Short-Living Defects and Recombination Processes in Li 6Gd(BO3)3 Crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium-gadolinium orthoborate Li6Gd(BO3)3 (LGBO) in the visible and ultraviolet spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li0 centers. At 290 K, the Li0 centers are involved in thermally stimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow TOA decay kinetics components, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-living Frenkel defect pairs in the LGBO cation sublattice have been analyzed. © 2009 Elsevier Ltd. All rights reserved

Search for Rare Nuclear Decays with HPGe Detectors at the STELLA Facility of the LNGS

Results on the search for rare nuclear decays with the ultra low background facility STELLA at the LNGS using gamma ray spectrometry are presented. In particular, the best T1/2 limits were obtained for double beta processes in 96Ru and 104Ru. Several isotopes, which potentially decay through different double beta decay channels, including also possible resonant double electron captures, were investigated for the first time (156Dy, 158Dy, 184Os, 192Os, 190Pt, 198Pt). Search for resonant absorption of solar 7Li axions in a LiF crystal gave the best limit for the mass of 7Li axions (< 8.6 keV). Rare alpha decay of 190Pt to the first excited level of 186Os (Eexc = 137.2 keV) was observed for the first time.Comment: 4 pages, contribution to the MEDEX'13 Conference, corrected list of author

Searches for neutrinoless resonant double electron captures at LNGS

Several experiments were performed during last years at underground (3600 m w.e.) Laboratori Nazionali del Gran Sasso (LNGS) of the INFN (Italy) to search for resonant 2$\varepsilon0\nu$ captures in 96Ru, 106Cd, 136Ce, 156Dy, 158Dy, 180W, 184Os, 190Pt with the help of HP Ge semiconductor detectors, and ZnWO4 and 106CdWO4 crystal scintillators. No evidence for r-2$\varepsilon0\nu$ decays was found, and only T_{1/2} limits were established in the range of 10^{14}-10^{21} yr.Comment: Proceedings of TAUP 2011 Conferenc

Temperature dependence of the luminescence of Li6GdxY1-x(BO3)(3): Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li 6Gd x Y 1 - x (BO 3) 3:Eu (LGYBO:Eu) have been studied. The photoluminescence (PL) spectra upon selective photoexcitation to lower excited states of Gd 3+ and Eu 3+ ions and the temperature dependences of the photoluminescence intensity at different energies of excitation photons have been measured in a wide temperature range: 10-500 K. We observed a photoluminescence band at 3.97 eV, which is due to the 6 P J → 8 S 7/2 transitions in Gd 3+ ions, and a characteristic line spectrum in the range of 1.6-2.2 eV, which is related to the radiative f-f transitions in impurity Eu 3+ ions, occurring mainly from the lower excited 5 D 0 level to the 7 F J states (J = 0, 1, ⋯, 6). The influence of the O-Eu charge transfer states and the vibrational relaxation between the 6 I J and 6 P J levels of the Gd 3+ ion on the temperature dependences of the intrinsic and impurity luminescence intensities is discussed. © 2012 Pleiades Publishing, Ltd

Transformation-assisted consolidation of Y2O3:Eu3+Y_{2}O_{3}:Eu^{3+} nanospheres as a concept to optical nanograined ceramics

A concept is proposed to produce Y2O3:Eu3+ optical nanograined ceramics by the transformation-assisted consolidation of nanospheres under 8 GPa pressure. The nanoceramics were prepared by high-pressure low-temperature sintering (~0.04–0.2Tm, where Tm is the melting temperature) accompanied by the cubic-to-monoclinic phase transition. The effects of sintering conditions upon phase composition, grain size evolution, density, morphology, optical and luminescent properties of sintered ceramics have been studied. It has been shown that Y2O3:Eu3+ nanograined ceramics consisting of individual (cubic or monoclinic) phases or their mixture can be obtained by variation of the sintering temperature. The use of transformation-assisted consolidation makes it possible to prepare Y2O3:Eu3+ nanoceramics with average grain size three times smaller (12 nm) than that of the starting nanopowders (37 nm), which corresponds to extremely low grain growth factor of 0.3. The grain size refinement is related to numerous nucleation events in the parent phase of cubic yttrium oxide. The preparation conditions of translucent (T=50%) composite Y2O3:Eu3+ nanograined ceramics with a relative density of 99±1 % have been determined. The obtained two-phase ceramics show high optical transparency due to negligible birefringence at extremely small (~12 nm) average grain size