Evidence for the reduction of nuclear level density away from the $β$-stability line

Abstract

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited 116 Sn and 116 Te nuclei. These nuclei are populated via p + 115 In and 4 He + 112 Sn 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 β-stable 115 Sn and neutron-deficient 115 Te as residues for the two cases. Theoretical analysis of the experimental spectra have 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 β-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Wood-Saxon mean field. The results have strong implication on the estimation of the level density of unstable nuclei, and calculation of astrophysical reaction rates relevant to r-and rp-processes.