Mass yield distributions in the Th232(n,f) reaction with fast neutrons

Abstract

The cumulative yields of 48 fission products in the mass range of 77--153 have been measured for the $^{232}\mathrm{Th}(n,f)$ reaction with the average neutron energies of 7.46, 13.4, and 18.0 MeV using an off-line \ensuremath{\gamma}-ray spectrometric technique. Fast neutron beams were generated using the $^{9}\mathrm{Be}(p,n)$ reaction with the proton energies of 25, 35, and 45 MeV from the MC-50 Cyclotron at the Korea Institute of Radiological and Medical Sciences. Post-neutron mass chain yields were obtained from the cumulative fission yields using the charge distribution correction of the medium energy. From the mass yield data, the peak-to-valley ratio, average light mass, and heavy mass, as well as the average neutron numbers were obtained. The fine structure of the mass yield distribution in the $^{232}\mathrm{Th}(n,f)$ reaction is explained by considering the nuclear structure effects such as the even--odd effect and shell closure proximity. The effect of deformed shells closure proximity arising due to the octupole deformation of fragments is discussed. The difference in the mass yield distribution between the $^{232}\mathrm{Th}(n,f)$ and $^{238}\mathrm{U}(n,f)$ reactions at various excitation energies is explained based on the different types of potential energy surfaces between the two fissioning systems. The role of excitation energy on the standard I and standard II asymmetric mode of fission is also discussed. In the $^{232}\mathrm{Th}(n,f)$ reaction with the excitation energies of 11--13 MeV, the onset of symmetric product yields, peak-to-valley ratio, average light mass, and heavy mass, as well as the average neutrons number have been observed, which are explained from the point of second chance fission.