Real-time Imaging of the Electric field Distribution in CdZnTe at low temperature
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1164-L03-03
Real-time Imaging of the Electric field Distribution in CdZnTe at low temperature P.J. Sellin1, G. Prekas1, A. Lohstroh1, M. Ozsan1, V. Perumal1, M. Veale1,2, P. Seller2 1
Department of Physics, University of Surrey, Guildford GU2 7XH, UK STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK 2
ABSTRACT Real time imaging of the electric field distribution in CZT at low temperature has been carried out using the Pockels electro-optical effect. CZT detectors have been observed to show degraded spectroscopic resolution at low temperature due to so-called ‘polarization’ phenomena. By mounting a CZT device in a custom optical cryostat, we have used Pockels imaging to observe the distortion of the electric field distribution in the temperature range 240K - 300K. At 240K the electric field has a severely non-uniform depth distribution, with a high field region occupying ~10% of the depth of the device under the cathode electrode and a low field in the remainder of the device. Using an alpha particle source positioned inside the vacuum chamber we have performed simultaneous alpha particle transient current (TCT) measurements. At low temperatures the alpha particle current pulses become significantly shorter, consistent with the reduced electron drift time due to a non-uniform electric field. These data provide useful insights into the mechanisms which limit the spectroscopic performance of CZT devices at reduced temperature.
INTRODUCTION In this paper we present a study of the non-uniform electric field distribution in cadmium zinc telluride (CZT) radiation detectors at low temperatures. For many years CZT had been actively developed for use in high resolution portable gamma ray and X-ray detectors, and it has a number of desirable properties including good quantum efficiency for gamma rays and X-rays, good charge transport properties, and the ability to operate at, or close to, room temperature [13]. However various groups have reported stability issues in CZT, such as the buildup of a nonuniform electric field when devices are operated under irradiation with high X-ray fluence. Such ‘polarization’ phenomena are believed to be associated with the accumulation of trapped space charge in the detector active volume which produces an internal electric field inside the detector. This internal field tends to oppose the externally applied field. Polarization effects generally cause a deterioration of the spectroscopic performance of the detector over time, with a shift of photopeaks to lower energy and a widening of peak widths [4-6]. The spectroscopic performance of CZT is known to improve with moderate cooling of the detector (eg. to 0 ºC) due to the reduction in thermal leakage current in the device. It can therefore be advantageous to mount portable CZT spectrometers onto miniature solid-state coolers such as Peltier devices. However lowering the temperature of a compensated semiconductor such as CZT can cause more complex effects due to the presence of deep states and
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