Effect of neutron composition and excitation energy on the primary fragment yield distribution in multifragmentation reaction

The isotopic properties of the primary and secondary fragment yield distribution in the multifragmentation of $^{58}$Fe + $^{58}$Ni and $^{58}$Fe + $^{58}$Fe reactions are studied with respect to the $^{58}$Ni + $^{58}$Ni reaction at 30, 40 and 47 MeV/nucleon. The reduced neutron and proton densities from the observed fragment yield distribution show primary fragment yield distribution to undergo strongly secondary de-excitations. The effect is small at the lowest excitation energy and smallest neutron-to-proton ratio and becomes large at higher excitation energies and higher neutron-to-proton ratio. The symmetry energy of the primary fragments deduced from the reduced neutron density is significantly lower than that for the normal nuclei at saturation density, indicating that the fragments are highly excited and formed at a reduced density. Furthermore, the symmetry energy is also observed to decrease slowly with increasing excitation energy. The observed effect is explained using the statistical multifragmentation model.