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A depth profile of14C in the lunar rock 12 002
EARTIt AND I'LANETARY SCIENCE LETTERS 16 (1972~ 269-272. NORTH-HOLLAND PUBLISIIING COMPANY
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A DEPTH PROFILE OF 14C IN THE LUNAR ROCK 1 2 0 0 2 Richard S. BOECKL* Cttemistry Department, Unircrsity o f California. San Diego, La Yolla. Ca. 9203 7, US.4
Received 16 December 1971
A depth profile o~ 14C in lunar rock 12 002 has been measured using six specimens with well-defined depth below the exposed surface o|" the rock. Tile re'mlt~obtained ,dlov¢an exces,~surface activity attributed to solar particle tlux.
1. Introduction Radioactive spallation products in hmar material from the landing site of Apollo 12 were studied by Rancitelli et al. [2], D'Amico et al. [31, Stoenner et al. [41 and other workers. A thorough investigation of cosmogenic radionuclei at different depths in rock 12 002 was carried out by Finkel et al. [1]. During the course of this work the attthor was afforded an opportunity by Professor J.R. Arnold to study samples of carbon from this rock for 14C activity.
2. Procedure Tile chemical procedure for tile recovery of carbon from lunar samples is described in Finkel et al. [1 ]. Briefly the procedure involved separating the rock sample into depth fractions, designated O P I - O P 6 , grinding and dissolving each sample in a teflon still. The crushed sample was placed in the still with 15 cm 3 of water and a mixture of 3 parts concentrated HF and 1 part concentrated HNO 3 were added step-wise to effect dissolution. Tile solution was distilled under a continuous stream of dry N~. In order to aid tile recovery of 14C from tire smnples 200 cm 3 STP of CO~ carrier (prepared by burning anthracite) were added to tile N~ stream in portions of 20 cm 3, Tire gases leaving tile still were passed through three ice cooled traps filled with acidified water where * Prement address: Lehrstuhl t[ir Kernchemie Darnlstadt, D6I Darmstadt, W. Germany.
Cl, I and Si were collected. Tile gases were then passed into tile 14C collection system which consisted of a CuO furnace at 550°C followed by two bubblers containing a carbonate free solution of [M SrCI~ in 39% NIt4OH. Tile exit gases were collected in an evacuated tank. The carbon precipitated as SrCO 3 in tile bubblers and these solutions were taken to dryness to collect the sample. Tire CO-, was recovered in a high vacuum system by adding _ 0 , HCI solution to the SrCO 3. This CO 2 was purified over charcoal and the yield determined for each sample by measuring tire amount of carrier collected ( table 11. We intended to carry out a total combustion analy'sis for 14C in ,:t separate sample of 12 002 to check the completeness of carrier equilibration in the extraction described above. It was not possible to carry out this experiment. However, tire consistency of our results with tile 14C measurements ofWfinke et al. [(~] and Begemann et al. [7] and with solar costrric ray determinations using nuclides with other half-lives (Finkel ct al. [1] ) leads us to conclude that tiffs is a small problem. Tire purified CO~ was converted to acetylene for counting by a procedure developed at tile La Jolla Radiocarbon and Tritium Laboratory [5]. The main reactions involved are: 10 Li + 2CO 2 --+ Li2C 2 + 4Li20 • Li2C 2 + 2H20 ---*C2H 2 + 2LiOH. The yield for all samples in this process was{80 -+ 5)~. The 14C was counted as C~H~ in a 100 cm 3 proportional counter constructed at the University of Bern,
-1"7
R.S. B()cckl. l)el?th pro, file ~;1 14(, iH lu;zar r¢~ck
_, 0
Table 1 14C production rates in lunar rock 12 002. Sample I1 u l l l 1) k'l
OP I 01'2
O1'3 01'4
'
O1'5 O1'6
Depth bclov, surface in mm
Sample
Yield ol
m a s ' ; Ill g
C O 2 II1 ':
I) 1 2 4 9 20
9.87 9.5o 17.65 25.32 18.15 50.43
65.o 64.3 61.1 38.7 62.5 65.4
I 2 4 9 20 40
dpm/k: 71.6* II 50.3 . 7 43.6 _~ 6
60.6 + 9 28.7 * 4 26.7 • 3
*-[hls sample \,.as exposed to :ur whtlc being pr~cessed. Bern, Switzerland. Tile coutlter was operated ill a twochannel m o d e in antler)incidence with a guard ling. T w o discriminators were used. Tlley were set to cotrect any counter drift and allow all the samples to be counted at the same gas gain, vaD,'ing the COtLnter gas pressure. Blank ~ampleb were prepared by' treating anthracite-produced CO-~ in tile same manner as tile lunar samples. These blanks were counted and agreed within statistical error with the background count rate of C-,H+ {prepared by burning anthracite) which underwent n o chenfistry. The background was 10.020 -+ 0.13) cpnl. The c o u n t e r efficiency was determined to be 70.1': using the NBS oxalic acid 14C standard.
