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A detailed analysis of the luminescence behaviour of Yb 3+ -doped and Yb 3+ -Tb 3+ co-doped strontium aluminates powders: Sr 1-x Yb x Al 4 O 7 (x=0.002–0.07) and Sr 1-x-y Yb x Tb y Al 4 O 7 (x=0.03; y=0.002–0.02) were performed. The studies of singly doped samples show that direct excitation of Yb 3+ by means of 2 F 7/2 - 2 F 5/2 absorption at 900–980 nm leads to Stokes Yb 3+ emission in the range of 970–1130 nm as well as bluish-green Yb 3+ cooperative luminescence (CL) whose energy doubles that of the NIR one. The effect of activator concentration and charge compensation through Na + co-doping on both Yb 3+ emissions were also studied. It was found that Na + addition enhanced Stokes Yb 3+ photoluminescence brightness, while the cooperative emission intensity appeared to be lower. In doubly Yb 3+ -Tb 3+ doped materials excitation at 980 nm led to cooperative sensitization of the Tb 3+ 5 D 4 level giving rise to its green 5 D 4 → 7 F J (J= 7 F 6 - 7 F 3 ) up-conversion luminescence with the dominant component around 542 nm. The cooperative energy transfer (CET) mechanism was proposed basing on the results obtained from emission and absorption spectra, decay kinetics as well as the dependence of UC luminescence intensity on NIR excitation power. [ABSTRACT FROM AUTHOR]

Abstract: Two series of calcium gallate phosphors: Ca1−x Eu x Ga4O7 and Ca1−2x Eu x Na x Ga4O7 (x=0, 0.002, 0.01, 0.02, 0.03, 0.05) were synthesized by a modified Pechini method and their optical properties at 298 and 77K were investigated. In undoped CaGa4O7 upon 255nm excitation a bluish white emission (λ max=500nm) followed by an afterglow of the same color lasting for 10–20s was observed. Eu3+-doping quenched the host-related luminescence and the characteristic red emission of the dopant with maximum at 613nm appeared. Its excitation spectrum consisted of a broad band assigned to ligand to metal, O2−→Eu3+, charge transfer absorption and narrow lines arising from intraconfigurational transitions within the 4f6 states of Eu3+ ion. The effects of Eu3+ concentration and Na+ co-doping on the luminescence properties and decay kinetics were studied. Low temperature emission spectra showed that Eu3+ ions are positioned in environments of different symmetries. Their relative populations changed with the activator content. Co-doping with Na+ ions led to a remarkable reduction of the number of Eu3+ sites as well as to noticeable improvement of the luminescence brightness though it did not affect the decay time of the emission. The quantum efficiencies of singly doped CaGa4O7:Eu3+ were very low (in the range of 1–3.7%). Na+ co-doping improved this parameter leading to the highest efficiency of 11% for CaGa4O7:3%Eu3+,3%Na+. [Copyright &y& Elsevier]

Scintillator materials are widely used in X-ray medical imaging detector applications, coupled with available photoreceptors like radiographic film or photoreceptors suitable for digital imaging like a-Si, charge-coupled devises (CCD), complementary metal-oxide-semiconductors (CMOS) and GaAs). In addition, scintillators can be utilized in non-medical imaging detectors such as industrial detectors for non-destructive testing (NDT) and detectors used for security purposes (i.e. airport luggage control). Image quality and dose burden in the above applications is associated with the amount of optical photons escaping the scintillator as well as the amount of optical photons captured by the photoreceptor. The former is characterized by the scintillator efficiency and the latter by the spectral matching between the emission spectrum of the scintillator and the spectral response of the photoreceptor. Recently, a scintillator material, europium-activated lutetium oxide (LuO:Eu), has shown improved scintillating properties. LuO:Eu samples of compact nanocrystalline non-agglomerated powder were developed in our laboratory using homogeneous precipitation from a water-toluene solution in the presence of polyvinyl alcohol as a surfactant. In order to test their light-emission properties, experimental measurements under the excitation of X-ray spectra with X-ray tube voltages between 50 kVp and 140 kVp were performed. This range of applied voltages is appropriate for X-ray radiology, NDT and security applications. LuO:Eu was evaluated with respect to output yield and spectral compatibility of digital imaging photoreceptors (CCD-based, CMOS-based, amorphous silicon a:Si flat panels, ES20 and GaAs). High light yield and spectral compatibility increase the performance of the medical detector and reduce the dose burden to the personnel involved. In addition a theoretical model was used to determine the values for the LuO:Eu optical photon light propagation parameters. The inverse diffusion length was found to be equal to 33 cm/g. In addition LuO:Eu was found to match well with several photoreceptors capable of digital imaging (i.e. GaAs). [ABSTRACT FROM AUTHOR]

Abstract: Two series of calcium aluminates: Singly doped CaAl4O7:Yb(x%) and co-doped CaAl4O7:Yb(x%),Na(x%) (x=0.002, 0.01, 0.02, 0.04, 0.05, 0.06, 0.07, 0.1) were synthesized by a modified Pechini citrate process and their optical properties at 298K and 77K were investigated. Direct excitation of Yb3+ by means of 2F7/2→2F5/2 absorption at 940–980nm leads to a reversed 2F5/2→2F7/2 transition giving Yb3+ emission in the range of 970–1130nm as well as bluish-green up-converted (UC) luminescence whose energy doubles that of the near-IR (NIR) one. The UC luminescence was characterized by absorption and emission spectra, decay time measurements as well as its intensity dependence on the pumping power. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and luminescent properties were also studied. The co-doping affected the luminescence behavior but had no influence on the powders morphology. Na+ addition led to narrowing of NIR luminescence bands, suggesting smaller disturbance of Yb3+ ions local symmetries and also significantly enhanced NIR and UC luminescence of Yb3+. [Copyright &y& Elsevier]

Abstract: The Optical Microscope (OM) on the Phoenix Mars lander (operated from May through October 2008) was used to search for visible-wavelength luminescence of soil particles excited by ultraviolet (UV) illumination (λ=360–390nm). No luminescent particles were found, with the possible exception of a few potentially luminescent features comprising about 0.02% of the total soil volume. The luminescence quantum efficiency of bulk soil as well as individual soil particles at the Phoenix site is constrained to less than 0.04%. A similar UV experiment will be performed by the Mars Hand Lens Imager (MAHLI) on the upcoming Mars Science Laboratory (MSL) mission. We compare OM and MAHLI UV experiments to each other and suggest a strategy to search for UV-excited luminescence with MAHLI. [Copyright &y& Elsevier]

Abstract: Comparative analysis of the luminescent properties of Y3Al5O12:Ce (YAG:Ce) transparent optical ceramics (OС) with those of single crystal (SC) and single crystalline film (SCF) analogues has been performed under excitation by pulsed synchrotron radiation in the fundamental absorption range of YAG host. It has been shown that the properties of YAG:Ce OC are closer to the properties of the SCF counterpart, where YAl antisite defects are completely absent, rather than to the properties of SC of this garnet with large concentration of YAl antisite defects. At the same time, the luminescence spectra of YAG:Ce OC show weak emission bands in the 200–470nm range related to YAl antisite defects and charged oxygen vacancies (F+ and F centers). YAG:Ce ОС also possesses significantly larger contribution of slow components in the Ce3+ luminescence decay under high-energy excitation in comparison with SC and SCF of this garnet due to the involvement of antisite defects, charged oxygen vacancies as well as boundaries of grains in the energy transfer processes from the host to the Ce3+ ions. [ABSTRACT FROM AUTHOR]