330 Earth and Planetary Science Letters, 88 (1988) 330-336 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [6] Nuclide analyses of rare earth elements of the Oklo uranium ore samples: a new method to estimate the neutron fluence Hiroshi Hidaka * and Akimasa Masuda Department of Chemistry, University of Tokyo, Tokyo 113 (Japan) Received February 23, 1987; revised version received January 19, 1988 Isotopic ratios and abundances of all REE in two Oklo ore samples have been measured. We have succeeded in developing a new method to estimate the neutron fluence, the conversion factor of uranium and the average neutron energy (or temperature) based on Gd and U isotopic ratios. This new calculation is found to be useful in evaluating those parameters for the natural nuclear reactors at Oklo. Comparison is made between the neutron fluence values evaluated by our new method employing Gd isotopes and a previous one [11] employing Sm and Nd isotopes. The relative agreement becomes better with the increase of fluence. A relationship between the abundances of fissiongenic nuclides of La, Ce, Nd, Sm, and Gd and their mass numbers is also presented. 1. Introduction After the discovery of the natural reactor at Oklo, many studies have been performed. Most of them have been published in reports issued by IAEA [1,2]. Since 235U is consumed in the nuclear fission reaction induced by the thermal neutron, 235U is depleted relative to 238U. (In an unusual case, however, 235U can be enriched [3].) And for many other elements, isotopic anomalies are found due to the nuclear reactions of fission and neutron capture [4-9]. Production rates for many nuclides of rare earth elements (REE) in the natural reactor should be relatively large because of their high fission prod- uct yields and large cross sections for neutron capture. As REE are non-volatile and less soluble elements, retentions of them in the reactor must have been considerably good, even if not perfect in the reactor. Therefore, the effect of fission can be particularly remarkable for REE. We investi- gated both isotopic abundance ratios and con- centrations, of REE and U for two Oklo samples. The first aim of our study was to evaluate the working parameters of the Oklo reactor by a new * Now at Toshiba Research and Development Center, Kanagawa 210, Japan. 0012-821X/88/$03.50 © 1988 Elsevier Science Publishers B.V. calculation system, employing isotopic composi- tion of Gd. The second was to examine the behav- iors of REE. It is hoped that the knowledge about the behaviors of fission products can be of much help for the studies concerning the disposal of radioactive waste. In this paper, however, empha- sis is placed on the first point. This is the first research to investigate the abundances of all of stable nuclides for all REE in the same samples. Besides, we have measured the isotopic compositions of all of "ordinary" REE by ourselves, employing the commercially available reagents. Loubet and All6gre carried out a similar examination [10], but they did not present the data on 138Ce, 144Smi 152Gd, 154Gd, 156Dy, 158Dy, 16°Dy, 162Er, 164Er, tV°Er and 168yb. Holliger and Devillers [11] presented the complete isotopic composition data for four Oklo samples, but there are no observations for La and Ce. Major concern of these researchers was to estimate the average equilibrium temperature of the neutrons at the time of the nuclear reactions, based on 176Lu/ 175Lu ratio. 2. Experimental procedures 2.1. Samples Two Oklo samples were used in this work. For convenience, we call them A and B. Sample A is