A. A. Masalov

The nature and mechanism of charging of electron traps in Lu2SiO5:Ce3+ crystals

Investigation of the thermoluminescence (TL) properties depending on the temperature, UV irradiation dose, and activator ion concentration in Lu2SiO5:Ce3+ (LSO:Ce) crystals and measurements of the decay of recombination radiation during photostimulated release of the excitation energy accumulated in these crystals showed that a part of Ce3+ impurity ions exhibit ionization leading to the injection of electrons into the conduction band. The conduction band of a LSO:Ce crystal is involved in two opposite processes: the charging of electron traps and the recombination of electrons with Ce4+ ions. The transport of electrons to the traps has a diffusion character: electrons possess a significant mobility [D(315 K)=10−3 cm2/s] and can diffuse away from the donor ions to distances exceeding the lattice parameter of the crystal. The results of experiments with controlled atomic packing of LSO:Ce nanoclusters unambiguously demonstrated the key role of the structure of oxyorthosilicates in the formation of electron traps. The existing two-center model does not provide adequate description of the properties of electron traps and TL in LSO:Ce crystals.

Spectroscopic Properties of Nanoceria Allowing Visualization of Its Antioxidant Action

Two distinct luminescence centers were revealed in ceria nanocrystals: first one – Ce3+ ions with 5d-4f luminescence at 390 nm, and second one – Ce4+–O2− complexes showing charge transfer (CT) luminescence at 630 nm. Intensity of Ce3+ luminescence depends directly on the concentration of oxygen vacancies in nanoceria and can be varied by means of change of both heat treatment atmosphere from oxidizing to reducing and the size of nanocrystal. Ce3+ luminescence can be used for visualization of the processes of interaction between ceria nanoparticles and reactive oxygen species using relative intensity of Ce3+ band as a measure of Ce4+/Ce3+ ratio during oxidation reaction.

Effect of coactivation with Dy3+ and Yb3+ ions on the efficiency of energy storage in Lu2SiO5:Ce3+ crystals

Cerium-activated lutetium oxyorthosilicate Lu2SiO5:Ce3+ (LSO:Ce) and coactivated LSO:Ce,Dy and LSO:Ce,Yb crystals have been synthesized by the sol-gel technique. It is shown that the introduction of coactivator (Yb and Dy) ions influences the energy storage in LSO:Ce, thus making it possible to control the afterglow and thermoluminescence in these crystals. The observed effect is related to the electron properties of coactivator ions (donor against acceptor), which determine the recharge of electron traps in LSO crystals.

Luminescent and scintillation properties of composites based on sol-gel SiO2 matrices and organic scintillators

Luminescent composites based on SiO2 matrices synthesized using the sol-gel method and organic scintillators PPO and o-POPOP are produced, and their optical, luminescent, and scintillation characteristics are studied. It is shown that these composites generate an intense photoluminescence signal, possess a nanosecond decay time, and have a transparency in the range of 400–700 nm of no less than 70%. The absolute light output during excitation by α radiation with an energy of 5.46 MeV is 4400–5100 photon/MeV, and the amplitude resolution is 27–32%.

Improving of LSO(Ce) Scintillator Properties by Co-Doping

The influence of the co-doping of cerium-doped lutetium oxyorthosilicate (LSO) with ytterbium on the output characteristics of this scintillator material were investigated by studying X-ray luminescence spectra, emission decay kinetics, afterglow level, Vickers indentation hardness, thermally stimulated luminescence, and radiation stability of luminescence parameters. It has been demonstrated that the optimal doping contents are ~ 0.1 mol.% and ~ 0.5 mol.% for cerium and ytterbium, respectively. A considerable increase in the radiation stability of light output, decrease in the afterglow level by more than two orders of magnitude and a 20% decrease in indentation hardness at the optimal concentration of Yb have been observed, however, at the expense of the light output.

Processes of excitation energy transport in EuPO4 and EuP3O9 nanocrystals

Processes of excitation energy transport were investigated for EuPO4 and EuP3O9 nanocrystals, in which Eu3+ ions form three- and one-dimensional subsystems, respectively. It was determined that for EuPO4 nanocrystals 5D0 luminescence quenching manifested itself even at low temperatures due to effective phonon-assisted energy migration. At the same time, for EuP3O9 nanocrystals no influence of energy migration on the luminescence quenching was observed in the whole temperature range from 10 to 293 K that can be ascribed to modification of phonon spectra for EuP3O9 nanocrystals as compared to their bulk counterparts.

Development of Nanocomposite Alpha-Detectors Based on Silica Matrices and Organic Scintillators

New materials based on highly-porous SiO2 matrices with incorporated molecules of organic scintillators (POPOP, PPO) were fabricated. The composite scintillators are characterized by decay times in the nanosecond range, a high transparency in the visible region (about 70 %) and an intensive photoluminescence. At excitation by 5.46 MeV alpha particles the light yield of SiO2 (POPOP, PPO) was equal to 4,400–5,100 photon/MeV, the resolution was 27–32 %.

Temperature-dependent segregation of Pr3+ impurity ions in Y2SiO5:Pr3+ and YPO4:Pr3+ nanocrystals

Stationary and time-resolved spectroscopic methods are used to show that the impurity ions in Y2SiO5:Pr3+ and YPO4:Pr3+ nanocrystals are distributed nonuniformly. This nonuniform distribution is found to be caused by the temperature-dependent segregation of Pr3+ ions near the surface of a nanocrystal. The motion of the activator ions from the bulk of a nanocrystal to the near-surface layer is traced when the activator concentration and the heat-treatment parameters are varied over wide ranges, and the main parameters of this effect (impurity redistribution intensity and time, diffusion coefficient) are estimated.

Temperature-dependent segregation of Pr{sup 3+} impurity ions in Y{sub 2}SiO{sub 5}:Pr{sup 3+} and YPO{sub 4}:Pr{sup 3+} nanocrystals

Stationary and time-resolved spectroscopic methods are used to show that the impurity ions in Y2SiO5:Pr3+ and YPO4:Pr3+ nanocrystals are distributed nonuniformly. This nonuniform distribution is found to be caused by the temperature-dependent segregation of Pr3+ ions near the surface of a nanocrystal. The motion of the activator ions from the bulk of a nanocrystal to the near-surface layer is traced when the activator concentration and the heat-treatment parameters are varied over wide ranges, and the main parameters of this effect (impurity redistribution intensity and time, diffusion coefficient) are estimated.