Pre- and post-scission neutron multiplicities in $¹⁹F$  + $²⁰⁹Bi$ reaction at 108 MeV

Abstract:

The pre- and post-scission neutron multiplicities have been determined from the fragment-neutron angular-correlation measurements in ¹⁹F + ²⁰⁹Bi reaction at bombarding energy of 108 MeV. The average pre-scission neutron multiplicity ( ν^pre) is significantly larger than that calculated on the basis of the statistical model. The excess in ν^pre value is used to estimate the dynamical delay in the fusion-fission time scale for the ²²⁸U compound system. For a level density parameter a _n = A/10 MeV^-1 and a _n = a _f, the fusion-fission dynamical time was established to be 120 ± 10 × 10^-21 s. We also observe that ν^pre is higher for asymmetric mass splits as compared to symmetric mass splits. The enhancement in the ν^pre for the asymmetric region could be due to strong variations in the fragment mass distribution at different stages in multiple chance fission at medium excitation energies.