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 ( νpre) is significantly larger than that calculated on the basis of the statistical model. The excess in νpre 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 νpre is higher for asymmetric mass splits as compared to symmetric mass splits. The enhancement in the νpre 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.