1.School of Science, Xi’an Jiaotong University, Xi’an 710049, China 2.Ion Beam and Optical Physical Laboratory, Xianyang Normal University, Xianyang 712000, China
Abstract: The de-excitation process of vacancy in the inner shell of the target atom caused by collision ionization produces the characteristic X-ray or Auger electrons. The precise measurement of ionization cross sections plays an important role in many basic research fields, as well as in practical fields, such as chemical analysis of Particle Induced X-ray Emission (PIXE), atomic and nuclear processes, and X-ray fluorescence (XRF) spectroscopy. As we know, when ionization cross sections are measured precisely, whether the emission of X-ray is isotropic in collision process must be considered. However, there have been few experimental results for angular dependence of Kβ/Kα intensity ratios in the literature until now. Therefore, this study aims to verify that the Kα and Kβ X-rays originated from filling of the K shell vacancies with total angular momentum quantum number 1/2 (J = 1/2) are isotropic. In this work, the typical K-shell X-ray spectra for Fe and V, which induced by bremsstrahlung with central energy of 13.1 keV, have been measured at emission angles varied from 120° to 170° at intervals of 10°. The characteristic X-ray spectra obtained by the detector are fitted by Gauss function, where the absorption of incident X-rays by the detector, the absorption of emitted X-rays by the atmosphere and the self-absorption correction factor of incident and emitted X-rays by the target are all taken into account. The experimental results of Kβ/Kα intensity ratio in this experiment coincide with those of theoretical calculation, as well as the Ertu?ral’s experimental result. The experimental results show that the intensity ratio of Kβ/Kα is a constant at different detection angles. Therefore it can be concluded that the emission of Kα and Kβ is isotropic in the detection range. Since the K shell has no sub-shell, there is no Coster-Kronig transition in the collision ionization process. In the process of photoionization, the vacancies in the K shell are produced by direct ionization. As a result, the cross section ratio of K shell X-ray generation is independent of the K shell photoionization cross section. In addition, the experimental results show that the Kβ/Kα characteristic X-ray intensity ratio of target Fe is 8% higher than that of target V, which are consistent with the theoretical analysis results that the characteristic X-ray intensity ratio depends on the target atomic number Z. Keywords:angular distribution/ photoionization/ isotropy
3.实验结果与讨论图2给出了Mini X射线光管出射中心能量为13.1 keV韧致辐射垂直入射靶材, 探测角度为30°时Fe和V靶的特征X射线能谱图, Fe的特征X射线Kα1(K1 L3)和Kα2 (K1 L2)的中心能量分别为6.403和6.39 keV, V的特征X射线Kα1 (K1 L3)和Kα2 (K1 L2)的中心能量分别为4.952和4.94 keV, 而探测器在5.9 keV峰处分辨率为125 eV, 所以无法分辨Fe的特征X射线Kα1, Kα2(Kα1和Kα2的能量差是13 eV)与V的特征X射线Kα1, Kα2 (Kα1和Kα2的能量差是12 eV), 我们统一记做Kα. 对实验结果进行高斯拟合, 从图2可以看出, 拟合曲线和特征峰形状符合得非常好. 图 2 150°探测角下Fe靶和V靶的能谱分布图 (a) Fe靶; (b) V靶 Figure2. Characteristic K X-ray spectrum of Fe and V induced by impact with bremsstrahlung with central energy of 13.1 keV and measured at the emission angle of 150°: (a) Target Fe; (b) target V.
图 1 实验装置示意图
图 2 150°探测角下Fe靶和V靶的能谱分布图 (a) Fe靶; (b) V靶
图 3 探测角为120°—170°时, Fe靶和V靶特征X射线强度比Kβ/Kα的角分布关系
图 4 Kβ/Kα的实验值、Ertu?ral[24]的实验结果和理论计算值[25–27]对比图
