S. Assmann

PHOTOSTIMULATED LUMINESCENCE IN A RARE EARTH-DOPED FLUOROBROMOZIRCONATE GLASS CERAMIC

Photostimulated luminescence at room temperature was observed in a fluorozirconate glass which was doped with 1% Eu2+ or 1% Ce3+ and 5% Br− ions. Small crystals of BaBr2 (high-pressure phase) form in the glass on cooling. The Eu2+ and Ce3+ luminescence spectra comprised two emission lines each: for Eu2+ one line at 413 nm and a broader band centered at 485 nm, and for Ce3+ one line at 320 nm and a broader band at 425 nm. The luminescence is attributed to the 5d14fn−1→4fn emissions of the rare earth ions. In both cases only one of the emission bands showed photostimulated luminescence. The stimulation band for both rare earth ions was centered at 570 nm.

RbBr and CsBr doped with Eu2+ as new competitive X-ray storage phosphors

Abstract After X-irradiation of the alkali halides RbBr and CsBr doped with divalent Eu a significant photostimulated luminescence (PSL) effect was found. In both systems the electron trap centre is an F centre, the hole trap centre is still unknown. The Eu 2+ aggregation with cation vacancies plays an important role. Although all of the different Eu 2+ aggregates seem to show PSL effects, they have different PSL efficiencies, decay times and spectral characteristics. In the case of RbBr:Eu 2+ it was possible to investigate the different Eu 2+ -cation vacancy aggregate states by magnetic circular dichroism of the optical absorption (MCDA) and MCDA-detected electron paramagnetic resonance. In Eu 2+ -doped CsBr no paramagnetic Eu 2+ aggregate nor isolated Eu 2+ was found, in spite of intense and typical Eu 2+ -related PSL effects.

Photoluminescence and crystallization in europium-doped fluorobromozirconate glass-ceramics

Abstract We have investigated barium bromide crystallization in glasses which have an overall composition of 53% ZrF 4 , 20% BaF 2 , 5% NaF, 15% NaBr, 1% LaF 3 , 3% AlF 3 , 2% YF 3 , and 1% EuF 2 . The glasses were prepared by melting at 850°C in an inert atmosphere and quenching into a brass mold. Subsequent thermal treatment of the glass yields a transparent or translucent fluorozirconate glass containing BaBr 2 crystals; this glass-ceramic material has potential applications as an X-ray storage phosphor. The glasses and glass-ceramics were investigated using photoluminescence, optical density, X-ray diffraction, and differential scanning calorimetry methods. Glasses which have been quenched to temperatures below 260°C are transparent and show no photoluminescence from the europium ions. When the glasses are subsequently heated above about 263°C, crystals of BaBr 2 in the hexagonal phase start to precipitate and photoluminescence from Eu 2+ ions is observed. It is possible to prepare the resulting glass-ceramic in transparent form by heating to temperatures just below the temperature of most rapid crystallization, T x =280 C. The photoluminescence is attributed to Eu 2+ ions replacing Ba 2+ ions in the BaBr 2 crystals.

Photostimulated luminescence process in the x-ray storage phosphor CsBr:Ga+

CsBr doped with Ga+ has previously proved to be an efficient X-ray storage phosphor with a figure of merit comparable to that of the commercially used BaFBr:Eu2+. By measuring the magnetic circular dichroism of the optical absorption (MCDA), the MCDA-detected electron paramagnetic resonance and the photostimulated luminescence (PSL), the paramagnetic defect centers generated upon x-irradiation involved in the storage and read-out process were investigated and found to be particularly simple: The electron traps are F centers, the hole trap centers are Ga2+ centers with a hyperfine interaction with 69Ga of 69A=6.5 GHz. No superhyperfine interaction was resolved. It is proposed that the PSL-active Ga2+ center is an isolated Ga2+ on a Cs+ site.

