Pulse filtering for thick mercuric iodide detectors

Mercuric iodide (HgI/sub 2/) is a semiconductor material of interest as a gamma-ray spectrometer due to its large band gap and high atomic number, properties which allow room temperature operation and provide high gamma-ray cross section. At EG and G Energy Measurements, Inc. (EG and G/EM), single crystals of mercuric iodide are grown by vapor sublimation and fabricated into detectors of 0.2-12.0 mm thickness. Thin detectors (< 1 mm) produce a good energy resolution (--1%-5% at 662 keV) and work well a x-ray detectors but have low gamma-ray efficiency. Detectors of 1 mm-1 cm thickness provide higher gamma ray efficiency but are limited by poor hole transport properties which degrade detector performance. When conventional pulse shaping is used, energy resolutions of 6%-12% and peak-to-valley ratio < 3:1 are typical for detectors of 1-5 mm thickness. Detectors of -- 1-cm thickness typically produce > 10% resolution and peak-to-valley ratios < 2:1. The authors have investigated a number of electronic pulse filtering techniques for thick mercuric iodide detectors and have achieved improved spectral response for detectors of 1-5 mm thickness and gamma ray energies up to 1.5 MeV. Conventional gaussian and gated integration filters were considered as well as more sophisticated filter combinations in order to develop methods of reducing charge measurement errors caused by poor hole collection. When using these techniques, energy resolutions of 3%-6% and peak-to-valley ratios of 5:1 are common. In this paper they discuss these approaches and present results with thick detectors