Anomalous behavior of the level density parameter in neutron and charged particle evaporation

The compound nucleus 76Kr* was populated at the excitation energy of 75 MeV and angular momentum of 39(ℎ/2π) in fusion reactions with two complementary, mass-symmetric (31P+45Sc) and mass-asymmetric (12C+64Zn) entrance channels. The neutron evaporation spectra were measured and compared with the predictions of statistical model calculations using the transmission coefficients for the spherical nuclei in the inverse absorption channel, the rotating liquid drop model moment of inertia, and the normal systematic value of A/8 for the level density parameter a. The results for the mass-asymmetric reaction are found to be consistent with the predictions of the statistical model calculations. However, for the mass-symmetric reaction (31P+45Sc), the experimental spectra are found to be harder than the theoretical neutron spectra and the statistical model calculations require a lower value of A/10 for the parameter a to reproduce the shape of the experimental spectra, indicating the neutron to be evaporated at higher temperature for the same excitation energy and angular momentum in symmetric system. According to the dynamical model, the formation time (37x10-22 sec) of the compound nucleus for the symmetric 31P+45Sc system is significantly higher than that (29x10-22 sec) for the asymmetric 12C+64Zn system. This may probably lead to the formation of a temperature-equilibrated dinuclear complex that may be responsible for neutron emission at higher temperature in the case of the symmetric system