密封Cf源的成分测定和中子发射率修正

当密封Cf源寿命超过20年后,其他同位素和子体核素尤其Cf-250和Cm-248的贡献不断增大,使得Cf源中子发射率不能简单按照Cf-252半衰期进行估算。基于Cf源中同位素及其子体核素的衰变和自发裂变规律,提出了一种测定密封Cf源中Cf-252和Cf-250成分的方法,达到快速估算寿命超过20年的Cf源中子发射率的目的。采用长计数器相对测量得到一个使用近30年Cf源在某一时刻的中子发射率,确定了该源初始时刻Cf-252和Cf-250原子核数分别为4.452×1016和7.746×1015,据此给出了Cf源的中子发射率随时间的变化关系。同时在另外两个时刻的推算结果与长计数器测量结果符合得非常好,相对偏差小于0.3%,进一步验证了该方法的可靠性。此外还对Cf-252,Cf-250和Cm-248对中子发射率的贡献与中子源寿命的关系进行了分析,结果表明当Cf源寿命达到30年时,Cf-252对总中子发射率的贡献已经降至67.65%,而Cf-250和Cm-248的贡献已经达到29.32%和2.94%。

Component determination of sealed californium source and the correction of neutron emission rate

When the age of a Cf source exceeds 20 years, the relative contribution of Cf-250 and Cm-248 isotopes will increase gradually. Then the total neutron emission rate will not follow the simple index law determined by Cf-252. Based on the laws of radioactive decay and spontaneous fission for every isotope in Cf source, we put forward a method of determining isotope components (Cf-252 and Cf-250) of the sealed Cf source to rapidly estimate the total neutron emission rate of a Cf source over 20 years. The neutron emission of a Cf source about 30 years old was measured relatively by a long counter at one point. Then isotope components of the sealed Cf source were determined. The numbers of Cf-252 nuclei and Cf-250 nuclei at the beginning time of the source were 4.4521016 and 7.7461015. The relations between the total neutron emission rate and the age of the Cf source is calculated based on its components. The calculated results are in accordance with the measured results, and the relative difference between calculation and experiment results is less than 0.3%. Furthermore, the relative contribution of each isotopes, including Cf-252, Cf-250 and Cm-248, to the total neutron emission rate is analyzed. The result shows that the relative contribution of Cf-252 is reduced to 67.65%, while those of Cf-250 and Cm-248 are increased to 29.32% and 2.94% respectively when the age of the Cf source has reached 30 years.