Entrance channel effects in fission of 197TI

The pre- and post-scission neutron multiplicities are measured for 16 O + 181 Ta and 19 F + 178 Hf systems where the same compound nucleus 197 Tl is formed at the same excitation energies (E* = 72, 76, and 81 MeV). The measured pre-scission neutron multiplicities are found to be different for the two reactions and this difference in neutron yield increases with the excitation energy of the compound nucleus. The experimental pre-scission neutron yield is compared with predictions from the statistical model of compound nuclear decay containing the strength of nuclear viscosity as a free parameter. The magnitude of nuclear viscosity required to fit the experimental yield is found to be different for the two reactions. Because the two systems under consideration lie on the two sides of the Businaro-Gallone point, this observation indicates that the entrance channel mass asymmetry plays an important role in determining the number of neutrons emitted prior to scission in fusion-fission reactions.