measurements of cosmogenic nuclide production profiles in well documented materials are very important. Apollo drill cores are the best samples available for study of the G C R production profile. Lunar samples have the advantage over meteorites that the heliocentric radius (1 A.U.) and shielding depth are accurately known. While it is true that the surface of lunar soil has been disturbed by meteorite impact gardening effects, only the upper few centimeters of the regolith are disturbed on a the million years time scale. Among 0012-821X/84/$03.00

© 1984 Elsevier Science Publishers B.V.

three deep drill cores, Apollo 15, 16 and 17, the Apollo 15 core is the best sample for long-lived nuclide studies because it is the least disturbed sample in terms of cratering and m a n m a d e processes. As part of a program to complete measurements of several cosmic ray produced nuclides in this long core, we have measured l°Be ( t i t 2 = 1.6 X 106 years) and 36C1 (tl/2 = 3.0 × 105 years) in the Apollo 15 drill core by tandem accelerator mass spectrometry. Results for 26A1 ( t i t 2 = 7.05 × 105 years) are reported elsewhere [4].

2. Experiment and results Nine soil samples were selected from the Apollo 15 drill core. The sample depth covered was from the surface to almost 400 g / c m 2 (218 cm), and the sample sizes ranged from 50 to 380 mg. Be, CI and A1 were chemically separated from the soil samples. We used relatively large samples to maximize the precision. After adding 2.0 mg of Be and 4.0 mg of C1 carrier, the sample was dissolved with a mixture of H N O 3 and HF. About 2-3% of the solution was taken as an aliquot for chemical

158 20

analysis by atomic absorption spectroscopy. After separating C1 by AgC1 precipitation [2], Be was separated from the other elements and purified by hydroxide precipitation, cation exchange and Beacetylacetone solvent extraction. Finally, Be(OH)2 precipitate was ignited to BeO at 950 o C [3]. The 1°Be and 36C1 measurements were carried out using the University of Rochester MP tandem Van de Graaff accelerator. The apparatus and method for accelerator mass spectrometry are essentially the same as described previously [2,5,6]. We selected an 8 MV terminal voltage for 1°Be measurements and 10 MV for 36C1 measurements. BeO was mixed with Ag powder ( - 1 : 4 by weight) for the ion source. We measured l°Be/9Be ratios in the range 4-8 x 10 -12 with experimental errors of 3-5%, and 3 6 C 1 / C 1 ratios of 2-4 x 10 -12 with errors of 1.5-3% (with one exception). Both l ° B e / 9 B e and 3 6 C 1 / C 1 measured values were norrealized to standards after blank correction. We have prepared a new 1°Be standard (ICN Chemical & Radioisotope Division) for this experiment and compared it with our old 1°Be standard, which we have used for more than 15 years at La Jolla and in previous accelerator mass spectrometry experiments [3,7]. The two 1°Be standards agreed with each other to within 1%. The 1°Be and 36C1 activities in the Apollo 15 drill core are shown in

o~

~'hr-~~;

~0

o

5

'°Be

b

"a

....

>

I 100

I 0

"---

I 200

DfiDth

I 300

( g / e r a 2) .

Fig. 1. Measured 1°Be activities in the Apollo 15 drill core. The filled circles were obtained by this experiment. The experimental errors are within the circles except for one showing a vertical bar. The open circles are results from previous accelerator mass spectrometry experiments in the same core sample [7]. The other points are previous measurements of 1°Be in lunar rock 12002 [9], 14310 [10] and 14321 [10]. The solid line indicates the best fit curve to the experimental data.

Table 1 along with the measured concentrations of K, Ca and Fe. Fig. 1 shows 1°Be activities in twelve samples of the Apollo 15 drill core along with previous measurements of 1°Be in lunar rocks 12002, 14310 and

TABLE 1 Result of 1°Be and 36CI measurements in Apollo 15 drill core Sample

Depth ( g / c m 2)

15006,267 15006,266 15005,463 15005,462 15004,214 15003,673 15002,582 15002,581 15001,361

4,2 41.9 68.1 110.1 162.2 220.4 290.8 336.3 387.8

15005,292 15004,168 15001,249

102.9 198.7 361.1

a Kohl

[8].

b Nishiizumi et al. [7].

