Influence of Neutron Shell Closure (N_c = 126) on Prescission Particle Emission of Fissioning Systems ^216,224Th

The effect of neutron shell closure N_c = 126 on the prescission particle emission of ¹²⁶Th and ²²⁴Th nuclei is investigated within the framework of an extensive Smoluchowski equation. It is found that there is a large difference in the prescission neutron multiplicity for the two Th nuclei, indicating a strong shell effect in neutron emission. Moreover, shell effects on particle emission are also investigated as functions of excitation energy, angular momentum and nuclear viscosity. The results show that with increasing excitation energy shell effects in prefission neutron evolve from continual strongness to gradual weakness. Both high angular momenta and low viscosity weaken the shell effects on the particle emission.