X-Rays

Abstract

Energetic beam electrons stimulate the atoms of the specimen to emit “characteristic” X-ray photons with sharply defined energies that are specific to each atom species. The critical condition for generating characteristic X-rays is that the energy of the beam electron must exceed the electron binding energy, the critical ionization energy E _c, for the particular atom species and the K-, L-, M-, and/or N- atomic shell(s). For efficient excitation, the incident beam energy should be at least twice the critical excitation energy, E ₀ > 2 E _c. Characteristic X-rays can be used to identify and quantify the elements present within the interaction volume. Simultaneously, beam electrons generate bremsstrahlung, or braking radiation, which creates a continuous X-ray spectrum, the “X-ray continuum,” whose energies fill the range from the practical measurement threshold of 50 eV to the incident beam energy, E ₀. This continuous X-ray spectrum forms a spectral background beneath the characteristic X-rays which impacts accurate measurement of the characteristic X-rays and determines a finite concentration limit of detection. X-rays are generated throughout a large fraction of the electron interaction volume. The spatial resolution, lateral and in-depth, of electron-excited X-ray microanalysis can be roughly estimated with a modified Kanaya–Okayama range equation or much more completely described with Monte Carlo electron trajectory simulation. Because of their generation over a range of depth, X-rays must propagate through the specimen to reach the surface and are subject to photoelectric absorption which reduces the intensity at all photon energies, but particularly at low energies.