Background: Recent observation of mass-asymmetric fission in neutron-deficient Hg and Pt nuclei has reignited interest in fission fragment mass distributions close to Pb. Investigations at energies close to the fission barrier, where mass-asymmetric fission is expected to be most obvious and the sensitivity to shell effects is maximized, are limited in this mass region.Purpose: To measure fission mass distributions for $^{205,207,209}\mathrm{Bi}$ nuclei at the lowest possible excitation energies to determine how the mass distributions change with excitation energy and the neutron number of the compound nucleus.Method: Proton beams bombarding targets of $^{204,206,208}\mathrm{Pb}$ were used to study the fission of $^{205,207,209}\mathrm{Bi}$ at energies from just above to 10 MeV above their fission barriers. Fission fragments were measured using the CUBE fission spectrometer. Fission fragment mass distributions were determined using a newly developed time difference analysis method. Mass distributions were characterized by triple-Gaussian fits to determine the systematic trends across each isotope with excitation energy.Results: Measured mass distributions of all three Bi isotopes exhibit a component of mass-asymmetric fission at all energies studied. The probability of mass-asymmetric fission decreases significantly with increasing excitation energy, from $\ensuremath{\approx}70$ to $\ensuremath{\approx}40%$ over a 10-MeV range. Comparisons between the three Bi isotopes hint at an increase in the mass-symmetric fission yield with increasing neutron number, which could be due to a decrease in the difference between the symmetric and asymmetric fission barriers. The centroids of the mass-asymmetric peaks suggest that several deformed shell gaps in the fission fragments could be contributing to the presence of the mass-asymmetric fission mode with ${Z}_{\mathrm{light}}\ensuremath{\simeq}38$, ${Z}_{\mathrm{heavy}}\ensuremath{\simeq}45$, and ${N}_{\mathrm{light}}\ensuremath{\simeq}56$ all present in the fission fragments.Conclusions: Measurements of fission mass distributions at the lowest possible excitation energies above the fission barrier provide an excellent platform to investigate the origins of the mass-asymmetric fission mode. Further systematic measurements at these energies offer an opportunity to rigorously test new models of fission in this mass region.

中文翻译：

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使用 Pb204,206,208 的质子轰击在接近裂变势垒的能量下 Bi205,207,209 的质量不对称裂变

背景：最近对缺中子 Hg 和 Pt 核中质量不对称裂变的观察重新点燃了人们对接近 Pb 的裂变碎片质量分布的兴趣。在接近裂变势垒的能量上进行的研究是有限的，其中质量不对称裂变最明显，对壳效应的敏感性最大，在这个质量区域中是有限的。 目的：测量裂变质量分布 $^{205,207,209}\ mathrm{Bi}$ 原子核在尽可能低的激发能量下，以确定质量分布如何随激发能量和复合核的中子数而变化。 方法：质子束轰击 $^{204,206,208}\mathrm{Pb}$ 的目标是用于研究 $^{205,207,209}\mathrm{Bi}$ 在能量刚好高于其裂变势垒至 10 MeV 时的裂变。使用 CUBE 裂变光谱仪测量裂变碎片。裂变碎片的质量分布是使用新开发的时间差分析方法确定的。质量分布由三重高斯拟合表征，以确定每个同位素与激发能的系统趋势。结果：所有三种 Bi 同位素的测量质量分布在所研究的所有能量下都表现出质量不对称裂变的组成部分。质量不对称裂变的概率随着激发能量的增加而显着降低，在 10-MeV 范围内从 $\ensuremath{\a​​pprox}70$ 到 $\ensuremath{\a​​pprox}40%$。三种铋同位素之间的比较表明，随着中子数的增加，质量对称裂变产额会增加，这可能是由于对称和非对称裂变势垒之间的差异减小了。质量不对称峰的质心表明裂变碎片中的几个变形壳间隙可能导致质量不对称裂变模式的存在，${Z}_{\mathrm{light}}\ensuremath{\simeq} 38$、${Z}_{\mathrm{heavy}}\ensuremath{\simeq}45$ 和 ${N}_{\mathrm{light}}\ensuremath{\simeq}56$ 都存在于裂变中碎片。结论：在裂变势垒以上的最低可能激发能量下测量裂变质量分布为研究质量不对称裂变模式的起源提供了一个极好的平台。对这些能量的进一步系统测量提供了一个机会，可以在这个质量区域中严格测试新的裂变模型。质量不对称峰的质心表明裂变碎片中的几个变形壳间隙可能有助于 ${Z}_{\mathrm{light}}\ensuremath{\simeq} 38$、${Z}_{\mathrm{heavy}}\ensuremath{\simeq}45$ 和 ${N}_{\mathrm{light}}\ensuremath{\simeq}56$ 都存在于裂变中碎片。结论：在裂变势垒以上的最低可能激发能量下测量裂变质量分布为研究质量不对称裂变模式的起源提供了一个极好的平台。对这些能量的进一步系统测量提供了一个机会，可以在这个质量区域中严格测试新的裂变模型。质量不对称峰的质心表明裂变碎片中的几个变形壳间隙可能有助于 ${Z}_{\mathrm{light}}\ensuremath{\simeq} 38$、${Z}_{\mathrm{heavy}}\ensuremath{\simeq}45$ 和 ${N}_{\mathrm{light}}\ensuremath{\simeq}56$ 都存在于裂变中碎片。结论：在裂变势垒以上的最低可能激发能量下测量裂变质量分布为研究质量不对称裂变模式的起源提供了一个极好的平台。对这些能量的进一步系统测量提供了一个机会，可以在这个质量区域中严格测试新的裂变模型。