正电子湮没寿命测量中干扰信号对谱仪和测量结果的影响

Influence of interference signals on spectrometer and measurement results in positron annihilation lifetime measurements

正电子湮没信号的精准采集与关联符合技术是寿命谱灵敏表征材料微观缺陷的基础。测量环境中放射性射线对正电子湮没信号采集的影响,制约着寿命谱方法在复杂辐射背景中应用,特别是在核结构材料中子辐照损伤研究中,中子活化诱发的放射性核素形成的γ射线本底,将影响正电子寿命谱仪的测量结果。为探究γ本底对正电子湮没寿命测量的影响规律,本文基于^(60)Co、^(137)Cs源设计了辐射背景仿真实验,结果显示:^(60)Co源产生的双高能γ射线是影响寿命谱形状及湮没寿命的主要因素;通过对比高、低两种典型活度比(^(60)Co/^(22)Na为3.3和1.9)下的测量结果,并经活化反应堆压力容器钢样品放射性本底真实情况检验,结果发现:在低活度比下,辐射本底导致的偶然符合概率增大,寿命谱峰谷比显著变差;在高活度比下,除偶然符合外,信号错误符合概率急剧增加,谱形明显畸变且寿命值迅速减小。基于本文辐射背景放射源模拟方法及干扰γ的影响规律,可进一步探索正电子湮没寿命测量中γ本底排除的新技术和新方法。

[Background]The accurate acquisition and correlation coincidence of positron annihilation signal form the basis of the lifetime spectrum sensitive characterization of microscopic defects in materials.The complex radiation background interferes with the acquisition of positron annihilation signals,particularly in the study of neutron radiation damage of nuclear structural materials.Theγray background generated by radionuclides induced by neutron activation affects the measurement results of positron lifetime spectrometer.[Purpose]This study aims to investigate the effect ofγbackground on positron annihilation lifetime measurement.[Methods]First of all,the positron lifetime measurement system is built in a"fast-fast coincidence"manner,and radiation background simulation experiments are designed by selecting two typical nuclides,^(60)Co and ^(137)Cs sources,with nearby featureγphoton energy for measuring positron annihilation lifetime.Then,the spectra under two typical activity ratios are compared with the activated neutron-irradiated samples.[Results]The simulation results indicate that the double high energyγrays generated by ^(60)Co sources are the primary factors affecting the spectrum shape and lifetime components.When the ^(60)Co/^(22)Na activity ratio is relatively low,1.9,the peak-to-valley ratio of the spectrum significantly degrades,with the increase of random coincidence probability caused by radiation background.Further,at high activity ratio of 3.3,besides random coincidence,the false coincidence probability increases sharply,and the spectral shape is evidently distorted.For neutron-irradiated RPV steel,the lifetime value is reduced by 17%and 46%at low and high activity,respectively,compared with the non-irradiated samples.[Conclusions]Using the simulation method of radiation background sources and the influence rules of interferenceγin this study,new techniques for eliminatingγbackground could be further explored in positron annihilation lifetime measurement.