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 (6τ) 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 I project is to design 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
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Radiation Monitoring Devices, Inc.
44 Hunt Street Watertown, MA 02472-4699
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