The vacancy-cluster mechanism of photocurrent degradation in TlBr detectors under γ-irradiation

The transient photocurrent kinetics of pure and doped TlBr detectors under γ-irradiation with ¹³⁷Cs gamma-ray in dose rate range 0,033-3,84 Gy/min has studied. The crystals grew by Bridgman-Stockbarger directional solidification technique. Mass fraction of Pb or Ca introduced in the doped TlBr crystals were 1-10ppm and 150ppm respectively. The crystals grew in various atmospheres: vacuum, in the vapor of bromine, in an atmosphere of hydrogen and air. The observed kinetics at the structures were a rather complicated and were associated with the electron-ion processes during irradiation. It was significant decay of photocurrent in doped crystals, crystals grown with an excess of thallium and crystals grown in hydrogen. The relaxation time constants were 30-1400s and were proportional to the resistivity of the crystals. The photocurrent decay in TlBr crystals irradiated by gamma radiation of high dose rate was associated with the photolysis and the formation of new recombination centers. The rate of photolysis was proportional to the concentration of Tl⁺ mobile ions and anionic V_a⁺ vacancies. The current decay in doped crystals was connected with the dissociation of the complexes {Me²⁺V_c^-}. The model was discussed to explain the complex behavior of transient photocurrent kinetics.