正电子湮没寿命测量统计精度研究

Study on the precision of positron lifetime measurements

正电子湮没寿命与正电子所在位置的电子密度密切相关,因此能反映材料内部原子尺度的微观结构信息,是研究材料缺陷的灵敏探针。正电子湮没寿命可利用时间符合谱仪进行测量,而寿命谱仪的时间分辨函数一般在200-300 ps之间,因此在实际测量中得到的正电子寿命的精确程度是人们广泛关注的问题。本文通过对几种典型样品进行长时间重复测量,并通过改变谱仪的工作状态,研究了各种因素如谱仪的分辨函数、寿命谱总计数、样品中寿命成分个数及各寿命之间的间隔对所分析出的正电子寿命值的影响。研究发现,在谱仪保持状态稳定的情况下,寿命谱的统计测量计数为100万左右时,对于单晶Si中的单个正电子寿命(218 ps),经过重复测量得到的标准偏差小于0.5 ps。对于存在三个寿命分量的高分子材料聚甲基丙烯酸甲酯(Poly(methyl methacrylate),PMMA)中,其平均寿命的标准偏差也小于1 ps,其中长寿命成分?3(1.83 ns)的标准偏差仅为0.016 ns。研究还表明,谱仪时间分辩函数的大小对正电子寿命测量值的影响很小。我们的结果证明正电子寿命测量具有较高的统计精度,能准确反映材料中微观结构细小的变化。

[Background] The precision of positron lifetime measured in the experiment is of great concern. [Purpose] Some typical materials were chosen to repeat the positron lifetime measurements over a long time to assure the precision. [Methods] By adjusting the working conditions of the spectrometer, we studied the factors affecting the precision of the measured positron lifetime, such as the time resolution, total count of the lifetime spectrum, number of lifetime components in the sample and also the interval between each lifetime components. [Results] It was found that with stable working condition of the spectrometer and a total count of 1 million for each spectrum, the positron bulk lifetime in Si single crystal is about 218 ps, and the standard deviation is only 0.5 ps after several repeated measurements. In poly（methyl methacrylate） （PMMA）, where positrons have three lifetime components, the standard deviation of the average lifetime is also only 1 ps, and the standard deviation of the longest lifetime r3 （1.83 ns） is 0.016 ns. Our experiment also shows that the time resolution of the spectrometer has little effect on the measure of positron lifetime. [Conclusion] The results indicate the positron lifetime measurement has high precision, and is a reliable tool to detect the change of the microstructure of materials.