Results 1 - 10 of 8397
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[en] Recently discovered scintillators Cs4SrI6:Eu and Cs4CaI6:Eu have shown promising properties for gamma-ray detection in homeland security applications, with <4% energy resolution (at 662 keV) and light yields above 50,000 ph/MeV. However, a thorough investigation of the effects of europium concentration has not yet been conducted. In this work, Eu2+ concentration was varied from 0.5 mol% to 9 mol%, and Ø7 mm single crystals were grown from the melt via the vertical Bridgman method. Scintillation performance was evaluated as a function of Eu2+ concentration to determine the optimal amount. The observed trend was improved energy resolution and higher light yield with increasing Eu2+ concentration up to 7 mol%. The best energy resolution achieved was 3.2% for Cs4SrI6:Eu 7% and 3.6% for Cs4CaI6:Eu 7%. Their respective light yields were 71,000 ph/MeV and 69,000 ph/MeV.
[en] An extension of the Judd-Ofelt theory, a theoretical model to the field of Luminescence intensity ratio method in phosphor thermometry has been presented, that enables a calculation of thermometric figures of merit: sensitivity, maximum sensitivity and temperature resolution, by the Judd-Ofelt intensity parameters. The model is applicable for 7 out of 14 lanthanides, and as a testing ground for the model's adequacy, Y2O3 doped with three different Eu3+ concentrations had been chosen. PL spectra were recorded up to 480 °C and photoluminescence and thermometric properties have been obtained. Conventionally estimated thermometric figures of merit have been compared to those obtained by a Judd-Ofelt thermometric model, with a good matching. Consequently, a novel method for calculation of magnetic dipole strength of the mixed induced electric dipole and magnetic dipole transition has been developed. Additionally, an interactive application software has been developed for the quick evaluation of the sensitivity and temperature resolution from Judd-Ofelt intensity parameters. © 2019 Elsevier B.V.
[en] Fluorapatite doped with rare-earth elements has a wide-range of biomedical applications. Here, a new type of fluorapatite nanocrystals doped with praseodymium (FAP-Pr) with excitation-emission profiles in visible part of the spectrum is fabricated. Energy levels of Pr3+ activator ion contain metastable multiplet states that offer the possibility of efficient multicolor emission lines in FAP nanocrystals. Three types of FAP-Pr nanocrystals with 0.1%, 0.5% and 1% atomic percent of Pr3+ (along with the undoped FAP control sample) are studied. Their novel chemical production method is described, the FAP-Pr nanocrystals structure, biocompatibility and the suitability for cell imaging are analyzed. Physicochemical characterization confirms crystals down to nanometer size. In addition, quantum-chemical calculation predicts that Pr3+ ions are incorporated into the FAP crystal lattice at Ca2 (6 h) sites. In vitro viability results shows that FAP-Pr nanocrystals are nontoxic to live cells. Additionally, the cell uptake of the FAP-Pr nanocrystals is studied using fluorescence-based widefield and confocal microscopy. The nanocrystals show characteristic green emission at 545 nm (3P0→3H5 transition of Pr3+ ion) and orange emission at 600 nm (1D2→3H4), which we use to discriminate from cell autofluorescence background. Orthogonal projections across 3D confocal stacks show that the nanocrystals are able to enter the cells positioning themselves within the cytoplasm. Overall, the new FAP-Pr nanocrystals are biocompatible and of the tested types, the 0.5% Pr3+ doped nanocrystals show the highest promise as a tracking nanoparticle probe for bioimaging applications. © 2019
[en] The cathodoluminescence (CL) induced in four oxide single crystals (α-Al2O3, ZrO2: Y or YSZ, MgAl2O4, and TiO2) by high-energy electrons from 400 keV to 1250 keV was studied as a function of beam parameters (flux and energy). The main CL bands are related to F center (oxygen vacancy) formation by elastic collisions above the threshold displacement energy of oxygen atoms. The beam-intensity dependence is interpreted on the basis of a kinetic-rate model involving F-center formation and annihilation. The temperature effect was also followed from 110 K to 300 K. A broad maximum is found for all bands at about 200 K for sapphire, whereas a monotonous increase with temperature is observed for YSZ. The plots of CL intensity versus temperature are mainly interpreted by the interplay between the thermal dependence of thermalized free-carrier trapping rates and luminescence efficiency. Finally, the dependence of CL intensity on the primary electron energy for F centers in YSZ showing a maximum at about 600 keV is explained on the basis of the interplay between point-defect formation and secondary-electron energy spectra production. (authors)
