Multi-element, large-volume CdZnTe detectors

Many applications using gamma rays, such as the determination of the elemental composition of planetary bodies, require the detection of gamma rays in the energy range 100 keV–10 MeV in a light, compact, room-temperature system. The photopeak efficiency of a single CdZnTe detector is limited because the volume currently available is limited to about 3.4 cm3. To achieve adequate efficiency, we have combined the signals from several large-volume, coplanar-grid CdZnTe detectors. At low energy, the signals are summed without requiring coincidence; at high energy, multi-fold coincidence is required to suppress the Compton continuum. To minimize noise, a signal from a single detector is only included in the sum if it exceeds a threshold set just above the noise. We describe the electronics, the single detectors, their packaging in thin-walled boxes, and the resulting spectra. The energy resolution for the combined spectrum is only slightly worse than the resolution of the single detectors. The effectiveness of the Compton suppression depends on the spectrum. The photopeak efficiency of the multi-element detector when the signals are summed without requiring coincidence is approximately the sum of the photopeak efficiencies of the single detectors, whereas the photopeak efficiency with Compton suppression is a more complicated function of the incident gamma-ray energy. These results indicate that a 27-element (3×3×3) array of 15 mm×15 mm×15 mm single detector elements can provide good gamma-ray data for planetary bodies and terrestrial applications requiring high-efficiency, good-resolution portable instruments.