A contactless, microwave-based radiation detector

The performance of conventional radiation spectrometers is currently limited by fundamental problems associated with extracting secondary particles such as electrons, holes or scintillation photons from within a detection medium. Some of these problems are impurity attachment, recombination, non-uniform electric fields, grid shielding inefficiency, hole trapping, inefficient light collection and self-absorption. All of these problems can be reduced or even eliminated if the ionization process can be measured directly within the bulk medium without actually collecting the secondary particles. We have detected the absorption of single, high-energy photons in CdZnTe and HPGe crystals using a contactless, microwave cavity perturbation technique. Photo-induced transient changes in semiconductor conductivity in the microwave region (109 Hz) produce a momentary increase in the power reflected from a critically coupled resonant cavity of quality factor Q. The magnitude of the reflected microwave pulse is a measure of the excitation energy and the duration of the pulse is related to the semiconductor carrier lifetime. Here we present an overview of the technique including an analysis of the factors affecting sensitivity and response time and the feasibility of performing pulse-height spectroscopy on the reflected microwave pulses.