R4D  TlBr

Thursday, Nov. 5  16:30-17:55  California

Session Chair:  Arnold Burger, Fisk University, United States

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(16:30) R4D-1, invited, Development of TlBr Gamma-Ray Detectors with Single Polarity Charge Sensing Configurations

K. Hitomi1, N. Nagano1, T. Onodera2, C. Disch3, S.-Y. Kim1, T. Ito1, K. Ishii1

1Tohoku University, Sendai, Japan
2Tohoku Institute of Technology, Sendai, Japan
3Freiburg Materials Research Center, Freiburg, Germany
Thallium bromide (TlBr) is a compound semiconductor attractive for fabrication of room-temperature gamma-ray detectors. It has high photon stopping power originating from the high atomic numbers of the constituent atoms (81 and 35) and its high density (7.56 g/cm3). Zone-purified TlBr crystals exhibit mobility-lifetime products of >10-3 cm2/V and >10-4 cm2/V for electrons and holes, respectively, which are comparable to those for CdTe crystals. In this study, single polarity charge sensing devices including pixelated detectors, double-sided strip detectors, and coplanar grid detectors were fabricated from TlBr crystals that were grown by the traveling molten zone method using zone-purified material. The electrodes of the devices were constructed by the vacuum evaporation of Tl using metal-shadow masks. The polarization phenomena in TlBr detectors are significantly suppressed by employing Tl electrodes. A pixelated TlBr detector exhibited an energy resolution of 1.1% FWHM for 662-keV gamma rays for single-pixel events with the depth correction at room temperature. An energy resolution of 4.5% FWHM was obtained from one strip of a double-sided TlBr strip detector without the depth correction at room temperature.

(16:50) R4D-2, invited, Room Temperature Performance of TlBr Detectors

S. Motakef, A. Datta

CapeSym, Inc, Natick, MA, USA
Thallium Bromide (TlBr) is a wide bandgap compound semiconductor with high gamma-ray stopping power and promising high resolution energy discrimination. However, performance degradation and the eventual failure of TlBr devices show a strong dependence on the operating temperature. Recent results indicate that the spectroscopic degradation of TlBr detectors at room temperature is not related to the standard polarization effect associated with the ionic conductivity of TlBr. The electro-migration of Br- ions and their reaction with the anode metal appear to be the primary factor controlling room temperature stability of these devices. We present the experimental observations on the reaction of Br- ions and contact metals, and report on approaches aimed at improving the stability of the semiconductor-metal interface, including the selection of contact metal.

(17:05) R4D-3, Signal Processing for TlBr Detectors: Accounting for the Motion of Holes and Using the Tl X-Ray

W. Koehler1, S. O'Neal1, Z. He1, H. Kim2, L. Cirignano2, K. Shah2

1University of Michigan, Ann Arbor, MI, United States
2Radiation Monitoring Devices, Watertown, MA, United States
Thallium-bromide (TlBr) has high atomic numbers (Tl: 81, Br: 35) and density (7.56 g/cm^3) giving it a large stopping power for X-rays and gamma-rays. Additionally, it has a large bandgap, making it a suitable alternative to CZT for room-temperature spectroscopy. Five millimeter thick TlBr detectors can achieve about 1% FWHM energy resolution at 662 keV, but these results are mostly limited to stable operation at -20°C. In addition to high µt electron (~103 cm2/Vs) some TlBr detectors show relatively high µt hole (up to 0.15*µt electron). High hole mobility-lifetime products can affect depth reconstruction when single-polarity charge sensing is assumed. In this work, we use digital signal processing to account for the motion of holes and improve depth reconstruction at all depths. For detectors with very high hole mobility, we use a combination of drift-time and cathode-to-anode signal ratio (CAR) to correctly reconstruct the depth. Additionally, this work uses the characteristic Tl X-ray to improve two pixel energy resolution from 3.84% to 3.44% FWHM at 662 keV on new 11x11 pixelated TlBr detectors. When the overall energy resolution approaches 1% FWHM at 662 keV, better filtering can be applied and this improvement will increase.

(17:20) R4D-4, Development of Larger Thallium Bromide Gamma-Ray Spectrometer Arrays

H. Kim1, K. Shah1, A. Churilov1, Y. Ogorodnik1, A. Kargar1, G. Ciampi1, L. Cirignano1, A. Gueorguiev1, S. Kim1, Z. He2, W. Koehler2, S. O'Neil2

1Radiation Monitoring Devices Inc., Watertown, MA, USA
2Dept of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI, USA
Thallium bromide (TlBr) is of interest as a gamma-ray detector material due to its high stopping power, wide band gap and relative ease of crystal growth. In addition, mobility lifetime products of electrons and holes are similar to those of cadmium telluride (CdTe) as a result of improvements in purification and crystal growth over the past several years. Single carrier devices such as small pixel arrays and Frisch collar devices originally applied to CZT, have also been applied to TlBr. Energy resolution of < 1% FWHM at 662 keV has been obtained for single pixel events with small (e.g. 3 x 3 pixels, 1-mm pitch) arrays [ref]. Because TlBr is an ionic conductor, long term detector stability under continuous bias at room temperature has been an issue. In addition to modest cooling which has been demonstrated to stabilize detector performance over many days of continuous operation, material selection and improvements in detector fabrication have also extended long term stability at room temperature. In this paper results from larger TlBr arrays (11 x 11 pixels, 1-mm pitch, 5 – 10 mm thickness) and progress in long term detector stability under continuous bias at room temperature are presented.

(17:35) R4D-5, First Principles Study of Semiconductor-Metal Contact Interactions in TlBr Room Temperature Radiation Detectors

K. G. Ray1, J. B. Varley1, C. R. Leão2, V. Lordi1

1Lawrence Livermore National Laboratory, Livermore, CA, USA
2Universidade Federal do ABC, Santo Andre, Brazil
TlBr is a promising semiconductor material for room temperature radiation detection. The figure of merit, µt, is large for TlBr, due to very long carrier lifetimes, and the intrinsic resistivity is very high. TlBr radiation detectors have achieved 1-2% energy resolution at 662 keV and a high absorption cross-section, however, the performance degrades over time. This process, depending on the device preparation, can take from hours to up to months. There are many possible causes of the degradation, including the incorporation of metal impurities from the contacts and the accumulation of charged vacancies at the ends of the device. Experimentally it has been shown that TlBr surface treatments and the choice of metal contact affect device longevity. Here we report on first principles density functional theory studies of metal impurities in TlBr with different surface treatments as well as the effects of intermetallic compounds that may form between TlBr and the contacts. In particular, the solubility and migration barriers of metal impurities in TlCl (HCl treated TlBr surfaces) and the formation energy, band gap, and band offsets of compounds of the form Tl-Br-Cl-Metal will be presented. We will review recent experimental results in light of our theoretical predictions and identify treatment and contact metal combinations with desirable properties.