Weight (mg) 47.7 50.4 52.6 63.4 91.9 154.0 251.8 308.6 376.7 9380 9390 14000

I 400

K (%)

Ca (%)

Fe (%)

aoBe (dpm/kg)

36CI (dpm/kg)

0.225 0.248 0.292 0.236 0.192 0.211 0.237 0.262 0.154

7.30 6.92 7.17 7.13 7.52 8.23 7.23 7.61 6.17

11.8 12.2 11.7 11.5 12.3 12.0 12.1 12.0 14.7

10.21 + 0.34 11.14 + 0.60 9.46 5:0.43 8.73 5:0.33 7.22 5:0.38 4.82 5:0.34 3.42 5:0.14 2.670 5:0.079 1.915 5:0.067

10.85 + 0.24 11.21 + 0.23 11.91 5:0.26 10.83 + 0.21 9.86 + 0.22 7.39 5:0.11 4.92 5:0.42 4.18 + 0.13 2.370 + 0.044

-

7.8 a 7.2 a 7.2 a

11.4 a 12.3 a 11.3 a

9.53 5:0.83 b 5.82 4- 0.43 b 2.205:0.11 b

_ _ --

159

I

I

~

I

36CI

20

3. Discussion

I

APOLLO 15 Drill Core

3.1. Model comparison of t°Be data

~ ..,['T"~,~ ('~ ~ • ~ ' ~ " " 10 -I

~

Reedy -Arnold theoretical GCR profile

T

o ~D ~3

5 >

• This work ~

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,

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Kohl (1975)

•

Wahlen et al. (1972) 14310, 14321 1

~

0

I

100

i

I

i

I

200 300 Depth ( g/cm2)_

i

I

400

Fig. 2. Measured 36C1 activities in the Apollo 15 drill core. The filled circles were obtained by this experiment. The experimental errors of this experiment are within the circles except for one showing a vertical bar. The other points are results from previous experiments [8,10]. The solid curve is the Reedy-Arnold G C R calculated profile [1]. The G C R component of this curve has been normalized to our data.

14321 by decay counting [9,10]. All rock data are normalized to the Apollo 15 chemical composition using the Reedy-Arnold model [1]. Fig. 2 shows 36C1 activities in nine samples with previous measurements of 36C1 in 14310 and 14321 by decay counting [10]. In both figures, experimental errors are within the circles except for one showing a vertical bar. The figure also shows an unpublished 36C1 result in 15004 by decay counting [8]. Although the errors are larger, the result agreed with the present experiment. Three a°Be samples from about 10 g of the Apollo 15 drill core had been prepared for beta counting previously [8]. The beta counting data were not reliable because of the low activities. An aliquot of the counting sample was diluted with 9Be and measured by accelerator mass spectrometry [7]. Results for these three samples were in excellent agreement with those for the nine samples prepared directly for this work. Both sets of Apollo 15 data are shown in Table 1 and Fig. 1.

