Independent isomeric yield ratio of 148Pm in fission of the moderately excited 236U compound nucleus as a measure of fragment angular momentum

The independent yield ratio of 41.3-day 148Pm/sup m/ (6-) and 5.37-day 148Pm/sup g/ (1-) has been measured radiochemically for fission of 233U and 235U induced by thermal neutrons and for fission of 232Th by 4He ions of 26--42 MeV. The values of the 148Pm isomer were 3.0 +- 0.5 and 2.6/sup +0.9//sub -0.5/ for the thermal-neutron-induced fission of 233U and 235U, respectively, and 6.8 +- 1.2, 6.6 +- 0.7, 5.3 +- 0.5, 8.5 +- 0.9, and 8.5 +- 0.9 for fission of 232Th by 4He ions of 25.8, 29.1, 31.8, 37.0, and 41.4 MeV, respectively. On the basis of the measured independent yield ratios the average intrinsic angular momentum of the primary fission fragments (which lead by the emission of prompt neutrons and γ rays to the secondary fragment 148Pm) was estimated by means of a simple statistical-model analysis based on the formalism developed by Huizenga and Vandenbosch. The following values for the average intrinsic spin of the primary fragment were obtained: (10.8 +- 0.9) h and (10.0/sup +1.6//sub -1.1/) h for thermal-neutron-induced fission of 233U and 235U, respectively, and (16.5 +- 1.5) h, (16.3 +- 0.9) h, (14.6 +- 0.7) h, (19.9 +- 1.1) h, and (19.9 +- 1.1) h for fission of 232Th by 4He ions of 25.8, 29.1, 31.8, 37.0, and 41.4 MeV, respectively. The results show that a fraction of the angular momentum of the fissioning nucleus appears as intrinsic spin of the primary fragments when the angular momentum of the fissioning nucleus is increased as in the case of the 4He-ion-induced fission. A summary of all published independent isomer yield ratio studies which were used for deducing fission fragment angular momenta is given.