M. Secu

Photostimulated luminescence in Eu-doped fluorochlorozirconate glass ceramics

We report the synthesis of Eu2+- and chlorine-doped fluorozirconate glass-ceramics that show an intense photostimulated luminescence (PSL) after x-ray irradiation at room temperature. The PSL efficiency is up to 80% of that found in the well-known crystalline x-ray storage phosphor BaFBr:Eu2+, and it is the largest thus far reported for a glass-ceramic. We attribute the PSL to crystallites of orthorhombic BaCl2 that are formed after annealing above the glass temperature. Hexagonal BaCl2 crystallites are also observed after short annealing times, but they do not provide a measurable PSL signal. The photoluminescence peak from glass-ceramics containing orthorhombic BaCl2 crystallites occurs at 402 nm, and the stimulation band is centered at about 560 nm.

Photostimulated luminescence from fluorochlorozirconate glass ceramics and the effect of crystallite size

We report the results from a study of europium-doped fluorochlorozirconate glasses that have been thermally processed to induce nucleation and crystallization of BaCl2 crystallites. The resulting glass ceramics show a photostimulated luminescence (PSL) effect with a conversion efficiency which is up to 80% of that found in the commercial crystalline x-ray storage phosphor material BaFBr:Eu2+. Thermal processing for 20 min in the range 240–260 °C produces small (∼6–11nm) hexagonal BaCl2 crystallites, while temperatures in the range 270–290 °C lead to the formation and growth of larger (∼15–100nm) orthorhombic BaCl2 crystallites, as well as additional unidentified phases. We observe only weak PSL from glass ceramics containing hexagonal BaCl2 (∼0.1% conversion efficiency compared to BaFBr:Eu2+ at room temperature), but orthorhombic phase crystallites give rise to a much larger relative efficiency that increases with particle size, and reaches ∼80% for 100 nm diameter particles. The PSL is attributed to the ...

Photostimulated luminescence from a fluorobromozirconate glass-ceramic and the effect of crystallite size and phase

We report a systematic study of the photoluminescence (PL), photostimulated luminescence (PSL) and thermostimulated luminescence (TSL) from europium-and bromine-doped fluorozirconate glass-ceramics. Eu2+ ions in the as-prepared glass show no PL, but after suitable thermal annealing hexagonal phase and orthorhombic phase barium bromide crystallites are precipitated and PL is observed from Eu2+ ions in these crystallites. Room temperature PSL is observed from the orthorhombic phase, with an efficiency which is up to 9% of the well known crystalline storage phosphor BaFBr:Eu2+. The emission is at 404 nm, and there is a maximum in the stimulation at 580 nm. We associate the PSL with an optically quenchable peak in the glow curve, which has an activation energy of 1.20 eV and attribute this feature to a perturbed F centre. Room temperature PSL from glass-ceramics containing predominantly the hexagonal phase of BaBr2 has a relative efficiency of less than 0.07%. The resultant trap depth determined from the glow curve is 0.60 eV, which suggests that this trap does not arise from a perturbed F centre. We find that the PSL efficiency systematically decreases with decreasing hexagonal BaBr2 nanocrystallite size and there is no detectable PSL from nanocrystallites smaller than 10 nm. However, below 100 K, the PSL efficiency for the hexagonal phase is nearly as high as for the orthorhombic phase and the concomitant electron traps are perturbed F centres. The measured low temperature emission was at 409 nm and an additional stimulation peak was observed at 700 nm. The PSL in this case is thermally bleached above about 150 K.

Structural phase changes in barium bromide nano-crystals in a fluorobromozirconate glass-ceramic x-ray storage phosphor

The structural changes in a fluorobromozirconate glass ceramic containing a ratio of 5% bromine to fluorine ions, following thermal annealing in the range 240-300 °C, are reported. The changes were monitored through x-ray diffraction, and the photoluminescence (PL) and electron paramagnetic resonance of Eu2+ dopant ions. In the range of 240-275 °C, the barium and bromine ions in the glass precipitate to form the metastable hexagonal phase of barium bromide. The Eu2+ PL spectrum comprises a narrow band at 410 nm, and a weaker broad band centred at 485 nm. The 410 nm band is assigned to two unresolved 4f65d1→4f7 emissions from Eu2+ ions at the two Ba2+ sites in this phase, whilst the 485 nm band is assigned to an impurity associated site or cluster. On annealing at 290 °C, the hexagonal phase transforms to the stable orthorhombic phase, and the PL spectrum comprises a single narrow band centred at 404 nm, assigned to a 4f65d1→4f7 emission from Eu2+ ions at the single Ba2+ site in the orthorhombic phase.

