When studying samples without destruction by x-ray fluorescence analysis, the thickness of the investigated sample is limited by the depth of total absorption of the outgoing and excitation radiation. When analyzing complex geological objects containing high-Z atoms, the excitation energy can reach several tens of kiloelectronvolts, which can generate x-ray fluorescent light from a depth of several hundred micrometers. However, focused on the front plane of the sample the beam of excitation radiation is defocused at such depth, and its cross-section under the conditions discussed in the report can become significantly larger than the focus size. This effect is especially significant when the x-ray source is small, for example, for the SKIF synchrotron radiation source. Based on the geometric model of the actinic voxel, the conditions for optimal choice of the excitation energy and focusing conditions of the initial x-ray beam are determined.
This work was supported in part by RFBR Grant Number 17-45-540618.