Study of thick CZT detectors for X-ray and Gamma-ray astronomy

CdZnTe (CZT) is a wide bandgap II–VI semiconductor developed for the spectroscopic detection of X-rays and γ-rays at room temperature. The Swift Burst Alert Telescope is using an 5240cm2 array of 2mm thick CZT detectors for the detection of 15–150keV X-rays from Gamma-ray Bursts. We report on the systematic tests of thicker (⩾0.5cm) CZT detectors with volumes between 2cm3 and 4 cm3 which are potential detector choices for a number of future X-ray telescopes that operate in the 10keV to a few MeV energy range. The detectors contacted in our laboratory achieve Full Width Half Maximum energy resolutions of 2.7keV (4.5%) at 59keV, 3keV (2.5%) at 122keV and 4keV (0.6%) at 662keV. The 59keV and 122keV energy resolutions are among the world-best results for ⩾0.5cm thick CZT detectors. We use the data set to study trends of how the energy resolution depends on the detector thickness and on the pixel pitch. Unfortunately, we do not find clear trends, indicating that even for the extremely good energy resolutions reported here, the achievable energy resolutions are largely determined by the properties of individual crystals. Somewhat surprisingly, we achieve the reported results without applying a correction of the anode signals for the depth of the interaction. Measuring the interaction depths thus does not seem to be a pre-requisite for achieving sub-1% energy resolutions at 662keV.