Defect-Driven Radioluminescence Sensitization in Scintillators: The Case of Lu2Si2O7:Pr

A prompt transport of radiation induced free carriers toward luminescence centers is a key factor for an efficient conversion of high energy radiation into light in scintillator materials. However, the transport stage of the scintillation process can be hampered by the presence of lattice imperfections. In this study, we investigated the increase of radioluminescence (RL) efficiency after prolonged material irradiation. The general character of the phenomenon was revealed by its occurrence in several scintillator crystals like Lu2Si2O7:Pr, Bi4Ge3O12, Lu3Al5O12:Ce, and Lu0.3Y0.7AlO3:Ce. A detailed investigation was carried out for Lu2Si2O7:Pr. We demonstrated that the RL efficiency increase is due to the progressive filling of traps responsible for the thermoluminescence glow peaks at 460 and 515 K, which compete with Pr3+ centers in free carrier trapping during irradiation. Correspondingly, their emptying results in the recovery of the initial lowest RL efficiency. Spatial correlation between traps and Pr3+ ions was evidenced by the detection of an a-thermal tunneling afterglow between trap levels and 4f excited states of Pr3+. Such hysteresis phenomenon represents a memory of previous irradiations that remains stored in the crystal. It is a further effect caused by traps, which deserves attention like other manifestations of defects in scintillators like the reduction of light yield and the occurrence of slow tails in the scintillation time decay.