Electronic Characterization of Mercuric Iodide Gamma Ray Spectrometers *
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ELECTRONIC CHARACTERIZATION OF MERCURIC IODIDE GAMMA RAY SPECTROMETERS*
VERNON M. GERRISH EG&G Energy Measurements, Inc., Santa Barbara Operations, Goleta, CA 93117
ABSTRACT During the past four years the yield of high-resolution mercuric iodide (HgI2) gamma ray spectrometers produced at EG&G/EM has increased dramatically. Data are presented which demonstrate a strong correlation between starting material and spectrometer performance. Improved spectrometer yields are attributed to the method of HgI 2 synthesis and to material purification procedures. Data are also presented which show that spectrometer performance is correlated with hole mobility-lifetime products. In addition, the measurement of Schottky barrier heights on HgI 2 spectrometers has been performed using I-V curves and the photoelectric method. Barrier heights near 1.10 eV have been obtained using various contacts and contact deposition methods. These data suggest the pinning of the Fermi level at midgap at the HgI 2 surface, probably due to surface states formed prior to contact deposition.
INTRODUCTION In recent years important advances have been made in the production of high-resolution HgI 2 spectrometers as a result of progress in the areas of material purification, crystal growth, and device fabrication. Methods have been developed for synthesizing high-purity mercuric iodide' which have led to increased yields of large volume, spectrometer grade detectors2 . Control of the crystal growth angle allows the production of crystals whose shape can be optimized for particular detector requirements. For example, crystals grown with a large C-face can be used for large area imaging arrays3 . In the area of detector fabrication, the long-term stability of HgI 2 detectors has been greatly improved due to advances in encapsulation methods4 . Also, there has been progress in developing suitable transparent electrical contacts for HgI 2 photodetectors. (HgI 2 photodetectors can be used as a replacement for photomultiplier tubes in scintillation spectrometers'.) Electronic characterization techniques have played a central role in the development of methods for improving the performance of HgI2 spectrometers. These techniques have also provided a better understanding of the physical mechanisms which effect detector response. For example, the correlation of starting materials and purification methods with detector performance and charge transport parameter measurements suggests that impurities greatly affect the electronic properties of HgI 2. Spectral response and hole lifetime measurements have allowed
"The submitted manuscript has been authored by a contractor of the U.S. Government under Contract No. DE-AC08-88NV10617. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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Z of Detectors with FWHM < 5% at 662 keV
1 - 4 mm Thickness 40
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