Spectroscopic evaluation of n-type CdZnTe gamma-ray spectrometers

Abstract

This work focuses on the evaluation of the spectroscopic performance of n-type CdZnTe gamma-ray spectrometers, grown by a modified horizontal Bridgman Technique developed at IMARAD Imaging Systems Ltd. Two types of devices are studied: (i) detector arrays grown and produced by IMARAD and employing ohmic indium contacts and (ii) detectors and arrays fabricated at Technion in crystals provided by IMARAD, employing different types of contacts. Alpha particle spectroscopy as well as gamma-ray spectroscopy is used to evaluate and characterize the energy resolution of gamma-ray spectrometers fabricated on n-type CdZnTe grown by a modified horizontal Bridgman and doped with indium. The electron and hole mobility lifetime products of the n-type CdZnTe material grown by IMARAD are estimated by measuring the dependence of charge collection efficiency upon the bias voltage, using a calibrated multichannel analyzer. The measured results indicate that the average electron and hole mobility-lifetime products are, respectively, of the order of μnτn=(1–2)·10−3 cm2/V and μpτp=6·10−6 cm2/V. The measured energy resolution of 122 keV photons is −(5–6)% when the source is not collimated and is reduced to −4.5% when the source is collimated. These results are obtained with ohmic cathode as well as with a rectifying cathode. A statistical model for the calculation of the pulse height spectra as a function of photon energy, electron and hole mobility-lifetime products and applied electric field, which has been recently reported in Applied Physical Letters, is used to determine the role of incomplete charge collection in the spectral performance of the n-type CdZnTe spectrometers. The comparison between the measured and modeled results indicates that the dark noise, cross talk and non-uniformity are the main limiting factors of the spectral performance of the n-type spectrometers rather than incomplete charge collection. The good spectroscopic performance of the arrays under study is attributed to an adequate hole mobility lifetime for the geometry of the pixilated arrays. The study indicates that the n-type CdZnTe spectrometers are useful for a wide range of imaging applications.