Symposium on Solid State Conductivity I. Radiation-induced Conductivity

Abstract

Photoconducting crystals such as cadmium sulphide are finding increasing use as detectors for X and γ-radiation, either in simple form or as components of solid state image amplifiers. An appreciation of the fundamental processes of free carrier production by radiation, factors influencing carrier life-time and those affecting recombination of carriers can prove valuable in assessing the optimum performance likely for such crystals. Such processes can be revealed by relatively simple measurements of relations between photocurrents or induced currents and excitation intensity of incident radiation, between current and voltage, current and temperature and photon energy, and of decay of current with time after removal of stimulus. Spectral response curves reveal the presence of surface states deleterious to long carrier life-times. Linear relations between current and radiation intensity reveal the presence of trapping states and a performance below optimum although the linearity may be a very desirable feature for dosimetry.Single crystals of cadmium sulphide show better behaviour than powder form layers with respect to response speed. A major problem is the quality control of crystals in production and the ability to introduce desired types of recombination centres for carriers. This prompts turning of attention to other possible photoconductors. Some suggestions include cadmium telluride and perhaps mercury sulphide which offer in addition higher stopping powers. Cadmium telluride can be synthesized and doped in the molten form and grown into relatively large crystals. A further possibility is the stabilization of silver halide conductivity counters by introduction of appropriate divalent impurities.An appendix gives some practical data on cadmium sulphide.