Guido Ciampi

Developing Larger TlBr Detectors—Detector Performance

Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/cm3) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 cm2/V. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 times 10-3 cm2/V have been measured. Devices with small pixel design have been built with 3, 5, and 10 mm thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector.

Thallium Bromide Nuclear Radiation Detector Development

Thallium bromide (TlBr) is a dense, high-Z, wide bandgap semiconductor that has potential as an efficient, compact, room temperature nuclear radiation detector. In this paper we report on our recent progress in TlBr nuclear detector development. In particular, improvements in material purification have led to an order of magnitude increase in the mobility-lifetime product of electrons, (mutau)e, to as high as 5 times 10-3 cm2/V. This has enabled much thicker detectors with good charge collection to be fabricated. We fabricated and tested small pixel TlBr arrays up to 10 mm thick. The energy resolution ~2% FWHM at 662 keV was recorded with 5-10 mm thick devices without 3-D spectral correction. We also investigated the long-term detector stability and were able to constantly operate a thin (0.5 mm) detector for five months at -18degC, under an electric field and with irradiation.

The Defect and Transport Properties of Donor Doped Single Crystal TlBr

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 T1 vacancies at 20°C.

Purification, crystal growth and detector performance of TlBr

TlBr is a promising semiconductor for gamma-ray detection at room temperature, but it has to be extremely pure to become useful. We investigated the purification and crystal growth of TlBr to improve the mobility and lifetime of charge carriers, and produce TlBr detectors for radioisotopic detection. Custom equipment was built for purification and crystal growth of TlBr. The zone refining and crystal growth were done in a horizontal configuration. The process parameters were optimized and detector grade material with an electron mobility-lifetime product of up to 3x10-3 cm2/V has been produced. The material analysis and detector characterization results are included.

Recent Progress in Thallium Bromide Gamma-Ray Spectrometer Development

In recent years, progress in processing and crystal growth methods have led to a significant increase in the mobility-lifetime product of electrons in thallium bromide (TlBr). This has enabled single carrier collection devices with thickness greater than 1-cm to be fabricated. In this paper we report on our latest results from pixellated devices with depth correction as well as our initial results with Frisch collar devices. After applying depth corrections, energy resolution of approximately 2% (FWHM at 662 keV) was obtained from a 13-mm thick TlBr array operated at -18°C and under continuous bias and irradiation for more than one month. Energy resolution of 2.4% was obtained at room temperature with an 8.4-mm thick TlBr Frisch collar device.

The defect and transport properties of acceptor doped TlBr: role of dopant exsolution and association

The role of acceptor dopants (S and Se) in controlling the ionic conductivity of single crystal TlBr, grown by the vertical Bridgman method, was examined as a function of temperature with the aid of impedance spectroscopy. Several features in the conductivity were identified and related to acceptor dopant-Br vacancy association, acceptor dopant exsolution, and Br vacancy mobility. The corresponding enthalpies for these processes were extracted from the data and were found to be equal to H(a) = 0.42 ± 0.07 eV, H(sol) = 1.55 ± 0.18 eV and H(m,Br) = 0.31 ± 0.02 eV respectively, the latter consistent with earlier studies on donor doped and undoped TlBr. A long term conductivity decay in the extrinsic region, attributed to S or Se exsolution, was observed. The time constant associated with exsolution was found to be thermally activated with an activation energy of 0.47 ± 0.1 eV. Estimates for Se solubility at different temperatures are provided.

Electronic effects of Se and Pb dopants in TlBr

Deep levels in Se- and Pb-doped bulk TlBr detectors were characterized with photo-induced conductivity transient spectroscopy (PICTS) and cathodoluminescence (CL). Se-doped TlBr revealed two traps with energies of 0.35 and 0.45 eV in PICTS spectra. The Pb-doped material revealed three levels with energies of 0.11, 0.45, and 0.75 eV. CL measurements in both materials correlate with optical transitions involving some of the identified levels. The ambipolar carrier lifetimes of Se-doped and Pb-doped TlBr were measured with microwave reflectivity transients and found to be significantly lower than the lifetime of undoped TlBr.Deep levels in Se- and Pb-doped bulk TlBr detectors were characterized with photo-induced conductivity transient spectroscopy (PICTS) and cathodoluminescence (CL). Se-doped TlBr revealed two traps with energies of 0.35 and 0.45 eV in PICTS spectra. The Pb-doped material revealed three levels with energies of 0.11, 0.45, and 0.75 eV. CL measurements in both materials correlate with optical transitions involving some of the identified levels. The ambipolar carrier lifetimes of Se-doped and Pb-doped TlBr were measured with microwave reflectivity transients and found to be significantly lower than the lifetime of undoped TlBr.

Thallium bromide and thallium bromoiodide gamma ray spectrometer development

Thallium bromide (TlBr) is of interest as a material for room temperature gamma ray spectroscopy due to its high density, high Z and wide bandgap. In addition, its cubic crystal structure and relatively low melting point facilitate crystal growth by melt techniques. Recent advances in material purification, crystal growth and device processing have led to mobility-lifetime products of electrons in the mid 10−3 cm2/V range enabling working detectors up to 15-mm thick to be fabricated. Although performance of TlBr devices is promising, long term detector stability at room temperature is an issue. We are investigating various compositions of the ternary compound, thallium bromoiodide (TlBrxI1−x) to vary the band gap and determine the effect of added thallium iodide (TlI) on detector performance. In this paper we report on our recent progress in TlBr gamma-ray spectrometer development as well as our initial results from TlBrxI1−x devices. Results from TlBr detectors up to 15-mm thick will be presented including depth corrected pulse height spectra. Detector stability will also be discussed. This work is being supported by the Domestic Nuclear Detection Office (DNDO).

Kinetics of Schottky defect formation and annihilation in single crystal TlBr.

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.

First principles and experimental study of the electronic structure and phase stability of bulk thallium bromide

We apply state-of-art first principle calculations to study the polymorphism and electronic structure of three previously reported phases of TlBr. The calculated band structures of NaCl-structure phase and orthorhombic-structure phase have different features than that of commonly observed CsCl-structure phase. We further interpret photoluminescence spectra based on our calculations. Several peaks close to calculated band gap values of the NaCl-structure phase and the orthorhombic-structure phase are found in unpolished TlBr samples.