Measurement and interpretation of the polarization of the x-ray line emission of heliumlike Fe XXV excited by an electron beam

The linear polarization of the 1s2p 1P1→1s2 1S0 resonance line, the 1s2p 3P1,2→1s2 1S0 intercombination lines, and the 1s2s 3S1→1s2 1S0 forbidden line was measured in heliumlike Fe XXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of +0.56-0.08+0.17 was determined for the polarization of the 1P1 line, -0.53-0.02+0.05 for the 3P2 line, -0.22-0.02+0.05 for the 3P1 line, and -0.076-0.007+0.007 for the 3S1 line. The measurements were compared with results from a relativistic distorted-wave calculation, which was carried out for a number of mid-Z heliumlike ions (Mg10+–Kr34+), and good agreement was found. By contrast, disagreement was noted with predictions based on Coulomb-Born calculations, allowing us to distinguish between theoretical approaches

Steps toward interstellar silicate mineralogy

Crystalline olivines are an important component of silicate dust particles in space. ISO observations revealed the presence of crystalline silicates in comets, protoplanetary accretion disks, and outflows from evolved stars. For the interpretation of astronomical spectra, the relevant material data at a variety of temperatures and over a broad wavelength range, are urgently needed. In contrast to this need, optical properties of the astronomically interesting olivines are scarcely available at present. In order to close this gap, we studied the optical properties of three minerals of the olivine group by reflection spectroscopy on single crystals in the infrared spectral range. We measured the iron endmember (fayalite, $\rm Fe_2SiO_4$), an Mg-rich olivine ($\rm Mg_{1.9}Fe_{0.1}SiO_4$), and the magnesium endmember (forsterite, $\rm Mg_2SiO_4$) of the $\rm (Mg,Fe)_2SiO_4$ series. For a direct comparison with astronomical observations, we present calculated mass absorption coefficients in the Rayleigh limit for different shapes and varying iron content of the dust particles. The laboratory data together with a set of ISO data for envelopes around evolved stars (Molster 2000) are used to constrain the properties of circumstellar silicates. We find that essentially all band positions are shifted to larger wavelengths with increasing iron content. The particle shape influences very significantly the strong bands such as the B$_{1u}$:$\nu_3$ mode that appears as the "11.4 μm" band of forsterite, whereas e.g. the two FIR modes longward of 40 μm remain practically unaffected by the particle shape but shift due to increasing iron content. The comparison with the band positions in ISO spectra points to the presence of olivine crystals strongly elongated along the crystallographic c-axis. In addition, we apply the calculated mass absorption coefficients to evaluate transmission measurements of particles embedded in a matrix -a technique which is frequently used in laboratory astrophysics. All data shown in this paper will be made available in digital form via the electronic database http://www.astro.uni-jena.de

Analyzer-based phase contrast imaging and phase retrieval using a rotating anode x-ray source

We have performed an analyzer crystal based phase contrast imaging (ABI) experiment using a rotating anode x-ray source. The use of such an incoherent source demonstrates the potential of ABI as a quantitative characterization tool for the laboratory environment. A phase contrast image of a plastic phantom was recorded on high resolution x-ray film and the projected thickness was retrieved from a single image. The projected thickness recovered from the phase contrast image was shown to quantitatively agree with a reference optical microscope measurement