TIBr Spectrometers with Improved Long Term Stability at Room Temperature
Department of Homeland Security
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Radiation Monitoring Devices, Inc.
44 Hunt Street, Watertown, MA, 02472-4699
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AbstractThe ideal semiconductor detector for the nuclear non-proliferation application should have good energy resolution, high detection efficiency, compact size, light weight, easy portability, low power requirments and low cost. In the proposed effort, we plan to continue our development of thallium bromide (TIBr), a wide band gap semiconductor that recently has shown great promise as a gamma-ray detector material. In addition to high density (7.5 g/cm2), high atomic number constituents (81,35) and wide band gap (2.68 eV) the material melts congruently at a modest temperature (480°C) and does not undergo a phase change as the crystal cools to room temperature, which allows use of melt-based crystal growth approaches to produce large volume TIBr crystals. The cubic crystal structure of TIBr also simplifies crystal growth and device processing. As a result of recent progress in purification, crystal growth and processing, TIBr detectors with mobility-lifetime products of mid 10^-3 cm2/V for electrons and mid 10^-4 cm2/V for holes has been achieved. This has enabled the development of TIBr gamma-ray spectrometers with thickness exceeding 1 cm. TIBr detectors fabricated in our lab have exhibited < 1% energy resolution (FWHM) at 662 keV with cooling and depth correction. To date, to obtain excellent long term performance of thick TIBr detector arrays, modest cooling (to ~-20 C) has been requied. We have demonstrated stable TIBr detector performance exceeding 9 months with the detector continuously biased and operated at ~18°C. This level of cooling is easily achieved with thermoelectric cooler. Cooling however, does increase the power budget of a detector system. In addition to cooling as a method to obtain long term TIBr detector stability, research at RMD and elsewhere has shown that surgace processing, electrode materials and thermal annealing significantly influence the long term stability of TIBr detectors operated at room temperature. RMD and its affiliated research teams have demonstrated thin TIBr detectors with long term stability exceeing 50 days at room temperature. It is our goal in this program to further investigate the effects of surface processing, electrodes and annealing on long term stability of TIBr detectors operated at room temperature. In addition, doping will be investigated as a method for modifying ionic conductivity. Dr. Harry Tuller's group at the materials science department of MIT will collaborate with RMD on this aspect of the project. Ultimately our goal is to develop TIBr spectrometers that are stable for more than 1 year at room temperature. Such as efficient, high resolution detector will find applications in nuclear monitoring areas such as nuclear treaty verificiation, dafeguards, environmental monitoring, nuclear waste cleanup, and border security. Nuclear and particle physics as well as astrophysics are other fields of science were gamma-ray spectrometers are used. The developed detectors should have the following advantages: · Efficient detection of gamma-rays (better than CZT per unit volume ·Energy resolution < 1% (FWHM) at 662 keV at room temperature · Lower cost than CZT-based system due to lower cost crystal growth
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