Detection

Abstract

The fourth phase of the x-ray spectrometric analysis (Section 3.2.5) is the detection and measurement of the characteristic secondary x-rays. Detection is the subject of this chapter; measurement is considered in Chapter 7. The detector transduces the x-rays into an electrical, luminous, or other analog that can be measured or otherwise used. Most commonly used x-ray detectors are based on the indirect effect of x-rays on the electronic structure of matter. X-rays having photon energy < 1 MeV interact with matter by one of three processes : photoelectric bsorption and coherent and incoherent (Rayleigh and Compton) scatter. At energies <50 keV, the initial interaction is usually photoelectric absorption, each x-ray photon producing a photoelectron having kinetic energy equal to the incident photon energy minus the binding energy of the electron. These photo-electrons expend their energy in various ways depending on their environment: In photographic emulsion, they render grains of silver halide developable—that is, labile to chemical reduction to silver—by an electronic process not fully understood. In fluorescent screens and scintillation phosphors, they elevate valence-band electrons to higher levels ; when these electrons return to their unexcited state, the energy is reemitted in the visible and ultraviolet regions. In gas-filled detectors, they ionize gas atoms, producing ion-electron pairs. In solid-state semiconductor detectors, they elevate valence-band electrons to the conduction band, creating electron-hole pairs.