Heavy mass yields in the slow neutron fission of 233U and 235U

Abstract

The chain yields of masses 157, 159 and 161 in the thermal neutron fission of 233U were determined by radiochemical methods. The last radioactive member of the mass chain was isolated and its activity measured relative to 99Mo. With this ratio and pertinent data from the thermal neutron fission of 235U, the mass chain yield was calculated. The chain yields are as follows: mass 157 (15·4 hr 157Eu), 6·4 × 10 −3 per cent; mass 159 (18·5 hr 159Gd), 9·1 × 10 −4 per cent; and mass 161 (6·9 day 161Tb), 1·2 × 10 −4 per cent. These yields are higher than was to be expected from analogy with the relative yields from thermal neutron fission of 235U. A possible explanation is given. The chain yields of masses 91, 141, 143, 144, 147, 149, 151 and 153 in the thermal neutron fission of 233U and 235U were also determined in the same manner as above. The values, when compared to the literature values for the chain yields, provided an estimate of the accuracy of the radiochemical measurement. The accuracy is estimated to be ±10 per cent. Improved values have been obtained for the chain yields of mass 156 in the thermal neutron fission of 233U and of mass 157, 159 and 161 in the thermal neutron fission of 235U.