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The Ce-doped (Lu y Gd 1Àx ) 3 (Ga y ,Al 1Ày ) 5 O 12 single crystals were grown by the micropulling down method. Their structure and chemical composition were checked by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) techniques. Optical, luminescent, and scintillation characteristics were measured by the methods of time-resolved luminescence spectroscopy, including the light yield and scintillation decay. Balanced Gd and Ga admixture into the Lu 3 Al 5 O 12 structure provided an excellent scintillator where the effect of shallow traps was suppressed, the spectrally corrected light yield value exceeded 40 000 photons/MeV, and scintillation decay was dominated by a 53 ns decay time value which is close to that of Ce 3+ photoluminescence decay. This study provides an excellent example of a combinatorial approach where targeted single-crystal compositions are obtained by a flexible, time saving, and cost-effective crystal growth technique.

The effects of organic moieties on the luminescence properties of organic−inorganic layered perovskite-type compounds were investigated. Three single crystals were fabricated, namely, (C 4 H 9 NH 3 ) 2 PbBr 4 {C4}, (C 6 H 5 CH 2 NH 3 ) 2 PbBr 4 {Ben}, and (C 6 H 5 C 2 H 4 NH 3 ) 2 PbBr 4 {Phe}. Among the single crystals, the exciton emission of Phe showed the highest quantum efficiencies. The quantum efficiencies of C4 and Ben only were 0.02 and 0.17 times that of Phe, respectively. The radiative and nonradiative decay rates were calculated from the quantum efficiencies and luminescence lifetimes. The relative values of the quantum efficiencies were in accordance with the values of the radiative decay rates. The results indicate that the luminescence properties of the organic−inorganic hybrid compounds may be governed by the excitonic properties of the inorganic layer and not by the concentration of any structural defects. Focusing on the geometry of the inorganic layers, the Pb−Br−Pb bond angles between the adjoining PbBr 6 2− octahedra of each compound were 150°(Ben), 152°(Phe), and 155°(C4). In addition, only Phe showed structural distortion inside the PbBr 6 2− octahedron with Br−Pb−Br bond angles of 171°. The increase in the radiative decay rate can be attributed to the increase in the reduced mass of the excitons from these structural distortions that lead to a decrease in the Bohr radius of the excitons. The results indicate that the luminescence properties of the organic−inorganic hybrid compounds are governed by the structural geometry of the inorganic layer.

To evaluate the X-ray-induced afterglow phenomenon, we developed an ionizing-radiation-induced luminescence characterization system equipped with a pulse-width-tunable X-ray source. The system consists of a pulse X-ray tube and a detector system based on photon counting. The excitation pulse width was tunable from nano- to millisecond ranges, and the dynamic range of the X-ray-induced afterglow was 106. Conventional scintillators for X-ray CT or security systems, namely, Bi4Ge3O12, CdWO4, Tl-doped CsI, and Tb and Pr-codoped Gd2O2S, were evaluated for the performance test. Results show that the afterglow time profiles of these scintillators are consistent with generally known results with high accuracy.

Scintillation materials and detectors that are used in many applications, such as medical imaging, security, oil-logging, high energy physics and non-destructive inspection, are reviewed. The fundamental physics understood today is explained, and common scintillators and scintillation detectors are introduced. The properties explained here are light yield, energy non-proportionality, emission wavelength, energy resolution, decay time, effective atomic number and timing resolution. For further understanding, the emission mechanisms of scintillator materials are also introduced. Furthermore, unresolved problems in scintillation phenomenon are considered, and my recent interpretations are discussed. These topics include positive hysteresis, the co-doping of non-luminescent ions, the introduction of an aimed impurity phase, the excitation density effect and the complementary relationship between scintillators and storage phosphors.