S. Hesse

CsEuBr3: Crystal structure and its role in the photostimulation of CsBr:Eu2+

CsBr:Eu2+ has recently been investigated as a photostimulable x-ray storage phosphor with great potential for application in high-resolution image plates. In a recent paper Hackenschmied et al. [J. Appl. Phys. 93, 5109 (2003)] suggested that segregations of CsEuBr3 or Cs4EuBr6 formed within CsBr:Eu2+ during annealing are responsible for an increase in the photostimulated luminescence (PSL) yield. In this work single crystals of CsEuBr3 were prepared by a one step synthesis and identified by x-ray diffraction (XRD) analysis as single phase perovskites. It was concluded that, after preparation, CsEuBr3 degrades in normal atmosphere into at least two phases, one of which is the orthorhombic structure of Cs2EuBr5∙10H2O. The XRD powder diffraction pattern of this compound is very similar to that of the segregations observed within CsBr:Eu2+ and reported by Hackenschmied et al. However, the increased PSL yield in CsBr:Eu2+ after annealing cannot be due to the segregations, because the trivalent nature of the eu...

Synthesis and functionality of the storage phosphor BaFBr:Eu2+

The present paper relates to a method for synthesizing the photostimulable storage phosphor BaFBr:Eu2+ that features high stimulation characteristics for use in image plates. All previous synthesis routes consist of a multistep process whereas the presented route is a one-step synthesis. Starting compounds are BaCO3, NH4F, and NH4Br, as well as EuF3 where the ammonium compounds are highly volatile at temperatures below 300°C. Their reaction products initiate the decomposition of BaCO3 and form BaFBr. The incorporation of F-centers and europium ions occurs at temperature exceeding 550°C. The individual reaction steps are monitored by simultaneous thermal analysis (thermogravimetry/differential thermal analysis) and the gaseous reaction products are detected by in situ mass spectroscopy. Intermediate and final products are characterized by x-ray diffraction techniques to determine the involved phases. Photoluminescence spectra reveal the typical emission peak of Eu2+ accompanied by a redshifted emission ban...

Influence of Li-codoping on the radiation hardness of CsBr:Eu2+

The poor radiation hardness of the otherwise excellent x-ray storage phosphor CsBr:Eu2+ constitutes a problem for its commercial application in medical diagnostics. X-ray induced vacancy centers such as M-centers enhance the diffusion of Eu2+ activators resulting in a formation of photostimulated luminescence (PSL) inactive europium clusters or second phases of europium compounds. The present study investigates the influence of Li-codoping on the radiation hardness of CsBr:Eu2+. It is reported that the integration of Li+ into the CsBr:Eu2+ suppresses the generation of M-centers during x-irradiation and thereby partially improves the radiation hardness.

Preparation-induced F-centre transformation in BaFBr : Eu2+

In this paper the transformation of one type of photostimulable colour centre (F-centre) into another, depending on the preparation conditions of the storage phosphor BaFBr : Eu2+, is investigated. Despite the fact that the quasi-binary phase diagram of BaF2-BaBr2 shows no detectable solid solution for either BaF2 or BaBr2 in BaFBr, it will be demonstrated that an imbalance induced on purpose in the bromine/fluorine ratio can alter the shape of the stimulation spectrum from F(F−)- to F(Br−)-centres and vice versa in a subsequent annealing process. It will be demonstrated that such an imbalance can be generated by exposing BaFBr : Eu2+ to an alcohol/water solution between a sintering step and an annealing step. Thereby the water dissolves BaFBr by removing BaBr2 from the surface of the grains, leaving an excess of BaF2 behind. Based on these findings a practical synthesis route will be offered, which can lead to improved phosphor properties. Such properties are a macroscopically single phase of BaFBr : Eu2+ without detectable BaF2 or BaBr2 residuals and solely photostimulable F(Br−)-centres.

Sensitization of the photostimulable x-ray storage-phosphor CsBr:Eu2+ following room-temperature hydration

The x-ray storage-phosphor CsBr:Eu2+ has been found to exhibit a strong increase in photostimulated-luminescence (PSL) intensity following hydration at room temperature. The sensitivity increases by a factor of 25 following a 60 min exposure to an atmosphere of 99% relative humidity, which is considered to be due to an incorporation of H2O molecules within the CsBr:Eu2+ matrix, which are orientated by and enhance the electric fields surrounding (Eu2+–O2−)-dipoles, increasing their charge trapping cross sections. Following irradiation the PSL increase is accompanied by the formation of mobile VK-centers, which decrease the F-center population by a recombination process, which appear as an intrinsic 360 nm emission from the CsBr matrix in low temperature thermoluminescence measurements.

The quasi-binary phase diagram BaF2-BaBr2 and its relation to the x-ray storage phosphor BaFBr : Eu2+

In order to understand the formation and stoichiometry of the x-ray storage phosphor BaFBr : Eu 2+ , the phase diagram of the quasi-binary BaF2–BaBr2 system has been investigated. The phase diagram was obtained by means of differential thermal analysis and temperature controlled x-ray diffraction experiments. The resulting phase diagram indicates that BaFBr forms a compound with no detectable solid solubility for neither BaF2 nor BaBr2. Experiments to obtain non-stoichiometric BaFBr via the synthesis route using BaF2 and NH4Br as proposed in the literature could not be verified. It will be shown that the type of colour centre created during x-ray irradiation is related to the non-stoichiometry of the starting compositions before sintering. A surplus of either barium fluoride or barium bromide during sintering allows the controlled formation of F(Br − )- and F(F − )-centres, respectively.

