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Home , Isotopes of xenon

Geochemical Journal, Vol. 26, pp. 1 to 20, 1992

The Shibata Prize Awardee's Lecture

Plutonium-244 in the early solar system and the Pre-Fermi natural reactor'

P. K. KURODA2

4191 Del Rosa Court, Las Vegas, Nevada 89121, U.S.A.

(Received December 2, 1991; Accepted February 17, 1992)

system and the natural reactors, many years later INTRODUCTION in the United States. It is therefore a great honor I express my deep gratitude to the and pleasure for me to have this opportunity to Geochemical Society of Japan for bestowing me speak to you on the current status of these ongo the highest award, which carries the name of the ing investigations. late Professor Yuji Shibata of the Imperial Uni versity of Tokyo, the founding father of THE PRE-FERMI NATURAL REACTOR Geochemistry in Japan. When I entered the University in 1936 and I graduated from the Imperial University of began attending a two-year course in Inorganic Tokyo in the same year Hahn and Strassmann Chemistry taught by Professor Shibata, what im (1939) discovered fission. Soon thereafter, Pro pressed me most was that a large-scale research fessor Kenjiro Kimura, under whom I did my project on volcanoes in Japan, supported by the thesis work, told me to initiate a research work Imperial Academy of Science, was being main on the radioactivity of the water of Masutomi tained in his laboratories in the Chemistry Springs in Yamanashi Prefecture, joint Department, when it was a general custom in ly with another graduate student named Shinya those days that chemistry professors seldom car Oana, who was doing his thesis work under the ried out researches based heavily on field work. I supervision of Professor Yuji Shibata on the den had the great fortune of being able to participate sity variations of natural waters associated with in Professor Shibata's research project as the volcanoes. Our goal was to see if there was a cer youngest and most inexperienced member of his tain relationship between the radioactivity and for two years just before his retirement the heavy water concentrations of the water. from the University, which coincided with the We found no clear relationship between them beginning of the war in the Pacific. after studying the water of Masutomi Radium What I learned from Professor Shibata and Springs for a of about two years from his co-workers about geochemistry more than 1939 through 1941, when the war began and our half a century ago gave me a strong incentive to joint work was terminated (Oana and Kuroda, undertake my studies on 244Pu in the early solar 1940, 1942). Although no definite conclusion ')This paper is a condensed version of the acceptance speech delivered by the author on the occasion of his being awarded the Shibata Prize, at the Annual Meeting of the Geochemical Society of Japan, held in Matsuyama, Ehime Prefecture, Japan, on October 1, 1991. 2jDistinguished Professor Emeritus , Department of Chemistry, University of Arkansas, Fayetteville, Arkansas, U. S.A.