3. R e s u l t s a n d d i s c u s s i o n
Table I shows the results obtained for tile 14C depth profile. The value for OP4 is strangely high and can reasonably be explained by' leakage of atmospheric 14C during the chemistry. The profile shows a deftnite decrease in 14C activity with increasing depth in tire rock. The upper series o f points in fig. 1 are a plot of the measured depth gradient of 14C activity in rock 12 002. The 14C c o n c e n t r a t i o n decreases sharply in tile top few m m and level> olT at depths greater than 20 ram. WSnke et al. [6] determined 14C in htnar fines from Mare Tranquilitatis and found 39 + 5 dpm/kg. The fines come from a depth corresponding to OPI OP4 in rock 12 002 and show about the same 14C vahles as were obtained in the present study'. A sttldy by this group on rock 12 053, Begemann et al. [7] 'also gives a 14C depth profile in reasonable agreemerit with this work.
d,,orn kg" 7050
I
,105-
I i~. I. l'he p r t t d u c u o n rate ot 14(. m lunar r~ck 12 rill2 a'~ a l u n c l l o n ot depth. Yhe upper ',erie', o I pomp, ,;hox,, ', the e \ perimentall_,, obser,,ed actt;itle',, l h e da,,hcd curve ,,hov, s the
~.olat proton reduced activity calculated on the basi'~ of a rlgidit_,, di,;tribution using a 4rr Ilu", ol 1011 p,:Clll2 ";eC~llld ;111 Ro 80 MV, the lov, er point,~ are the experimentally ob,,erved '. alues Ior tile ",tqar llare produced 14C v.,ith the galactic cont lll'.u t iotl ,.ub I r:.leled. In lunar material L4C is produced in a number of nuclear reactions. The radiation causing these reactions is o f two kinds and two sources [g]. The solar cosmic rays (SCR) are mostly low-energy protons, seldom over I O0 MeV, which are produced in solar flares. These particles are nUltlerous but are stopped so quickly by ionization losses that their p r o d u c t i o n of 14C is limited to the first few rnn~ of the rock's sur(ace. SCR 14C accounts for tile steeply falling profile in the surface regions. Tile main reaction for 14C production by SCR particles is 160 (p, 3p) l a c . The solar particle l]ux can be represented by an exponential rigidity distribution [8, 9] " ) exp ( R dR - k Ro ,
R.S. Bocckl. Depth pro tile o l [4C in lunar rock wltere R = Pc/'Ze is the rigidity in MV, k is a normalization constant and R 0 is tire shape paratneter. Both k and R n vary from flare to flare. The majority o f 14C at depth is p r o d u c e d by the other c o m p o n e n t of cosmic rays, tire galactic cosmic rays {GCR). These particles have much higher energies than the SCR and penetrate to greater depths. They have a spectral shape o f d Y - K { o ~ + / : ? ) 2.5. dE where K is a normalization constant and o~ is a shape parameter for the G C R [8]. According to this model the 14C p r o d u c e d by G C R should increase slightly' with depth to about 50 g/cm 2, due to the build up o f secondary neutrons which react with oxygen to produce 14C, and then gradually decrease b e y o n d this depth. The difference in G C R p r o d u c t i o n b e t w e e n the surface and 4 cm depth is not larger than 3 dpm/kg, which is smaller than our experimental error. It is possible, using a suitable model, to subtract the galactic c o n t r i b u t i o n to the 14C activity from the experimental numbers to give a profile of the solar flare p r o d u c e