Reedy and Arnold [1] modelled production rates of cosmogenic nuclides in the moon. They derived a flux of bombarding particles with E > 10 MeV, which decreases with depth, and a spectral shape parameter a (essentially a mean energy) which models the increase with depth of the relative number of lower energy particles. In Fig. 1 the solid line indicates the best fit curve to the experimental data. Below 150 g / c m 2, a least squares straight line is shown. The ReedyArnold calculated line bends down at below 300 g / c m 2 [1], but all seven experimental points fall on the straight line. This implies that the shape parameter a of the G C R flux below 300 g / c m 2 in the moon used in the Reedy-Arnold calculation should be changed. In fact a may steadily decrease with increasing depths below 300 g / c m z. In seems that the buildop of low-energy secondary particles relative to high-energy particles below 300 g / c m 2 is higher than predicted in the Reedy-Arnold model [1]. From this work, the half-attenuation length for ~°Be production is 120 g / c m 2 (150-400 g / c m 2 region), with a linear regression coefficient of r 2 = 0.997. The absolute calculated values of a°Be by the Reedy-Arnold model [1] are a factor 2.5 lower than the experimental values assuming a cross section ratio o ( a0B e ) / o ( 7Be) = 0.15. This ratio was derived from the measured cross section ratios of ]°Be to 7Be for the target elements C, N, O, Mg, Si and Fe [11-17, 23-25], obtained by proton bombardments on thin targets with incident energies from 135 MeV to 21 GeV. However, measured ]°Be cross section data are few, and well known 7Be cross sections; with an assumed o(l°Be)/o(VBe) ratio, had to be used for theoretical calculations [1]. The ~°Be/VBe ratio is an important parameter in the model, so it is useful to take its value from measurements in extraterrestrial materials. Evans et al. [18] measured 7Be in six recently fallen stony meteorites. The 7Be activities ranged from 38 to 230 d p m / k g meteorite. The short half-life of 7Be (t~/2 = 53.3 days) results in a 7Be content that reflects the temporal variation of G C R intensity,

160 since most of the 7Be is formed in a several month period before the meteorite falls on the earth. The G C R intensity varies directly with neutron monitor rate and inversely with sunspot numbers. Thus the recent neutron monitor rate shows that the minimum intensity of G C R occurred during 1968-1970 (solar maximum) and the intensity quickly increased from 1971 and stayed higher in intensity during the next several years. The VBe in Lost City, which fell in January 1970, was 38 d p m / k g [18]. As expected, the average 7Be in five meteorites which fell during 1973-1977 was much higher, 134 d p m / k g [18]. Assuming an average l°Be content of 20 d p m / k g , the 1°Be to 7Be ratios of the above two cases are 0.53 and 0.15. The measured a°Be to 7Be activity ratio in trench soil sample 78421 (22-40 g / c m 2 depth), which was collected in December 1972 (higher G C R intensity), was 0.13 _+ 0.03 [8], which agrees with the five meteorites. The conclusion is that the average production rate ratio of a°Be to 7Be is probably near the mean of 0.15 and 0.53. The proton cross section ratio of 0.15 which led to a factor 2.5 lower in the Reedy-Arnold production profile of l°Be, was at the lower end of the measured production rate ratio of 1°Be to 7Be. This discrepancy would be explained if the neutron cross sections were higher than the proton cross sections, as is expected. Recently, to fit the theoretical model to this experimental data, R.C. Reedy (personal communication) recalculated the l°Be production profile of the Apollo 15 drill core using new cross section ratios of 1°Be to 7Be: 0.3 for Mg, 0.2 for Al and Si, and 0.3-0.4 for O instead of 0.15 for all target elements. The results were still 30% lower than the measured values. The half-attenuation length for the new calculations was 120 g / c m 2 (150-400 g / c m 2 region), in good accord with the observed value of 120 g / c m 2, and much different from the 97 g / c m 2 value which was calculated earlier from the proton cross section data. The main improvement of Reedy's new calculation is that it significantly increased the cross section of oxygen at 80-few hundred MeV region. Our measured profile which shows a long half-attenuation length suggests that the neutron excitation function of oxygen at energies below 100 MeV must be

increased. This experiment and Reedy's new calculation indicate that oxygen is the most important target element for producing l°Be by GCR. The elemental production rate of 1°Be from oxygen is 4 - 5 times higher than that from Mg. This is the opposite conclusion from that of Pal et al. [19] who concluded that the l°Be production rate per target atom of Mg is approximately 2 times higher than that for oxygen. If we used their conclusion, the half-attenuation length would be much shorter than the experimental value.

