Study of the radiation charging of dielectrics by ions

Abstract

The radiation charging of dielectrics exposed to a flow of charged particles (electrons or ions) has been a subject of intensive investigation over the last two decades. Most of the studies have focused on electron irradiation, which has partly to do with the greater difficulty of generating ion beams and the consequent greater expense of studying dielectric charging by ions.This obviously makes numerical studies more important, since numerical methods make it possible to account for the various factors which influence the dynamics of the charging process.Along with experimental results, the authors of [1] presented a phenomenological analysis of the charging of a dielectric irradiated by protons with an energy of -10 MeV. Theoretical results were obtained only for a shorted dielectric (j" E(z)dz = 0, E, o where E is the strength of the field and a is the thickness of the dielectric), and the quasi-neutrality approximation that was used basically precludes proper description of carrier drift in the non-irradiated part.Here, we use the mathematical model in [2] to perform numerical studies of the charging of a dielectric irradiated by ions and neutral atoms. The model considers the dynamics of quasi-free charge carriers of each sign with allowance for ionization of the dielectric by the beam and charge recombination, as well as charge drift in an electric field. The effective mobility of the charge carriers is determined with allowance for its dependence on the dose rate.The dose rate, the distribution of thermalized particles, and the current of fast particles are found by solving the kinetic equation. The integral kinetic equation in the "continuous trajectories" model [3] was solved for the ions, this equation ignoring elastic scattering but allowing for fluctuations in the energy losses. The specific energy losses of the ions were calculated from the data in [4]. The effect of the electric field on ion transfer was ignored, since the Coulomb force acting on the ions is much weaker than the frictional force -which is equal to the specific energy losses.The kinetic equation in [3] for ions and electrons was solved by expanding the differential flux into a Fourier series in the space coordinate. The distributions thus obtained for the absorbed energy of the thermalized particles agree well with the results in [5, 6].In this article, we present results of numerical investigations of the charging of different dielectrics irradiated by protons and atoms of hydrogen with initial energies up to 100 MeV within the range of proton current densities j(b) = 10-9_10-7 A/era 2. Figure la shows the scheme of specimen irradiation. A flow of particles moving in a drift region of width l strikes the dielectric. The depth of penetration of the particles is determined by their path length inside the dielectric R o. The irradiated part (IP) of the dielectric is the region z <__ R 0, while the non-irradiated part (NIP) is the region z > R 0. The electrode that was placed on the non-irradiated surface functioned as a blocking elect...