Independent isomeric yield ratio ofPm148in fission of the moderately excitedU236compound nucleus as a measure of fragment angular momentum

The independent yield ratio of 41.3-day ${}_{}{}^{148}{}_{}{}^{}\mathrm{Pm}_{}^m{}_{}{}^{}$ (6-) and 5.37-day ${}_{}{}^{148}{}_{}{}^{}\mathrm{Pm}_{}^g{}_{}{}^{}$ (1-) has been measured radiochemically for fission of ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ induced by thermal neutrons and for fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ions of 26-42 MeV. The values of the ${}_{}{}^{148}{}_{}{}^{}\mathrm{Pm}$ isomer ratios were 3.0 ± 0.5 and ${2.6}_{-0.5\mathrm{}}^{+0.9\mathrm{}}$ for the thermal-neutron-induced fission of ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, 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 ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ 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 $\gamma$ rays to the secondary fragment ${}_{}{}^{148}{}_{}{}^{}\mathrm{Pm}$) 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: $(10.8\mathrm{}\pm 0.9)\hslash$ and $\left({10.0}_{-1.1\mathrm{}}^{+1.6\mathrm{}}\right)\hslash$ for thermal-neutron-induced fission of ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, respectively, and $(16.5\mathrm{}\pm 1.5)\hslash$, $(16.3\mathrm{}\pm 0.9)\hslash$, $(14.6\mathrm{}\pm 0.7)\hslash$, $(19.9\mathrm{}\pm 1.1)\hslash$, and $(19.9\mathrm{}\pm 1.1)\hslash$ for fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ 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 ${}_{}{}^4{}_{}{}^{}\mathrm{He}$-ion-induced fission. A summary of all published independent isomer-yield ratio studies which were used for deducing fission fragment angular momenta is given.