J. Zimmermann

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.

Hafnium oxide thin films: Effect of growth parameters on oxygen and hafnium vacancies

Thin films of hafnium oxide were grown by reactive molecular beam epitaxy. The growth parameters, substrate temperature, and oxidation conditions were varied in a wide range in order to investigate their influence on the thin film characteristics. The oxidation conditions during growth were decisive to hafnium oxide film orientation on c-cut sapphire substrates; it was possible to grow single oriented (00l) or (−111) oriented hafnium oxide films depending upon the oxidation conditions. The authors could successfully grow hafnium oxide thin films with oxygen or hafnium vacancies, depending on the oxidation conditions during growth, evident from optical band gap measurements. All the hafnium oxide thin films investigated in this study, irrespective of oxygen/hafnium vacancies, failed to show any ferromagnetic characteristics.

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.

Lanthanum-stabilized europium-doped cubic barium chloride: An efficient x-ray phosphor

We report the photoluminescence and x-ray luminescence of lanthanum-stabilized cubic barium chloride with europium (Eu2+) doping, of general composition Ba1−x−yEuyLaxCl2+x. X-ray diffraction patterns confirm that the material adopts a cubic fluorite structure for x=0.125 and 0<y<0.1. The photoluminescence shows an intense band comprising two components with maxima at 420 and 394 nm. The 394 nm band is assigned to Eu2+ substituting for Ba2+ giving a site of cubic symmetry, based on the excitation spectra which show a T2g band and Eg band similar to those in cubic SrCl2:Eu. The 420 nm band has a very different lifetime and excitation spectrum and is assigned to Eu2+ ions which have an associated chlorine interstitial ion. The two bands are also found in the x-ray luminescence spectra. For both bands, and an additional minor emission at 520 nm found only at low temperatures and for high Eu concentrations, there is a common excitation in the UV from 150 to 200 nm which we attribute to excitons associated with...

Influence of oxygen doping and hydration on photostimulated luminescence of CsBr and CsBr:Eu2+

Powdered samples of CsBr:O2− and CsBr:Eu2+,O2− with oxygen concentrations ranging from 0.01 to 5.00 mol% have been synthesized and are shown to have photostimulated luminescence (PSL) properties which are strongly influenced by the oxygen concentration. In europium free CsBr:O2−, it was found that weak PSL emission at 460 nm arises from oxygen impurities while with oxygen concentrations higher than 0.05 mol% the emission shifts to the 379 nm emission of the CsBr matrix, which is attributed to the nonexcitability of oxygen agglomerates. The F-center concentration in CsBr:O2− is observed to increase with oxygen concentration, due to an increase in Br-vacancies as charge compensation centers for the O2− ions. In all CsBr:Eu2+ samples studied, intense PSL emission arises only following room temperature hydration in an atmosphere of 99% relative humidity, and it is concluded that the presence of water molecules is essential for the PSL in CsBr:Eu2+. In CsBr:Eu2+,O2− this enhancement effect of PSL intensity is ...

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.