1 2 P. K. Kuroda could be drawn from the early studies on the in the remote past. The reaction site consisted of radioactive springs at Masutomi, I often several bodies of very rich ore, and wondered in those days whether or not a more than 500 tons of uranium had been involv uranium ore deposit located deep under the ed in the reactions with a quantity of energy ground might have been involved in some released equal to about 100 x 109 kWh. The in nuclear reactions in which the heavy water tegrated flux at certain points exceeded played an important role (for more details con 1.5 x 1021neutrons per cm2 (for more details, see "The Oklo Phenomenon" cerning the early history, see Kuroda, 1979, , Proceedings of a 1982, 1983, 1989, 1990a, b, 1991). Symposium, Libreville, 23-27 June 1975, In 1952, I moved to the University of Arkan published by the International Atomic Energy sas at Fayetteville, Arkansas, and started my Agency, Vienna, 1975; excellent review articles own research project, which was to measure the published by Roth, 1977; and by Hagemann and contents of various radioactive pro Roth, 1978; a book written in the Japanese duced by the of 238Uin pit language by Fujii, 1985; and several review ar chblende ores (Kuroda and Edwards, 1954; ticles by Kuroda, 1975, 1982, 1983). Kuroda et al., 1956, 1957; Ashizawa and In 1976, Shukolyukov and co-workers Kuroda, 1957; Parker and Kuroda, 1958; (Shukolyukov et al., 1976; Shukolyukov and Heydegger and Kuroda, 1959; Kenna and Min, 1977; Shukolyukov and Minh, 1977) Kuroda, 1960, 1961, 1964; Kuroda and Menon, reported that they found an anomalous fission 1961; Kuroda and Arino, 1964), with the component in the Oklo reactors. The ultimate objectives to establish the natural occur isotopic composition of this anomalous fission rences of element 43 () and element xenon is such that the abundances of 131Xeand 61 (), both of which had so far been 132Xerelative to 136Xe are markedly enhanced regarded as "artificial" elements (Kenna and when compared to the relative fission yields from Kuroda, 1961, 1964; Attrep and Kuroda, 1968). the thermal neutron-induced fission of 235U.The The papers concerning the theory of natural reac nature of this anomalous xenon from the Oklo tors (Kuroda, 1956a, b) were by-products of the reactor remained unexplained for many years, extensive radiochemical studies which were be but Kuroda (1990a, b) pointed out that it can be ing carried out at the University of Arkansas dur attributed to the fact that 131Xe,132Xe and 134Xe ing the 1950's. have fairly long-lived precursors: 8.04-day 1311, In 1956, I predicted that a large uranium ore 78.2-hour 132Te and 42-minute 134Te, respec deposit could have become an operating pile dur tively. ing the geological history of the earth (Kuroda, which have been retained and 1956a,b). During the 1950's and 1960's, the idea preserved at the site of nuclear reactors have of a critical uranium chain reaction occurring in high melting and boiling points. Gaseous nature was not taken seriously and it soon elements and elements with melting points lower became almost completely forgotten, but sixteen than that of (452°C) appear to have years later, on September 25, 1972, the world's mostly migrated out of the reactor. About one scientific community learned of an extraordinary percent of fission-produced xenon isotopes have discovery made by research workers at the been retained, however, and the isotopic com French Atomic Energy Establishment (Bodu et positions of small amounts of xenon released al., 1972; Neuilly et al., 1972; Baudin et al., from the Oklo reactors were found to be abnor 1972) that uranium had been found, in the mal in that the relative abundances of 131Xeand 132Xewere markedly enhanced deposit at Oklo in the Republic of Gabon, . Africa, with an abnormal isotopic composition The origin of this anomalous xenon can be at that led one to arrive at the conclusion that self tributed to a non-linear oscillatory mode of sustaining nuclear chain reactions had occurred operation (Bilanovic and Harms, 1985) of the 244pu in the early solar system and natural reactors 3 reactors at temperatures of about 400°C, from the chondritic (stone) meteorite Richard periodically being turned on and off, in a manner ton was heavily enriched in 129Xe. He concluded quite similar to the present-day geysers or inter that this almost certainly was formed mittent hot springs. The time period during from the of 1291with a half-life which the reactor was turned off was calculated of 16 million years, now extinct as a natural to be about 3 hours from the observed ratios of radioactivity but not so at the time of formation 132Xe , 134Xe and 136Xe in the anomalous xenon of the meteorite: (for more details, see Kuroda, 1990a, b; 1991). 1291 f _ 129Xe (stable) 1.6 x 107 year -244 IN METEORITES Immediately after the discovery of 1291in the In 1957, the Soviet Union launched the Sput early solar system, Kuroda (1960, 1961) pointed nik and newly elected President John F. Ken out that 244Puwith a half-life of 82 million years nedy declared in 1960 that the United States should have also been present in the early solar would send Man to the moon before the decade system and the experimental evidence for its ex was over. Reynolds (1960a, b) at Berkeley then istence could be secured by searching for the made the important discovery that the xenon presence in meteorites of excess heavy xenon

252Cf

oG 2.55 Y

248 Um

0G 4.7 x 105 y

244Pu 240PU

OL 8.2 x 107 y 7.3 m 60 m 240NP

oC 6760 y

P 14.lh

240U 2360

232Th J 2.39x107 y Spontaneous Fission (6.55 x 10 la y)

0(, 1.41 x 1010 y

( Series ) I

Stable Xenon Isotopes I (131X,' 132X,, 134X,, 136X,) 208Pb1 (stable)

Fig. 1. The beginning of the thorium series decay chain. 4 P. K. Kuroda

Table 1. The experimental confirmation of the ex (1964) reported that they discovered the presence istence of 244Pu spontaneous fission xenon in of roughly 50 x 10-12 (cc STP / g) of excess fission meteorites (Alexander et al., 1971) xenon (136fXe)in the carbonaceous chondrite

