Long and short term effects of X-rays on charge coupled devices

Abstract

EEV buried channel charge coupled devices (BC CDs) with technological variations have been studied with respect to their response to 70kVp X-rays. Process variations considered are the conventional BCCD, scintillator coated BCCDs ( (Gadox(Eu) and Csl(Tl)) and the inversion mode device. The work was made necessary by the use of these CCDs for dental X-ray imaging. Effects investigated include changes in device operating voltages and dark current. The dark current buildup has been characterised in terms of a prompt component seen immediately following irradiation, and a time dependent component which occurs gradually. A major part of this work was the determination of the location and concentration of the energy states responsible for this dark current buildup. Also a novel aspect of the work was the derivation of an expression describing the time dependent component as a function of time and temperature. Effects associated with the bias dependence of the BCCD have also been considered, with particular regard to the effect of a negative substrate bias, and the theoretical explanation has been developed. The findings of this work have demonstrated the suitability of these devices for the commercial application of imaging x-rays for dentistry.