Atomic Energy Science and Technology ›› 2022, Vol. 56 ›› Issue (5): 798-804.DOI: 10.7538/yzk.2022.youxian.0054

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Measurement of Post-neutron Mass Distribution in Thermal-neutron-induced Fission of 239Pu

LIU Chao;LIU Shilong;YANG Yi;ZHAO Kunling;CHEN Yongjing;SU Yang;ZHANG Kai;FENG Jing   

  1. China Nuclear Data Center, Key Laboratory of Nuclear Data, China Institute of Atomic Energy, Beijing 102413, China; Chengdu University of Technology, Chengdu 610059, China
  • Online:2022-05-20 Published:2022-05-20



  1. 中国原子能科学研究院 核数据重点实验室,中国核数据中心,北京102413;成都理工大学,四川 成都610059

Abstract: Nuclear fission is one of the most complex physical processes. Up to now, there is still a lack of theory that can well describe the prefission and postfission processes. Mass yields in the neutroninduced fission of 239Pu provide fundamental and significant data support for nuclear energy utilization, but the independent fission yields are scarce and far from precise. In addition, the current evaluation work for fission yields partially refers to the experimental data based on radiochemical methods and mass spectrometric, it will make sense to compare the evaluation data with the data obtained by direct systematic measurement. In order to determine independent mass yield distributions precisely, the fission fragments identification spectrometer (FFIS) device was developed. The axial grid ionization chamber and timing detectors based on microchannel plates were designed and tested for determining the kinetic energy and timeofflight of fission fragments, respectively. Specifically, the intrinsic energy resolution for the ionization chamber was measured to be about 0.5% for 80 MeV 63Cu particles, and the time resolution was about 157 ps (FWHM) determined by 241Am source. By directly coincident measurement of energy and velocity of the fission fragments on their flight path, the postneutronemission masses can be obtained according to the classical formula for kinetic energy. The mass yield distribution in the thermalneutroninduced fission of 239Pu was measured at the inhospital neutron irradiator (IHNI1). With the flight trajectory of 10045 cm, the relative uncertainty of the flight path length is better than 01%, and the time resolution is about 02%. When using the Ev method to measure mass distribution, two vital issues need to be resolved. For one thing, the nonlinear energy response of the detector affects the accuracy of the mass calculation. In this work, energy losses in the carbon foil of the stop timing detector and the silicon nitride window of the ionization chamber were corrected eventbyevent based on Geant4 calculation. For another thing, the scattering of heavy ions or the inhomogeneous of the detector materials may make the recorded energy to be underestimated. These abnormal data points are hard to eliminate in the valley region, which makes the mass yields of symmetric fission fragments being overestimated, and improving statistics may relatively reduce this kind of effect. Taking into account the correction uncertainty, the energy resolution for the light fission fragment is better than 07%, and that is approximately 1% for heavy fragments peak. Thus, the mass resolution is estimated to be 1 amu at 99 amu, and 15 amu at 138 amu.

Key words: 239Pu, mass distribution, Ev method, thermal-neutron-induced fission

摘要: 原子核裂变是最复杂的物理过程之一,至今仍缺乏可以统一描述裂变前和裂变后过程的理论。中子诱发239Pu裂变产额数据是重要的核数据,完整的初级裂变产物质量分布数据有助于完善裂变理论模型并提高产额评价数据的质量。本文研制了初级裂变产物鉴别谱仪(FFIS),通过屏栅电离室和微通道板时间探测器分别测量裂变碎片的动能和飞行时间,基于动能速度关联的方法直接获得碎片放中子后的质量分布,在BNCT医院中子照射器(IHNI1)上开展了热中子诱发239Pu裂变初级裂变产物的质量分布测量。测量结果表明,对轻峰碎片质量分辨约为1 amu,对重峰碎片质量分辨约为15 amu。239Pu(nth,f)初级裂变产物质量分布的精确测量可为裂变产额理论计算和评价提供重要的实验数据。

关键词: 239Pu, 质量分布, 动能-速度方法, 热中子诱发裂变