Abstract
The pre- and post-scission neutron multiplicities have been extracted for the 220,222,224Th nuclei for the excitation energy range of 40 MeV to 64 MeV using the National Array of Neutron Detectors (NAND). The Th isotopes are populated from the fusion reaction of 16O+204,206,208Pb systems in order to investigate the dynamics of fusion-fission reactions using the neutron multiplicity as a probe. The theoretical calculations were performed using the Bohr-Wheeler fission width as well as the dissipative dynamical fission width from Kramers prescription. It is observed that the Bohr-Wheeler fission width underestimates the pre-scission yields to a large extent. A large amount of dissipation is required in the Kramers width to fit the observed pre-scission neutron multiplicities.
Highlights
Considerable progress has been made in the last few years in the understanding of the fission of a highly excited compound nucleus formed in heavy-ion reactions, both experimentally and theoretically
The large excess of neutrons which are emitted before the nucleus undergoes fission immediately points to a slowing down of the fission process compared with the statistical model fission rate as given by Bohr and Wheeler [6]
It is interpreted as arising from the dynamical effects in the fission decay process
Summary
Considerable progress has been made in the last few years in the understanding of the fission of a highly excited compound nucleus formed in heavy-ion reactions, both experimentally and theoretically. The particles emitted during the fission process, and in particular the pre-scission ones represent a powerful tool to investigate the fission dynamics [1,2,3,4] These studies have resulted in the interesting observation of the substantially higher yield of pre-scission charged particles [1], neutrons [2,3] and gamma rays [4] than those predicted by the standard statistical model of fission [5]. These measurements represent the evidence of the effects of nuclear viscosity in the fission process. We give the nature and strength of nuclear viscosity by comparing the data with the dynamical models
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