Dependence of total kinetic energy of fission fragments on the excitation energy of fissioning systems
Dependence of total kinetic energy of fission fragments on the excitation energy of fissioning systems
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DOI：
10.1103/physrevc.104.054609
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发表时间：
2021-11
影响因子：
3.1
通讯作者：
K. Shimada;C. Ishizuka;F. Ivanyuk;S. Chiba
中科院分区：
文献类型：
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作者：
K. Shimada;C. Ishizuka;F. Ivanyuk;S. Chiba
从中子诱发裂变事件的实验数据出发，阐明了裂变碎片的平均总动能随裂变系统激发能的增加而减小的原因。为了探索这个问题，我们使用了一种基于我们开发的四维朗之万方程的方法。本文计算了在裂变势垒以上激发的裂变系统$^{236}$U$^{*}$和$^{240}$Pu$^{*}$的裂变碎片的TKE，并分别与n + $^{235}$U和n + $^{239}$Pu反应的实验数据进行了比较。从Langevin模型分析中，我们发现，丰富的重碎片的形状从低激发域的近球形到高激发能的高度长球形的变化，而轻碎片的形状没有明显的变化。重碎片的“形状“的变化导致随着激发能的增加，刚断裂后新生碎片的电荷中心之间的距离增加。相应地，两个碎片之间的库仑排斥随着激发能的增加而减小，这导致平均TKE减小。通过这种方式，我们发现，作为激发能的函数的重碎片的形状的变化是裂变碎片的TKE随裂变核的激发能的增加而减小的关键问题。换句话说，影响重碎片形状的洗出壳效应是裂变碎片平均TKE能量依赖性降低的关键原因。
We elucidated the reason why the average total kinetic energy (TKE) of fission fragments decreases when the excitation energy of the fissioning systems increases as indicated by experimental data for the neutron-induced fission events. To explore this problem, we used a method based on the four-dimensional Langevin equations we have developed. We have calculated the TKE of fission fragments for fissioning systems $^{236}$U$^{*}$ and $^{240}$Pu$^{*}$ excited above respective fission barriers, and compared the results with experimental data for n + $^{235}$U and n + $^{239}$Pu reactions, respectively. From the Langevin-model analysis, we have found that the shape of the abundant heavy fragments changes from almost spherical for low excitation domain to highly prolate shape for high excitation energy, while that of the light fragments does not change noticeably. The change of the"shape"of the heavy fragments causes an increase of a distance between the charge centers of the nascent fragments just after scission as excitation energy increases. Accordingly, the Coulomb repulsion between the two fragments decreases with an increase of the excitation energy, which causes the decrease of the average TKE. In this manner, we found that the change of the shape of the heavy fragment as a function of the excitation energy is the key issue for the TKE of fission fragments to decrease as the excitation energy of the fissioning nuclei increases. In other words, washing out of the shell effects which affect the shape of the heavy fragments is the key reason for the decreasing energy dependence of the average TKE of the fission fragments.