Simulation studies of CZT Detectors as Gamma-Ray Calorimeter

Several astrophysics and nuclear physics applications require the detection of photons in the energy range of keV up to several MeV with good position and energy resolution. For certain applications Cadmium Zinc Telluride (CZT) detectors might be the detector option of choice. Up to now, CZT detectors have mainly been used in the energy range between a few keV and ∼1 MeV. They operate at room temperature and achieve excellent position resolution and substantially better energy resolution than scintillation detectors. Furthermore, they can be built more compact and more economically than Ge detectors and do not require cryogenic cooling. In this paper, we describe the results of 3-D Monte Carlo simulations of a “CZT calorimeter” that can be used to detect photons in the keV to several MeV range. The main objective of these studies is to evaluate the feasibility of CZT calorimeters, to study their performance and detect and understand performance limiting factors. Such a calorimeter consists of many layers of closely packed pixellated CZT detector units. Our simulations of single detector units reproduce experimental results, indicating that our simulations capture the main factors that limit the performance of a detector unit. For a full calorimeter the limiting factors within a range from∼20 keV to∼10 MeV are: a) the fact, that the incident energy is not totally deposited within the detector area because secondary particles leave the detector against the direction from which the incident radiation enters, b) signal loss when the interaction is near the pixel edges and near the anodes. In this case signals which are induced in neighboring pixels are discarded when their intensities lie below the trigger threshold. c) the steep weighting potential gradient close to the anodes, which affects about 0.25 cm next to the anode and impairs there the correction of the depth of interaction (DOI). This effect dominates in thin detectors (0.5 cm). Understanding the limiting factors we come to the conclusion that 1 cm to 1.5 cm thick detector units can be used to build a calorimeter with good performance over the energy range from ∼20 keV to ∼10 MeV .