| Mercuric iodide (HgI2) detectors were studied as potential gamma-ray spectrometers that can operate at room temperature or above without the need for cooling. Similar to other compound semiconductor materials, HgI 2 suffers from poor hole molibity and severe hole trapping. Thus, poor spectroscopic results will occur if this detector material used with conventional planar electrodes. The use of single polarity charge sensing, whereby the energy deposited by the gamma-ray is determined solely by electron movement and collection, was attempted by incorporating pixelated anodes. The results showed remarkable spectroscopic improvement over conventional electrodes, with less than 2% energy resolution from single pixels on a 1 cm thick detector using a 662 keV Cs-137 gamma-ray source. The conventional planar cathode showed no photopeak due to the lack of hole movement in HgI2. Since the cathode signal has a relatively linear dependence on the depth of interaction, both the anode pixel and cathode signals were used to measure the depth of interaction. This was then used to correct for electron trapping and variations of the induced charge with the depth of interaction. Depth correction further improved the resolution for single pixels to as low as 1.4% (∼9 keV) energy resolution. Nearly three-quarters of all pixels tested on 1 cm thick detectors had resolutions less than 2% after depth correction was performed. Pixelated HgI2 detectors were also used to study the charge transport properties of the material. While the measured hole transportation properties were similar to previously measured values, measured (mutau)e values were on the order of 10-3 cm2/V, about an order of magnitude better than previously measured values. This demonstrates that the material quality may have improved over the past several years. The pixelated HgI2 detectors showed very little polarization, and remained stable over several months of operation. Continued development of these detectors will allow them to compete with current CdZnTe detectors, possibly improving on detection efficiency due to the higher atomic number of HgI2. |
