X‐Ray Fluorescence Imaging

Abstract

In Wilhelm Conrad Röntgen's cathode‐ray‐tube experiments, the lighting up of a screen made from barium platinocyanide crystals due to fluorescence led to his discovery of X rays in 1895. Subsequent investigations by Röntgen and by Straubel and Winkelmann showed that the radiation that emanated from a fluorspar, or fluorite crystal CaF 2 excited by X rays is more absorbing than the incident X rays. This so‐called fluorspar radiation is X‐ray fluorescence. Nowadays, it is known that X‐ray fluorescence is one of the by‐products of the interaction of electromagnetic (EM) waves with the atoms in matter in the X‐ray regime (0.1–500 Å). X‐ray fluorescence is a two‐step process; X‐ray absorption (the ejection of an inner electron) and X‐ray emission (the recapture of an electron). Both involve the electron configuration of atoms in the irradiated material. An X‐ray fluorescent image is usually obtained by following these steps: The incident X‐ray beam, after proper monochromatization and collimation, impinges on a sample of interest. An absorption spectrum is acquired by scanning the monochromator to identify the locations of the absorption edges of the element investigated. The incident beam is tuned to photon energies higher than the absorption edges so that the constituent elements of the sample are excited. Then, X‐ray fluorescence emanates. The spatial distribution of the fluorescent intensity, the fluorescent image, is recorded on a two‐dimensional detector or on a point detector using a proper scan scheme for the sample. The following items need to be considered for good quality images of high spatial and spectral resolution: (1) appropriate X‐ray optics for shaping the incident beam and for analyzing the fluoresced X rays and (2) a suitable scanning scheme for the sample to cross the incident beam. In addition, image reconstruction of intensity data versus position is also important for direct mapping of the trace elements in a sample.