Ga2+ hole centers and photostimulated luminescence in the x-ray storage phosphor RbBr:Ga+

It was previously shown that RbBr doped with Ga+ is an efficient x-ray storage phosphor with a figure of merit comparable to that of the commercially used BaFBr:Eu2+. The paramagnetic hole centers generated upon x-irradiation involved in the storage and read-out process were investigated by measuring the magnetic circular dichroism of the optical absorption (MCDA), the MCDA-detected electron paramagnetic resonance (MCDA-EPR) and the photostimulated luminescence (PSL). Two different Ga2+ hole centers with a hyperfine interaction with 69Ga of 69A=8.2 GHz and 69A=6.0 GHz, respectively, were found. No superhyperfine interaction was resolved. Therefore, the structural difference between the two Ga2+ centers could not be established from EPR. The PSL effect is correlated only to one of the two Ga2+ centers, namely that with A=8.2 GHz. The generation mechanism of the two hole centers is investigated. It is proposed that the PSL-active Ga2+ center is an isolated Ga2+ on a Rb+ site, whereas the other center is a G...

ENDOR Investigation of the Eu2+ Activator in the X-Ray Storage Phosphor BaFBr

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) have been used to investigate the surrounding of the activator ion Eu 2 in single crystals of the X-ray storage phosphor BaFBr. An analysis of the Eu 2+ superhyperfine interaction with the next fluorine neighbours showed that Eu 2+ substitutes for Ba 2+ and is not shifted from a regular position within experimental error, in spite of the large difference in ionic radii. EPR and ENDOR investigations on BaFBr:Eu 2+ powders established that different methods of producing these materials have no influence on the local environment of Eu 2+ .

Site of the Eu2+ Activator in the X-Ray Storage Phosphor SrFCl

With electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) the surrounding of the activator Eu 2+ was investigated in SrFCl single crystals. The analysis of the Eu 2+ superhyperfine interaction with the nuclei of the next fluorine shells yielded that Eu 2+ substitutes for Sr 2+ and is not shifted from a regular lattice site. This result settles previous conflicting conclusions about such a shift.

Radiation damage in rare-earth and bromine-doped fluorozirconate glass ceramics as basis for novel X-ray storage phosphors

Abstract We found a photostimulated luminescence (PSL) effect in Eu- and Ce-doped fluorozirconate glasses which were additionally doped with 5% Br − ions. Small crystallites of the high pressure phase BaBr 2 form in the glass on cooling. The PSL is attributed to the 5 d −4 f emission of Eu 2+ or Ce 3+ . The PSL excitation band peaking at 570 nm is due to radiation-induced electron trap centres which are located either in the Eu- or Ce-doped crystallites or in the glass; where exactly is not yet known.

Magneto-optical investigations of radiation defects in cerium-doped fluorozirconate glasses

Abstract Measurements of the magnetic circular dichroism of the optical absorption (MCDA) on X-irradiated Ce-doped fluorozirconate glass showed an increased paramagnetic band in the range from 300 nm to 700 nm and a new paramagnetic band peaking at 570 nm. Electron paramagnetic resonance (EPR) detected in these MCDA bands yielded two different defect centres having resonances at g =1.90 and g =1.98, respectively.

Excited states of F centres in alkali halides resonant with the conduction band

Abstract With magnetic circular dichroism of the optical absorption (MCDA) and MCDA-detected electron paramagnetic resonance (MCDA-EPR) it is possible to unambiguously identify the higher absorption bands (L1—L3) of F centres. In CsBr, similarly as found earlier for example KCI, there are several transitions beyond the K band to states resonant in the conduction band up to about 1.5 eV. It is observed that the peak-to-peak separation of the MCDA increases with increasing transition energy indicating the dominant role played by lifetime effects additionally to the spin-orbit coupling in the resonance states. In RbBr similar bands and features have been observed. However, the phase of the derivative-like MCDA-L band structures is opposite to that in CsBr. A preliminary theoretical approach to explain the resonance states is presented.