Kinetics of the current response in TlBr detectors under a high dose rate of $\gamma$-ray irradiation

The kinetics of the photocurrent response in doped and undoped TlBr samples subjected to irradiation with $\gamma$-ray photons from a $^{137}$Cs source with the dose rate 0.033 to 3.84 Gy/min are studied. The crystals were grown by the directional crystallization of the melt method using the Bridgman–Stockbarger technique. The Pb impurity mass fraction introduced into the doped TlBr crystals was 1–10 ppm and amounted to 150 ppm for the Ca impurity. The crystals were grown in a vacuum, in bromine vapors, in a hydrogen atmosphere, and in air. Decay of the photocurrent is observed for extrinsic semiconductor crystals doped with bivalent cations (irrespective of the growth atmosphere), and also for crystals grown in hydrogen and crystals grown in an excess of thallium. The time constant of photocurrent decay $\tau$ amounted to 30–1400 s and was proportional to resistivity. It is shown that the current response can be related to photolysis in the TlBr crystals during irradiation with $\gamma$-ray photons. The energy of hole traps responsible for a slow increase in the photo-current has been estimated and found to be equal to 0.6–0.85 eV.