Abstract The competition between the dominant mass-asymmetric and rarer narrow mass-symmetric fission modes in actinide nuclei are controlled by deformed and spherical shell effects. The low energy fission of 80 180 Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around 40 90 Zr, the system does not fission mass-symmetrically. Several theoretical approaches have been used to explain this unexpected result. To investigate the underlying mechanism, systematic measurements of fission mass distributions for isotopes of Os, Pt, Hg and Pb, formed in fusion reactions with p, 12C, 32S, 40,48Ca projectiles, have been made for excitation energies above the fission saddle-point (E eff ⁎ ) between 2.8 and 28.2 MeV. Evidence for mass-asymmetric fission is widespread, manifested as flat topped mass distributions or significant deviations from a single Gaussian shape. The systematic trends seen cannot be attributed to quasifission. Comparing two-Gaussian fits at a wide range of E⁎, it is concluded that the fit centroids reflect the low energy character of mass-asymmetric fission in the sub-lead region. Quantitative comparisons were made with microscopic calculations by Scamps and Simenel (2019) [33] of fission mass-asymmetries attributed to the influence of shell gaps in both neutrons (N=52, 56 for compact octuple deformations) and protons (Z=34 and Z=42, 44, 46 with large quadrupole deformations). For the predominant fission mode in the calculations, having one elongated and one compact fragment, the results are in extremely good agreement with all experimental values. This provides strong support for both the calculations, and the exploration of mass-asymmetric fission systematics through heavy ion fusion reactions. The total kinetic energy distributions for 176Pt and 180Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for 178Pt by Tsekhanovich et al. (2019) [39] .

中文翻译：

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从 176Os 到 206Pb 的激发核质量不对称裂变的系统学

摘要 锕系原子核中占主导地位的质量不对称和稀有的窄质量对称裂变模式之间的竞争是由变形和球壳效应控制的。最近观察到 80 180 Hg 的低能裂变具有强烈的质量不对称性，这表明尽管在 40 90 Zr 附近的碎片中有球壳间隙，但该系统不会发生质量对称性裂变。已使用多种理论方法来解释这一意外结果。为了研究潜在的机制，系统测量了 Os、Pt、Hg 和 Pb 同位素的裂变质量分布，这些同位素在与 p、12C、32S、40,48Ca 射弹的聚变反应中形成，已针对裂变鞍座以上的激发能量进行了点 (E eff ⁎ ) 介于 2.8 和 28.2 MeV 之间。质量不对称裂变的证据很普遍，表现为平顶质量分布或与单个高斯形状的显着偏差。所看到的系统趋势不能归因于准分裂。比较大范围 E⁎ 下的二高斯拟合，可以得出结论，拟合质心反映了子引线区域质量不对称裂变的低能量特性。Scamps 和 Simenel (2019) [33] 通过微观计算对裂变质量不对称进行了定量比较，裂变质量不对称归因于中子（N=52、56 用于紧凑八重变形）和质子（Z=34 和Z=42, 44, 46 具有大的四极变形）。对于计算中的主要裂变模式，具有一个细长和一个紧凑的碎片，结果与所有实验值非常吻合。这为计算和通过重离子聚变反应探索质量不对称裂变系统提供了强有力的支持。176Pt 和 180Pt 的总动能分布没有显示任何低 TKE 质量对称裂变模式的证据，正如 Tsekhanovich 等人报道的 178Pt 一样。(2019) [39]。