Center-trap aggregations in oxide scintillators

A prompt transport of radiation induced free carriers towards 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. Spatial correlation between defects and luminescent centers can occur due to the need of local charge compensation, or due to the presence of slight lattice distortions induced by a different ionic radius of the luminescent species with respect to lattice constituents. Such a spatial correlation between defects acting as traps and centers causes a strong competition between them in the capture of free carriers during the very final stage of their relaxation. The occurrence of center-trap aggregates can be put in evidence by the analysis of thermo- , radio-luminescence, and afterglow phenomena. Several examples will be shown concerning bulk oxide scintillators like lutetium ortho- and pyro-silicates, perovskites, and tungstates. The case of rare earth doped nanoparticles will also be presented as an example of particular spatial confinement between traps and centers. The influence of such aggregations at the atomic scale in the scintillation efficiency, timing, and radiation damage will be discussed.