Fission yield Renazzo and, although the amount of excess Source 131Xe 132Xe 134Xe 136Xe xenon found in Renazzo appeared to be greater than expected by at least a factor of 2, they Pasamonte 33 ± 3 93±8 91±2.5 100 noted that "...this discrepancy alone is not Pasamonte 25 ± 3 88.5±3 94±5 100 Whitlokite 31±8 97 ± 8 93 ± 1 100 sufficient to rule against the possibility of 244Pu from fission gas in Renazzo." St. Severin Three years later, Funk et al. (1967) re-exam Kapoeta 26±3 88±4 91±5 100 244Pu0 2 25.1±2.2 87.6±3.1 92.1 ±2.7 100 ined the xenon isotope data for the car bonaceous chondrites Murray (Reynolds, 1963) and Renazzo (Reynolds and Turner, 1964) and isotopes 131Xe, 132Xe, '34Xe, and 136Xe which are reported that they confirmed the presence of ex produced by the spontaneous fission of 244Pu as ceptionally large amounts of fission xenon shown in Fig. 1. postulated by Reynolds' group, as shown in The presence of excess 244Pu fission xenon Table 2. They reported to have demonstrated was first reported by Rowe and Kuroda (1965) in that three possible complications, namely, the the eucrite (pyroxene-plagioclase achondrite) presence of an atmospheric component, a Pasamonte and many supporting evidences for cosmic-ray component and diffusion effects the existence of excess 244Pu fission xenon in (mass-fractionation) were not capable of explain achondrites and ordinary chondrites have since ing away the large excesses of fission xenon ob been reported (see, for example, Rowe and served and stated that: "We have concluded, Bogard, 1966; Merrihue, 1966; Kuroda et al., with Reynolds et al., that the amounts of fission 1966; Hohenberg et al., 1967; Sabu and Kuroda, xenon in Renazzo and Murray are incompatible 1968; Meason and Rao, 1969; Wasserburg et al., with the model of continuous galactic nucleosyn 1969a, b; Reynolds et al., 1969). The final proof thesis (Burbidge et al., 1957), if the fission xenon that the fission xenon observed in the meteorites is from 2 Pu which decayed in situ." was in fact the spontaneous fission decay pro The presence of large excesses of fission duct of 244Pu was reported by Alexander et al. xenon was soon reported in other carbonaceous (1971). They used 13.0 mg of "pure" 244Pu (as chondrites, such as Mokoia (Rowe, 1968; Pu02) and measured the relative mass yields of Manuel et al., 1972a), Leoville (Manuel et al., the fissiogenic xenon isotopes with a mass spec 1970) and Allende (Manuel et al., 1972a) and at trometer. Their measurements agreed almost tempts to explain the origin of this strange xenon perfectly with the 244Puspontaneous fission mass component were made by many investigators. yields deduced from the meteorite data, as Manuel et al. (1972b), for example, argued that shown in Table 1. carbonaceous chondrites contained isotopically distinct components which could not be explain ed by the occurrences of nuclear or fractionation PLUTONIUM-244 IN CARBONACEOUS CHONDRITES processes within these meteorites and suggested Six months before the report by Rowe and that the large excesses of fission xenon found in Kuroda (1965) appeared in the February 1, 1965 these meteorites may be the product of galactic issue of the Journal of Geophysical Research, a , which had not been uniformly paper entitled "Rare Gases in the Chondrite mixed with the solar system materials (see also Renazzo" by Reynolds and Turner (1964) ap Sabu et al., 1974; Manuel and Sabu, 1975; Sabu peared in the August 1, 1964 issue of the same and Manuel, 1980). journal. In this paper, Reynolds and Turner Anders et al. (1975), on the other hand, z44Pu in the early solar system and natural reactors 5

Table 2. Estimation of the excess fission xenon (134fXe)contents and the initial 244Pu/238Uratios in the car bonaceous chondrites Murray and Renazzo (Funk et al., 1967) 13"Xe Meteorite (244Pu/ 238U)0 Reference (10-12 CCSTP/g) (atom/atom)

(1) Murray >52 > 0.26 Reynolds (1963) and Hohenberg et al. (1967) >90±40 > 0.45:i 0.17 Funk et al. (1967)

(2) Renazzo 40 > 0.19 Reynolds and Turner (1964) 15±6 0.08±0.02 Funk et al. (1967)

