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

TL;DR

This study investigates how nuclear level density depends on isospin by analyzing neutron spectra from excited nuclei near stability, revealing a decrease in level density parameter away from the beta-stability line, with implications for astrophysical processes.

Contribution

It provides experimental evidence and theoretical analysis showing the reduction of nuclear level density away from the beta-stability line, using both Hauser-Feshbach modeling and shell-model calculations.

Findings

Level density parameter decreases with deviation from beta stability

Experimental spectra explained only with decreasing level density parameter

Results impact estimation of unstable nuclei's level densities and astrophysical reaction rates

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 $\beta$-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 $\beta$-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.