AuBr 3 mechanism in the reduction of halide vacancies is not fully clear, however no Au could be detected in the crystals and it clearly segregates as pure gold rejected on top of the boules at the end of the growth process. Moreover, the reduction of halide vacancies and their effect in breaking up electron–hole pairs created by ionizing radiation suppresses the slow self-trapped holes migration and tunneling process involved in the BaBrCl:Eu scintillation to the benefit of prompt formation of self-trapped excitons followed by the picosecond-scale dipole–dipole energy transfer toward Eu 2+ ions. In a later paper 18 we reported picosecond transient absorption measurements which indicate that the addition of AuBr 3 reduces the concentration of native halide vacancies that can capture electrons to form F centers. 17 This additive strongly improves the scintillation characteristics of BaBrCl:Eu with light yield increases up to 3 times and remarkable reductions in long scintillation tails these improvements are accompanied by a clear reduction in thermally and optically stimulated features related to halide vacancies. In this context, we reported the use of AuBr 3 as additive in the melt during the growth of BaBrCl:Eu single crystals. Several synthesis strategies, from aliovalent (co-)doping 9–12 to band gap engineering and solid solution, 13–16 have been reported over time to deal with defects in scintillators. 6–8 Many R&D efforts are indeed dedicated to the minimization of the impact of defects on the final light emission from scintillators. The competition between charge carrier recombination and trapping is ultimately responsible for the degradation of the scintillator response with a reduction in the light yield, the presence of long scintillation decay tails and afterglow, and luminescence hysteresis phenomena. Free charge carrier thermalization and their transfer to the luminescence centers, in particular, can be strongly affected by the presence of defects acting as traps for the carriers. The light emission from a scintillator is in fact the last of a complex series of events occurring after the initial interaction of ionizing radiation with the material. For this reason, scintillators are currently the subject of intense research and development (R&D) efforts, concentrated on both the discovery of new materials and the optimization of the established ones. 1–3 The wide variety of ionizing radiation in terms of energy, type, and flux used in the various applications implies that no single scintillator is able to meet the specific requirements of all applications. 1 Introduction The ability of inorganic scintillators to efficiently emit visible or ultraviolet light as they are struck by ionizing radiation (be it high energy photons or particles) makes these materials fundamental in medical, industrial, high energy physics and astronomy, as well as security applications. In the case of Cs 2LiLaBr 6:Ce, no clear improvements in either the light yield or the scintillation decay time are visible in the case of low Ce content, while a reduction of light yield upon AuBr 3 addition caused by luminescence quenching phenomena is observed for high Ce concentration. These improvements are, however, associated with a reduction in the energy resolution of these crystals related to a worse energy response non-proportionality. The improvements detected in the case of BaBr 2:Eu are related to a substantial reduction in the long lived scintillation decay tails as well as in the thermally stimulated luminescence amplitude with respect to the crystal grown without AuBr 3 in the melt. Although AuBr 3 is not incorporated in the two crystalline matrices, it increases remarkably, by up to a factor 2, the light yield of BaBr 2:Eu in a similar manner to that observed for BaBrCl:Eu, but is at best ineffective in the case of Cs 2LiLaBr 6:Ce. The results indicate that such an additive has very different effects in the two investigated materials. Results for Cs 2LiLaBr 6:Ce and BaBr 2:Eu single crystals with various Ce and Eu concentrations are presented. Following the observation of large increases in light output of BaBrCl:Eu single crystals using AuBr 3 as an additive in the melt, the impact of this process on the scintillation properties of other Br-based scintillating materials is investigated in an attempt to assess its broader use.
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