Study On The Preparation And Modification Of TlBr Materials Used In Room-temperature Nuclear Radiation Detectors

Thallium bromide (TlBr) is one of the very promising materials used for next generation nuclear radiation detectors due to its high atomic number (Tl:81 and Br:35), wide bandgap (2.68eV), high resistivity (10¹¹Î©Â·cm) and high density(7.56g/cm³) . TlBr radiation detectors can operate at room temperature, and have high detection efficiency and energy resolution for X- and gamma rays. So TlBr detectors are widely used in X-ray astronomy, high-energy physics, nuclear medicine, safety inspection and environment monitoring.Presently, the studies oversea focus mostly on TlBr crystal growth and preparation of detectors. But TlBr powder purification and crystals modification have not been studied by the numbers. Impurities and defects in TlBr crystals result in poor quality which badly affects the performance of detectors. There is not publication on TlBr material research in our country. The aim of this paper is to purify TlBr powder by hydrothermal re-crystallization method, to get high quality TlBr crystals by the annealing at heat treatment, in atmosphere or under hydrothermal conditions, and to establish an experimental foundation for fabrication of high performance TlBr detectors.In this paper, hydrothermal re-crystallization method was used for TlBr powder purification. Based on the positive temperature coefficient of TlBr solubility, optimal parameters in re-crystallization process were ascertained through adjustment of the solution cooling rate in the autoclave, and good purification effect and high purification efficiency were obtained. After one time hydrothermal re-crystallization process, the impurities of Ca, Fe, Mg, K, Zn, Cu, Na and Si obviously decrease. The ratios of impurity concentration in raw material to that in purified powder are 67.26, 28.05, 26.76, 18.78, 11.08, 8.69, 5.86 and 5.40, respectively. The purity of purified TlBr powder is up to 99.999%.The influences of various annealing parameters (annealing temperature, heating rate, annealing time) on the optical properties were investigated. When heating rate changes from 40℃/h to 160℃/h in the annealing processes, there is a larger temperature gradient which point to the surface of wafers between the surface and the wafer center. The temperature gradient drove out the defects and Tl-rich phases from wafer inner to surface. Therefore, some important property indexes are improved evidently, such as T₄₀₀₀, T₁₀₀₀, T_400-4000, T₅₀₀, T₇₅₀. When annealing temperature changes from 200℃to 320℃, wafers' optical properties can be improved significantly. It can be explained that high temperature is valid for detects diffusing and migrating to wafer surface, and the critical size of which Tl precipitates can migrate may reduce. With the annealing time prolongs from 40h to 200h, optical characteristics enhanced slightly.Wafers were annealed in argon, oxygen atmosphere. The annealing effect of oxygen is better, and the transmissions of T_400-4000 and T₇₅₀ are 63.5% and 33.89%, respectively, which are closed to the theoretical value. In the oxygen annealing process, oxygen atoms can diffuse to the Tl precipitation locality and Tl precipitation phases are oxidized to Tl₂O. Because Tl₂O has good volatility and is very soluble in TlBr, the concentration of Tl precipitation phase in annealed wafer is decreased and optical characteristics are upgraded.Hydrothermal annealing of wafers was investigated. Hydrothermal treatment can effectually eliminate the Tl-rich phase, but the efficiency is low. Thermal annealing process with a combination of hydrothermal annealing can improve wafers' optical properties quickly and efficiently. Transmissions T₄₀₀₀, T₁₀₀₀, T_400-4000, T₅₀₀ and T₇₅₀ of the annealed wafer are 58.56%, 62.41%, 60.89%, 24.68% and 34.94%, respectively. The resistivity value of annealed wafer is 1.023×l0¹¹Î©Â·cm, which meet to the high-resistivity requirement for used as TlBr radiation detectors.