60Co in lunar samples

EARTH AND PLANETARY SCIENCE LETTERS 19 (1973) 315-320 © NORTH-HOLLANDPUBLISHINGCO , AMSTERDAM

m 6°C0 IN L U N A R S A M P L E S M WAHLEN, R C FINKEL, M IMAMURA *, C P KOHL and J R ARNOLD Department of Chemzstry, Umvers~tyof Cahforma, San Dtego, USA Received 6 October 1972 Revised version recewed 24 April 1973 Concentratmns of the (n, ~)-produced radmnuchde 60Co were measured m lunar samples at various depths from the surface down to 360 g/cm By comparing the data obtamed to calculated productmn rates (based on the work of Llngenfelter et al ~8]) we determined the present day lunar neutron producnon rate, which was found to be (12 -+3) neutrons/cm sec (E < 10 MeV)

1. lntroduct,on The interaction of cosmic ray partMes with the moon generates a great number of neutrons in the lunar regohth, most of them with energies below 10 MeV These neutrons are subsequently slowed down to lower energies by scattering Most of the present knowledge of the resulting neutron flux of thermal, eplthermal and higher energy (E < 10 MeV) has been derived from studies of stable isotope anomahes m lunar soils and rocks These changes m isotope ratios result from neutron capture by isotopes with extremely high capture cross sectmns For example, isotope anomalies have been reported for the rare earth elements Gd and S m [ 1,2, 3, 4] and for the heavy rare gases Kr and Xe [5,2, 6] Changed isotopic ratms in Gd and Sm result from neutron capture within the element ~tself The rare gas anomahes result from neutron capture by different elements (Br m the case of Kr and Ba m the case of Xe) to produce radmactlve daughters which decay to the rare gas ~sotope Lunar neutron fluxes are great enough to induce significant isotopic changes down to about 800 g/cm 2 m the lunar regohth Thus stable isotope anomalies, which integrate the total neutron flux experienced, reflect the entire history of a sample above this depth

In order to use these anomalies to reveal the detailed history of material while in the upper part of the lunar regollth (gardening, mLxmg, layering, cratermg, etc ), it is essential to know the absolute intensity and spectral shape of the eqmhbrlum neutron flux as a function of depth This reformation can be obtained either by m SltU measurements or by investigation of (n, ),)-produced radioactive isotopes Using advanced low-level techniques there are four (n, ),)-produced radioisotopes which might possibly be detected m lunar samples 6°Co(tl = 5 2 yr), lS2Eu(tl = 14 yr), 41Ca(tl = 8 X 104 yr), and a6Cl(tl = 3 X l0 s yr) I~ 60 Co, I~ I S 2 Eu, and 4 1 Ca are produced exclusively by neutron capture processes, whereas 36Cl lS also produced by high energy spallatlon reactions Therefore, for a6C1, only m specially selected samples with a high C1 content is it possible to separate the (n,),)contrlbutmn from the spaUatlon component [7] In this work we report results obtained for 6°Co m lunar soils and rocks of different depths These resuits are compared to a production calculation based on the work of Lmgenfelter et al [8], and conclusions are drawn about the absolute Intensity and shape of the present day lunar neutron spectrum

2 Experimental techniques 2 1 Samples and chemwal procedure

* Present address Dept of Chemistry, Umverslty of Tokyo, Tokyo, Japan

This study has been done on returned lunar sam-

316

M Wahlen et al, 6°Co m lunar samples

TABLE 1 Results

No

Sample

1 2 3 4

Soil 10084, 19 Rock 14321, 123 Soil 15031 37 Soll 15005,67-79 Soil 15001,89-133 Soil and Rocks, combined sample of 10084,19 10017 and 12002

5 6

Weight

Depth

g

g/cm 2

Co content ppm

0-5 <'20 60 98-109 349-386

35* 29** 45t 45* 45*

94 4 42 194 9 38 14 0

239 9

0-20

--

Measured activity Co for date of collection counted dpm/kg mg dpm/g Co sample 2 32 0 94 061 0 248 0 263

17 +- 8 26-+ 18 88 ± 22 114 -+41 78 ± 34

10 4

18 ± 6tt

0 59 +- 0 28 0 73 ± 0 52 40 ± 1 0 5 1 -+ 18 35 ± 1 5

Date of collection July Feb Aug Aug Aug

21, 1969 6, 1971 2,1971 2, 1971 2,1971

Sept 20, 1969

* SHRELLDALFF [9] ** Wanke et al [18] t Morgan et al [19] 1t O'Kelley et al [ 11 ] * Assumed identical to 15031

ples m which other cosmic ray produced radlonuclldes have been previously determined [9, 10], or are presently being investigated These samples are listed m table 1 They include an Apollo 11 surface soil sample (10084, 19), an Apollo 14 rock sample (14321, 123, FM-5), and three Apollo 15 sod samples of greater depths (trench sample 15031,37 and drill core samples 1 5 0 0 5 , 6 7 - 7 9 and 1 5 0 0 1 , 8 9 - 1 3 3 ) Sample No 6 in table 1 is a complex ensemble of mdwldual samples counted by O'Kelley et al [11] containing sample No 1 (10084) and the Co-fractions of the deeper samples of rocks 10017 and 12002, and essentially represents an additional surface sample Each sample was dissolved in H F - H N O 3 and Co was separated (carrier-free) using ion exchange techtuques Cobalt was radiochemlcally purified using standard radlochemlcal procedures (PbS-scavenge, mtrate anion exchange column, Fe(OH)3-scavenge) and was electroplated on Cu sample holders The amount of plated Co was determined by atomic absorption spectroscopy The details of the chemistry are given in SHRELLDALFF [9] and Wahlen et al

