Shell effect on fission fragment mass distribution at Ecn* up to 70 MeV: Role of multichance fission

Abstract

Mass distributions of fission fragments produced from the $^{249}\mathrm{Bk}$ compound nucleus (CN), populated by complete fusion of $^{11}\mathrm{B}$ with $^{238}\mathrm{U}$, were measured for the excitation energies in the range of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}36.7--69.7$ MeV. Nearly flat tops observed for these distributions indicate the presence of asymmetric fission, contrary to the pure Gaussian mass distributions expected at such high excitation energies. A fit to the mass distribution using three Gaussian functions provides an estimate of individual contributions from symmetric and asymmetric modes of fission. A significant contribution of the asymmetric component observed at CN excitation energies above 40 MeV can be understood only by invoking ``multichance fission'' in the calculations using the semiempirical model code gef. A systematic analysis of the mass distributions for several heavy transuranic nuclei reveals that the manifestation of the fragment shell effect in the integral mass distributions are visible even up to an initial compound nucleus excitation of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}70$ MeV due to their influence on the distributions of higher chance fissions.