Defects in Silver-Doped Mercuric Iodide Crystals and their Effect on X-Ray and Gamma-Ray Detector Performance
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DEFECTS IN SILVER-DOPED MERCURIC IODIDE CRYSTALS AND THEIR EFFECT ON X-RAY AND GAMMA-RAY DETECTOR PERFORMANCE R. B. JAMES*, X. J. BAO**, T. E. SCHLESINGER**, A. Y. CHENG***, AND V. M. GERRISH*** *Advanced Materials Research Department, Sandia National Laboratories, Livermore, CA 94550
**Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 ***EG&G Energy Measurements, Goleta, CA 93116 ABSTRACT
The processing steps associated with purification of source material, crystal growth, and attachment of electrical contacts can introduce defects into mercuric iodide (HgI2) that degrade the performance of detectors. We have employed low-temperature photoluminescence (PL) spectroscopy to study radiative recombination centers in the interfacial region between a thin semitransparent film of silver and mercuric iodide. The Ag film was found to introduce a new broad emission band centered at 5490 A in the photoluminescence spectrum of HgI2. This PL feature can be used as a signature to identify the existence of Ag as a contaminant in HgI2 crystals and detectors. Experiments were also conducted on mercuric iodide surfaces that had been doped with silver, and the results showed that Ag is a rapid diffuser in bulk HgI2. Detectors with silver electrodes were also fabricated and tested using an americium-241 gamma-ray source. Large increases in the leakage currents were observed for the Ag-doped HgI2 devices, indicated that Ag impurities are electrically active in HgI2. These measurements show that silver is unacceptable as an electrode material for mercuric iodide x-ray and gamma-ray detector applications. In addition, they reveal that caution must be taken during handling of mercuric iodide source material, crystals, and detectors to avoid contact with silver, silver compounds, or with any material that contains silver as a contaminant. INTRODUCTION
Mercuric iodide (HgI2) has several properties that make it desirable for use as an x-ray and gamma-ray spectrometer. [1-5] Both the Hg and I have high atomic masses, so the material has superior ability to attenuate energetic photons. In its red tetragonal from, HgI2 is photosensitive and the number of charge carriers produced in the crystal is proportional to the incident photon energy. HgI2 also has a high bulk resistivity (about 1013 fl-cm), which ensures a low dark current and allows for room temperature operation of detectors fabricated from the material. Although the potential of HgI2 for the manufacturing of state-of-the-art solid-state spectrometers has been clearly demonstrated [6], the production of consistently reliable sensors continues to present significant challenges related to defects in the as-grown crystals and defects introduced during detector processing. Most of the difficulties with crystal growth and processing of HgI2 for detector applications stem from the fact that the material is difficult to purify, is easily deformed, has a high vapor pressure at room temperature, and is chemically very reactive. The defects co
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