Fission product yields for 38 masses were determined for the fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ with essentially monoenergetic neutrons of 2.0, 3.0, 4.0, 5.9, 6.4, 6.9, 7.6, and 8.0 MeV. Fission product activities were measured by Ge(Li) $\gamma$-ray spectrometry of irradiated ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ foils and by chemical separation of the fission product elements followed by $\beta$ counting. The mass yield data for ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}(n, f)$ show a sensitive increase of fission yields in the near-symmetric mass region (valley) with increasing incident neutron energy $E_n$ and a pronounced dip in yield at the onset of second-chance fission just above the neutron binding energy (at ∼ 6 MeV) where the excitation energy is lowered by competition with neutron evaporation prior to fission. The effect of second-chance fission is also seen in the yields of asymmetric peak products. A distinct third peak is observed at symmetry in the valley of the mass distribution, and enhanced yields are observed in the asymmetric peaks at masses associated with even $Z$ (proton pairing effect). The fission yields of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}(n, f)$ are compared with those of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(n, f)$ and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}(n, f)$.

NUCLEAR REACTIONS, FISSION ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}(n, f)$ $E_n=2.0, 3.0, 4.0, 5.9, 6.4, 6.9, 7.6, \mathrm{and} 8.0$ MeV; measured mass yields.

• Received 15 February 1980

DOI:https://doi.org/10.1103/PhysRevC.22.152