Large Volume HgI2 Gamma-Ray Spectrometers

This paper demonstrates the enhanced capability of single polarity charge sensing, the 3-dimensional position sensing technique, developed at the University of Michigan and previously successfully demonstrated on CdZnTe detectors, to improve the spectroscopic performance of HgI2 and to extend its range for spectrometry to an unprecedented thickness of 10 mm. Energy resolutions of close to 1% FWHM at 662 keV gamma-ray energy were obtained from individual depth locations underneath pixel anodes, and 1.4-2.0% FWHM energy resolutions from 5 out of 6 tested pixel anodes on two 10 mm thick detectors. Introduction Hgl2 has properties of high atomic number (Z=80-53), high density (p= 6.3 g/cm), wide band-gap (2.13 eV) and high bulk resistance (10-10 Q). These properties make it a very attractive material for efficient gamma-ray detectors capable of roomtemperature operation. However, problems of charge trapping, material nonuniformity and temporary change of its properties result in poor spectral performance and limited thickness (not more than 3 mm) of a conventional detector using planarelectrodes. The 3-dimensional position-sensitive single-polarity charge sensing technique developed at the University of Michigan [1] should be able to eliminate the problem of trapping of holes, to correct for the trapping of electrons, and to mitigate the material non-uniformity to the scale of the position resolution (-1 mm in 3-dimensions) of the detector system. Figure 1 illustrates the principle of the 3-dimensional position sensing technique. A two dimensional array of anode pixels is fabricated on the anode surface and all pixels are biased at the same voltage potential. A large number of electron-hole pairs proportional to gamma-ray energy deposition are generated from the gamma-ray interaction. The electrons move towards the anode and are collected by one of the pixel anode directly located above the location of gamma-ray interaction. The induced signal E' on the pixel anode is dominated by the number of electrons collected, and has a slight dependence on the depth of interaction as shown in the left-bottom of Figure 1. The lateral position (*, y) of the gamma-ray interaction is identified from the location of the pixel anode, and the depth (z) of interaction can be obtained from the ratio of the cathode signal to the signal of the pixel anode [2]. The actual energy deposition EQ can then be deduced from the signal of the pixel anode E' and the depth (z) of interaction. CP632, Unattended Radiation Sensor Systems for Remote Applications, edited by J. I. Trombka et al. © 2002 American Institute of Physics 0-7354-0087-3/02/$ 19.00 113 Because of the promising results obtained on 5 mm thick HgI2 spectrometers [3,4], the 3-dimensional position-sensitive single polarity charge sensing technique has been applied to 10 mm thick detectors. Several prototype detectors have been fabricated by Constellation Technology Corporation [5], each detector has four pixel anodes with dimensions about 1x1 mm surrounded by a large anode [3]. Signals from each pixel anode and the cathode are readout using Amptek A250 preamplifiers and shaped using standard Canberra 243 amplifiers. A sample prototype HgI2 detector with dimensions of Ixlxl cm is shown in Figure 2. nnnnnnnnnna nnannaaanaa nnnnnnnnnna nnnnnnminnn nnnnnnnnnna nnnnnnnnnna nnnnnnnnnna nnnnnnnnnna nnnnnnnnnnn Cathode From pixel #i: (E', x, y) Depth z = C/A = C/E' (E0,x,y,z) Anode FIGURE 1. Illustration of the 3-dimensional position-sensitive technique. FIGURE 2. A sample 3-dimensional position-sensitive prototype HgI2 gamma-ray spectrometer. Detector performance Six pixel anodes, three on each 10 mm thick HgI2 detector, were tested. Energy spectra of 662 keV gamma rays obtained at three different interaction depths from