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.

The photoluminescence and X-ray-induced radioluminescence properties of TlBr crystals were analyzed. We compared the samples cut from two parts of the crystal that are empirically known to be appropriate and inappropriate for semiconductor detectors. In the X-ray-induced radioluminescence spectra, two bands were observed at 490 and 650 nm for both the samples. We observed an additional band at 560 nm only for the inappropriate part of the crystal. We observed an emission band at 520 nm in the photoluminescence spectra of both the samples and attributed this to some defects near the surface on which photoexcitation occurs. Interestingly, an emission band at 560 nm appeared after thermal cycle from room temperature to 5 K and back to room temperature. Assuming that the thermal cycle introduced some distortions in the sample, the emission band at 560 nm could be related to lattice defects, such as dislocation, that are relevant to distortion.

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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.

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