Gas filled prototype of a CdZnTe pixel detector
CdZnTe pixel structures are currently the most promising detectors for the focal planes of hard X-ray telescopes, for astronomical observation in the range 5–100 keV. In Sharma et al. (Proc. SPIE 3765 (1999) 822) and Ramsey et al. (Nucl. Instrum. Methods A 458 (2001) 55) we presented preliminary results on the development of prototype 4×4 CdZnTe imaging detectors operated under vacuum. These pixel detectors were installed inside vacuum chambers on three-stage Peltier coolers providing detector temperatures down to −40°C. A miniature sputter ion pump inside each chamber maintained the necessary vacuum of 10−5 Torr. At a temperature of −20°C we achieved an FWHM energy resolution of between 2% and 3% at 60 keV and ≈15% at 5.9 keV; however, the dependency on temperature was weak and at +20°C the respective resolutions were 3% and 20%. As the detectors could be operated at room temperature without loss of their good characteristics it was possible to exclude the sputter ion pump and fill the chamber with dry nitrogen instead. We have tested a nitrogen-filled CdZnTe (5×5×1 mm3) prototype having 0.65×0.65 mm2 readout pads on a 0.75 mm pitch. The interpixel resistance at an applied voltage of 10 V was higher than 50 GΩ and the pixel leakage currents at room temperature with a bias of 200 V between each pad and the common electrode did not exceed 0.8 nA. The pixel detector inside the microassembly, which also contained the input stages of the preamplifiers, was installed on a Peltier cooler to maintain the detector temperature at +20°C. To define real leakage currents of the pixels in their switched-on state we have checked the voltage on the preamplifiers feedback resistors. The resulting currents were 10–50 pA at a detector bias of 500 V. Under test, the typical energy resolution per pixel at +20°C was ≈3% at energy 59.6 keV and ≈20% at energy 5.9 keV, which are similar to the values obtained in the vacuum prototype at room temperature (Nucl. Instrum. Methods A 458 (2001) 55). Using the three stage Peltier cooler it is possible to cool the pixel detector in the nitrogen atmosphere to 0°C which would slightly improve these spectrometric characteristics. Thus the conclusion here is that the use of a nitrogen environment instead of a vacuum one did not cause any deterioration of the characteristics of the CdZnTe imaging detector.
Source: Radiation Physics and Chemistry