X-ray produced charge deposition and dose in dielectrics near interfaces including space-charge field and conductivity effects

When a dielectric material is exposed to x- or γ-radiation, charge buildup occurs in regions of the dielectric near interfaces with dissimilar media. The charge buildup results from the divergence of photo-Compton current near the interface. The charge density acts as a source for an electric field. If one assumes a transient dielectric conductivity equal to the bulk radiation-induced conductivity of the dielectric, one can predict a sufficiently large x-ray generated space-charge field at high exposures to either limit the photo-Compton current — hence perturb the depth-dose profile in the dielectric — or initiate dielectric breakdown. There also occurs at the interface, however, a gradient in dose which should produce a gradient in transient conductivity. This conductivity gradient, it is shown, profoundly modifies the charge buildup and space-charge field in the dielectric. Calculations of current density, charge density, and space-charge field are presented for a planar gold/polyethylene interface irradiated by 30, 100, 200, and 1000 keV x-rays. It is found that for the lower x-ray energies the saturation space-charge field is too low to significantly perturb the depth-dose profile or initiate breakdown, but for the 1000 keV x-rays the saturation space-charge field may be sufficiently high to significantly perturb the depth-dose profile. In all cases, the charge density profile is strongly perturbed by the conduction currents, even for relatively low exposures.