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 K_β/K_α intensity ratios in the literature until now. Therefore, this study aims to verify that the K_α and K_β 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 K_β/K_α intensity ratio in this experiment coincide with those of theoretical calculation, as well as the Ertuğral’s experimental result. The experimental results show that the intensity ratio of K_β/K_α is a constant at different detection angles. Therefore it can be concluded that the emission of K_α and K_β 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 K_β/K_α 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.