[论文解读] Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides

Guillaume Scamps, C. Simenel|arXiv (Cornell University)|Apr 10, 2018

Nuclear physics research studies被引用 1

一句话总结

该论文表明，锕系素中裂变碎片的八极（梨形）形变——尤其在Z = 52–56附近——在驱动质量不对称裂变中起着关键作用，解释了为何尽管在Z = 50处存在强烈的球形壳效应，但主导的裂变碎片仍以Z ≈ 52–56为主。通过采用超流体裂变动力学的量子多体模型，研究显示八极形变可稳定质量不对称裂变路径，而球形幻数核如132Sn则抗拒此类形变，从而阻碍其作为裂变碎片的生成。

ABSTRACT

Abstract Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass splits1. Without quantum shell effects, which can give extra binding energy to their mass-asymmetric shapes, these nuclei would fission symmetrically. The strongest shell effects appear in spherical nuclei, such as the spherical ‘doubly magic’ (that is, both its atomic and neutron numbers are ‘magic’ numbers) nucleus 132Sn, which contains 50 protons and 82 neutrons. However, a systematic study of fission2 has shown that heavy fission fragments have atomic numbers distributed around Z = 52 to Z = 56, indicating that the strong shell effects in 132Sn are not the only factor affecting actinide fission. Reconciling the strong spherical shell effects at Z = 50 with the different Z values of fission fragments observed in nature has been a longstanding puzzle3. Here we show that the final mass asymmetry of the fragments is also determined by the extra stability provided by octupole (pear-shaped) deformations, which have been recently confirmed experimentally around 144Ba (Z = 56)4, 5, one of very few nuclei with shell-stabilized octupole deformation6. Using a quantum many-body model of superfluid fission dynamics7, we find that heavy fission fragments are produced predominantly with 52 to 56 protons, which is associated with substantial octupole deformation acquired on the way to fission. These octupole shapes, which favour asymmetric fission, are induced by deformed shells at Z = 52 and Z = 56. By contrast, spherical magic nuclei are very resistant to octupole deformation, which hinders their production as fission fragments. These findings may explain surprising observations of asymmetric fission in nuclei lighter than lead8.

研究动机与目标

• 解决长期以来关于为何锕系素裂变碎片偏好Z ≈ 52–56而非Z = 50（尽管Z = 50处存在强烈的球形壳闭合效应）的谜题。
• 研究八极（梨形）形变在塑造裂变碎片质量不对称性中的作用。
• 确定为何双幻核如132Sn（Z = 50，N = 82）尽管具有高稳定性，却很少作为裂变碎片被观测到。
• 将观测到的裂变碎片Z分布与八极形变和形变壳效应之间的相互作用联系起来。

提出的方法

• 采用超流体裂变动力学的量子多体模型，模拟锕系素核的裂变路径。
• 追踪裂变路径上核形状的演化，重点关注八极（梨形）形变的出现。
• 分析Z = 52和Z = 56处形变壳效应对不对称裂变形状稳定性的影响力。
• 比较球形幻数核（如132Sn）与Z = 52–56区域形变核在抵抗八极形变方面的差异。
• 利用144Ba（Z = 56）中八极形变的实验证据作为模型验证的基准。
• 基于壳效应与形变能之间的竞争，计算不对称裂变的能量偏好。

实验结果

研究问题

• RQ1 为何锕系素裂变碎片主要具有Z ≈ 52–56，而非Z = 50，尽管Z = 50处存在强烈的球形壳闭合效应？
• RQ2 八极（梨形）形变在多大程度上影响锕系素核中裂变碎片的质量不对称性？
• RQ3 Z = 52和Z = 56处的形变壳效应如何稳定不对称裂变路径？
• RQ4 为何尽管具有高稳定性，双幻核如132Sn却很少作为裂变碎片产生？
• RQ5 观测到的裂变碎片Z分布能否通过八极形变与壳效应之间的相互作用来解释？

主要发现

• 裂变碎片中，特别是在Z = 56附近的八极（梨形）形变显著稳定了质量不对称裂变路径。
• Z = 52和Z = 56处的形变壳效应增强了不对称裂变形状的稳定性，从而有利于具有这些质子数的碎片生成。
• 球形幻数核（如132Sn）抗拒八极形变，这限制了其作为裂变碎片的生成，尽管其壳能较高。
• 该模型预测，由于八极形变与形变壳稳定性的共同作用，裂变碎片主要以52至56个质子生成。
• 144Ba（Z = 56）中八极形变的实验确认支持了该模型的预测，即此类形状是锕系素中不对称裂变的关键。
• 研究结果解释了为何在铅以下的核素中，尽管Z = 50处的球形壳效应不占主导，但不对称裂变仍出人意料地普遍。