TlBr Spectrometers with Improved Long Term Stability at Room Temperature

The 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 requirements and low cost. In the proposed effort, we plan to continue our development of thallium bromide (TlBr), 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/cm3), 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 TlBr crystals. The cubic crystal structure of TlBr also simplifies crystal growth and device processing. As a result of recent progress in purification, crystal growth and processing, TlBr 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 TlBr gamma-ray spectrometers with thickness exceeding 1 cm. TlBr 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 TlBr detector arrays, modest cooling (to ~ - 20 C) has been required. We have demonstrated stable TlBr detector performance exceeding 9 months with the detector continuously biased and operated at – 18 C. This level of cooling is easily achieved with a thermoelectric cooler. Cooling however, does increase the power budget of a detector system. In addition to cooling as a method to obtain long term TlBr detector stability, research at RMD and elsewhere has shown that surface processing, electrode materials and thermal annealing significantly influence the long term stability of TlBr detectors operated at room temperature. During Phase I RMD has demonstrated 5 mm thick TlBr detectors with long term stability exceeding 90 days at room temperature. It is our goal in Phase II to further investigate the effects of surface processing, electrodes and annealing on long term stability of TlBr 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 TlBr spectrometers that are stable for more than 1 year at room temperature. Such an efficient, high resolution detector will find applications in nuclear monitoring areas such as nuclear treaty verification, safeguards, 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