Mass distributions in monoenergetic-neutron-induced fission of /sup 232/Th

Fission product yields for 38 masses were determined for the fission of /sup 232/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 /sup 232/Th foils and by chemical separation of the fission product elements followed by ..beta.. counting. The mass yield data for /sup 232/Th(n,f ) show a sensitive increase of fission yields in the near-symmetric mass region (valley) with increasing incident neutron energy E/sub n/ and a pronounced dip in yield at the onset of second-chance fission just above the neutron binding energy (at approx. 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 yeilds of /sup 232/Th(n,f ) are compared with those of /sup 238/U(n,f ) and /sup 232/Th(..gamma..,f ).