[10]

2 2 Countmg

The counting of 6°Co (/3-, Ema x = 0 314 MeV, E.y = 1 173, 1 332 MeV) was performed using/3-7 coincidence techniques The detector system was slmdar to the one described m SHRELLDALFF [9] for the measurements of the positron emitters 26A1 and 22Na The detectors consisted of a flat Gelger counter for momtorlng the beta particles and a 4" × 4" NaIwell-crystal for detecting the coincident gamma rays The Gelger counter was operated reside the well of the NaI(T1) crystal Samples were mounted on both sides of the Gelger counter The whole assembly was shielded with 4 mches of lead and 2 inches of mercury The 7-spectrum of the coincident events was collected by a 256-channel pulse height analyzer. The count rates of the mdwldual detectors, as well as the signals m coincidence and antlcolncldence, were separately monitored by scalers m order to control the overall performance and stabdity of the system Backgrounds were determined using blank samples made from Indian Ocean basalt (with appropriate amounts of K, Th and U mixed m) which had under-

M Wahlenet al, 6°Co m lunar samples

gone the same chemical treatment as the actual lunar samples Background was determined periodically during the counting of the lunar samples and was found to be constant over several months The efficiency was determined and monitored by dady callbranons using standard samples made from commercially available 60 Co sources For the evaluahon of the 6°Co activity m the lunar samples the energy interval from 1 12 to 1 39 MeV m the coincident gamma ray spectrum was used This interval fully covers the two gamma lines (1 17 and 1 33 MeV) which oc. cur in coincidence with the/3- decay of 6°Co The efficiency obtained for this energy range was (0 065) cpm/dpm for a sample thickness of 0 3 mg/cm 2 The corresponding background amounted to (0 0051 -+ 0 0003) cpm Considerable counting time had to be devoted to each sample (up to 30,000 min) because of the extremely low ratm of signal plus background to background The ratio ranged from 1 20 in the worst case to 1 65 in the best case

3 Results

The results obtained are summarized in table 1 The errors quoted include (quadratically added) a one standard deviation counting error, a 3% error in the efficiency, a 5% error m the chemical yield and a 5% error in the self absorption correction which was applied where standards and samples &ffered in thickness The unfortunately large errors quoted mainly reflect the poor counting statistics which result from the extremely low actwltles involved The results of the surface samples, Nos 1, 2 and 6, can be compared to data or upper limits obtamed by other workers using non-destructive 3,-ray spectrometry on rock and soll samples of the Apollo 11 and 12 mlssmns [12, 13, 14] Within the large experimental errors all these surface samples agree The results from the deeper Apollo 15 sods are the first ones available at these greater depths

4 Discussion

The measurement of 6o Co actlvlhes at different depths can give reformation about the present day equilibrium neutron flux in the lunar regohth The

317

most maportant quantity which can be derwed from these data is the absolute present day lunar neutron production rate Because the mean life of 6°Co IS about 8 yr, the time averaged neutron production rate obtained does not depend much on solar modulation during a solar cycle We obtain the neutron production rate by comparing the measured 6°Co data to theoretically calculated capture rates as a function of depth These calculated capture rates were gained by folding the differential neutron flux at different depths with the capture cross section In carrying out this calculation we basically followed and used the work of Lmgenfelter et al [8], who give full details A summary of the procedure is given below 4 1 Neutron spectra and capture cross sectton

The neutron energy spectra which we used in generating the Co capture rates are those obtained by Llngenfelter et al [8], for a medium with the chemical composition of the Apollo 11 soil (corresponding effective total macroscopic capture cross section Neff = 0 010 cm 2 g-i) at a temperature of 0°K These spectra were calculated for 25 energy intervals (0 ~ 0 178 eV)and proportional to Ee -E/° 083 ( E < 0 178 eV) These "group cross sections" were derived from a 1/v-dependent capture cross section (37 2 barns at 0 025 eV), and in the energy regions of the resonances the resonance Integrals were evaluated using the Brelt-WIgner parameters in [ 16] 4 2 Co-capture rates, chemtstry and neutron producnon rate

The calculated capture rate for Co as a function of depth obtained by this procedure for an Apollo 11 sample is shown in fig 1 (curve 1), together with the expertmental results The normahzatmn for the neutron production rate IS 16 n/cm 2 sec The choice of

318

M Wahlen et al , 60Co m lunar samples

i I

/ /--/

APOLLO 14 1~ ~~3~ -APOLLO APOLLO 11 i'll

zx 1 0 0 8 , 4 0 143;21 o 1000,4

10017

12C)02--

• 1,~C131

r-11500-'~ • lm';c;)01

o PROI:II-ES

I

i

, i , ,

,

50 DEPTH

Fig 1

FOR

, i ,

,

I00

i~lll

18

N/CM"

iiIi

OEC

JllllLll

[ll~,l

....

