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

• Published in: Physics letters. B 2024-12, Vol.859, p.139101, Article 139101
• Main Authors: Roy, Pratap, Banerjee, K., Hung, N. Quang, Anh, N. Ngoc, Kundu, Samir, Manna, S., Sen, A., Ghosh, T.K., Rana, T.K., Mukherjee, G., Pandey, R., Mukhopadhyay, S., Pandit, Deepak, Mondal, Debasish, Pal, Surajit, Bhattacharya, C.
• Format: Article
• Language: English
• Subjects: Isospin effects; Neutron evaporation; Nuclear level density; TALYS
• ISSN: 0370-2693
• DOI: 10.1016/j.physletb.2024.139101

A systematic investigation of isospin dependence of nuclear level density (NLD) is performed for the Zn isotopes. Experimental level densities of 67Zn and 61Zn have been determined by analyzing the spectra of evaporated neutrons emitted from the excited 68Zn and 62Zn nuclei populated via 4He + 64Ni and 4He + 58Ni reactions, respectively. At the low excitation energies, the neutron spectra are predominantly contributed by the first-chance decay, leading to 67Zn and 61Zn as daughters for the two cases. A comparison of the present experimental data along with previously measured NLDs of 66Zn, 64Zn, and 60Zn is performed with state-of-the-art microscopic calculations. While the experimental NLDs of stable isotopes (i.e., 67Zn, 66Zn, and 64Zn) align excellently with theoretical predictions, a compelling contrast emerges for the unstable isotopes 61Zn and 60Zn, 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.