The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Munoz-Garcia et al. ( unoz-Garcia, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).

Luminescence and Luminescence Quenching in Gd3(Ga,Al)5O12 Scintillators Doped with Ce3+

Intrinsic emission of complex oxides with garnet structure was studied by means of the analyses of the time-resolved luminescence spectra and decay kinetic for single crystals (SC) and single-crystalline films (SCF) of Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) garnets under excitation by the pulse X-ray and synchrotron radiation at 8–300 K. YAG and LuAG SC and SCF are characterized by significantly different concentration of the YAl and LuAl antisite defects (AD) due to the substantial difference in the conditions of their crystallization. For this reason, LuAG and YAG SC and SCF are very useful model objects for the study of nature of the intrinsic emission of these oxides, consisting of the excitons and AD luminescence. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton and antisite defect-related luminescence in Lu3Al5O12 and Y3Al5O12 garnets

We have developed a new nonstoichiometric Gd 3+δ (Al,Ga) 5-δ O 12 :Ce garnet scintillator for an X-ray CT scanner. For commercially used scintillators, reproducible luminescence properties are of great importance. We have found that the afterglow properties of the garnet change drastically in the vicinity of the stoichiometric compositions. The most suitable properties are obtained at slightly excess (Gd,Ce) compositions from the stoichiometry. In order to investigate the afterglow properties, crystallography arrangements, phase equilibrium relationship between crystal phases, and luminescence characteristics of host material doped with different valence ions have been examined. The luminescence properties are explained by isoelectronic traps based on the difference in the electronegativity of the dopant and host ions, which would act as electron trap centers and hole trap centers within the band gap.

Characteristics of a Nonstoichiometric Gd3+δ(Al,Ga)5−δO12:Ce Garnet Scintillator

A cadmium tungstate crystal scintillator enriched in 106 Cd was developed for an experiment to search for double beta processes in 106 Cd. With this aim samples of cadmium with natural isotopic composition and enriched in 106 Cd were purified by vacuum distillation. Cadmium tungstate compounds were synthesized from solutions. The absolute isotopic composition of the enriched cadmium was accurately determined as 66.4% by thermal ionisation mass-spectrometry. A 106 CdWO 4 crystal boule was grown by the low-thermal-gradient Czochralski technique. The total irrecoverable loss of the enriched cadmium in all the stages of the crystal scintillator production does not exceed 2.3%. The produced 106 CdWO 4 crystal scintillator with mass of 216 g exhibits good optical and scintillation properties.

Development of enriched 106CdWO4 crystal scintillators to search for double β decay processes in 106Cd

Energy transfer to Ce3+ ions in Tb3Al5O12:Ce single crystalline films

Epitaxial structures of garnets as scintillation detectors of ionizing radiation

We consider the factors determining the energy yield of crystallophosphors based on single-crystal films of oxides of the system Al2−Y2O3−R2O3 (R is the rare-earth ions) that are used as different types of luminescent converters. Special features of producing the films by the method of liquid-phase epitaxy from Pb−B2O3 and Bi2O3-based fluxes are analyzed, and the advantages of this method for production of efficient luminophors, which are based on high-melting oxides doped with mercury-like impurities, are shown. It is established that the main factor that bounds the crystallophosphor luminous yield is the presence of donor-acceptor complexes of the Pb2+−Pb4+ and Fe3+−Fe2+-type that form different channels of dissipation of excitation energy. The means of minimizing the contribution of these ions to the processes of excitation-energy relaxation are discussed.

Factors determining the energy yield of luminophors based on single-crystal films of Al2−Y2O3−R2O3 oxides

We analyze the optical characteristics of crystals within the framework of a model representation that presupposes the entry of the background impurity of iron ions in two valence states, Fe3+ and Fe2+, into regular lattice nodes. The spectral manifestation of intrinsic defects of vacancy-type structure depending on the conditions of crystal growth and heat treatment is comprehensively investigated. With allowance for the specific features of the technology applied, we have tested the technique suggested in scientific periodicals for bleaching ZnWO4 and CdWO4 crystals by doping with nonisovalent ions, in particular, with Sb and Ag impurities.

Role of intrinsic defects and impurities in forming the optical characteristics of ZnWO4 and CdWO4 crystals

Antisite defects in fluorescence of crystallophosphors with garnet structure

Special features of the incorporation of a copper impurity into CsBr crystals are studied for different types of impurities (metallic copper, CuO, and CuBr2) and crystal-growth techniques (the Bridgman method or the method of isothermal evaporation from solution). The optical characteristics of these crystals (absorption, photoluminescence, and photostimulated-luminescence spectra) are investigated. The copper impurity is shown to enter CsBr:Cumet crystals in the monovalent state Cu+ and CsBr:CuO and CsBr:CuBr2 crystals in the divalent state Cu2+. It is found that the CsBr crystals doped with Cu+ and Cu2+ ions exhibit intense photostimulated luminescence and can be used as storage phosphors for visualization of X-ray images.

M. M. Batenchuk

Papers

Epitaxial structures of garnets as scintillation detectors of ionizing radiation

Factors determining the energy yield of luminophors based on single-crystal films of Al2−Y2O3−R2O3 oxides

Role of intrinsic defects and impurities in forming the optical characteristics of ZnWO4 and CdWO4 crystals

Antisite defects in fluorescence of crystallophosphors with garnet structure

Growth and luminescence of CsBr:Cu crystals