Elpasolite scintillators show great promise for dual mode neutron-gamma radiation detection and imaging but the impact of high dose radiation on performance characteristics has not been evaluated. Crystalline (Cs,Tl)2LiLaBr6:Ce (CTLLB) materials are grown. After growth the 1-inch diameter crystals are processed and packaged. Prior to the irradiation sequence at SRNL, the detector performance of the encapsulated crystal is measured with gamma-ray check sources including 137Cs and 152Eu. Properties such as energy resolution, light yield, proportionality behavior, and decay times are measured. The packaged crystal is irradiated four times at dose rates of 1, 10, 100 and 1,000 Gy of absorbed dose in a 60Co irradiation and the performance of the crystal is evaluated before and after each radiation treatment. Cumulative dose rates between irradiation were 1, 11, 111 and 1,111 Gy. Initial results show a decreased measured light yield after each radiation exposure, however further analysis on the packaging materials will be pursued.
Cadmium Zine Telluride (CZT) has been extensively studied as a room temperature semiconductor gamma
radiation detector. CZT continues to show promise as a bulk and pixelated gamma spectrometer with less than one
percent energy resolution; however the fabrication costs are high. Improved yields of high quality, large CZT
spectroscopy grade crystals must be achieved. CZT is grown by the Traveling Heater Method (THM) with a Te
overpressure to account for vaporization losses. This procedure creates Te rich zones. During growth, boules will often
cleave limiting the number of harvestable crystals. As a result, crystal growth parameter optimization was evaluated by
modeling the heat flow within the system. Interestingly, Cadmium Telluride (CdTe) is used as a thermal conductivity
surrogate in the absence of a thorough study of the CZT thermal properties. The current study has measured the thermal
conductivity of CZT pressed powders with varying Te concentrations from 50-100% over 25-800°C to understand the
variation in this parameter from CdTe. Cd0.9Zn0.1Te1.0 is the base CZT (designated 50%). CZT exhibits a thermal
conductivity of nearly 1 W/mK, an order of magnitude greater than CdTe. Further, the thermal conductivity decreased
with increasing Te concentration.
We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode
signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel
electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and
high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device.
Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and
expound on their relation to our material-characterization data.
We studied two separate as-grown CdMnTe crystals by Infrared (IR) microscopy
and Pockels effect imaging, and then developed an algorithm to analyze and
visualize the electric field within the crystals' bulk. In one of the two crystals the
size and distribution of inclusions within the bulk promised to be more favorable in
terms of efficiency as a detector crystal. However, the Te inclusions were arranged
in characteristic 'planes'. Pockels imaging revealed an accumulation of charges in
the region of these planes. We demonstrated that the planes induced stress within
the bulk of the crystal that accumulated charges, thereby causing non-uniformity of
the internal electric field and degrading the detector's performance.
Generally, mechanical polishing is performed to diminish the cutting damage followed by chemical etching to
remove the remaining damage on crystal surfaces. In this paper, we detail the findings from our study of the effects of
various chemical treatments on the roughness of crystal surfaces. We prepared several CdZnTe (CZT) and CdMnTe
(CMT) crystals by mechanical polishing with 5 μm and/or lower grits of Al2O3 abrasive papers including final polishing
with 0.05-μm particle size alumina powder and then etched them for different periods with a 2%, 5% Bromine-Methanol
(B-M) solution, and also with an E-solution (HNO3:H20:K2Cr2O7). The material removal rate (etching rate) from the
crystals was found to be 10 μm, 30 μm, and 15 μm per minute, respectively. The roughness of the resulting surfaces was
determined by the Atomic Force Microscopy (AFM) to identify the most efficient surface processing method by
combining mechanical and chemical polishing.
In the past, various virtual Frisch-grid designs have been proposed for cadmium zinc telluride (CZT) and other
compound semiconductor detectors. These include three-terminal, semi-spherical, CAPture, Frisch-ring, capacitive
Frisch-grid and pixel devices (along with their modifications). Among them, the Frisch-grid design employing a non-contacting
ring extended over the entire side surfaces of parallelepiped-shaped CZT crystals is the most promising. The
defect-free parallelepiped-shaped crystals with typical dimensions of 5x5x12 mm3 are easy to produce and can be
arranged into large arrays used for imaging and gamma-ray spectroscopy. In this paper, we report on further advances
of the virtual Frisch-grid detector design for the parallelepiped-shaped CZT crystals. Both the experimental testing and
modelling results are described.
The performance of current long-drift-length Cadmium Zinc Telluride (CZT) detectors principally is determined by
the material's quality. Hence, the material's limitations must be better understood and potential solutions identified to
grow CZT crystals with the required qualities. Our efforts have focused on developing novel techniques and testing
methods that will allow us to explore the correlations between the crystal's defects and the detector's properties. Local
stoichiometric variations and other local disordering make it very hard to systematically correlate performance and
material defects on a macroscopic scale. Therefore, to delineate the factors limiting the energy resolution of CZT
detectors, we directed our efforts towards micron-scale material characterization and assessments of the detectors using
the National Synchrotron Light Source (NSLS). The NSLS offers us a highly collimated high-intensity X-ray beam that
we employed to undertake detector-performance mapping, and to investigate the association between microscopic
defects and fluctuations in collected charge. In this paper, we illustrate our techniques and results.
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