[论文解读] Review on the progress in nuclear fission

Karl‐Heinz Schmidt, B. Jurado|arXiv (Cornell University)|Apr 27, 2018

Nuclear physics research studies被引用 52

一句话总结

这是对最近核裂变领域实验与理论进展的全面评述，着重介绍能够接触到更广范围裂变体系的新方法，以及三种前景看好的建模途径：自洽量子理论、随机模型以及半经验的 GEF 式框架。

ABSTRACT

An overview is given on some of the main advances in experimental methods, experimental results and theoretical models and ideas of the last years in the field of nuclear fission. New experimental approaches extended the availability of fissioning systems considerably and provided a full identification of all fission products in A and Z for the first time. In particular, the transition from symmetric to asymmetric fission around 226Th and some unexpected structure in the mass distributions in the fission of systems around Z = 80 to 84 as well as an extended systematics of the odd-even effect in fission fragment Z distributions have been measured [A. N. Andreyev et al., Rep. Progr. Phys. 81 (2018) 016301]. Three classes of theoretical descriptions of fission presently appear to be the most promising or the most successful ones: Self-consistent quantum-mechanical models fully consider the quantum-mechanical features of the fission process. Intense efforts are presently made to develop suitable theoretical tools [N. Schunck, L. M. Robledo, Rep. Prog. Phys. 79 (2016) 116301] for modeling the non-equilibrium, large-amplitude collective motion leading to fission. Stochastic, essentially classical models provide a fully developed technical framework. The main features of the fission-fragment mass distribution are well reproduced from mercury to fermium and beyond [P. Möller, J. Randrup, Phys. Rev. C 91 (2015) 044316]. In an alternative semi-empirical approach [K.-H. Schmidt et al., Nucl. Data Sheets 131 (2016) 107], considerable progress in describing the fission observables has been achieved by combining several general theoretical concepts. This new approach reveals a high degree of regularity and allows calculating high-quality data with a unique parameter set for a large number of systems and an extended excitation-energy range.

研究动机与目标

• 通过综合微观到统计水平的实验发现和理论思想，推动对裂变过程的更深入理解。
• 将实验观测（产额、TKE、瞬发辐射）与潜在物理机制联系起来，如势垒特性和裂解条件。
• 评估裂变理论与实验的现状，包括局限性与未来需要。
• 总结三种最具前景的模型族（自洽量子、随机、半经验 GEF）及其关键洞察。

提出的方法

• 综述裂变的全动力学量子力学与经典模型及其应用。
• 讨论随机传输方程及其在再现裂变产物分布中的应用。
• 介绍 General Fission Model (GEF) 及其组成部分，包括地形/拓扑考虑、壳效应/宏观影响以及热浴耦合。
• 描述结合多种理论思想、在无需对每个体系逐一拟合的情况下预测裂变观测量的半经验方法。
• 突出扩大可访问裂变体系、并实现裂变碎片的完整核素鉴定的实验创新。

实验结果

研究问题

• RQ1 自洽微观模型如何表征裂变势垒、耗散和大振幅集体运动？
• RQ2 随机传输方程在广泛核种中再现裂变碎片质量分布的效果如何？
• RQ3 如 GEF 等半经验方法在预测裂变产额和能量时，会出现哪些规律性和有效自由度？
• RQ4 哪些实验创新揭示了裂变观测量（产额、TKE、瞬发中子/伽玛）随激发能量和体系的演变？
• RQ5 推进统一裂变动力学描述所需的实际需求和理论扩展是什么？

主要发现

• 在若干实验装置中，新的实验方法首次实现了裂变产物在 A 和 Z 的完全鉴定。
• 在226Th附近，裂变从不对称转向对称的转变被观测到（例如在某些重核附近，如 226Th）。
• 裂变碎片质量和电荷分布中的细结构表明存在超出简单分支的规律，并随激发能量和体系而变化。
• Z 分布的奇偶起伏随不对称性增加而增大，而某些实验中 N 分布的细结构在能量依赖性上表现较弱。
• SOFIA 结果表明，裂变碎片的 N 分布结构对激发能量的依赖性很弱，与 Z 分布不同。
• GEF 模型在最少的系统特异性拟合下即可再现光谱和其他瞬发辐射特征。