Energy Dispersive X-ray Spectrometry: Physical Principles and User-Selected Parameters

Abstract

As illustrated in Fig. 16.1, the physical basis of energy dispersive X-ray spectrometry (EDS) with a semiconductor detector begins with photoelectric absorption of an X-ray photon in the active volume of the semiconductor (Si). The entire energy of the photon is transferred to a bound inner shell atomic electron, which is ejected with kinetic energy equal to the photon energy minus the shell ionization energy (binding energy), 1.838 keV for the Si K-shell and 0.098 keV for the Si L-shell. The ejected photoelectron undergoes inelastic scattering within the Si crystal. One of the consequences of the energy loss is the promotion of bound outer shell valence electrons to the conduction band of the semiconductor, leaving behind positively charged ``holes'' in the valence band. In the conduction band, the free electrons can move in response to a potential applied between the entrance surface electrode and the back surface electrode across the thickness of the Si crystal, while the positive holes in the conduction band drift in the opposite direction, resulting in the collection of electrons at the anode on the back surface of the EDS detector. This charge generation process requires approximately 3.6 eV per electron hole pair, so that the number of charge carriers is proportional to the original photon energy, Ep: n = E p / 3 . 6 e V $$ n={E}_p/3.6\eV $$ For a Mn K-L3 photon with an energy of 5.895 keV, approximately 1638 electron--hole pairs are created, comprising a charge of 2.6 {\texttimes} 10 16 coulombs. Because the detector can respond to any photon energy from a threshold of approximately 50 eV to 30 keV or more, the process has been named ``energy dispersive,'' although in the spectrometry sense there is no actual dispersion such as occurs in a diffraction element spectrometer.