(3) Steady State 5 0.05 Burbidge et al. (1957), Fowler Theory (personal communication, 1966)

speculated that one of the unknown superheavy xenon caused by the processes of a) mass-frac elements (element 115, 114, or 113) may have tionation, b) spallation and c) neutron-capture had an isotope with a half-life in the range of 107 reactions (plus the decays of 129Iand 244Puto pro to 108 years, which is too short to survive to the duce excesses of 129Xeand 131-'36Xe,respectively) present day, but long enough to leave detectable (see also, Kuroda et al., 1974, 1975; Kuroda, effects in meteorites, and this isotope may have 1976a, b, 1982; Kuroda and Sheng, 1986a, b; been present in the carbonaceous chondrites and Saebo and Kuroda, 1986a, b). It is important to decayed to 131-136Xeby spontaneous fission (see note here that Funk et al. (1967) reported earlier also Lewis et al., 1975). that the processes such as those mentioned above Srinivasan and Anders (1978) reported, seemed to play a relatively minor role in altering however, that the carbonaceous chondrite Mur the isotopic composition of xenon found in car chison contained a new type of xenon compo bonaceous chondrites, but a vast amount of ex nent highly enriched in five of nine stable perimental data which have been accumulated isotopes, mass numbers 128 to 132, and these pat since 1967 seem to indicate that it may not terns were highly suggestive of the s-process necessarily be the case. nucleosynthesis believed to take place in red Figures 2 and 3 compare the 3-isotope plots giants. It is also worthy of note that Lewis et al. of `Xe/ 136Xevs 134Xe/136Xe where i=124, 126, (1983) reported that and chromite frac 128, 129, 130, 131 and 132 for the isotopic com tions from the Allende meteorite that contain positions of xenon released from achondrites isotopically anomalous xenon-131 to xenon-136 (Myers and Kuroda, 1991a) and carbonaceous (carbonaceous chondrite fission or CCF xenon) chondrites (Myers and Kuroda, 1991c). The data at up to 5 x 1011 atoms per gram, showed no points for a total of 213 analyses of xenon releas detectable isotopic anomalies in -130 to ed from bulk samples and temperature fractions barium-138, and this seemed to rule out the of 35 achondrites and a total of 80 analyses in possibility that the CCF xenon was formed by in cluding xenon fractions released at different tem situ fission of an extinct superheavy element. peratures and the total release from bulk They concluded therefore that the CCF xenon samples of the carbonaceous chondrites and its carbonaceous carrier were relics from Renazzo, Mokoia and Groznaya are plotted in stellar nucleosynthesis (see also Swart et -al., Fig. 2 and Fig. 3, respectively. The point P in 1983; Wieler et al., 1991). these figures is Takaoka (1972)'s primitive Kuroda (1971) emphasized, however, that xenon, A is the atmospheric xenon, X is the so the xenon isotope data should be interpreted in called CCF xenon, and F is the 244Pu fission terms of the alterations of the relative abun xenon (Alexander et al., 1971). The curve PM is dances of each of the nine stable isotopes of the mass-fractionation line for the atmospheric 6 P. K. Kuroda

0.05 a) 11 0.04 1 T .l M 0.03 1 e

T

0.02 H

N

0.01

0.00 0. 60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136Xe

0.05 b) 1

0.04 T

M 0.03 KN 1

/ /

0.02 I81 e N i r

0.01 X M T 0.00 0. 60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136Xe

Fig. 2. Three isotope correlation plots for xenon released from achondrites (Myers and Kuroda, 1991c). Point A is the composition of atmospheric xenon, F is "Pu fission xenon (Alexander et al., 1971), P is the primitive xenon of Takaoka (1972) and X is the CCF xenon (Myers and Kuroda, 1989). The curve PM is the mass frac tionation line for atmospheric xenon. 244pu in the early solar system and natural reactors 7

0.8 C) e F -H 0.7

0 0.6 ta

0.5 b M

Fe-I F-$ .11 11 0.4 F$i161 ® F~ 0.3 0 Eei e T 0.2

11-. 1 0.1

-- 0.0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136Xe

10 d)

110 G

b M ~ 5

a N

M 0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136Xe ,

N 8 P. K. Kuroda

1.0

0.9 e)

