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Abstract Thallium bromide (TlBr) semiconductor detectors are being developed as promising candidates for high-detection-efficiency, high-energy-resolution, and room-temperature gamma-ray spectrometers. This study presents methods for evaluating TlBr crystal quality and carrier transport characteristics using neutron Bragg-dip imaging and the time-of-flight method for pulsed-laser-induced carriers, respectively. Neutron Bragg-dip imaging effectively determines the crystal orientation distribution, revealing crystal imperfections and grain boundaries. Time-of-flight measurements provide a spatial distribution of carrier mobility. In this study, two samples obtained from both the upstream and downstream region in the crystal ingot were evaluated. Although both samples show similar crystal quality, the upstream sample showed high carrier mobility across all areas, whereas the downstream sample exhibits low mobility in some areas. These findings suggest that, at least within the range of carrier mobility currently obtained, the effect of crystal integrity on carrier mobility is less significant than that of impurities. In conclusion, combining neutron Bragg-dip imaging with carrier mobility measurements offers a comprehensive approach to evaluating and improving TlBr detectors.

This study was carried out to examine the potential of antimony tri-iodide (SbI3) as a material for radiation detectors that operate at room temperature. SbI3 is a compound semiconductor with an AsI3-type crystal structure, high atomic number (Sb: 51, I: 53), high density (4.92 g/cm3), and a wide band-gap energy (2.2 eV). In addition, crystalline SbI3 is easy to grow by conventional crystal growth techniques from melting phase because the material exhibits a low melting point (171°C) and undergoes no phase transition in the range of its solid phase. In this study, SbI3 crystals were grown by the Bridgman method after synthesis of SbI3 from 99.9999% pure Sb and 99.999% pure I2. The grown crystals consisted of several large grains with red color and were confirmed to be single-phase crystals by X-ray diffraction analysis. SbI3 detectors with a simple planar structure were fabricated using the cleavage plates of the grown crystals, and the pulse-height spectra were recorded at room temperature using an 241Am alpha-particle (5.48 MeV) source. The detector showed response to the alpha-particle radiation.

To develop semiconductor detectors based on TlBr with excellent energy resolution at high yields, a variety of characterization methods for TlBr crystals are necessary. Among the various methods based on excitation by photons or ionizing radiation, we chose transient absorption measurements after the irradiation of pulsed electron beams to analyze the recombination dynamics of electron-hole pairs generated by the electron beams. The decay behavior of the transient absorption depended on the pulse intensity and was satisfactorily fitted with a decay function based on bimolecular recombination dynamics. The bimolecular recombination coefficients obtained from the fitting were similar for the samples cut from different parts of a crystal boule. Assuming that the recombination occurred through Shockley-Read-Hall bimolecular recombination via electronic levels of defects within the bandgap, the result indicates that the concentrations of the defects responsible for the recombination were similar for the samples out from different parts of the crystal boule. [ABSTRACT FROM AUTHOR]

The changes in thallium bromide (TlBr) surfaces induced by plasma etching were characterized by X-ray photoelectron spectroscopy (XPS). A TlBr crystal was grown by the traveling molten zone method with zone-purified material. TlBr wafers were obtained from the crystal. Ar plasma etching was performed on the TlBr wafers. XPS revealed that Tl metal (Tl0) was created in TlBr by the plasma etching. The Tl0 concentration increased with the irradiation time of the plasma. The net Tl0 concentrations of TlBr wafers etched by the plasma for 45 and 180 s were 0.7 and 2.6 wt%, respectively. [ABSTRACT FROM AUTHOR]

Thallium bromide (TlBr) is a candidate material for room-temperature semiconductor gamma-ray detectors with high detection efficiency and energy resolution. By adopting a pixelated detector configuration, a TlBr detector can determine not only the deposition energy but also the 3D positions of interactions between gamma-ray photons and the detector medium. A Compton imager, which can measure the directional distribution of incident gamma rays based on Compton scattering kinetics, can be realized by using the information of deposition energies and interaction positions in the gamma-ray detector. In this study, we demonstrated Compton imaging using a pixelated TlBr semiconductor detector. [ABSTRACT FROM AUTHOR]

