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.

中文翻译：

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N ≈ Z Zn 同位素与 N > Z 相比抑制核能级密度的谜题


对 Zn 同位素的核能级密度 （NLD） 的同位素依赖性进行了系统研究。通过分析分别通过 4He + 64Ni 和 4He + 58Ni 反应填充的激发的 68Zn 和 62Zn 原子核发射的蒸发中子的光谱，确定了 67Zn 和 61Zn 的实验能级密度。在低激发能量下，中子谱主要由第一次机会衰变贡献，导致 67Zn 和 61Zn 成为这两种情况的子级。将目前的实验数据与以前测量的 66Zn、64Zn 和 60Zn 的 NLD 与最先进的显微计算进行了比较。虽然稳定同位素（即 67Zn、66Zn 和 64Zn）的实验 NLD 与理论预测非常一致，但不稳定同位素 61Zn 和 60Zn 出现了令人信服的对比，表现出的 NLD 比预测的低一个数量级。讨论了观察到的 N ≈ Z 同位素抑制 NLD 的可能来源。远离稳定线的原子核的能级密度显著降低，可能对计算与重元素核合成相关的天体物理反应速率具有关键意义。