Photoluminescence analysis of TlBr crystals for radiation detector applications

Photoluminescence (PL) analysis is widely used for characterization of common semiconductor detector materials such as CdTe and HgI₂. Although Thallium Bromide (TlBr) emerged as a promising detector material recently, a few results of PL analysis of TlBr crystals were reported. In this study, PL analysis of TlBr crystals was performed in order to reveal correlation between the PL spectra of TlBr crystals and the detector performance. The TlBr powder with purity of 99.99% was purified by the vacuum distillation and zone melting method. After the purification, single zone pass was performed with the aim of improving the crystallinity. The TlBr crystal was cut into wafers with a diamond wire saw. Both surfaces of the wafers were polished mechanically. TlBr wafers from pure end and impure end of the TlBr ingot were analyzed with the PL method. The samples were placed in an electronic cryostat and kept at 4 K. The samples were excited by a He-Cd laser (325 nm). A PL spectrum obtained from a TlBr crystal from the pure end of the ingot exhibited PL peaks at 3, 2.6 and 2.5 eV and a PL band at 1.9 eV. The PL peak at 3 eV originates from direct transition emission. The PL peak at 2.6 eV is indirect transition emission relating to impurities. The origin and nature of the emission lines are still under investigation. PL spectra obtained from TlBr crystals from pure end and impure end are compared. Significant difference of the PL intensity ratio between 2.6 and 2.5 eV peaks was observed between the pure and impure crystals. The pure crystal exhibited the PL intensity ratio less than 1. A ²²Na gamma-ray spectrum with a clear 511 keV full-energy peak was obtained from a TlBr detector fabricated from a TlBr crystal at pure end which exhibited the PL intensity ratio less than 1. On the other hand, poor detector performance (no full-energy peak) was obtained from TlBr detectors fabricated from TlBr crystals exhibiting the PL intensity ratio more than 1.