d 14C. F r o m this curve we can calculate the spectral shape and flux for the SCR averaged over the half-life o f 14C. We originally thought that OPO contained only G C R p r o d u c e d 14C. But it was shown in refs. [1] and [10] that there is some SCR contribution to radionuclide p r o d u c t i o n in OP6. The net solar values shown as the lower points on fig. I, were obtained by the same process as that described in ref. [/1 for o t h e r nuclear species. The dashed curve represents an R 0 c f 80. However, the best fit to calculated parameters are with an R o of 100 MV and a 4Tr Flux ( t : ' > 10 MeV) of 200 p/cm 2 -sec. By measuring a radionuclide profile on surface exposed lttnar material one can determine the nrean solar particle flux and spectra[ shape in the e a r t h - m o o n region over a period of time comparable to the mean Table 2 Nuclide
tb2(y}
RoIMVI
22Na s5 Fe
2.6 2.6 7.4 "< l0 s 3.4 X 10('
85 100 10() l(10
26A1 53Mn
J(E > 10 MeV) (inprot./cm 110 100 80 90
2
";ec{4n))
271
life of the nuclide. Finkel et al. [1] have measured many nuclides in the same salnples we used for 14C and their results can be conrpared with ours. Comparing R 0"s and fluxes for nuclides of different halflives it is possible to deter(nine the constancy of tire average solar activity. The corrected values for solar parameters for various nuclides in rock 12 002 given in ref. [ I O] are shown in table 2. It is evident that the values for nuclides o f very different half-lives are quite similar. This indicates that tire averaged solar a c t M t y has been essentially constant fur the last nrillion years. Our 14C activity results o f R 0 = 100 a n d J = 200 p / c m 2 .sec show a somewhat hi~lcr flux. The discrepancy may well be due to inaccuracies in the excitation functions used in tire calculations of 14C activity.
Acknowledgements
The author is most grateful to Professor James Arnold tbr his permission to modil~, the extraction procedure in a way which made it possible to include 14C ill the investigated radionuclides. D e t e r m i n a t i o n of 14C activities was carried out in the La Jolla R a d i o c a r b o n Laboratory of H.E. Suess. The author also wishes to give special thanks to Candace Kohl for her help in writing this manuscript. Financial support was obtained through N A S A Grant NGL-05-009-005, Supl. 5 and NGL-05-009-48.
References [ 1 ] R.('. I inkel, J.R. Arnold, R.C. Reedy, J.S. Fruchter, tI.H. Loosli, J.C. Fvans, J.l'. Shedlovsky and A.C. De(any, Depth Variation of Cosmogenic Nuclides in a Lunar Surface Rock and Lunar Soil, Proceedings ol the Second Lunar Science Conference, 2 (1971 ) 1773. [21 L.A. Rancitelli, R3,V. Perkins, W.D. I elix and N.A. Wogman, Erosion and mixing of tile hmar surface from cosnloge[lic and primordial radlonuclide measurements in Apollo 12 hmar samples. Proceeding of tire Second Lunar Scmnce Conference, 2 ~1971) 1757. [3] J. D'Amico, J. l)eFelice, F.L. Fireman. C. Jones and G. Spannagcl, Tritium and argon radioactivities and lhmr depth satiations in Apollo 12 samples, Proceedings of the Second Lunar Science Conference, 2 {197l} 1825. [41 R.W. Stoenner, W.J. Lyman and R. Davis, Jr.. Radioactive rare gases and tritmm in hmar rocks as in the sample return container, Proceedings of the Second Lunar Scmnce Conference, 2 11971 ) 1813.