3.2. Model comparison of 36C1data In Fig. 2, the solid line shows the Reedy-Arnold theoretical curve for 36C1 [1] normalized to our experimental values. The absolute calculated value is about 15% lower than this line. The top - 3 g / c m 2 or more were lost during or subsequent to sampling [20]; the theoretical curve has taken this loss into account. The measured points are in good agreement with the Reedy-Arnold profile except for the deepest sample (15001,361) and the 10-70 g / c m z region. The 36C1 production in lunar samples is a complex process. The main target elements for production of 36C1 on lunar samples are K, Ca, Ti and Fe. The Reedy-Arnold calculation [1] for 36C1 production from each of these four target elements in material with the Apollo 15 drill core average chemical composition are shown in Fig. 3. Our chemical analysis shows that the average composition of the core sample is 7.10% of AI, 0.238% of K, 7.39% of Ca, 0.150% of Mn, and 12.0% of Fe. At depth below 50 g / e r a 2 effectively all 36C1 is produced by low-energy reactions from K and Ca, such as 39K(n, a) and 4°Ca(n, 3p2n), even though Fe is the most abundant target element. High-energy nuclear reactions such as Fe(n,x)36C1 and Ti(n,x)36C1, require neutrons or protons with energies over 100 MeV/nucleon. Thus, the production rate for these targets decreases quickly with increasing depth. The deepest sample 15001,361 shows a quite different chemical composition from the average. It contains relatively low A1 (5.43%), K (0.154%), Ca (6.17%) and high Mn (0.175%) and Fe (14.7%). If we normalize the 36C1 in 15001,361 to the

161 20

1

10 ~

T

i

o

t

a

I

average chemical c o m p o s i t i o n using the elemental p r o d u c t i o n r a t i o c a l c u l a t e d b y R e e d y - A r n o l d [1], it will b e increased b y 22%. I n Fig. 4, all d a t a were so n o r m a l i z e d to average c h e m i c a l c o m p o s i t i o n as follows:

i

l

o

36 C l n o r m a l i z e d = 36 E 1 m e a s u r e d

(

_o × \ '~3

0

,O

-

%

0.5

o.,

0.1

,

0

I 1

,

100

200 Deoth

I

300 (g/cm 2 )

400

Fig. 3. The Reedy-Arnold calculations of 36C1 production from target elements, K, Ca, Ti and Fe [1]. All curves were calculated for the average chemical compositions of the Apollo 15 drill core. i

"~20 \

I

I APOLLO

36C~

15

I Drill

Core

,,f

E ¢k

"o10 O

,O m

5

.p

2

1

Normalized to average chemical composition

I

0

100

J

I

J

I

200 300 Depth ( g / c m 2)

I

I

400

Fig. 4. 36C1 activities in the Apollo 15 drill core measured by tandem accelerator mass spectrometry. The experimental data have been normalized to the Apollo 15 average chemical composition using the Reedy-Arnold model [1]. The solid curve is the Reedy-Arnold calculated profile which has been normalized to the data. The dashed line shows the summation of spallation and (n,7) contribution for producing 36C1 in the core. The absolute value is normalized to the experimental data.

[0.24PK+7.39Pca+l.OPTi-1-12.0PFe]

)

