Pixelated TlBr Detectors Research Articles

Accurate Determination of the Ionization Energy in Pixelated TlBr Correcting for Charge Collection Efficiency

Thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. Performance of better than 1% full-width at half maximum at 662 keV can be achieved when TlBr detectors are cooled to −20 °C. The theoretical limit of energy resolution is determined by the ionization energy in semiconductor detectors, and accurately measuring it is important for determining the best possible performance. One method to determine the ionization energy of semiconductor detectors compares pulse heights obtained from the semiconductor to pulse heights from a silicon detector. Due to their higher trapping, the charge collection efficiency (CCE) of TlBr is significantly lower than it is in silicon, therefore a correction to the ionization energy must be made. In this paper, we present the theory and measurement for accurately determining the CCE using the Shockley-Ramo theorem and apply it to measurement of ionization energy in pixelated TlBr detectors. The ionization energy of two TlBr detectors is measured and found to be 4.83(8) and 5.49(10) eV for the two samples at room temperature and −20 °C, respectively.

Analysis of High-Energy Tailing in TlBr Detectors

Thallium bromide (TlBr) is an attractive material for room-temperature semiconductor radiation detection due to its high atomic number and density. Performance of better than 1% full-width at half maximum at 662 keV has been measured on $\sim 5 \times 5 \times 5$ mm3 pixelated TlBr detectors. Though most TlBr detectors show expected performance, a few detectors have high-energy tails on their photopeaks which are caused by nonflat tails on anode waveforms. The properties of these anode tails are studied and the generation of extra electrons by the movement of holes is proposed to explain the observations. The detector is operated in reverse bias (with holes drifting toward the pixelated electrode) to help confirm these observations and a mechanism for correcting preamplifier decay from long collection time digital waveforms is developed and used in the analysis.

Quantification of the Conditioning Phase in Cooled Pixelated TlBr Detectors

Thallium-bromide (TlBr) is currently under investigation as an alternative room-temperature semiconductor gamma-ray spectrometer due to its favorable material properties (large bandgap, high atomic numbers, and high density). Previous work has shown that 5 mm thick pixelated TlBr detectors can achieve sub-1% FWHM energy resolution at 662 keV for single-pixel events. These results are limited to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ - 20{^ \circ }{\rm C}$</tex></formula> operation where detector performance is stable. During the first one to five days of applied bias at <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ - 20{^ \circ }{\rm C}$</tex> </formula> , many TlBr detectors undergo a conditioning phase, where the energy resolution improves and the depth-dependent electron drift velocity stabilizes. In this work, the spectroscopic performance, drift velocity, and freed electron concentrations of multiple 5 mm thick pixelated TlBr detectors are monitored throughout the conditioning phase. Additionally, conditioning is performed twice on the same detector at different times to show that improvement mechanisms relax when the detector is stored without bias. We conclude that the improved spectroscopy results from internal electric field stabilization and uniformity caused by fewer trapped electrons.

Characterization of pixelated TlBr detectors with Tl electrodes

A 4.36-mm-thick pixelated thallium bromide (TlBr) detector with Tl electrodes was fabricated from a crystal grown by the traveling molten zone method using zone-purified material. The detector had four 1×1mm2 pixelated anodes. The detector performance was characterized at room temperature. The mobility-lifetime products of electrons for each pixel of the TlBr detector were measured to be >2.8×10−3cm2/V. The four pixelated anodes of the detector exhibited energy resolutions of 1.5–1.8% full width at half maximum (FWHM) for 662-keV gamma rays for single-pixel events with the depth correction method. An energy resolution of 4.5% FWHM for 662-keV gamma rays was obtained from a reconstructed energy spectrum using two-pixel events from the two pixelated anodes on the detector.

Pixellated TlBr detectors with the depth sensing technique

The depth sensing technique has been introduced to a 1.8 mm thick pixellated thallium bromide (TlBr) detector in order to enhance the performance of the device. Significant improvement of the energy resolution has been achieved with the device. An energy resolution of 2% FWHM at 662 keV was recorded with the device after the depth correction and rejection.

Spectroscopic performance of pixellated thallium bromide detectors

The purpose of this study is to evaluate spectroscopic performance of pixellated thallium bromide (TlBr) radiation detectors. Pixellated TlBr detectors have been fabricated from TlBr crystals grown by the traveling molten zone (TMZ) method with zone-refined materials. Charge transport properties of the crystals were measured by performing the Hecht analysis. The TlBr crystals have exhibited the mobility-lifetime product of around 10/sup -3/ cm/sup 2//V for electrons, which is three times larger than the previously reported value. The pixellated TlBr detectors with /spl sim/2 mm thickness have 4 pixellated electrodes (0.57 mm /spl times/ 0.57 mm each) on the anode surface. In order to evaluate the spectroscopic performance, the detectors have been tested with /sup 57/Co, /sup 137/Cs, and /sup 60/Co gamma-ray sources at room temperature. The TlBr detector 1.9 mm thick has demonstrated energy resolutions of 6.7 keV (5.5%) and 22.3 keV (3.4%) for 122 and 662 keV gamma-rays, respectively. Full-energy peaks of 1.17 and 1.33 MeV gamma-rays from the /sup 60/Co source have also been detected with the detector.

Pixellated thallium bromide detectors for gamma-ray spectroscopy and imaging

Recently, pixellated semiconductor detectors exhibit high-energy resolution, which have been studied actively and fabricated from CdTe, CZT and HgI 2. Thallium bromide (TlBr) is a compound semiconductor characterized with its high atomic numbers (Tl=81, Br=35) and high density (7.56 g/cm 3). Thus, TlBr exhibits higher photon stopping power than other semiconductor materials used for radiation detector fabrication such as CdTe, CZT and HgI 2. The wide band gap of TlBr (2.68 eV) permits the detectors low-noise operation at around room temperature. Our studies made an effort to fabricate pixellated TlBr detectors had sufficient detection efficiency and good charge collection efficiency. In this study, pixellated TlBr detectors were fabricated from the crystals purified by the multipass zone-refining method and grown by the horizontal traveling molten zone (TMZ) method. The TlBr detector has a continuous cathode over one crystal surface and 3×3 pixellated anodes (0.57×0.57 mm 2 each) surrounded by a guard ring on the opposite surface. The electrodes were realized by vacuum evaporation of palladium through a shadow mask. Typical thickness of the detector was 2 mm. Spectrometric performance of the TlBr detectors was tested by irradiating them with 241Am (59.5 keV), 57Co (122 keV) and 137Cs (662 keV) gamma-ray sources at temperature of –20°C. Energy resolutions (FWHM) were measured to be 4.0, 6.0 and 9.7 keV for 59.5, 122 and 662 keV gamma-rays, respectively.