272
R.S. Bocckl. Depth pro.O'le o f t4(. in lunar rock
[5J ('.L, [lubbs and G.S. Bien, l_a Jolla radmcarbon measurements V, Radiocarbon 9 11967) 261. [6] H. Wiinke, F. Begemann. 1.. Vllcsek, R. Rieder, I . Teschke, re. Born, M. Quijano-Rico, 1,. Wlotzka, Major and trace element,; and co,stoic-ray produced radioisotopes in lunar samples, Scmnce, 167 { 197(I) 523. 17] I . Begemann, W, Born, H, Pahne, F, Vilcsek and H. W/ink< Cosmic ray produced radioisotopes in Apollo 12 and Apollo 14 ,,ample,< Revised abstracts of papers presented at the Ttnrd Annual Lunar Science Contcrence, tlouston, Texas, January, 1972. Page 53.
[8] R.('. Reedy and J.R. Arnold, lnteractu_m t;t" solar and
galactic cosmic-ray parucles with tile moon. J. Geophys, Re,,., 77, no. 4 , 5 3 7 . [9] D. Lal and V.S. Vcakatavaradan, Acuvation of cosmic dnst by co,,nlic ra) particles, Earth Planet. Sci. Letters 3 ~1967) 299. [Iq)[ M. Wahlcn, M. Honda. ,M. lmamura, J.S. Fruchter, R.('. l:mkel. ('.P. Kohl, J.R. Arnold and R.C. Reed)', ('<~smogenic nuclides in |ootball-sized rocks, submitted to Proceedings ~t" tile Third Annual Lunar Science Conference, ltou:,ton, Texas, Januar.~, 1972.
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A DEPTH PROFILE OF 14C IN THE LUNAR ROCK 1 2 0 0 2 Richard S. BOECKL* Cttemistry Department, Unircrsity o f California. San Diego, La Yolla. Ca. 9203 7, US.4
Received 16 December 1971
A depth profile o~ 14C in lunar rock 12 002 has been measured using six specimens with well-defined depth below the exposed surface o|" the rock. Tile re'mlt~obtained ,dlov¢an exces,~surface activity attributed to solar particle tlux.
1. Introduction Radioactive spallation products in hmar material from the landing site of Apollo 12 were studied by Rancitelli et al. [2], D'Amico et al. [31, Stoenner et al. [41 and other workers. A thorough investigation of cosmogenic radionuclei at different depths in rock 12 002 was carried out by Finkel et al. [1]. During the course of this work the attthor was afforded an opportunity by Professor J.R. Arnold to study samples of carbon from this rock for 14C activity.
2. Procedure Tile chemical procedure for tile recovery of carbon from lunar samples is described in Finkel et al. [1 ]. Briefly the procedure involved separating the rock sample into depth fractions, designated O P I - O P 6 , grinding and dissolving each sample in a teflon still. The crushed sample was placed in the still with 15 cm 3 of water and a mixture of 3 parts concentrated HF and 1 part concentrated HNO 3 were added step-wise to effect dissolution. Tile solution was distilled under a continuous stream of dry N~. In order to aid tile recovery of 14C from tire smnples 200 cm 3 STP of CO~ carrier (prepared by burning anthracite) were added to tile N~ stream in portions of 20 cm 3, Tire gases leaving tile still were passed through three ice cooled traps filled with acidified water where * Prement address: Lehrstuhl t[ir Kernchemie Darnlstadt, D6I Darmstadt, W. Germany.