0.8 -r T 1 1 4) 0.7 El

0.6 le

0.5

0.4 M 0 2 0.3

0.2 - i M 0.1

0.0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136 Xe

5 f)

4

11 3 0 M

T

1

2

M

1 M

X 0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

1 34Xe/ 136Xe 244Pu in the early solar system and natural reactors 9

5 g)

4 .gyp i T la 3 M T ..r I 0

2 I-< N M

1 F x 0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

114Xe/ 136Xe

xenon. points is less pronounced as compared with the Figure 2 shows that many of the data points case of achondrites shown in Fig. 2 and the data tend to lie along the lines connecting the points P points in the plots of 1Xe/ 136Xevs 134Xe/136Xe (primitive xenon) or A (atmospheric xenon) and ratios for i= 130, 131 and 132 tend to lie along F (2`14Pufission xenon) in the plot of 1Xe/ 136Xevs the line PX and this creates an impression that 134Xe/13'Xe ratios for i= 132 , demonstrating the there exists a xenon component whose isotopic fact that we are dealing here mostly with the composition X is different from that of the 244Pu mixtures of 244Pu fission xenon and a mass fission xenon. It is to be noted here, however, fractionated primitive xenon P and it is possible that the lines PX and PM lie close to each other to calculate the approximate amounts of 244Pu in these plots and there is a clear trend that the fission xenon present in the samples from these data points tend to deviate upward from the line data. In the cases of the plots of 1Xe/ 136Xevs PM in the cases of the plots for i=130 and 131, 134Xe/ 13'Xeratios for i= 124 , 126 and 128, most while the opposite appears to be the case for of the data points lie far above the line PF, i=132, reflecting the fact that the relative abun indicating that the relative abundances of 124Xe, dances of 13'Xe and 131Xecan be more easily 126Xeand 128Xeare severely altered by the proc altered than that of 132Xeby the above-mentioned esses of a) mass-fractionation, b) spallation and processes a), b) and c). This means that the fact c) neutron-capture. In the cases for the plots for that the data points lie along the line PX in these i=129, 130 and 131, the scatter of the data plots does not necessarily support the view that points is less pronounced and, although many of the carbonaceous chondrites contain excess them lie along the lines of PF and PM, it is fission xenon whose isotopic composition is virtually impossible to estimate the amounts of different from the 244Pufission xenon. It is more 244Pu fission xenon in the samples from these likely that we are dealing here with mixtures of xenon isotope data. 244pu fission xenon and a mass-fractionated Figure 3 shows that a scatter of the data primitive xenon P, just as it was the case for the Y. K. Kuroaa

0.02 a) P o 1 (Ai 4)

A / pA L

0.01 / / M a) / T A A

N

M 0.00 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134X' ./ 136Xe

0.02 0 b) 1 /

141 -TA 0)

/

0.01 M 4) A

M

0.00 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e Fig. 3. Three isotope correlation plots for xenon released from the carbonaceous chondrites Renazzo (0), Mokoia (+) and Groznaya (Lx) (Myers and Kuroda, 1991a). Point A is the composition of atmospheric xenon, F is 244Pufission xenon (Alexander et al., 1971), P is the primitive xenon of Takaoka (1972) and X is the CCF xenon (Myers and Kuroda, 1989). The curve PM is the mass fractionation line for atmospheric xenon.

N 244pu in the early solar system and natural reactors 11

0.50 C)

0.40

,

0.30 M .r

0.20 1-~ 00 A N M 0.10 i

x 0.00 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e

10 d) 9 + 8 HH 0 7 0

M 6 0 .r

i 5 A. +A

4 a ~+ N 3 0 2 M 1

0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e

0 12 P. K. Kuroda

1.00

0.90 e)

0.80 4) 0.70 P

0.60 i 0.50

M 0 0.40 M . i ..r 0.30

0.20 i 0.10 _/ F/

0.00 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e

5 f)

4

a) P

3

M W 2

M V 1 M ---~%

X FI 0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e 244Pu in the early solar system and natural reactors 13

5 g)

