IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 50, NO. 4, AUGUST 2003 1031 Temporal Response of CZT Detectors Under Intense Irradiation Yanfeng Du, James LeBlanc, George E. Possin, Brian D. Yanoff, and Snezana Bogdanovich Abstract—The temporal response of CdZnTe (CZT) detectors is measured under different X-ray flux, spectra, and detector bias conditions. A comprehensive model has been developed to investigate the detector response under these conditions. The calculations have been compared with our measured results. Reasonable qualitative agreement is shown between the model and measurement results. This model provides a powerful tool to understand the detector temporal response, photocurrent de- pendence on the irradiation intensity, bias voltage, and defect characteristics. Understanding the detector response from a mi- croscopic level can provide a guide to improve material properties and detector device design. Index Terms—CdZnTe detector, medical imaging, polarization, temporal response. I. INTRODUCTION W IDEBAND compound semiconductors, such as CdZnTe (CZT) and CdTe, are attractive room temperature radi- ation detectors due to their high sensitivity and X-ray stopping power. High densities of impurities and defects that lead to se- vere charge carrier trapping can be major issues affecting their performance. Single polarity charge sensing, such as coplanar grid or pixellated detector geometry, is an effective approach to overcoming the hole trapping problem. Better than 2% energy resolution at 662 keV has been achieved with large volume CZT detectors [1], making it feasible to use CZT detectors for many spectroscopic and low flux applications. For some medical or industrial imaging applications, the typ- ical X-ray flux could be as high as 200–2000 MHz/mm , while the detector pixel area has to be around 1 mm to minimize the charge sharing. For these applications, it is very challenge to operate the detector in pulse mode. Instead, for these high flux applications, the detector can be operated in charge integration mode or current mode. Some of the techniques used to overcome charge trapping problems cannot be applied in charge integra- tion mode. Furthermore, under high irradiation conditions, the radiation induced polarization effect caused by the build up of trapped charges can also affect the detector stability [2], [3]. In this paper, the temporal response of CZT detectors is measured under varying levels of irradiation. A comprehensive model is developed to investigate the physical mechanisms that could affect the detector response. Manuscript received November 8, 2002; revised May 5, 2003. The authors are with the GE Global Research Center, Niskayuna, NY 12309 USA (e-mail: duy@crd.ge.com). Digital Object Identifier 10.1109/TNS.2003.815348 Fig. 1. Temporal response for a 3 3 pixellated CZT detector. The detector is 3 mm thick and each pixel is 0.7 mm 0.7 mm in area. Bias voltage of 200 V is applied to the cathode electrode. The exposure time at 60 kVp, 160 mA X-ray is 2 s. II. MEASUREMENT OF CZT DETECTOR RESPONSE The temporal response of several CZT pixellated detectors from eV products and Imarad has been measured under irra- diation characteristic of medical imaging: 100–640 mA and 60–120 kVp. All the samples have 3 3 pixels surrounded by a guard ring on the anode surface. The Imarad detectors have two configurations: 2.46 mm 2.46 mm in pixel area, 5 mm thick or 1 mm 1 mm in pixel area and 3 mm thick, with In/In or In/Pt contacts. The eV detector pixel area is 0.5 mm 0.5 mm on a 0.6 mm pitch, 5 mm thick, with Pt/Pt contacts. Although these detectors have different contact materials and are manufactured using different crystal growth processes, the behavior under intense irradiation has similar unstable temporal response, represented by a sharp peak at the beginning of the exposure followed by decay to a steady state at a much lower level. The fall off time characteristics, peak magnitude, and steady-state photocurrent show large detector to detector and pixel to pixel variations. Fig. 1 shows the temporal response for one corner pixel of a 3 3 pixellated Imarad CZT detector, with top and bottom con- tacts consisting of indium (In/In contacts). The detector pixel area is 0.7 mm 0.7 mm on a 1 mm pitch. The thickness is 3 mm. A bias voltage of negative 200 V was applied to the common cathode of the detector using a Keithley 6517A elec- trometer. The anode pixel current was amplified and converted to voltage output using a Keithley 428 current amplifier. Its 0018-9499/03$17.00 © 2003 IEEE