Trap Engineering Approach for a Theory of X-ray Induced Memory Effects in Scintillators and Phosphors

Radiation induced sensitization in scintillator materials (also commonly called “bright burn” or “memory effect”) represents an issue for those applications, like medical imaging and real-time radioluminescence dosimetry, which strongly rely on high reproducibility and dose linearity of the luminescence output. This effect is caused by defects which act as traps for radiation-induced free carriers and compete with the recombination centres along the recombination path, thus lowering the luminescence efficiency of the scintillation materials. Considering that the competition between these two processes is driven by the capture cross sections and by the relative concentration of empty traps with respect to that of the recombination centres, the progressive trap filling during ionizing irradiation results in an increase in the radiative recombination probability and, thus, in an enhancement in the RL intensity. Despite such effect might have profound consequences on scintillator performance, it has rarely been studied and only few reports are present in the literature [1-3]. Indeed, the use of standard scintillators (like LSO, YAG, …) may prove difficult for a more thorough investigation on this effect mainly because of the critical dependence on material reproducibility in term of traps which are related to uncontrolled impurities and defects.We recently proposed YPO4:Ce,Nd as a model material for such a study [4], since the main electronic trap is represented by Nd3+ ions [5] whose content can be easily selected during the sample synthesis. The obtained results clearly showed that RL efficiency increase is strongly dependent on the Nd content and on measurement temperature. On the basis of the experimental results, a model based on thermoluminescence trap filling and able to accurately describe the memory effect was proposed. In this communication, we extend our work on the memory effect performed on YPO4:Ce,Nd by considering two other ions (namelyPr andDy) in place of the Nd ones. The various codopants gives rise to electronic traps with different characteristics. The results obtained with the Dy and Pr-codoped crystals are compared with those on the Nd-codoped ones in order to put in evidence the role of trap thermal stability, and doping ions in determining the memory effect amplitude and shape. The obtained experimental results are the base for the further refinement of the trap filling model recently proposed for YPO4:Ce,Nd. The model validity and generality will also be discussed.