TlBr semiconductor detectors are candidate high-detection-efficiency, high-energyresolution, and room-temperature gamma-ray spectrometers. We evaluate the effective volume of a small-anode TlBr detector with a guard electrode through basic experiments and simulations, which are conducted for various voltages applied to the guard electrode. We confirm that the detection efficiency increases with decreasing guard voltage compared with the anode and saturates at a certain value. Under saturation conditions, the effective volume is evaluated to be 56% by simulation. In addition, we experimentally confirm that the energy resolution can be improved for a small-anode TlBr detector with a guard electrode by correcting the signal pulse height by pulse shape discrimination. [ABSTRACT FROM AUTHOR]

Because thallium bromide (TlBr) semiconductor detectors with a wide bandgap show high detection efficiency and energy resolution, room-temperature gamma-ray spectrometers based on such detectors have been developed. In this study, we developed a system for evaluating the carrier transport properties in a TlBr detector, such as the mobility and the mobility-lifetime product (μτ product), in which the time-of-flight method is applied to charge carriers generated by a pulsed laser. In the developed system, the recombination effect and internal electric field distortion were experimentally confirmed to be negligible under the experimental conditions in this study. We determined the electron mobility and μτ product in the TlBr detector to be 25.3 cm²/Vs and 1.3 × 10-3 cm²/V, respectively. Because the laser spot diameter is less than 0.5 mm, the constructed measurement system allows the evaluation of the mobility and μτ product at a point on the crystal. It is also possible to evaluate the 2D distribution of the mobility and μτ product by scanning the laser irradiation position. [ABSTRACT FROM AUTHOR]

Thallium bromide (TlBr) semiconductor detectors are promising candidates for highdetection-efficiency, high-energy-resolution, and room-temperature gamma-ray spectrometers. In this study, we conducted neutron Bragg dip and electron backscatter diffraction (EBSD) imaging of TlBr crystals to measure the crystal orientation distribution. We confirmed that crystal grains were continuous over a certain distance along the solidification direction for samples fabricated with the current growth procedure. Finally, we compared the crystal orientation maps obtained with the two techniques. The two types of maps showed similar patterns. We concluded that EBSD, which can observe only surfaces, can be utilized to assess the uniformity of the bulky TlBr crystal, especially when observing the crystal surface perpendicular to the solidification direction. [ABSTRACT FROM AUTHOR]

We analyzed the transient absorption properties of TlBr crystals using pulsed electron beams as excitation sources. We observed transient absorption spectra and temporal profiles on the pico- and nanosecond scales and compared the results obtained for TIBr crystals that are empirically appropriate with those that are inappropriate for semiconductor detectors. The results showed negligible differences in properties between the two types of crystal, which indicates that their trap center concentrations were similar. A transient absorption band was observed at approximately 1160 nm on the nanosecond scale, while its short-wavelength tail was observed on the picosecond scale. The absorption band is attributed to the localized holes at Tl+ that are stabilized by some defects. In contrast, no absorption band attributable to localized electron centers was observed, indicating that while hole transport is hindered by defects, electron transport is not [ABSTRACT FROM AUTHOR]

Thallium bromide (TlBr) has been one of the promising gamma-ray detector materials because of its high gamma-ray attenuation length, availability of room-temperature operation, and relatively high energy resolution. However, the high-resolution detector has been limited to a relatively small size. Hence, the next task in the TlBr development is to establish fabrication processes of large detectors. As one of the candidates for evaluating the TlBr crystal quality, we demonstrated neutron Bragg-dip imaging, which is one of the neutron diffraction techniques and is based on wavelength-resolved neutron imaging. All the studied samples seemed to be imperfect crystals and slightly distorted. Although differences in crystal orientation distributions among the samples cannot be obtained from the present data, it is suggested that one of the crystals has a boundary, or possibly a small crystal grain near the crystal center, which may result in a low-quality electron mobility and lifetime (µτ) product. [ABSTRACT FROM AUTHOR]