Ultranarrow x-ray lines radiated by an electron beam in a solid-state superlattice

Abstract

We show that anomalous dispersion in the vicinity of absorption edges of the material can greatly enhance x-ray transition radiation of a low-energy electron beam passing through the multilayer solid-state structure. We showed earlier1 that the critical energy of the electron beam required to obtain soft x-ray radiation in such a structure with a 50-100-Å spatial period can be as low as a few hundred keV. We demonstrate here that the anomalous dispersion near the absorption edge of one of the solid-state materials forming the multilayer structure yields radiation with very narrow spectrum in the vicinity of that edge. In addition to the resonant condition which determines the spatial period of the system, one must choose one of the materials (radiator) to have a frequency of the absorption edge (K, L, M … series) close to the required frequency (usually, for short wavelength, it must be a heavy element), whereas another one (spacer) is a light material, e.g., B, Be, and C. The analysis of the theoretical and experimental data on atomic scattering factors2 available to date reveals that the proposed technique may produce a universal method for generation of very narrow x-ray lines with Δω/ω ≃ 10-4-10-5 in the range between 1 and 200 Å with most of the elements of the periodic table. For example, Ba/Be multilayer structure with 1000 individual layers, each of which is 100 Å thick, irradiated by 1-MeV electron beam, yields 10-6 photons/eV/electron at a wave length of 16 Å with the linewidth Δω/ω ≃ 10-4.