4 11

3 M / A / 2 . / 5 / N M / M / 1 F X 0 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40

134Xe/ 136X e

xenon found in chondrites and achondrites, the 134fXe/ g) and (15±6)x 10-12 (cc STP 134fXe/ g) only difference being that the carbonaceous chon reported by Reynolds and Turner (1964) and drites contain far greater amounts of trapped Funk et al. (1967), respectively, for the car (mass-fractionated primitive) xenon relative to bonaceous chondrite Renazzo (see Table 2). the 244Pu fission xenon than the xenon found in Initial ratios of plutonium to uranium the chondrites and achondrites. (244Pu/ 238U)o,were calculated from the contents These results indicate that it is virtually im of 244Pufission xenon (1361Xe)and uranium. The 244pu ages were then calculated as described by possible to determine the contents of 244Pu fission xenon in meteorites without taking into Kuroda (1989) using the value of (244Pu/ 238U)o= consideration the alterations of relative abun 0.0117 ± 0.0014 (atom /atom) for the Kapoeta dances of all the stable isotopes of xenon. We meteorite as a reference standard and accepting have therefore developed a new method in which its age 4560 million years to be correct (Rowe, a computer is used to calculate the isotopic com 1970). The values of (244Pu/ 238U)othus calculated positions of trapped xenon and the amounts of for six carbonaceous chondrites range from 244Pu fission xenon found in each of the xenon 0.114 to 0.170 (atom / atom) and they appear to fractions released from the meteorites (for have started to retain their xenon 4800 to 4900 details concerning the method of calculations see million years ago. The values of (2lPu/238U)o for Kuroda, 1989; Kuroda and Myers, 1989, 1991a, four chondrites range from 0.0070 to 0.0130 b, c, d, e; Myers and Kuroda, 1991a, b, c, d). (atom /atom) and their 244Pu ages range from Table 3 shows that the amounts of the 244Pu 4500 to 4573 million years. The values of fission xenon thus calculated for six car (244Pu/238U)ofor four achondrites range from bonaceous chondrites are all in the range of 22 to 0.0018 to 0.0117 (atom/ atom) and their 244Pu 34 x 10-12 (cc STP 136fXe/g)and are intermediate ages range from 4337 to 4560 million years. between the values of 40 x 10-12 (cc STP The 244Pu ages thus obtained for the car 14 P. K. Kuroda

Table 3. Plutonium-244 fission xenon in meteorites

U '36rXe y 244Pu age Sample (244Pu/ 238U)o (ppm) (10-'2 CCSTP/g) (atom/ atom) (million years)

(A) Carbonaceous Chondrites +7 Mokoia 0.014 34±2 0.170±0.010 4881 -7

+29 Renazzo 0.0115 26±2 0.158±0.042 4872 -38

+17 Groznaya 0.019 36±4 0.139±0.022 4857 -20 +10 Murray 0.013 25±1 0.134±0.012 4853 -11

+10 Murchison 0.013 23±1 0.123±0.011 4843 -11 +10 Allende 0.0134 22±1 0.114±0.005 4833 -11 (B) Chondrites +40 Guarena 0.0091 1.69±0.67 0.0130±0.0052 4573 -61

St. Severin (light) +28 0.0083 1.49±0.39 0.0126±0.0033 4570 -37 +49 Pultusk 0.010 1.37±0.69 0.0096±0.0048 4536 84 +39 Olivenza 0.0109 1.10±0.44 0.0070±0.0027 4500 -59 (C) Achondrites

Kapoeta 0.064 10.8±0.3 0.0117±0.0014 =4560 +11 Angra dos Reis 0.198 20.9±2.1 0.0074±0.0007 4505 -13

