Processing and Characterization of HgBr x I 2-x Radiation Detectors
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PROCESSING AND CHARACTERIZATION OF HgBrxl 2.x RADIATION DETECTORS C. ZHOU, M. R. SQUILLANTE, L. P. MOY, P. BENNETT Radiation Monitoring Devices, Inc., 44 Hunt St., Watertown, MA 02172
ABSTRACT This paper reports our recent work on the crystal processing, structural and optical characterization of HgBrxI2_x nuclear radiation detectors. To understand the electrical and optical properties of the detectors, we measured the energy gap of HgBrxI2_x as a function of the Br/I ratio. The energy band of this ternary semiconductor compound can be modulated from 2.1eV (HgI 2 ) to 3.4eV (HgBr 2 ) by adjusting its chemical composition. This energy scope covers a wavelength spectrum between 365nm and 596nm, much of the visible spectrum. Nuclear and photoconductive detectors were fabricated from HgBrxI2_x single crystals and the responses of these devices were investigated with different radiation sources (24 1 Am, 13 7 Cs).
INTRODUCTION Semiconductor detectors (both nuclear radiation and photoconductive detectors) require an appropriate bandgap (Eg - 2 eV), a high electrical resistivity and high mobility - lifetime product (ptr) of minor carriers. These properties are necessary for a large quantum yield and large signal to noise ratio. In addition, a nuclear radiation detector must have a high stopping power, which suggests high density and high atomic numbers of elements for the semiconductor device. HgI2 , meeting all the necessary conditions described above, has been a leading contender as a radiation detector material for about two decades. Although it has been recently used in several commercial products, HgI 2 has deficiencies such as weak mechanical strength, chemical instability and a disagreeable allotropic transformation. Desire to overcome and circumvent these problems leads directly to research on similar compounds while maintaining semiconducting properties. One way to achieve this is to add another element to the compound, which is in the same group as one of the constituent elements. For example, both Br and I belong to VIIB group, addition of 2t 20 mole%HgBr 2 to HgI 2 can stabilize HgI 2 crystals and sidestep the solid state transformation at 127 0C as shown in the HgBr2 - HgI 2 phase diagram [1](Figure 1.) This chemical modification increases the physical strength and chemical stability while it circumvents the allotropic transformation. The band gap of HgBrxI 2.x can be modulated from 2.1 eV (HgI2) to 3.4 eV (HgBr 2 ) by varying the chemical composition. Since all semiconductor materials show maximum photo response near X __hc/Eg, the spectral response of HgBrxI 2. photodetectors can be tuned (by adjusting the composition) to match almost all common inorganic scintillators.
PURIFICATION AND CRYSTAL GROWTH HgI2 and HgBr2 powders were purchased from AESAR (99.9995%). These starting materials were purified at a greater degree by using a multiple step sublimation process [2]. HgI2 and HgBr 2 were loaded in quartz ampoules, respectively, (14mm ID, 19mm OD) and sealed under vacuum. The ampoules were then placed in
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