Modeling of CZT Response to Gamma Photons Using MCNP and Garfield
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Modeling of CZT Response to Gamma Photons Using MCNP and Garfield Jonathan Lassiter1, Randy Robinson1, Latressa Williams1, Stephen Babalola1, Claudiu Muntele2 1 2
Alabama A&M University, Normal, AL Cygnus Scientific Services, Huntsville, AL
ABSTRACT CZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, technological difficulties in production of large size defectfree CZT crystals are yet to be overcome. The most common problem is accumulation of tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. The common work-around solutions are to fabricate pixelated detectors by either grouping together many small volume CZT crystals to act as individual detectors, or to deposit a pixelated grid of electrical contacts on a larger, but defective, crystal, and selectively collect charge. These solutions are satisfactory in an R&D environment, but are unsuitable for mass production and commercial development. Our modeling effort is aimed at quantifying the various contributions of tellurium inclusions in CZT crystals to the charge generation, transport, and collection, as a function of inclusions size, position, and concentration. We model the energy deposition of gamma photons in the sensitive volume of the detector using LANL’s MCNP code. The electron-hole pairs produced at the energy deposition sites are then transported through the defective crystal and collected as integral charge at the electrical contact sites using CERN’s Garfield software package. The size and position distribution of tellurium inclusions is modeled by sampling experimentally measured distributions of such inclusions on a variety of commercially-grown CZT crystals using IR microscopy and image processing software packages. INTRODUCTION Cadmium Zinc Telluride (CZT) is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors [1]. However, to date, technological difficulties in production of large size defect-free CZT crystals are yet to be overcome [2]. The most common problem is accummulation of Tellurium precipitates as microscopic inclusions. These inclusions have been experimentally demonstrated to influence the charge collection through charge trapping and electric field distortion [3, 4, 5]. The common work-around solution is to fabricate pixelated detectors by either grouping together many small volume CZT individual detectors, or by depositing a pixelated grid of electrical connects on a larger, but defective, crystal and selectively collect charge [4, 6]. These solutions work well in an research and development environment, but are unsuitable for mass production and commercial development. Our modeling effort is aimed at quantifying the various contributions of tellurium inclusions in CZT crystals to the charge generation, transport, and collection, as a function of inclusions size, position, and concentration. The software packag
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