Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.
Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy due to high density, high Z and wide bandgap of the material. Furthermore, its low melting point (460 °C), cubic crystal structure and congruent melting with no solid-solid phase transitions between the melting point and room temperature, TlBr can be grown by relatively simple melt based methods. As a result of improvements in material processing and detector fabrication over the last several years, TlBr with electron mobility-lifetime products (μ<sub>e</sub>τ<sub>e</sub>) in the mid 10<sup>-3</sup> cm<sup>2</sup>/V range has been obtained. In this paper we are going to report on our unipolar charging TlBr results for the application as a small animal imaging. For SPECT application, about 5 mm thick pixellated detectors were fabricated and tested. About 1 % FWHM at 662 keV energy resolution was estimated at room temperature. By applying the depth correction technique, less than 1 % energy resolution was estimated. We are going to report the results from orthogonal strip TlBr detector for PET application. In this paper we also present our latest detector highlights and recent progress made in long term stability of TlBr detectors at or near room temperature. This work is being supported by the Domestic Nuclear Detection Office (DNDO) and the Department of Energy (DOE).
Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development for room
temperature gamma ray spectroscopy due to several promising properties. Due to recent advances in material
processing, electron mobility-lifetime product of TlBr is close to Cd(Zn)Te's value which allowed us to fabricate large
working detectors. We were also able to fabricate and obtain spectroscopic results from TlBr Capacitive Frisch Grid
detector and orthogonal strip detectors. In this paper we report on our recent TlBr and related ternary detector results
and preliminary results from Cinnabar (HgS) detectors.
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.
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<sup>-3</sup> cm<sup>2</sup>/V
has been produced. The material analysis and detector characterization results are included.
Single crystals of LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>: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 LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>: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 LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>:1% Pr scintillators were from about 400 to 800 nm. The
CeBr<sub>3</sub>:1% Pr scintillator had a dominating emission peak due to CeBr<sub>3</sub> 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 LaBr<sub>3</sub> and Pr-doped CeBr<sub>3</sub>
crystals. We will also describe the special handling and processing procedures developed for these scintillator