~(252)Cf自发裂变中子发射率符合测量的回归分析

~(252)Cf同位素源具有标准的自发裂变中子能谱,但由于其半衰期较短,应用中常需要对源强进行标定修正.随着源年龄增加,来自源中~(250)Cf和~(248)Cm自发裂变的影响愈加凸显,不能简单按~(252)Cf的衰变规律计算源中子发射率,而通过锰浴活化的间接测量方法周期较长,且在源强低于10~4 n/s时误差较大.最近,基于中子多重性计数的源强绝对测量算法已得到验证.本文进一步从点模型假设的测量方程出发,在将符合计数率与总中子计数率关联的基础上,分别对符合计数率随源位置、符合门宽的变化关系进行回归分析,提取变化过程的特征系数,建立了两种避规效率变化的~(252)Cf中子发射率测量方法,并基于JCC-51型中子符合测量装置开展实验验证.结果表明:两种回归分析方法的测量值均与标称值的修正结果在2%的偏差范围内一致;反推求得装置中轴线上的探测效率也与基于MCNPX程序的蒙特卡罗模拟计算值相符.研究结果可为活度信息不明的~(252)Cf源强标定及符合测量装置的效率刻度提供便携准确的实验方法.

Regression analysis of coincidence measurements for determinating the neutron emission rate of 252Cf spontaneous fission

The ²⁵²Cf isotope sources have a recommended standard neutron spectrum of spontaneous fission, and have been widely used in scientific researches, such as the detection efficiency calibration of neutron detectors, the characterization of neutron dose equivalent meters, the active analysis of special nuclear materials, etc. However, it is often necessary to correct the neutron emission rate due to its short half-life of 2.645 years. As the source age increases the contributions from ²⁵⁰Cf and ²⁴⁸Cm spontaneous fission become more significant, thus the neutron emission rate cannot be calculated simply according to the ²⁵²Cf decay law. In addition, the indirect measurement method by manganese bath activation needs a long period more than 8 hours; and it will have a large uncertainty while the source strength is lower than 10⁴ n/s. In order to develop a more portable measurement method for larger suitable dynamic range, the comprehensive algorithms based on the neutron multiplicity counting are studied in this paper. On the basis of the measurement equations under the point model assumption, the neutron coincidence counting rate is correlated with the total neutron counting rate, and then the regression analyses with different coincidence gates and different source locations in the counter are performed. On the assumption that the average neutron die-away time is constant in the sensitive range of detection system, therefore the characteristic coefficient from the changing process can be extracted, and two kinds of methods of measuring the neutron strength are established, which are independent of the efficiency variation. The verification experiments are carried out by the JCC-51 neutron coincidence counter. It is shown that the values measured by the two regression methods are consistent with the corrected results of the nominal value within 2% deviation. Furthermore, the detection efficiency is inversed by dividing the total neutron counting rate with the neutron emission rate when the source is placed at the central axis, which accords with the result of Monte Carlo simulation by using the MCNPX code well. It can provide an accurate method of determining the neutron emission rate of ²⁵²Cf spontaneous fission, and also an approach to calibrating the detection efficiency of neutron coincidence counter while the source strength is unknown.