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.
Lithium Indium Selenide (LiInSe2) has been under development in RMD Inc. and Fisk University for
room temperature thermal neutron detection due to a number of promising properties. The recent advances
of the crystal growth, material processing, and detector fabrication technologies allowed us to fabricate
large detectors with 100 mm2 active area. The thermal neutron detection sensitivity and gamma rejection
ratio (GRR) were comparable to 3He tube with 10 atm gas pressure at comparable dimensions. The
synthesis, crystal growth, detector fabrication, and characterization are reported in this paper.
Degradation of room temperature operation of TlBr radiation detectors with time is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. Scanning Auger electron spectroscopy (AES) in combination with sputter depth profiling was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage and create a TlBr1-xClx surface layer prior to metal contact deposition. Auger compositional depth profiling results reveal non-equilibrium interfacial diffusion after device operation in both air and N2 at ambient temperature. These results improve our understanding of contact/device degradation versus operating environment for further enhancing radiation detector performance.
TlBr radiation detector operation degrades with time at room temperature and is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. X-ray photoemission spectroscopy (XPS) was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage prior to Mo or Pt contact deposition. High-resolution photoemission measurements on the Tl 4f, Br 3d, Cl 2p, Mo 3d and Pt 4f core lines were used to evaluate surface chemistry and non-equilibrium interfacial diffusion. Results indicate that anion substitution at the TlBr surface due to the HCl etch forms TlBr1-xClx with consequent formation of a shallow heterojunction. In addition, a reduction of Tl1+ to Tl0 is observed at the metal contacts after device operation in both air and N2 at ambient temperature. Understanding contact/device degradation versus operating environment is useful for improving radiation detector performance.
Four thallium bromide planar detectors were fabricated from materials grown at RMD Inc. The TlBr samples were prepared to investigate the effect of guard ring on device gamma-ray spectroscopy performance, and to investigate the leakage current through surface and bulk. The devices’ active area in planar configuration were 4.4 × 4.4 × 1.0 mm3. In this report, the detector fabrication process is described and the resulting energy spectra are discussed. It is shown that the guard ring improves device spectroscopic performance by shielding the sensing electrode from the surface leakage current, and by making the electric filed more uniform in the active region of the device.
Device-grade TlBr was subjected to various chemical treatments used in room temperature radiation detector fabrication
to determine the resulting surface composition and electronic structure. Samples of as polished TlBr were treated
separately with 2%Br:MeOH, 10%HF, 10%HCl and 96%SOCl2 solutions. High-resolution photoemission measurements
on the valence band electronic structure and Tl 4f, Br 3d, Cl 2p and S 2p core lines were used to evaluate surface
chemistry. Results suggest anion substitution at the surface with subsequent shallow heterojunction formation. Surface
chemistry and valence band electronic structure were further correlated with the goal of optimizing the long-term
stability and radiation response.
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 (μeτe) in the mid 10-3 cm2/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).
TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV
and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV,
demonstrating the potential of this material system. However, these detectors are known to polarize using conventional
configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors
based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical
starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma
spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By
incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the
detectors remains unchanged. 241Am spectra are also shown to be more stable for extended periods; detectors have been
held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the
long term degradation mechanism being related to the contact metal is presented.
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 3x10-3 cm2/V
has been produced. The material analysis and detector characterization results are included.
Thallium bromide (TlBr) is a compound semiconductor with high density, high atomic numbers and wide
bandgap. In addition, recent results indicate that the mobility-lifetime product of electrons can be quite high,
approaching the values for CdTe and CZT. These properties make TlBr a very promising material for nuclear radiation
detector at room temperature. In this paper we report on our investigation of the performance of planar TlBr detector
under high flux x-rays irradiation. This study proposes an alternate contact method that reduces the polarization effects,
and the afterglow for a wide range of high flux applications.
Recent results have shown that capacitive Frisch grid structures significantly improve spectroscopic performance of planar CZT detectors especially at higher energies. This paper presents results obtained with larger detectors than those previously reported on. Devices with various aspect ratios and grid length-to-device thickness ratios were fabricated and evaluated. A FWHM energy resolution of approximately 2% at 662 keV was obtained for a device with dimensions of 5 mm x 5 mm x 9.2 mm.