中子反应总截面测量系统关联不确定度的模拟

Correlated uncertainties of measuring neutron total cross-sections with simulation

在15 MeV电子加速器驱动的白光中子源装置上进行了铍材料的中子总反应截面测量,测量结果与已有实验数据和ENDF/VII.1数据库的评价数据基本符合。为了得到由于加速器束流波动、飞行路径测量、靶厚度测量以及获取系统死时间等引起的测量系统的关联不确定度,利用Geant4模拟软件对整个实验装置、实验过程进行了模拟,经过对模拟中使用的中子反应总截面与模拟计算得到截面的比较分析,得到了实验系统关联不确定度的估计值。在模拟中采用了局部区域增加权重的方法来降低模拟运算时间。对模拟得到的中子飞行时间谱与实验测量得到的飞行时间谱进行了比较,并且对探测器处的中子产额也进行了比较,以此来验证模拟程序的可靠性。

A system for measuring neutron total cross section,the Photo-Neutron Source(PNS,phase 1),has been developed for the acquisition of nuclear data from the Thorium Molten Salt Reactor(TMSR)at the Shanghai Institute of Applied Physics(SINAP).It is also used in the study of neutron detector and the irradiation material about reactor.The neutron total crosssections of beryllium were measured in the neutron energy region from 0.007 to 0.100 eV.The present measurement result was compared with the existed experimental and the evaluated data of ENDF/VII.1.The correlated uncertainties were determined by using Geant4 Monte Carlo simulation method.In order to reduce the calculation time,the local weighted method was used in the simulation.The neutron time of flight spectrum and the neutron yield at the detector position in experiment and simulation were compared.