Researchers from Shanghaitech University, Shanghai Institute of Microsystem and Information Technology, Institute of Geology and Geophysics, Chinese Academy of Sciences have conducted measurements of X-ray fluorescence spectra excited by X-rays and electrons based on microcalorimeters. It was demonstrated that there are obvious differences between the fluorescence spectral lines excited by electrons and photons. This is a good example for elemental analysis based on fluorescence spectra in electron scanning microscopes. It also provides clear guidance for choosing X-ray excitation sources (such as Fe55 or X-ray tubes) or electron excitation sources (such as Sr90) in space exploration.
In the exploration of celestial bodies, such as Mars, the Moon, and asteroids, X-ray fluorescence analysis has emerged as a critical tool for elemental analysis. However, the varying selection rules and excitation sources introduce complexity. Specifically, these discrepancies can cause variations in the intensities of the characteristic spectral lines emitted by identical elements. These variations, compounded by the minimal energy spacing between these spectral lines, pose substantial challenges for conventional silicon drift detectors (SDD), hindering their ability to accurately differentiate these lines and provide detailed insights into the material structure. To overcome this challenge, a cryogenic X-ray spectrometer based on transition-edge sensor (TES) detector arrays is required to achieve precise measurements. This study measured and analyzed the K-edge characteristic lines of copper and the diverse L-edge characteristic lines of tungsten using a comparative analysis of the electron and X-ray excitation processes. For the electron excitation experiments, copper and tungsten targets were employed as X-ray sources, as they emit distinctive X-ray spectra upon electron-beam bombardment. In the photon excitation experiments, a molybdenum target was used to produce a continuous spectrum with the prominent Mo Kα lines to emit pure copper and tungsten samples. TES detectors were used for the comparative spectroscopic analysis. The initial comparison revealed no substantial differences in the Kα and Kβ lines of copper across different excitation sources. Similarly, the Lα lines of tungsten exhibited uniformity under different excitation sources. However, this investigation revealed pronounced differences within the Lβ line series. The study found that XRF spectra preferentially excite outer-shell electrons, in contrast to intrinsic spectra, owing to different photon and electron interaction mechanisms. Photon interactions are selection-rule-dependent and involve a single electron, whereas electron interactions can involve multiple electrons without such limitations. This leads to varied excitation transitions, as evidenced in the observed Lβ line series.