+17 Pasamonte 0.106 6.4±0.6 0.0042±0.0004 4431 -20

+17 Stannern 0.176 4.6:L:0.4 0.0018±0.0003 4337 -19

bonaceous chondrites are much greater than the and 0.087--L0.011 (atom /atom) for two fine value of (4550± 70) million years for the earth grained inclusions of the Allende meteorite. and meteorites reported by Patterson (1956) and also the values of (4511:L42) and (4496±10) PLUTONIUM-244 IN LUNAR SAMPLES million years for the 207Pb/206Pbages reported by Tatsumoto et al. (1973) for the Murray and In the first report on the examination of the Allende meteorites, respectively. It is extremely samples brought to earth from the 20 July 1969 difficult, however, to determine the -isotope Apollo 11 landing on the moon, members of the ages of bulk samples of carbonaceous chondrites Lunar Sample Preliminary Examination Team because the isotopic compositions of lead found (LSPET, 1969) wrote that the isotopic composi in this group of meteorites are quite similar to tions of xenon found in the fines and in a breccia that of the primordial lead. Chen and Wasser resembled those of trapped xenon found in car burg (1981) reported that the isotopic composi bonaceous chondrites and that decay products tions of lead found in acid-soluble phases of the of the extinct nuclides 129Iand 244Pu appeared to Allende coarse-grained inclusions gave a mean be absent in lunar samples. These early results 207Pb/206Pb model age of (4559± 15) million seemed to indicate that the moon was not quite years, while Droz et al. (1977) reported the as old as it had been thought earlier and the values of (244Pu/ 238U)0as high as >0.14±0.02 results from the age determinations of lunar 244Pu in the early solar system and natural reactors 15 samples by the K-Ar and Rb-Sr dating methods an exception and the presence of 244Pu fission seemed to support this view. xenon in lunar samples was reported by several Soon after the lunar samples were returned investigators (see, for example, Drozd et al., from the 5 February 1971 Apollo 14 landing on 1972; Behrmann et al., 1973; Drozd et al., 1975, the moon, the Lunar Sample Preliminary Ex 1976; Swindle et al., 1985). These investigators amination Team (LSPET, 1971) reported that have concluded that the excesses of 2`l4Pufission the lunar brecchia 14301, one of the five samples xenon were not due to in situ decay of 244Pu,but whose isotopic compositions of xenon were deter they were "parentless". Which means that the mined mass-spectrometrically, contained as daughter products were incorporated onto grain much as 150 X 10-12 cc STP 136sXeper gram of surfaces following decay of the parent nuclide fissiogenic xenon. The uranium content of 14301 elsewhere (Swindle et al., 1985). was 3.5 ppm and the amount of excess Table 4 shows the contents of uranium, 244Pu fissiogenic xenon which they found in this sam fission xenon, and the values of (244Pu/ 238U)oand ple was clearly much more than could have been 244Pu ages of a total of 11 samples of fines, produced by the spontaneous fission of 238Uin regolith, breccias and rock (Kuroda and Myers, 4.5 billion years. They reported, however, that 1971c). These values were calculated in the same there appeared to be no need to invoke extinct ra manner as in the cases of meteorites shown in dionuclides to explain the observed abundances Table 3. Uranium contents of lunar samples are of 129Xeand '36Xe, with the possible exception of generally much higher than those of the sample 14301. meteorites and, accordingly, the amounts of The case of sample 14301 soon ceased to be 244Pufission xenon are also considerably higher

Table 4. Plutonium-244 fission xenon in lunar samples

U 136fXe 244Puage Sample (244Pu/ 238U) (ppm) (10-12 cc STP/g) 0 (atom/atom) (million years) (A) Lunar Fines and Regolith

1608-3-26 0.25 738:1--155 0.206±0.040 4904 +23 28 +23 10084-12 0.46±0.10 602 0.091±0.020 4808 -30 +23 10084-59 0.46 ±0.10 336 0.051 ±0.011 4738 -30 +25 10084-29 0.46±0.10 141 0.021 ±0.005 4631 34 (B) Lunar Breccias

14313 3.23 1019 ±200 0.022±0.004 4636 +21 26 +17 67455 0.05 6.7 ±1.0 0.0083 ±0.0014 4533 -19 +11 14055, 3 3.68 476 ±46 0.0090±0.0009 4529 -12 +12 14047 3.39 393±41 0.0081 ± 0.0008 4516 _ 13 +5 14301 3.63 282 ± 13 0.0054±0.0002 4468 _ 6 +5 14318 4.0 236±11 0.0041±0.0002 4435 6 (C) Lunar Rocks