G/~..IV~ 2

6°Coactivity or capture rates as a function of depth expertmental results and calculated capture rate profiles

temperature has no significant effect on the capture rates because of the lack of resonances at thermal energies A separate evaluation of the capture rates due to 1/v-capture and resonance capture yielded the followlng relat]ve contributions at depth of peak capture rates (100 - 150 g/cm 2) 30% (l/v) and 70% (resonances) The relative contrlbut]ons are rather constant over a broad range (30 - 250 g/cm2), only changing at the surface to 27% and 73% and below 250 g/cm 2 to 37% and 63% The neutron capture at a g~ven location m the regohth depends on the chemical composition of that spot Therefore, before a meaningful comparison could be made between the calculated capture rates and our data, which were determined on samples from different Apollo missions representing locahtles of different chemical composmons, we had to investigate the influence of the chemistry on the neutron spectra and the resulting capture rates Unfortunately, m the work of Lmgenfelter et al [8], no explicit neutron spectra were available for an Apollo 14 or Apollo 15 soil chemistry, and we therefore had to use an extra-

polatlon As demonstrated by Lmgenfelter et al [8], the effect of a varying chemical composition on the capture rate can, for a given isotope, be reduced to a vanatlon of the total effective macroscopic capture cross section ~eff Llngenfelter calculated peak capture rates (depth ~ 150 g/cm 2) as function of Y'eff for &fferent isotopes Because the relative shape of the depth dependence of the capture rates is quite insensitive on the observed vanatxon m chemistry, we can rehably normahze the depth profile of Co-capture rates obtained for an Apollo 11 chemistry (~eff = 0 010 cm2/g) to the ones for Apollo 14 (~eff = 0 0088 cm2/g) and Apollo 15 softs (~eff = 0 0072 cm2/g) by using the ratios of the peak capture rates Although Llngenfelter does not give peak capture rates for 6°Co, we can match the behavior of Co as a function of Y'eff by comparing with other isotopes From Fig 6b m Llngenfelter et al [8], we find an mcrease m the peak capture rate m going from an Apollo 11 to an Apollo 15 soft chemistry of 33% for a pure 1Iv capture isotope and 5% for a mainly eplthermal resonance capture isotope hke 81Br For an Apollo 14 chemistry these increases are 16% and 2% respectively

M ICahlen et al, 6°Co m lunar samples

Applying these variations to the respectwe contributions by l/v- and resonance capture of Co, we find that we must increase the Apollo 1 1 capture rate profile by 6 3% for an Apollo 14 chemistry and by 13 5% for an Apollo 15 chemistry, as indicated m fig 1, curves 2 and 3 As can be seen, the effect of the variation of chemistry among the samples on the capture rate profiles is rather small, and therefore the uncertainty introduced by this procedure is of minor importance The measured 6°Co depth profile is well represented by the theoretical capture rate profile calculated as described above Although the experimental errors are large, after taking account of the different chemical compositions, the measured data are internally consistent and do not disagree with the calculated curves, which were normahzed to 16 neutrons/ cm 2 sec for E n < 10 MeV (fig 1) Using the two measurements w~th the smallest experimental errors, we derive a best estimate of the lunar neutron productlon rate of (12 -+ 3) neutrons/cm 2 sec ( E n < 10 MeV) This is the average over the mean hfe of 6°Co, about the last 8 yr The error quoted Is the total error of the experimental procedure and is dominated by the 1 o counting error (cf sect 3) The value obtained, (12 + 3) neutrons/cm 2 sec, is m good agreement with the theoretical estimate of (16 + 5) neutrons/cm 2 sec made by Lmgenfelter et al [8] It also compares well with the independent value of 17 5 neutrons/cm2 sec ( E n < 15 MeV) as derxved by Armstrong and Alsmlller [17] using a Monte Carlo technique Neutron m o m t o r experiments anticipated by other workers for the Apollo 17 mission wdl provide an additional evaluation of the lunar neutron produchon rate At present, the 6°Co results gwen here provide the first unambiguous experimental evaluation of the neutron flux m the moon The consistency between the measurements and the predlct~ons of theoretical calculations give mcreased confidence m the use of the theory for evaluating neutron effects m other ~sotopes

Acknowledgements We thank R E Lmgenfelter for the detailed data on neutron spectra and for helpful discussions, and

319

R C Reedy for provldmg a computer program We also acknowledge comments and valuable d~scusslons by J Gexss, S Gugglsberg, H Oeschger and G P Russ III L F m n m , N Fong and F Klrchner provided valuable support and asslstence throughout this work This work was supported by NASA grants NGL 05009-148 and NGL 05-009-004

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M Wahlen et al , 6°Co m lunar samples

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[19]

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