Investigation of the electric field distribution in x-ray detectors by Pockels effect

Abstract

In x- and γ-ray semiconductor detectors, the distribution of the internal electric field strongly affects their charge-collection properties. In planar detectors, the electric field distribution depends not only on the semiconductor characteristics, but mainly on the nature of the electrical contacts. This is an issue in CdTe and CdZnTe (CZT) detectors, where many efforts have been devoted to the study and the improvement of contacts, with the aim of suppressing dark current to low values and enhancing the collection of the photo-generated carriers. This work explores the effect of recently developed electrodes for CdTe detectors on the internal electric field distribution by exploiting the voltage-induced birefringence, which is shown by zincblende-structured materials such as CdTe. We used the transverse Pockels effect, in which near-infrared radiation impinges on the side of the CdTe detector parallel to the contact planes, and therefore perpendicular to the electric field. The implemented set-up and the procedure to reconstruct the electric field distribution is presented. The system allows us to extract maps of the electric field with a good spatial resolution, and to study its dependence on the applied voltage. The use of the Pockels effect appears to be a powerful tool, not only to gain information on the electric field, but also to analyse the time evolution observed, at room temperature, in the performance of these detectors.