Spectral characteristics of small- and large-volume CdTe detectors: comparison among hemispheric, planar, and pixellated structure

Progress achieved in the quality of II-VI compounds over the last few years offers new possibilities in the development of large-volume radiation detectors. Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) are the semiconductor materials of great interest for room temperature application in X-ray and γ-ray detectors in a number of fields. The performance of CdTe (and CdZnTe) depends largely not only on the semiconductor material, but also on the contacts as well as the surface preparation procedures. In this paper, the comparison of spectral properties (⁵⁷Co and ¹³⁷Cs) among different types of detector, planar, hemispheric, and planar pixellated structure, is reported. The detectors under investigation were of 65- and 500-mm³ active volume with a sensitive thickness of 2.5 and 5 mm, respectively. The bias voltage applied varied from 100 to 300 V and from 100 to 1200 V, respectively, for small- (5 × 5 × 2.5 mm³) and large-volume (10 × 10 × 5 mm³) detectors. The lowest leakage current was observed for the planar pixellated detectors, and in the case of the hemispheric, it was more than twice lower than in the planar detectors. The large-volume planar detectors did not support high-voltage applications contrary to the hentispheric ones. The energy resolution of the photo-peak at 122 keV (⁵⁷Co) was less than 7%, and at 662 keV (¹³⁷Cs), it was less than 5% (typically 34%) for small- and large-volume hemispheric detectors. The peak-to-valley ratio (PN) varied only slightly with increasing voltage in the small planar detectors, while in the hemispheric detectors, this value was close to 12 for ⁵⁷Co and twice higher for ¹³⁷Cs gamma source. The photo-peak efficiency was three to four times higher in the hemispheric than in planar devices. The highest values of photo-peak intensity as well as photo-peak efficiency have been found for 4 × 4 matrix elements. The total spectrum efficiency and PN ratio values for the latter are situated between hemispheric and planar detectors.