Monte Carlo simulation on the electron beam incident angle with spherical particles applied to the energy loss in ZnS phosphor powders

Abstract

During electron beam irradiation of ZnS phosphor powders, a non-luminescent ZnO layer is formed on the powder due to electron-beam-stimulated surface reactions. As the thickness of the oxide layer increases, the energy loss in the ZnS bulk decreases with a subsequent degradation in cathodoluminescence (CL). Owing to the morphology of the phosphor powder, growth of the oxide layer leads to varying effective ZnO thicknesses. In this paper the effect of the interaction between the electron beam and the ZnO/ZnS powder particles on the energy loss in the ZnS is studied using Monte Carlo simulation techniques. In the simulation, an electron beam with a certain profile is spread according to a Gaussian distribution function over the phosphor particles. These particles consists of both flat and spherical-shaped grains. The incident angle between the electron beam and the particle's surface is determined by using simple vector analysis. If the beam contains a sufficient number of electrons, a distribution of incident angles can be obtained. It was found that this distribution is independent upon the profile of the electron beam as long as the full width at half-maximum (FWHM) beam diameter is larger than the size of the irradiated particles. Using these results a comparison was made between the energy loss in the ZnS particles as a function of the ZnO thickness with and without the angular distribution. A publicly available Monte Carlo code for electron trajectory simulations in solids, called CASINO, was used for this purpose. When the distribution is considered, there is a decreased energy loss in the ZnS bulk. Because the light generation during electron beam irradiation is proportional to this energy loss, the morphology of the phosphor powder also contributes to the CL degradation. Copyright © 2000 John Wiley & Sons, Ltd.