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Bishop S.R.,Massachusetts Institute of Technology | Higgins W.,RMD Instruments LLC | Ciampi G.,RMD Instruments LLC | Churilov A.,RMD Instruments LLC | And 2 more authors.
Journal of the Electrochemical Society | Year: 2011

Thallium bromide (TlBr) is attractive for high energy radiation detection, given its large molecular weight and wide energy bandgap. However, TlBr exhibits levels of ionic conductivity that can lead to an undesirable leakage, or dark current, thereby reducing sensor performance. To investigate the role of dopants in controlling the ionic conductivity, single crystals of TlBr were grown using zone refining and/or vertical Bridgman methods with controlled levels of donor (Pb) dopants. Their electrical properties were examined as a function of temperature (20-300°C) with frequency dependent impedance spectroscopy. A Schottky-based defect equilibria model was fitted to the resulting conductivity data, and enthalpies of Schottky defect formation (0.91±0.03 eV), cation migration (0.51±0.03 eV), and anion migration (0.28±0.05 eV) were extracted. Br vacancies were found to posses about 5 orders of magnitude higher mobility than that of Tl vacancies at 20°C. © 2010 The Electrochemical Society.

Bishop S.R.,Massachusetts Institute of Technology | Bishop S.R.,Kyushu University | Tuller H.L.,Massachusetts Institute of Technology | Kuhn M.,Massachusetts Institute of Technology | And 3 more authors.
Physical Chemistry Chemical Physics | Year: 2013

The kinetics for Schottky defect (Tl and Br vacancy pair) formation and annihilation in ionically conducting TlBr are characterized through a temperature induced conductivity relaxation technique. Near room temperature, defect generation-annihilation was found to take on the order of hours before equilibrium was reached after a step change in temperature, and that mechanical damage imparted on the sample rapidly increases this rate. The rate limiting step to Schottky defect formation-annihilation is identified as being the migration of lower mobility Tl (versus Br), with an estimate for source-sink density derived from calculated diffusion lengths. This study represents one of the first investigations of Schottky defect generation-annihilation kinetics and demonstrates its utility in quantifying detrimental mechanical damage in radiation detector materials. © 2013 the Owner Societies.

Bishop S.R.,Massachusetts Institute of Technology | Higgins W.,RMD Instruments LLC | Churilov A.,RMD Instruments LLC | Ciampi G.,RMD Instruments LLC | And 6 more authors.
ECS Transactions | Year: 2010

TlBr is an ionic material with good potential for use in high energy radiation detectors because of its relatively large band gap and heavy elements. In these sensors, incident radiation excites electron hole pairs that are collected as the response, and the mobility-lifetime product for electrons and holes, as well as low dark current are common figures of merit. TlBr reportedly displays ionic conductivity that leads to undesirable leakage, or dark current thereby reducing sensor response. This work focuses on the development of a defect and transport model appropriate to TlBr. The derived enthalpies of migration and Schottky defect formation are compared with literature values. ©The Electrochemical Society.

Kuhn M.,Massachusetts Institute of Technology | Bishop S.R.,Massachusetts Institute of Technology | Bishop S.R.,Kyushu University | Ciampi G.,RMD Instruments LLC | And 3 more authors.
Solid State Ionics | Year: 2013

Thallium bromide (TlBr) is an attractive material for high-energy radiation detection applications due to its high molecular weight and relatively large band gap. Previous studies on acceptor doping with Se and S revealed limited solubility of the acceptor dopant in the TlBr crystal, leading to long term changes in dark ionic conductivity, and, presumably, detector sensitivity and performance. Here, the ionic conductivity of TlBr acceptor doped with the larger ionic radius Te2 - (2.21 Å versus 1.98 Å for Se 2 -) is studied using impedance spectroscopy. Due to the low concentration of Te, in contrast to Se or S doping, acceptor dopant-defect association was not observed. Acceptor exsolution was confirmed at temperatures below 100 C by following changes in ionic conductivity with time. The time constant and activation energy (0.27 ± 0.14 eV) for exsolution of Te are considerably lower than that of Se, with the mechanistic origin of the activation energy related to Br migration, in marked contrast to that of Se exsolution, which is controlled instead by Tl migration, as discussed here. This long term, time-dependent change in dark resistance could negatively impact radiation detector sensitivity and therefore acceptor doping should be avoided. © 2013 Elsevier B.V.

Dilon Technologies Inc. and RMD Instruments Corporation | Date: 2011-04-19

medical apparatus, namely, gamma positioning system, comprised of collimators, probes and handpieces, for detection and quantification of gamma radiation from gamma-emitting isotopes in the body or body tissue.

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