低能正电子致原子内壳层电离的截面测量研究

The measurement of atomic inner-shell ionization cross-sections impacted by positron at low energies (i.e., U≤4, where U is the ratio between the incident positron energy and the atomic inner-shell ionization threshold energy) is significant in both theoretical researches and practical applications. However, so far, the experimental data of atomic inner-shell ionization for positron impact at low energies are very scarce, only seldom inner-shell ionization data of Cu-K，Ag-K， Ag-L，In-L，Sn-L，Au-L3, Ti-K have been reported. Therefore, we apply for the project for the measurement of inner-shell ionization cross-sections for low-Z, medium-Z and higher-Z elements by positron impact up to 30keV. The objectives of the study are to set up the measurement platform of atomic inner-shell ionization cross-sections by low-energy positron impact and improve data processing method. The methods mainly include: (1) The X-rays emitted from the targets will be detected using a Silicon Drifted Detector (SDD). The crystals' thickness of the SDD is an order of magnitude thinner than those in conventional Si(Li) detector. Use of this detector can suppress the large background produced in the crystal resulting from the positron annihilation. (2) Developing a method for a thin target on a thin carbon grid substrate is to acquire the characteristic X-ray peak counts more accurately. (3) In the case of adopting the thin film on the thin carbon grid substrate, the incident positrons will suffer some scattering and lose energy within the thin film. In addition, bremsstrahlung photons and the annihilation γ rays will increase the characteristic X-ray peak counts. So the characteristic X-ray peak counts need to be corrected. The effects of the target film's thickness on the measured values are corrected by means of Monte Carlo simulation. (4) For on-line monitoring the incident positron beam stability and intensity, the annihilation γ rays will be detected by a HPGe detector in the experiments and simulated by Monte Carlo method. (5) The detection efficiency calibration of the SDD in low-energy region down to several keV is carried out by determining from the ratio of the experimental and theoretical thick-carbon-target bremsstrahlung spectra. Based on these techniques, the experimental data of some atomic inner-shell ionization cross sections by positron impact up to 30keV will be given for the first time. The experimental data of atomic inner-shell ionization cross-sections by low-energy positron impact will compare with the theoretical models recently developed and the reported experimental data of atomic inner-shell ionization cross-sections by electron impact for better understanding of the positron-atom and the electron-atom collision process. For example, the Coulomb interaction and the exchange interaction, which are important but not yet understood completely in low-energy incident regions.

能量在原子阈能附近的正电子致原子内壳层电离截面实验数据非常缺乏。本项目拟建立正电子引起靶原子内壳层电离的截面测量平台，并采用准确的数据处理技术，以获得可靠的截面数据。本项目使用晶体厚度较通常X射线探测器薄得多的硅漂移探测器收集X能谱，以抑制正电子产生的湮灭光子对本底计数的贡献，提高X射线特征峰净计数的获取精度；采取薄膜薄碳网格衬靶技术，以减少因膜厚、衬底存在对X射线特征峰净计数的修正；用蒙卡模拟估算膜厚对电离截面测量值的影响大小；由HPGeγ谱仪对湮灭光子数的记录并结合蒙卡模拟以实现对入射正电子束流稳定性及强度的在线监测；用相对效率刻度法提高低能端硅漂移探测器的效率刻度精度等。应用上述技术，完成30keV以下能量的正电子碰撞多个低、中、高Z原子内壳层电离截面在国际上的首次测量,并将结果与电子碰撞原子的实验数据及一些理论模型进行比较，以促进理论的进展，了解正负电子与原子间的作用机制。

低能正负电子致原子内壳层电离截面的测量研究在理论和实际应用方面都有着非常重要的意义。理论方面，低能正负电子由于电荷极性的差异使得二者碰撞原子的作用机制存在差别，至今还没有可靠的理论描述，因此，需要可靠的实验数据检验并促进理论的发展。实际应用方面，低能正电子碰撞原子内壳层电离截面数据在材料科学、辐射物理、等离子体物理、大气物理、天体物理等许多领域都是不可或缺的。然而，由于实验技术方面存在困难，低能正电子致原子内壳层电离截面实验数据非常缺乏。本项目使用SDD（硅漂移探测器）X射线能谱仪收集低能正电子碰撞靶样品时产生的X射线能谱，进而获得低能正电子致原子内壳层电离的截面数据。项目完成的主要内容有：①搭建了低能正电子与靶样品碰撞的先进实验平台；②利用SDD收集19keV电子碰撞纯厚碳靶产生的轫致辐射实验谱，并结合蒙卡模拟得到理论轫致辐射谱，二者相比即得到SDD的相对效率刻度曲线，再利用241Am标准源的13.9keVX射线能量的绝对探测效率值将相对效率刻度曲线绝对化，从而完成对SDD低至0.7 keV以上能区的准确探测效率刻度；③发展了蒙卡模拟结合HPGe探测器对湮灭光子在线监测的技术准确获取X能谱收集过程中与靶碰撞的入射正电子数；④发展了正负电子与靶样品碰撞过程中，光致电离、散射、次级粒子等效应对电离截面测量结果的影响等算法研究；⑤首次采用薄网格衬靶方法开展了低能正负电子致原子内壳层电离的截面测量研究，并证实了其可行性；⑥获得了低能电子致Se（Z=34）、Ag（Z=47）和Sn（Z=50）的L壳层X射线产生截面，低能电子致W（Z=74）、Bi(Z=83)等多个高Z原子的M壳层X射线产生截面，以及低能正电子致Al（Z=13）、Si（Z=14）、Ti（Z=22）的K壳层，Zn（Z=30）、Nb（Z=41）、Mo（Z=42）、Ag（Z=47）的L壳层以及Ta（Z=73）、W（Z=74）、Pb（Z=82）、Bi（Z=83）的M壳层X射线产生截面（正电子实验结果中的绝大多数都是首次获得），并将实验结果与DWBA等先进理论模型做了比较，以帮助人们正确认识正负电子与原子的作用机制。