[en] In this study, we present photoluminescence (PL) analyses under UV and X-ray excitations of thin layers of CaWO4 and CdWO4 deposited on SiO2/Si substrates, by radio-frequency sputtering method. The main objective was to determine the efficiency of PL emissions in the case of these specific SiO2/Si substrates. Polycrystalline CaWO4 and CdWO4 phases were used as standards for PL emission analyses. Characterizations of films were carried out by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The PL experiments were carried out under monochromatic UV and polychromatic X-ray excitations. The PL intensities varied with tungstate film thicknesses. In the case of X-ray excitation, oscillations of PL intensities were observed. These oscillations corresponded to interferences of PL emissions, strongly correlated with the thickness of intermediate SiO2 layers. A Fabry-Perot model simulating these oscillations is applied allowing differentiating the PL responses of these films. (authors)
[en] A theoretical model for approximate evaluation of CIE chromaticity coordinates from the Judd-Ofelt intensity parameters has been presented. By reviewing the photoluminescence properties of all the lanthanides, the model's applicability was restricted to Nd3+, Sm3+, Eu3+, Tb3+ and Dy3+. Explicit equations for calculation of CIE coordinates for each of the allowed lanthanides were tested on Y2O3:Eu3+, P2O5·ZnO·Al2O3·BaO·PbO:Tb3+, LiNbO3:Dy3+, LaF3:Sm3+, LaF3:Eu3+, Ca3Sc2Si3O12:Eu3+, and TeO2·TiO2·Nb2O5:Nd3+ by comparing the CIE coordinates of the actual spectra to the coordinates obtained from the Judd-Ofelt parameters, with overall adequate matching. © 2019 Elsevier B.V.
[en] In this paper we will be presenting JOES (Judd-Ofelt from Emission Spectra), an application software for calculation of the Judd-Ofelt intensity parameters and derived quantities from the emission spectra of Eu3+ doped materials. The program is written to be user friendly and it requires no previous experience in the field of study. This Free and Open-Source program written in JAVA, works on Windows, Linux and MAC OS operating systems. Program has been tested on three europium doped oxides with good luminescent properties: TiO2:Eu3+, ZrO2:Eu3+ and Nb2O5:Eu3+. We wish to give to the researchers this theoretical tool which can make the calculations easier, faster and more reliable. © 2018 Elsevier B.V.
[en] By time-resolved spectrsscopy the intrinsic emission spectrum of UO2MoO4 at 4.2 K is obtained. The main progressions in the vibronic structure are identified as couplings with the Asub(g) correlation field components of the symmetric and asymmetric UO2 stretching modes. The intrinsic zero-phonon line in the emission and excitation spectrum is shown to be split both by the crystal field and correlation field. The steady-state emission spectrum at 4.2 K ist dominated at emission from traps. The vibronic structure of the trap emission reveals that all traps are distorted uranyl groups. (orig.)
[en] The effect of 1 MeV electron irradiation and a 100sup(o)C annealing stage on the intensity of the U2 line at 1.1182eV is presented. Evidence is provided from intensity vs. excitation power measurements tha the U2 radiative recombination involves only a single exciton. Eight models are discussed, with a complex isoelectronic center being favored. (orig.)
[en] Ca(Tb)3Ga2Ge3O12 and Sr3[Y(Tb)]2Ge3O12 garnets provide a cubic lattice with trivalent terbium, a luminescent rare earth, in dodecahedral (8 oxygen neighbors) and in octahedral (6 oxygen neighbors) sites, respectively. In the materials examined, Tb3+ in octahedral sites has a narrower fluorescence emission spectrum, higher luminescence efficiency and a longer lifetime than in dodecahedral sites. The luminescence properties and the unit cell size of the Ca3Y2Ge3O12 garnet, where Tb3+ is expected to be located on octahedral sites, are between those of the CaGa and SrY garnets. All these garnets are phosphorescent. On the basis of the variation with temperature of the phosphorecence decay law, it is proposed that the radiative electron-hole recombination process proceeds via a tunneling mechanism at temperatures below the thermal glow peaks, and via a free carrier diffusion mechanism at temperatures above the thermal glow peaks. All three Tb3+-doped germanate garnets have sufficiently high efficiency and persistence under electron beam excitation to be suitable for practical phosphor applications. The strontium yttrium germanate-Tb material has a cathodoluminescence decay time nearly three times that of the commercial P53 garnet phosphor. (orig.)