10057 0.78 ±0.16 8.1*1 0.00051±0.00014 4181+30 39

*~ This includes about 2.3 x 10-12 (cc STP/ g) of 136fXefrom the spontaneous fission of 238U. 16 P. K. Kuroda than those of the meteorites. The values of were encountered in explaining the origin of the (244Pu/ 238U)ofor lunar fines and regolith range so-called CCF (carbonaceous chondrite fission) from 0.021 to 0.206 (atom/atom) and their 244Pu xenon since the 1960's. Results from recent ages from 4631 to 4904 million years. The values studies indicate, however, that this strange of (244Pu/238U)ofor six samples of lunar brec xenon component is a mixture of 244Pufission c ias range from 0.0041 to 0.022 (atom /atom) xenon and mass-fractionated trapped xenon. A and their 'Pu ages from 4435 to 4636 million new dating method for the early solar system years. The lunar rock 10057 contains 8.1 x 10-12 based on the decay of 244Puto the stable xenon (cc STP 136fXe/g)of excess fission xenon, which isotopes 131Xe,132Xe, 134Xe and 136Xehas recently includes about 2.3 x 10-12 (cc STP 136fXe/g)of been established. The 244Pu ages of meteorites fission xenon from the spontaneous fission of and lunar samples cover a period of nearly one 238U(Kuroda and Myers , 1991e) and its 244Puage thousand million years, from about 4000 million is 4181 million years. to almost 5000 million years ago. The 244Pu ages of lunar samples thus calculated are in general agreement with their Acknowledgments-I wish to express my deep lead-isotope ages. For example, according to gratitude to Professor Koh Sakamoto of Kanazawa University for his very kind introduction; to Professor Tatsumoto and Rosholt (1970), the 206Pb/ 238U, 207Pb/ 235Uand 207Pb/206 Pb ages of lunar rock Kunihiko Watanuki of the University of Tokyo, Presi dent of the Geochemical Society of Japan, for bestow 10057 are 4089, 4146 and 4173 million years, re ing me this highest honor. spectively, in excellent agreement with the 244Pu age of 4181 million years. They also reported the REFERENCES 206Pb/ 238U, 207Pb/ 235Uand 207Pb/16 Pb ages of lunar fine 10084 to be 4685, 4668 and 4659 Alexander, E. C. Jr., Lewis, R. S., Reynolds, J. H. million years, while the 244Puage of 10084 ranges and Michel, M. C. (1971) Plutonium-244: Confirma from 4631 to 4808 million years as shown in tion as an extinct radioactivity. Science 172, 837 840. Table 4. According to Tatsumoto (1973), the 206Pb/ 238U Anders, E., Higuchi, H., Gros, J., Takahashi, H. and , 207pb/ 235U, 207pb/206 Pb and Morgan, J. W. (1975) Extinct superheavy element 208Pb/232 Th ages of Luna 20 soil are 5130 , 5020, in the Allende meteorite. Science 190, 1262-1271. 4980 and 4950 million years, respectively. Ashizawa, F. T. and Kuroda, P. K. (1957) The occur Although the xenon isotope data for the Luna 20 rence of the short-lived isotopes in natural soil sample are not available, the 244Puage of the and in depleted uranium salts. J. Inorg. Nucl. Chem. 5, 12-22. Luna 16 regolith 1608-3-26 is 4904 million Attrep, M. Jr. and Kuroda, P. K. (1968) Promethium years, as shown in Table 4. It is also interesting in pitchblende. J. Inorg. Nucl. Chem. 30, 699-703. to recall that (1970a, b) pointed out that Baudin, G., Blain, C., Hagemann, R., Kremer, M., the found in the Apollo 11 fines and Lucas, M., Merlivat, L., Molina, R., Nief, G., breccia are mixtures of extraordinarily Prost-Marechal, F., Regnaud, F. and Roth, E. (1972) Quelques donnees nouvelles sur les reactions heterogeneous components having 207Pb/ 206Pb nucleaires en chaine qui se sont produites dans le ratios which vary by more than 50 percent and gisement d'Oklo. C.R. Acad. Sc. Paris 275, 2291 the 207Pb/206 Pb age of the oldest component may 2294. be as great as (4950± 100) million years. Behrmann, C. J., Drozd, R. J. and Hohenberg, C. M. (1973) Extinct lunar radio-activities: Xenon from 244Pu and 1291in Apollo 14 breccias. Earth Planet. SUMMARY Sci. Lett. 17, 446-455. Bilanovic, Z. and Harms, A. A. (1985) The nonlinear A brief outline of the sequence of events dynamics of the Oklo natural reactor. Nuclear which led to the discoveries of the Pre-Fermi Science and Engineering 91, 286-292. natural reactor and the occurrence of 244Pu in the Bodu, R., Bouzigues, H., Morin, N. and Pfiffelmann J. P. (1972) Sur 1'existence d'anomalies isotopiques early solar system is presented. Great difficulties 244Pu in the early solar system and natural reactors 17

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