The puzzle of suppression of nuclear level density in N ≈ Z Zn isotopes compared to N > Z

This is the authors' abstract. We don't add key points for this paper.

A systematic investigation of isospin dependence of nuclear level density (NLD) is performed for the Zn isotopes. Experimental level densities of 67 Zn and 61 Zn have been determined by analyzing the spectra of evaporated neutrons emitted from the excited 68 Zn and 62 Zn nuclei populated via 4 He + 64 Ni and 4 He + 58 Ni reactions, respectively. At the low excitation energies, the neutron spectra are predominantly contributed by the first-chance decay, leading to 67 Zn and 61 Zn as daughters for the two cases. A comparison of the present experimental data along with previously measured NLDs of 66 Zn, 64 Zn, and 60 Zn is performed with state-of-the-art microscopic calculations. While the experimental NLDs of stable isotopes (i.e., 67 Zn, 66 Zn, and 64 Zn) align excellently with theoretical predictions, a compelling contrast emerges for the unstable isotopes 61 Zn and 60 Zn, exhibiting NLDs an order of magnitude lower than predicted. The possible origins of the observed suppression of NLDs for the N ≈ Z isotopes are discussed. A significant reduction of level density for nuclei situated away from the stability line could have critical implications for the calculation of astrophysical reaction rates pertinent to heavy-element nucleosynthesis.