High Efficiency TlBr Gamma-Ray Detector Module

The ideal semiconductor detector for nuclear monitoring should have good energy resolution, high detection efficiency, compact size, light weight, easy portability and low cost. In the proposed effort, we plan to develop a detector module for nuclear monitoring based on thallium bromide (TlBr), a wide band gap semiconductor that recently has shown great promise as a gamma-ray detector material. TlBr has a number of very promising properties. It has high density (7.5 g/cm3) and high atomic number constituents (81, 35), which promises high sensitivity. The electrical resistivity of the material is high (>1010 -cm) without deep level doping. Furthermore, 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 such as Bridgman and Czochralski 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 (μ) 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. In fact, TlBr detectors fabricated at RMD have exhibited < 1 % energy resolution (FWHM) at 662 keV upon depth correction. These detectors were cooled to -20C to achieve stable operation. The goal of this Phase II project is to build a cooled, compact TlBr gamma-ray detector module using the 3-dimensional position-sensitive readout technology pioneered by the group at the University of Michigan. The key advancement is to develop a lower power charge sensing ASIC that can digitally sample the outputs of an array of preamplifiers. By sampling the preamplifier outputs, the induced charges on the detector electrodes can be obtained as a function of time, so that digital signal processing can be used to perform gamma-ray spectroscopy, to determine the depth of interaction of individual gamma-ray energy depositions, as well as to measure charge drift time, electric field distribution within TlBr and lifetimes of electrons and holes. The digital ASIC readout system will enable both fundamental research on TlBr detectors and practical operation to perform gamma-ray imaging and spectroscopy outside the laboratory. Since TlBr detectors can operate in a stable manner at 20C, power consumption of the digital ASIC system should be minimized so that the system can be cooled to required temperature using Peltier coolers. Such an efficient, high resolution, 3-D position sensitive detector module will find application 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  -Lower cost than CZT-based system due to lower cost crystal growth