Hans Kraus

Abstract:

The results of in situ temperature-resolved powder diffraction studies of CaWO4 scheelite using both synchrotron radiation and neutron scattering are reported. The studies performed over a broad temperature range of 5-1773 K confirm the scheelite type of structure for calcium tungstate over the whole temperature range. The anisotropy of thermal expansion in calcium tungstate as well as the rigidity of WO4 complexes have been analysed in terms of bond distances, interatomic angles and anisotropic displacement parameters. The WO4 2- complex anions showed a remarkable robustness in the whole studied temperature range, thus pointing out that the layered structure formed by two-dimensional CsCl-type arrangements of Ca cations and WO4 complexes is the primary reason for the anisotropy of thermal expansion in calcium tungstate. © 2011 International Union of Crystallography Printed in Singapore - all rights reserved.

Abstract:

Minimising intrinsic radioactivity of crystal scintillators is of particular importance for experiments searching for rare events. We studied the impact of the crystal production process (recrystallisation) on the level of radioactive contamination of CaWO4 crystal scintillators. Several samples of single crystal scintillators were produced using the recrystallisation procedure. It is shown that this has a significant effect on the radioactive contamination of the crystals. Depending on the stage of recrystallisation the activity due to 210Po (product of 210Pb decay) varies in the range 0.031.32 Bq kg-1 while the activity of 238U varies from 0.04 to 0.33 Bq kg-1. We found that uranium is rejected by the crystal with a segregation coefficient ≈0.3. The improvement in radiopurity of CaWO4 by one order of magnitude due to recrystallisation has been demonstrated. The additional benefit of this process is the improvement in the energy resolution. A programme to develop radiopure CaWO4 crystal scintillators is discussed briefly. © 2010 Elsevier B.V. All rights reserved.

Abstract:

The variation of luminescence and excitation spectra of titanium doped Al2O3 for the concentration of Ti ranging from 10 to 1000 ppm was investigated using synchrotron radiation. In the lightly doped Al 2O3-Ti (<100 ppm) samples we identified several emission bands. These are the emission of the excitons localized at Ti (290 nm), the emission due to F centers (325 nm), the band around 420 nm traditionally attributed to F center emission, and the luminescence of Ti3+ ions at 720 nm. The emphasis in this study is on the clarification of the nature of the blue emission band in the samples with high concentration of Ti (≥100 ppm), where the luminescence and excitation spectra of the blue emission exhibit noticeable variability. This is explained by a model of the luminescence process of Ti4+-F centers that includes the photoionization of Ti 3+, the subsequent capture of electrons at F-centers, formation of excited F-centers and, finally, the emission of a blue photon. The quenching of the blue emission with increasing Ti concentration is interpreted in terms of competition between oxygen vacancies and Ti4+ centers in the capture of the electron. © 2011 American Institute of Physics.