The features of deformation-stimulated RbI luminescence

This paper studies deformation-stimulated features of radiative relaxation of self-trapped excitons and recombination assembly of exciton-like luminescence in RbI crystal. Methods of research were luminescence and thermal activation spectroscopy. The identity of the mechanism of manifestation of the X-ray luminescence, tunnel luminescence and thermally stimulated luminescence spectra were found in the elastically deformed RbI crystal, interpreted by the luminescence of self-trapped exciton, tunnel recharge of F′, VK -pairs and thermally stimulated recombination of e−, VK -centres, respectively.The temperatures of the maximum destruction peaks of thermally stimulated luminescence, their spectral composition and activation energies were determined experimentally, on the basis of which the mechanisms of recombination assembly of exciton-like luminescences in a RbI crystal were interpreted. Uniaxial elastic deformation leads to the effective formation of point radiation defects ( F′, HA, VK -centers) in comparison with an unbroken lattice, where the predominant mechanism is the association of interstitial atoms ( H -centres) with the formation of I3−-centres.

A Photo and Thermally Stimulated Luminescence Study of BaCl2:Eu2+ with Application to Neutron Imaging

<p>This thesis presents the results of a photo and thermally stimulated luminescence study of europium-doped barium chloride in relation to its potential application as a storage phosphor in glass ceramics for radiation imaging, particularly for neutron imaging. Previous work done on lithium borate (LiBO) glasses containing BaCl2:Eu2+ nanocrystals at Victoria University of Wellington had demonstrated comparable imaging capability with commercially available BaFBr:Eu2+ based imaging plates, though the sensitivity and spatial resolution were inferior to that material, and there was a substantial afterglow during the read-out process which degraded any image. These problems are addressed here. The effect of various different co-dopants on the storage properties was examined using the thermally stimulated luminescence (TSL) technique, with dopants primarily chosen from the alkali and alkaline earth elements. The resulting glow curves have been analysed to determine the activation energies associated with the various traps, and tentative assignments of structural defects to the various glow curve peaks are proposed. It was found that Li+ and Na+ gave small increases (20% and 50% respectively) in efficiency, though other dopants tended to reduce the overall output. In particular, K+ and Rb+ were found to substantially reduce the output efficiency. It was found that Li+ co-doped BaCl2:Eu2+ contained thermally unstable traps which at room temperature could result in the observed afterglow in LiBO/BaCl2 glass ceramics through a read-out induced phototransfer process. The experimental measurements required substantial hardware and software development of the existing VUW facilities for TSL, and these improvements are also described here. The most significant improvements are an extension of the operating range at the lower end of the existing TSL spectrometer to -50 degrees C through a cooled gas flow system, and the engineering of a completely new system to record TSL from 25 K to 400 K.</p>

A Photo and Thermally Stimulated Luminescence Study of BaCl2:Eu2+ with Application to Neutron Imaging

<p>This thesis presents the results of a photo and thermally stimulated luminescence study of europium-doped barium chloride in relation to its potential application as a storage phosphor in glass ceramics for radiation imaging, particularly for neutron imaging. Previous work done on lithium borate (LiBO) glasses containing BaCl2:Eu2+ nanocrystals at Victoria University of Wellington had demonstrated comparable imaging capability with commercially available BaFBr:Eu2+ based imaging plates, though the sensitivity and spatial resolution were inferior to that material, and there was a substantial afterglow during the read-out process which degraded any image. These problems are addressed here. The effect of various different co-dopants on the storage properties was examined using the thermally stimulated luminescence (TSL) technique, with dopants primarily chosen from the alkali and alkaline earth elements. The resulting glow curves have been analysed to determine the activation energies associated with the various traps, and tentative assignments of structural defects to the various glow curve peaks are proposed. It was found that Li+ and Na+ gave small increases (20% and 50% respectively) in efficiency, though other dopants tended to reduce the overall output. In particular, K+ and Rb+ were found to substantially reduce the output efficiency. It was found that Li+ co-doped BaCl2:Eu2+ contained thermally unstable traps which at room temperature could result in the observed afterglow in LiBO/BaCl2 glass ceramics through a read-out induced phototransfer process. The experimental measurements required substantial hardware and software development of the existing VUW facilities for TSL, and these improvements are also described here. The most significant improvements are an extension of the operating range at the lower end of the existing TSL spectrometer to -50 degrees C through a cooled gas flow system, and the engineering of a completely new system to record TSL from 25 K to 400 K.</p>

Radiation defects in NaCl matrix with reduced lattice symmetry caused by light cation doping and elastic uniaxial deformation

The processes of radiation defect creation and radiative relaxation of electronic excitations under applied local or/and uniaxial elastic deformation have been studied in NaCl crystals by means of optical absorption, luminescence and thermoactivation spectroscopy methods. In NaCl:Li at 80 K, X-ray-induced absorption bands peaked around 3.35 and 4.6 eV have been detected and ascribed to interstitial halide atoms located nearby Li impurity cations, HA(Li) centres. Subsequent thermal annealing of HA(Li) centres leads to the formation of polyhalide centres responsible for the absorption band at 5.35 eV. In an X-irradiated and stressed NaCl:Li crystal (degree of uniaxial elastic deformation of ε = 0.9%), the peak of thermally stimulated luminescence at ~115 K is composed of the ~2.7-eV emission appearing, in our opinion, due to the recombination of the electron, thermally released from an F′ centre, with a hole-type HA(Li) centre. The applied uniaxial elastic stress facilitates the self-trapping of anion excitons in regular regions of a NaCl lattice and impedes the energy transfer by mobile excitons to impurities/defects and, in turn, attenuates the Br-related luminescence peaked at 3.95 eV with respect to the π-emission of self-trapped excitons (~3.35 eV). The 3.95 eV emission has been detected in a natural NaCl crystal containing homologous Br impurity ions.

New Perspective on Thermally Stimulated Luminescence and Crystallization of Barium Borate Oxyfluoride Glasses

The demand for modern materials, especially glasses, used in different applications, such as radiation sensors and spectral converters, requires a detailed study of their properties. The incorporation of fluoride compounds in borate glasses and their crystallization at the nanometric scale allows the properties of these materials to be further enhanced. Although many works showed improvements in some of these properties, some critical aspects, such as the crystallization mechanism and the role of the fluorine phase, need more investigation. We worked with xNaF (100 − x)BaO·2B2O3 glasses with x = 0, 5, 10, 15, 20, 25, 30, and 35% (in mol) to increase the knowledge in this field. The structural modifications and the thermally stimulated luminescence of the glasses were studied, and their crystallization was analyzed by thermal analysis and X-ray diffraction. A continuous trap distribution was found, which was responsible for its very good luminescent signal, especially in glasses with 20% NaF. By selecting a suitable amount of NaF, it is possible to obtain nanocrystals of BaF2. These promising results we reached show the applicability of these materials.