Optical,magneto-optical and structural properties of F centres in BaFBr

Abstract

In BaFBr, a material which can be used as a storage phosphor for X-rays, two types of F centres can be created by additive coloration or by X-irradiation. The properties of both F centres which play an important role in the storage phosphor mechanism were investigated with optical and magneto-optical methods. The absorption bands, the emission bands, production, and stability of the F centres are discussed.     

Photo-stimulated emission of X-irradiated BaFBr:Eu

Abstract

The photo-stimulated luminescence (PSL) of Eu²⁺-doped BaFBr is excited in the two F centre absorption bands produced by X-irradiation at room temperature. The PSL intensity decreases with decreasing temperature by about a factor of 10. After bleaching at low temperatures the PSL is nearly zero, it can be regenerated by warming the crystal to room temperature. The results are discussed with a new model of the PSL process.     

On the convergence of “Threshold Accepting”

Simulated Annealing (SA) has become a very popular tool in combinatorial optimization since its introduction in 1982. Recently Dueck and Scheuer proposed another simple modification of local search which they called Threshold Accepting (TA). In this paper some convergence results for TA are presented. The proofs are not constructive and make use of the fact that in a certain sense SA belongs to the convex hull of TA.     

A gas chromatography/combustion/isotope ratio mass spectrometry system for high-precision delta13C measurements of atmospheric methane extracted from ice core samples

Past atmospheric composition can be reconstructed by the analysis of air enclosures in polar ice cores which archive ancient air in decadal to centennial resolution. Due to the different carbon isotopic signatures of different methane sources high-precision measurements of delta13CH4 in ice cores provide clues about the global methane cycle in the past. We developed a highly automated (continuous-flow) gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) technique for ice core samples of approximately 200 g. The methane is melt-extracted using a purge-and-trap method, then separated from the main air constituents, combusted and measured as CO2 by a conventional isotope ratio mass spectrometer. One CO2 working standard, one CH4 and two air reference gases are used to identify potential sources of isotope fractionation within the entire sample preparation process and to enhance the stability, reproducibility and accuracy of the measurement. After correction for gravitational fractionation, pre-industrial air samples from Greenland ice (1831 +/- 40 years) show a delta13C(VPDB) of -49.54 +/- 0.13 per thousand and Antarctic samples (1530 +/- 25 years) show a delta13C(VPDB) of -48.00 +/- 0.12 per thousand in good agreement with published data.     

Investigations of As-antisite-related defects in GaAs

Optically Detected Magnetic Resonance (ODMR) is a powerful tool to study paramagnetic defects in semiconductors. As an example for the successful application of various methods based on ODMR, investigations of As antisite-related defects in GaAs are presented. Information about defect properties such as their microscopic structure, their metastability and thermal stability can be obtained by ODMR which was measured via the Magnetic Circular Dichroism of the optical Absorption (MCDA).Low temperature irradiation of GaAs (4.2 K) with 2 MeV electrons produces the Ga vacancy and some new As antisite-related defects. Annealing steps were investigated at 77 K and above. At 77 K the isolated As antisite defect is detected if semi-insulating (si) GaAs was irradiated. Between 200 K and 300 K the Ga vacancy decays. Its decay is correlated to the formation of an anti-structure pair. A second annealing step was found at about 520 K. There the anti-structure pair decays. EL2 is formed at that temperature if si material was irradiated, but this EL2 formation is not correlated with the anti-structure pair decay. We performed also magneto-optical measurements to investigate the metastability properties of the three As antisite-related defects. They all show metastability.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994