Alexei V. Churilov

Selected Properties of Cs

Homeland security applications often require detection of both neutron and gamma radiation. A combination of two detectors registering neutrons and gammas separately is typically used. Recently, a number of scintillators from the elpasolite crystal family were proposed, that provide detection of both types of radiation. The most promising are Cs2LiYCl6, Cs2LiLaCl6, and Cs2LiLaBr6. All are doped with Ce3+. They are capable of providing very high energy resolution. The best values achieved for each material are 3.9%, 3.4%, and 2.9% at 662 keV (FWHM), respectively. Since 6Li has an acceptable cross-section for thermal neutron capture, these materials also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak typically appears above 3 MeV gamma equivalent energy. Thus very effective pulse height discrimination can be implemented with these materials. The CLLC and CLYC emissions consist of two main components: Core-to-Valence Luminescence (CVL; 220 nm to 320 nm) and Ce emission (350 to 500 nm). The former is of particular interest since it appears only under gamma excitation. It is also very fast and decays with less than 2 ns time constant. The CVL provides a significant difference to temporal responses under gamma and neutron excitation thus it may be used for effective pulse shape discrimination.

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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.

LiYCl

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.

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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.

, Cs

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.

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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.

LiLaCl

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.

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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).

, and Cs

Single crystals of LaBr3:1% Pr and CeBr3:1% Pr have been grown by the vertical Bridgman technique. Crystals of these scintillators can be used in the fabrication of gamma-ray spectrometers. The LaBr3:1% Pr and CeBr3:1% Pr crystals we have grown had light outputs of ~73,000 and ~50,000 photons/MeV, respectively, and principal decay constants of 11μs and 26 ns, respectively. There were a number of emission peaks observed for these compounds. The emission wavelength range for the LaBr3:1% Pr and CeBr3:1% Pr scintillators were from about 400 to 800 nm. The CeBr3:1% Pr scintillator had a dominating emission peak due to CeBr3 at 390 nm. These two materials had energy resolutions of 9 and 7% FWHM, respectively, for 662 keV photons at room temperature. In this paper, we will report on our results to date for vertical Bridgman crystal growth and characterization of Pr-doped LaBr3 and Pr-doped CeBr3 crystals. We will also describe the special handling and processing procedures developed for these scintillator compositions.

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Many materials used in radiation detectors are environmentally unstable and/or fragile. These properties are frustrating to researchers and add significantly to the time and cost of developing new detectors as well as to the cost of manufacturing products. The work presented here investigates the properties of HgS. This material was selected for study based partly on its inherent stability and ruggedness, high density, high atomic number, and bandgap. HgS is found in nature as the mineral cinnabar. A discussion of the physical properties of HgS, experimental characterization of natural cinnabar, and initial radiation detection results are presented along with a discussion of potential crystal growth techniques for producing crystals of HgS in the laboratory.