Entrance channel effects in the fusion-fission time scales from studies of prescission neutron multiplicities.

Abstract

Prescission neutron multiplicities in fusion-fission reactions of $^{11}\mathrm{B}$${+}^{237}$Np, $^{11}\mathrm{B}$${+}^{232}$Th, $^{12}\mathrm{C}$${+}^{232}$Th, and $^{16}\mathrm{O}$${+}^{232}$Th, lying on either side of the Businaro-Gallone mass asymmetry (${\mathrm{\ensuremath{\alpha}}}_{\mathrm{BG}}$), have been measured. The present data along with those available in literature for compound systems spanning the fissility range from 0.70 to 0.84 were analyzed in a consistent manner to deduce fusion-fission time scales for all systems. From the systematic behavior of all the data, the three components of total dynamical fusion-fission delay, namely, transient delay, saddle-to-scission delay, and formation delay, have been deduced. It is found that the formation delay depends on the entrance channel mass asymmetry relative to Businaro-Gallone point. The variations of the fusion-fission time scales with fissility, ratio of fission barrier to temperature, and entrance channel mass asymmetry have been studied.