[K(~) VK + Ca(~) Pc~ + 1.0e~, + F~(~) P~o ],~m~,o

where PK, PCa, PTi a n d PFe are p r o d u c t i o n rates of K, Ca, Ti a n d F e c a l c u l a t e d b y R e e d y - A r n o l d [1] a t a p a r t i c u l a r depth. T h e n o r m a l i z e d values are in a g r e e m e n t with the R e e d y - A r n o l d theoretical curve, except in the 1 0 - 7 0 g / c m 2 region a n d at 162 g / c m 2, where there are small b u t significant differences. F r o m this experiment, the h a l f - a t t e n u a t i o n length for 36C1 p r o d u c t i o n was c a l c u l a t e d to be 132 g / c m 2 ( 1 5 0 - 4 0 0 g / c m 2 region), with a linear regression coefficient of 0.998. This h a l f - a t t e n u a t i o n length is slightly greater t h a n that for 53Mn (123 g / c m 2, 1 5 0 - 4 0 0 g / c m 2 region) [4] a n d 26A1 (122 g / c m 2) [4] a n d significantly longer t h a n for 1°Be (120 g / c m 2) as d e s c r i b e d above. T h e flat profile n e a r the surface region seen in Fig. 4 can b e e x p l a i n e d as follows. T h e t h e r m a l n e u t r o n c a p t u r e cross section for 35C1 (75.55% a b u n d a n c e ) is 43 barns. T h e C1 c o n t e n t in the core s a m p l e is n o t available, so we will use the value of 20 p p m that R e e d et al. [21] o b t a i n e d for A p o l l o 15 soil samples. If we a s s u m e that the 35Cl(n,y)36Cl r e a c t i o n is similar to 59Co(n,'y)6°Co, which has a t h e r m a l n e u t r o n cross section of 37 barns, we can e s t i m a t e that the p r o d u c t i o n rate for 36C1 n e u t r o n s is a n a l o g o u s to that for 6°Co [22] after a d j u s t m e n t for 6°Co e p i - t h e r m a l n e u t r o n c o n t r i b u t i o n s . This t h e r m a l n e u t r o n c a p t u r e c o n t r i b u t i o n is a b o u t 2 - 3 d p m 3 6 C l / k g s a m p l e at 5 0 - 2 5 0 g / c m 2. I n Fig. 4 with d a s h e d line, we a d d e d this c o n t r i b u t i o n to the R e e d y - A r n o l d c a l c u l a t i o n a n d n o r m a l i z e d to the m e a s u r e d value. A l m o s t all d a t a p o i n t s fit on the theoretical curve, except at the t o p of the s a m p l e which p r o b a b l y has a solar c o s m i c r a y c o n t r i b u tion. It therefore seems r e a s o n a b l e to believe that the flatter a n d s o m e w h a t d e e p e r p r o d u c t i o n m a x i m u m seen in the d a t a o f Fig. 4 can be u n d e r s t o o d in terms of real p h e n o m e n a , a n d this m a y p o i n t the

162 w a y to i m p r o v e m e n t s in the R e e d y - A r n o l d t h e o r e t i c a l m o d e l . W e p l a n to s u p p l e m e n t the m e a s u r e m e n t s w i t h 10 Be a n d 36 C1 d e t e r m i n a t i o n s in m i n e r a l s e p a r a t e s to p r o v i d e p r o d u c t i o n - d e p t h p r o f i l e s for the separate major target dements. These should permit a more detailed check of the model.

Acknowledgements W e are d e e p l y i n d e b t e d to R . C . R e e d y for m a n y u s e f u l s u g g e s t i o n s . W e t h a n k L.E. T u b b s , M . W a h l e n , I. B r i s s a u d , R. T e n g , M. Singh, a n d t h e N S R L s t a f f f o r t e c h n i c a l assistance. W e w i s h t o t h a n k C.P. Kolal f o r h e r c o n t r i b u t i o n to t h e e a r l y p a r t o f this w o r k . W e a r e g r a t e f u l to M. I m a m u r a f o r h e l p i n g us w i t h t h e a t o m i c a b s o r p t i o n s p e c t r o s c o p y . W e t h a n k F. K i r c h n e r for t y p i n g t h e m a n u s c r i p t o f this p a p e r . T h i s w o r k w a s s u p p o r t e d b y N A S A g r a n t N G L 005-009-148 a n d NSF grant PHY-82-40321.

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24 G.M. Raisbeck and F. Yiou, The 1°Be problem revisited, in: Proc. 15th Int. Cosmic Ray Conf., Plovdiv, Bulgaria, Vol. 2, pp. 203-207, 1977. 25 G.M. Raisbeck and F. Yiou, The application of nucler cross section measurements to spallation reactions in cosmic rays, in: Spallation Nuclear Reactions and Their Applications, B.S.P. Shen and M. Merker, eds., pp. 83-111, Reidel, Dordrecht, 1976.