Evidence For The Reduction Of Nuclear Level Density Away From The Beta-Stability Line

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited Sn-116 and Te-116 nuclei. These nuclei are populated via p +In- 1(15) and He-4 + Sn-112 reactions in the excitation energy range of 18-26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the beta-stable Sn-115 and neutron-deficient Te-115 as residues for the two cases. Theoretical analysis of the experimental spectra has been performed within the Hauser-Feshbach formalism by employing different models of the level-density parameter. It is observed that the data could only be explained by the level-density parameter that decreases monotonically when the proton number deviates from the beta-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Woods-Saxon mean field. The results have strong implications on the estimation of the level density of unstable nuclei and calculation of astrophysical reaction rates relevant to r and rp processes.