Chylii M. Recombination luminescence and size effects in scintillation materials

The work is devoted to the problem of development of new scintillation materials based on the properties of nanoparticle. The aim of the research is to study the features of the radiative relaxation processes of excitation energy in nanocrystals possessing intrinsic and impurity luminescence depending on temperature. It was found that the luminescent properties of nanoparticles depend on their sizes. In the case of band-to-band or high-energy excitation, luminescence quenching was observed at decrease of nanoparticle sizes. Luminescence quenching is caused by excitation energy losses during the free charge carriers migration and intracenter quenching. It was shown that the luminescence intensity of nanoparticles depends less on their sizes as the temperature decreased. We assume that this is due to the electron thermalization length shortening with temperature decreasing. The luminescence quenching model that takes into account the excitons diffusion to the surface was proposed. Using this model, the diffusion lengths of self-trapped excitons in SrF2, CaF2 and the diffusion length of the core holes in BaF2 were estimated. In CaF2:Eu nanoparticles the transformation of Eu3+ → Eu2+ under continued X-ray irradiation was detected and the excitation of Eu2+ ions mainly by the absorp¬tion of singlet excitons emission in CaF2 was found. During the synthesis of SrF2:Ce nanoparticles the aggregation of Ce3+ impurity ions and the formation of CeF3 nanophase are observed. In SrF2:Ce nanoparticles, the excitation energy transfer from single cerium centers to cerium centers in the CeF3 nanophases was revealed. The excitation energy transfer by non-radiative channel due to the multipole interaction is found. The study of the dependence of impurity luminescence intensity from nano-particle sizes was performed in YVO4:Eu system. The method of modeling the depend¬ence of luminescence intensity of YVO4:Eu nanoparticles under X-ray excita¬tion from nanoparticle sizes, which takes into account the dependence of the effective mass of electrons on their kinetic energy, is proposed. The dependence of effective mass of electrons on their kinetic energy for YVO4 was obtained, from which the value of mean effective mass for electrons in conduction band with kinetic energy in the range [0, Eg] was estimated to be 1.5 me. The distribution of secondary electrons by the thermalization lengths is obtained, from which it is estimated that the average electron thermalization length in YVO4 is about 6 nm.