09. October 2015. at 16:15 in Physicum's aud B103 Raul Laasner will defend his doctoral theses "Excited state dynamics under high excitation densities in tungstates" .
Dr Vitali Nagirnõi, University of Tartu, Institute of Physics
Dr Deniss Grjaznovs, Latvian University, Institute of Solide State physics
Tungstate crystals are widely used as scintillators for the detection of ionizing radiation, with applications ranging from medical imaging to security systems to high energy physics. In order to improve the efficiency of scintillators, a better understanding of their scintillation mechanisms is required. This includes the phenomenom of scintillator nonproportionality, which is one of the major unsolved problems for scintillators. Nonproportionality means that the total light output of the scintillator is not proportional to the energy of the absorbed high-energy particle and leads to a considerable worsening of the energy resolution. This work includes the theoretical and experimental investigation of the excited state dynamics that constitute the scintillation process. We develop several models describing the excitation, thermalization, and the luminescence stage. The models are successfully tested on experimental data obtained for the selected tungstate crystals. Most importantly, we are able to provide an accurate description of nonproportionality, which is based on the dipole-dipole interaction of excitons and is applicable to