HgI2 detector with a virtual Frisch ring

We investigated the performance of the modification of a Frisch-ring device design applied to the bar shaped HgI2 detector. In this device, the ring electrode, separated from the detector surface by thin insulating layer, is extended to cove the entire area of side surfaces of the crystal, and connected to the cathode. To fabricate this device the side surfaces of 3 x 3 x 6 mm(3) HgI2 crystals were wrapped in a Cu foil separated from the crystal by a thin layer of a Teflon tape. These bar-shaped HgI2 detectors can be used in large-area high-efficiency detector arrays that are in demand for medical imaging and nuclear materials detection. We obtained an energy resolution of similar to 5% FWHM at 511 keV with our tested detectors. This is a very good result for the HgI2 detector of such thickness. However, this resolution is far away from the theoretical limit predicted for HgI2 material. This paper analyzes several factors affecting the energy resolution of this type of the virtual Frisch-grid devices, of which the fluctuation of charge loss due to electron trapping by local defects with high concentration of traps seems is a dominant factor.