Cl, I and Si were collected. Tile gases were then passed into tile 14C collection system which consisted of a CuO furnace at 550°C followed by two bubblers containing a carbonate free solution of [M SrCI~ in 39% NIt4OH. Tile exit gases were collected in an evacuated tank. The carbon precipitated as SrCO 3 in tile bubblers and these solutions were taken to dryness to collect the sample. Tire CO-, was recovered in a high vacuum system by adding _ 0 , HCI solution to the SrCO 3. This CO 2 was purified over charcoal and the yield determined for each sample by measuring tire amount of carrier collected ( table 11. We intended to carry out a total combustion analy'sis for 14C in ,:t separate sample of 12 002 to check the completeness of carrier equilibration in the extraction described above. It was not possible to carry out this experiment. However, tire consistency of our results with tile 14C measurements ofWfinke et al. [(~] and Begemann et al. [7] and with solar costrric ray determinations using nuclides with other half-lives (Finkel ct al. [1] ) leads us to conclude that tiffs is a small problem. Tire purified CO~ was converted to acetylene for counting by a procedure developed at tile La Jolla Radiocarbon and Tritium Laboratory [5]. The main reactions involved are: 10 Li + 2CO 2 --+ Li2C 2 + 4Li20 • Li2C 2 + 2H20 ---*C2H 2 + 2LiOH. The yield for all samples in this process was{80 -+ 5)~. The 14C was counted as C~H~ in a 100 cm 3 proportional counter constructed at the University of Bern,
-1"7
R.S. B()cckl. l)el?th pro, file ~;1 14(, iH lu;zar r¢~ck
_, 0
Table 1 14C production rates in lunar rock 12 002. Sample I1 u l l l 1) k'l
OP I 01'2
O1'3 01'4
'
O1'5 O1'6
Depth bclov, surface in mm
Sample
Yield ol
m a s ' ; Ill g
C O 2 II1 ':
I) 1 2 4 9 20
9.87 9.5o 17.65 25.32 18.15 50.43
65.o 64.3 61.1 38.7 62.5 65.4
I 2 4 9 20 40
dpm/k: 71.6* II 50.3 . 7 43.6 _~ 6
60.6 + 9 28.7 * 4 26.7 • 3
*-[hls sample \,.as exposed to :ur whtlc being pr~cessed. Bern, Switzerland. Tile coutlter was operated ill a twochannel m o d e in antler)incidence with a guard ling. T w o discriminators were used. Tlley were set to cotrect any counter drift and allow all the samples to be counted at the same gas gain, vaD,'ing the COtLnter gas pressure. Blank ~ampleb were prepared by' treating anthracite-produced CO-~ in tile same manner as tile lunar samples. These blanks were counted and agreed within statistical error with the background count rate of C-,H+ {prepared by burning anthracite) which underwent n o chenfistry. The background was 10.020 -+ 0.13) cpnl. The c o u n t e r efficiency was determined to be 70.1': using the NBS oxalic acid 14C standard.
3. R e s u l t s a n d d i s c u s s i o n
Table I shows the results obtained for tile 14C depth profile. The value for OP4 is strangely high and can reasonably be explained by' leakage of atmospheric 14C during the chemistry. The profile shows a deftnite decrease in 14C activity with increasing depth in tire rock. The upper series o f points in fig. 1 are a plot of the measured depth gradient of 14C activity in rock 12 002. The 14C c o n c e n t r a t i o n decreases sharply in tile top few m m and level> olT at depths greater than 20 ram. WSnke et al. [6] determined 14C in htnar fines from Mare Tranquilitatis and found 39 + 5 dpm/kg. The fines come from a depth corresponding to OPI OP4 in rock 12 002 and show about the same 14C vahles as were obtained in the present study'. A sttldy by this group on rock 12 053, Begemann et al. [7] 'also gives a 14C depth profile in reasonable agreemerit with this work.
