Evaluation of Polarization Effects of e-Collection Schottky CdTe Medipix3RX Hybrid Pixel Detector

Abstract

Abstract-This paper focuses on the evaluation of operational conditions such as temperature, exposure time and flux on the polarization of a Schottky electron collection CdTe detector. A Schottky e − collection CdTe Medipix3RX hybrid pixel detector was developed as a part of the CALIPSO-HIZPAD2 EU project. The 128 x 128 pixel matrix and 0.75 mm thick CdTe sensor bumpbonded to Medipix3RX readout chips enabled the study of the polarization effects. Single and quad module Medipix3RX chips were used which had 128 x 128 and 256 x 256 pixel matrices, respectively. This study reports the sensor-level and pixel-level polarization effects of the detector obtained from a laboratory Xray source. We report that the sensor-level polarization is highly dependent on temperature, flux and exposure time. Furthermore, the study of pixel-level polarization effects led to identification of a new type of pixel behaviour that is characterised by three distinct phases and, thus, named "tri-phase" (3-P) pixels. The 3-P pixels were the dominant cause of degradation of the flat-field image uniformity under high flux operation. A new method of identifying the optimum operational conditions that utilises a criterion related to the 3-P pixels is proposed. A generated optimum operational conditions chart under the new method is reported. The criterion is used for bias voltage reset depolarization of the detector. The method successfully represented the dependency of polarization on temperature, flux and exposure time and was reproducible for multiple sensors. Operating the detector under the 3-P pixel criterion resulted in the total efficiency not falling below 95%.

Characterization of the Uniformity of High-Flux CdZnTe Material

“…Temporal variations in the performance of CdTe and CdZnTe detectors have commonly been reported. These phenomena are typically caused by flux-induced [ 2 , 32 ] and bias-induced polarization [ 33 , 34 ] in which a buildup of space charge, caused by the trapping of carriers, leads to a progressive modification of the electric field. These changes in electric field result in a reduction in both the counting efficiency and the spectroscopic performance of detectors.…”

“…The presence of bias-induced polarization is normally addressed through the use of a bias refresh scheme, where the detector bias is periodically set to 0 V for a short time of the order of a second in order to allow the space charge to recombine. The period of bias refreshing can be on the order of minutes or hours depending on the detector conditions [ 16 , 33 , 34 , 35 ]. In the case of flux-induced polarization, the large number of photo-generated carriers results in excessive numbers of trapped holes that form the space charge within the sensor.…”

Characterization of Seamless CdTe Photon Counting X-Ray Detector

“…A CdTe detector under operation is subject to the polarization effect. This effect depends on the amplitude of bias voltage, but also the temperature and total time that bias applied [34]- [36]. A brief study of the polarization effect is presented in section IV.…”

Observation of radiation damage in CdTe Schottky sensors created by 20 keV photons

“…Depending on the temperature of the sensor, the normalized number of photons hits decreases, globally over the pixel array, over time. This is a known effect which is commonly attributed to bias induced polarization in CdTe Schottky sensors [25]. As can be seen in the normalized photon hit maps, this global decrease has strong local variability.…”