Temperature and n-p asymmetry dependencies of the level-density parameter in Ni+Mo fusion reactions

Properties of evaporation residues and the accompanying light particles have been measured in ⁶⁰Ni +^92,100Mo fusion reactions at bombarding energies from E/A = 5 to 9 MeV. The data indicate that these reactions are essentially complete-fusion reactions with only a small amount of nonequilibrium emission at the highest bombarding energy studied. The measured kinetic-energy spectra of evaporated n, p, d, t, ³He, and α particles are compared to statistical-model predictions. It is found that a constant excitation-energy independent level-density parameter is not able to reproduce these spectra over the compound-nucleus excitation-energy range from 90 to 250 MeV. The kinetic-energy spectra for all particles were fit using the level-density parametrization a = A / (7 + 1.3×U / A) MeV^-1, where U is the thermal excitation energy. The Coulomb barriers for charged-particle emission were reduced from the standard values to reproduce the peak energy and multiplicity for α particles. Using these ingredients, the measured mass, velocity, and angular distributions of evaporation residues are also reproduced. The average Z and N of the evaporation residues deduced from the light-particle multiplicities are in agreement with the predicted location of the evaporation attractor line. A neutron-proton asymmetry dependence of the level-density parameter is shown to have potentially important consequences for the neutron or proton richness of the evaporation residues. However, no evidence for such an effect is found in the measured data; and from the experimental multiplicity ratio of t / ³He it was deduced that any such dependence in the excitation-energy regime 100<E*<250 MeV is very small.