d,,orn kg" 7050
I
,105-
I i~. I. l'he p r t t d u c u o n rate ot 14(. m lunar r~ck 12 rill2 a'~ a l u n c l l o n ot depth. Yhe upper ',erie', o I pomp, ,;hox,, ', the e \ perimentall_,, obser,,ed actt;itle',, l h e da,,hcd curve ,,hov, s the
~.olat proton reduced activity calculated on the basi'~ of a rlgidit_,, di,;tribution using a 4rr Ilu", ol 1011 p,:Clll2 ";eC~llld ;111 Ro 80 MV, the lov, er point,~ are the experimentally ob,,erved '. alues Ior tile ",tqar llare produced 14C v.,ith the galactic cont lll'.u t iotl ,.ub I r:.leled. In lunar material L4C is produced in a number of nuclear reactions. The radiation causing these reactions is o f two kinds and two sources [g]. The solar cosmic rays (SCR) are mostly low-energy protons, seldom over I O0 MeV, which are produced in solar flares. These particles are nUltlerous but are stopped so quickly by ionization losses that their p r o d u c t i o n of 14C is limited to the first few rnn~ of the rock's sur(ace. SCR 14C accounts for tile steeply falling profile in the surface regions. Tile main reaction for 14C production by SCR particles is 160 (p, 3p) l a c . The solar particle l]ux can be represented by an exponential rigidity distribution [8, 9] " ) exp ( R dR - k Ro ,
R.S. Bocckl. Depth pro tile o l [4C in lunar rock wltere R = Pc/'Ze is the rigidity in MV, k is a normalization constant and R 0 is tire shape paratneter. Both k and R n vary from flare to flare. The majority o f 14C at depth is p r o d u c e d by the other c o m p o n e n t of cosmic rays, tire galactic cosmic rays {GCR). These particles have much higher energies than the SCR and penetrate to greater depths. They have a spectral shape o f d Y - K { o ~ + / : ? ) 2.5. dE where K is a normalization constant and o~ is a shape parameter for the G C R [8]. According to this model the 14C p r o d u c e d by G C R should increase slightly' with depth to about 50 g/cm 2, due to the build up o f secondary neutrons which react with oxygen to produce 14C, and then gradually decrease b e y o n d this depth. The difference in G C R p r o d u c t i o n b e t w e e n the surface and 4 cm depth is not larger than 3 dpm/kg, which is smaller than our experimental error. It is possible, using a suitable model, to subtract the galactic c o n t r i b u t i o n to the 14C activity from the experimental numbers to give a profile of the solar flare p r o d u c e d 14C. F r o m this curve we can calculate the spectral shape and flux for the SCR averaged over the half-life o f 14C. We originally thought that OPO contained only G C R p r o d u c e d 14C. But it was shown in refs. [1] and [10] that there is some SCR contribution to radionuclide p r o d u c t i o n in OP6. The net solar values shown as the lower points on fig. I, were obtained by the same process as that described in ref. [/1 for o t h e r nuclear species. The dashed curve represents an R 0 c f 80. However, the best fit to calculated parameters are with an R o of 100 MV and a 4Tr Flux ( t : ' > 10 MeV) of 200 p/cm 2 -sec. By measuring a radionuclide profile on surface exposed lttnar material one can determine the nrean solar particle flux and spectra[ shape in the e a r t h - m o o n region over a period of time comparable to the mean Table 2 Nuclide
tb2(y}
RoIMVI
22Na s5 Fe
2.6 2.6 7.4 "< l0 s 3.4 X 10('
85 100 10() l(10
26A1 53Mn
J(E > 10 MeV) (inprot./cm 110 100 80 90
2
";ec{4n))
271
life of the nuclide. Finkel et al. [1] have measured many nuclides in the same salnples we used for 14C and their results can be conrpared with ours. Comparing R 0"s and fluxes for nuclides of different halflives it is possible to deter(nine the constancy of tire average solar activity. The corrected values for solar parameters for various nuclides in rock 12 002 given in ref. [ I O] are shown in table 2. It is evident that the values for nuclides o f very different half-lives are quite similar. This indicates that tire averaged solar a c t M t y has been essentially constant fur the last nrillion years. Our 14C activity results o f R 0 = 100 a n d J = 200 p / c m 2 .sec show a somewhat hi~lcr flux. The discrepancy may well be due to inaccuracies in the excitation functions used in tire calculations of 14C activity.