A new fluorozirconate glass-ceramic X-ray storage phosphor

A new fluorozirconate glass-ceramic has been discovered that displays an X-ray storage phosphor effect with an efficiency of up to 8% of that observed in the established crystalline storage phosphor, BaFBr:Eu 2+ . The glass-ceramic is based on the well-known ZBLAN20 formulation, but the replacement of Na by Rb and Li, partial substitution of F by Br, as well as doping with Eu 2+ , results in the formation of RbBa 2 Br 5 :Eu 2+ microcrystallites. X-ray diffraction, photostimulated luminescence and EPR measurements on the as-made glass and the annealed glass-ceramic show that the X-ray storage phosphor effect arises from the RbBa 2 Br 5 :Eu 2+ microcrystallites.

Comparison of the luminescence properties of the x-ray storage phosphors BaCl2: Ce3+ and BaBr2:Ce3+

The single-crystalline x-ray storage phosphor BaCl2:Ce3+ has been investigated by photo- and x-ray luminescence, and also by observing the afterglow and the photostimulated luminescence (PSL). A single active Ce3+ site has been observed with a characteristic emission doublet at 349 and 373 nm at room temperature. The PSL stimulation spectrum shows a clear resemblance to F-type absorption spectra in undoped BaCl2, indicating that the PSL-active electron centres are F-type centres. The results are compared with previous photoluminescence, PSL and new x-ray luminescence data on BaBr2:Ce3+. The integral PSL efficiency in the chloride is found to be somewhat smaller than in the bromide, which is still large enough for storage phosphor applications, while the efficiency in the bromide may be even larger or comparable to that of the commercial x-ray storage phosphor BaFBr:Eu2+.

Luminescence properties of the x-ray storage phosphor BaBr2:Ce3+

Photoluminescence studies on single-crystal samples of the x-ray storage phosphor orthorhombic BaBr2:Ce3+ show the existence of three luminescent Ce sites with distinct emission and excitation spectra. The isolated Ce3+ emits only at low temperatures, while the two other Ce3+ sites are charge compensated and active also at room temperature. One of the charge compensators was identified as a monovalent cation impurity, which was confirmed by the investigation of BaBr2:Ce3+ co-doped with potassium. The same charge-compensated sites serve as efficient emission sources during the read-out of stored images following x-ray exposure. Read-out is a photostimulated luminescence (PSL) process initiated by the excitation of x-ray-induced F-type trapped electron centres in the visible region. There are two different PSL processes, corresponding to the two kinds of charge-compensated Ce centre, each involving trapped electron and trapped hole centres of a slightly different kind in the vicinity of the charge-compensated Ce sites.

Radiation-induced defects and their recombination processes in the x-ray storage phosphor BaBr2:Eu2+

The recombination processes in the x-ray storage phosphor BaBr2:Eu2+ were investigated by optical and magneto-optical methods. A structure-sensitive investigation of the defects involved in the recombination processes was performed by detecting the microwave-induced changes in the recombination luminescence in a high magnetic field. F centres as well as VK hole centres are created after x-irradiation at low temperatures. The low-energy recombination band peaking at about 460 nm is due to F–VK centre recombinations, whereas the two high energy bands at 282 and 315 nm are probably due to recombinations of self-trapped excitons.

Development of X-ray storage phosphor glass-ceramics

Europium- and bromine-doped fluorozirconate glass-ceramics show a significant photostimulated luminescence (PSL) effect after X-irradiation at room temperature. The PSL is assigned to the 5 d -4 f emission of Eu 2+ ions incorporated in barium bromide crystallites which are formed in the glass matrix upon suitable thermal annealing. The BaBr 2 crystallites grows in both the hexagonal and orthorhombic phases. The PSL efficiency of the glass-ceramic containing predominantly the hexagonal phase is only about 0.0025% of the well-known crystalline storage phosphor BaFBr:Eu used as a standard, whereas the glass-ceramic containing the orthorhombic phase has a relative efficiency of a few percent. Using the same formulation for the fluorozirconate glass-ceramic, but replacing Na by Rb and Li, results in the formation of RbBa 2 Br 5 microcrystallites after suitable annealing. The relative PSL efficiency increases to about 10%. The structural changes in these two fluorozirconate glass-ceramics were monitored by X-ray diffraction, photoluminescence, and electron paramagnetic resonance.