Non-stoichiometric BaFBr : Eu2+: a study on phase compositions and their relationship to F-centre formation

Even though image plate technology is successfully applied in x-ray detection, many questions concerning its detailed functional mechanisms have still not been answered. In this paper it will be shown that a previously suggested non-stoichiometric monophase of BaF1.12Br0.88 : Eu2+ resulting from the BaF2/NH4Br preparation route cannot be supported. It will be demonstrated by x-ray analysis that depending on the starting ratio of BaF2 to NH4Br during sintering or the already synthesized BaFBr : Eu2+ to BaF2 during annealing, stoichiometric BaFBr : Eu2+ with or without a BaF2 phase will always be obtained. Furthermore, it will be shown that in the case of a molar stoichiometric starting ratio of BaF2/NH4Br, partial sublimation of NH4Br during sintering can be hold responsible for the appearance of BaF2 as a second phase. Rietveld refinement is utilized to derive the quantitative amount of the individual phase compositions of BaFBr : Eu2+ and BaF2. In terms of image plate properties, it will be shown that the composition of BaFBr : Eu2+ and BaF2 after the high temperature treatment determines the type of photostimulable colour centre generated during x-irradiation. Finally, the results obtained will be interpreted in terms of published photostimulated luminescence formation models.

Photoluminescence and photostimulated luminescence of oxygen impurities in CsBr

Oxygen impurities have been detected in undoped CsBr by photoluminescence (PL) spectroscopy and their contribution to photostimulated luminescence (PSL) properties of powdered CsBr is discussed. When excited at 200 nm, PL is observed from CsBr which consists of three separate emission peaks. The intrinsic luminescence of CsBr at 379 nm is accompanied by two emission peaks at 395 and 460 nm which arise from oxygen impurities, the latter of which is also PSL active following x-irradiation. Sintering of CsBr with the reducing agent NH4Br removes the oxygen impurities so that the 395 and 460 nm emissions are no longer detectable, and subsequently the PSL emission is significantly reduced. PSL storage time measurements of these materials show that oxygen impurities favorably increase both the PSL sensitivity and radiation induced charge-center stability in CsBr:Eu2+. The oxygen impurities and their associated luminescence properties can be reintroduced to the CsBr matrix with a controlled concentration by doping with CsOH and then subsequently sintering the resultant CsBr:OH− with NH4Br, which in this system reduces the OH− centers to O2−.Oxygen impurities have been detected in undoped CsBr by photoluminescence (PL) spectroscopy and their contribution to photostimulated luminescence (PSL) properties of powdered CsBr is discussed. When excited at 200 nm, PL is observed from CsBr which consists of three separate emission peaks. The intrinsic luminescence of CsBr at 379 nm is accompanied by two emission peaks at 395 and 460 nm which arise from oxygen impurities, the latter of which is also PSL active following x-irradiation. Sintering of CsBr with the reducing agent NH4Br removes the oxygen impurities so that the 395 and 460 nm emissions are no longer detectable, and subsequently the PSL emission is significantly reduced. PSL storage time measurements of these materials show that oxygen impurities favorably increase both the PSL sensitivity and radiation induced charge-center stability in CsBr:Eu2+. The oxygen impurities and their associated luminescence properties can be reintroduced to the CsBr matrix with a controlled concentration by dopi...

Preparation and optimization of ceramic neutron image plates based on BaFBr : Eu2+ and GdF3

Commercially available neutron image plates (NIPs) consist of a mixture of a powdered x-ray storage phosphor and a neutron converter, both embedded in an organic binder supported on a polymer sheet. The initiation of the storage mechanism in the phosphor is caused by conversion electrons generated in the neutron converter due to neutron absorption and activation and its subsequent decay. The organic binder phase just provides mechanical stability to the NIP but reduces its efficiency through two effects: first by the absorption of low energy electrons and second by introducing an inactive volume fraction to the layer. Avoiding the organic fraction, for example by preparing a ceramic NIP without binder, could increase the efficiency and spatial resolution without a loss in mechanical stability. In the following, two processes for preparation of ceramic NIPs are reported, both delivering ceramic NIPs consisting solely of GdF3, as the neutron converter and BaFBr : Eu2+, as the storage phosphor. The correlation between the sintering parameters and volume fraction of the neutron converter is investigated with respect to high efficiency and high spatial resolution. The generally observed antidromic behaviour between these two quantities was observed in this study also.

Structures of CsEuBr3 and its degradation product Cs2EuBr5.10H2O.

CsEuBr(3), caesium europium tribromide, crystallizes in an orthorhombic perovskite-type structure with an a(-)a(-)c(+) octahedral tilting scheme (GdFeO(3) type). CsEuBr(3) is unstable in air and one of the degradation products was identified as Cs(2)EuBr(5).10H(2)O by single-crystal X-ray analysis and synchrotron powder diffraction. The Eu(3+) ions on twofold rotational axes are coordinated by nine water molecules, and each water O atom is linked to two Br atoms by hydrogen bonds. The tricapped trigonal [EuO(9)] prisms are separated from each other by infinite {Cs(2)Br(5).H(2)O} chains; the description Eu(OH(2))(9)Cs(2)Br(5)(OH(2)) might therefore be more appropriate. The oxidation of Eu(2+) to Eu(3+) during the degradation of CsEuBr(3) is further confirmed by changes in the magnetic properties from the as-prepared material into the degraded product.