Fission Yields of Some Isotopes in the Fission of Th232 by Reactor Neutrons

The fission yields of the longer-lived isotopes produced in the fission of Th232 are not very well known; existing data show rather large discrepancies and/or uncertainties. Since we feel that at least some of these discrepancies arise from difficulties in measuring the absolute activities of the fission products, we measured the fission yield of 10 selected isotopes whose decay schemes are well understood. The thorium foils were irradiated in a position at the edge of the core of the SAPHIR swimming pool reactor. Following irradiation, the thorium was dissolved after addition of appropriate carriers. The fission products of interest were determined by conventional radiochemical methods that had to be modified slightly to ensure good decontamination from the abundantly formed Pa233 . The chemical yields were determined by gravimetric methods. Counting was done preferentially on a γ-spectrometer that had been calibrated at 11 different energies by standards either obtained from the IAEA or prepared by 4πβ-counting. In the case of Sr90, Ru106 and Ce144 a β-proportional counter was used that had been calibrated for these isotopes. In addition to the sought elements, Mo99 was isolated from each foil to serve as an internal monitor for the number of fissions taking place. The experiment thus gave the ratio of the yield of the sought element to the yield of Mo99. This ratio ''R'' was obtained for Sr90, Ru103, Ru106, Ag111, Pd112, I131, Cs137, Ba140, Ba141, Ce141 and Ce144, Results indicate the existence of a third peak in the yield mass curve in the region of symmetric fission. Yields of fission products relative to the Mo99 yields are given, and the absolute yields calculated by assuming y Mo99 = 2.78%. This number was derived from the work of Iyer et al., and was obtained by normalizing the area under the yield mass curve to 200%. (author)