Acknowledgements
The author is most grateful to Professor James Arnold tbr his permission to modil~, the extraction procedure in a way which made it possible to include 14C ill the investigated radionuclides. D e t e r m i n a t i o n of 14C activities was carried out in the La Jolla R a d i o c a r b o n Laboratory of H.E. Suess. The author also wishes to give special thanks to Candace Kohl for her help in writing this manuscript. Financial support was obtained through N A S A Grant NGL-05-009-005, Supl. 5 and NGL-05-009-48.
References [ 1 ] R.('. I inkel, J.R. Arnold, R.C. Reedy, J.S. Fruchter, tI.H. Loosli, J.C. Fvans, J.l'. Shedlovsky and A.C. De(any, Depth Variation of Cosmogenic Nuclides in a Lunar Surface Rock and Lunar Soil, Proceedings ol the Second Lunar Science Conference, 2 (1971 ) 1773. [21 L.A. Rancitelli, R3,V. Perkins, W.D. I elix and N.A. Wogman, Erosion and mixing of tile hmar surface from cosnloge[lic and primordial radlonuclide measurements in Apollo 12 hmar samples. Proceeding of tire Second Lunar Scmnce Conference, 2 ~1971) 1757. [3] J. D'Amico, J. l)eFelice, F.L. Fireman. C. Jones and G. Spannagcl, Tritium and argon radioactivities and lhmr depth satiations in Apollo 12 samples, Proceedings of the Second Lunar Science Conference, 2 {197l} 1825. [41 R.W. Stoenner, W.J. Lyman and R. Davis, Jr.. Radioactive rare gases and tritmm in hmar rocks as in the sample return container, Proceedings of the Second Lunar Scmnce Conference, 2 11971 ) 1813.
272
R.S. Bocckl. Depth pro.O'le o f t4(. in lunar rock
[5J ('.L, [lubbs and G.S. Bien, l_a Jolla radmcarbon measurements V, Radiocarbon 9 11967) 261. [6] H. Wiinke, F. Begemann. 1.. Vllcsek, R. Rieder, I . Teschke, re. Born, M. Quijano-Rico, 1,. Wlotzka, Major and trace element,; and co,stoic-ray produced radioisotopes in lunar samples, Scmnce, 167 { 197(I) 523. 17] I . Begemann, W, Born, H, Pahne, F, Vilcsek and H. W/ink< Cosmic ray produced radioisotopes in Apollo 12 and Apollo 14 ,,ample,< Revised abstracts of papers presented at the Ttnrd Annual Lunar Science Contcrence, tlouston, Texas, January, 1972. Page 53.
[8] R.('. Reedy and J.R. Arnold, lnteractu_m t;t" solar and
galactic cosmic-ray parucles with tile moon. J. Geophys, Re,,., 77, no. 4 , 5 3 7 . [9] D. Lal and V.S. Vcakatavaradan, Acuvation of cosmic dnst by co,,nlic ra) particles, Earth Planet. Sci. Letters 3 ~1967) 299. [Iq)[ M. Wahlcn, M. Honda. ,M. lmamura, J.S. Fruchter, R.('. l:mkel. ('.P. Kohl, J.R. Arnold and R.C. Reed)', ('<~smogenic nuclides in |ootball-sized rocks, submitted to Proceedings ~t" tile Third Annual Lunar Science Conference, ltou:,ton, Texas, Januar.~, 1972.