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27Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2016
FEATURES IN MICA AND QUARTZ SPHERICALLY BENT CRYSTAL REFLECTIVITY APPEARED IN EXPERIMENTS WITH RELATIVISTIC LASER
PLASMA
M.A. Alkhimova1'2, S.A. Pikuz1,2, I.Yu. Skobelev1,2, A.Ya. Faenov3,1
1-Joint Institute for High Temperatures RAS, Moscow, Russia, e-mail: MAAlkhimova@mephi.ru 2-National research nuclei university MEPHI, Moscow, Russia.
3-Institute for Academic Initiatives, Osaka University, Japan
Spherically bent crystals are widely applied in X-ray spectroscopy of high energy density plasma since the ability to cover a wide range of photon energy. For diagnostic of high power x-ray radiation in the wavelength range 4 - 9 A a-quartz and mica bent crystals usually used. SiO2 crystal properties of x-ray reflectivity and absorption are well investigated byXANES approach [1,2] showing Si K-edge maximum of absorption in the vicinity of 6.65 - 6.75 A wavelengths. However, precise determination of Si K-edge position is strongly important for further spectra physical interpretation because even a small shift of the defined point about Ak edge ~ ± 0.001 A results in changing of a plasma parameters interpretation.
Here we demonstrate the results of experimental measurements of Si k-edge position measurement in SiO2bent crystal irradiated by dense laser plasma emission. Our measurements were based on the comparison of X-ray spectrameasured by two focusing spectrometers with spatial resolution (FSSR) for Al, Si and Fe foil targets.In the case of relativistic laser intensity I laser = 2 • 1021 W/cm2the spectra composedof strong recombination continuum and a set ofresonance lines which werechosen as reference pointsallowing to determine the k-edge position with AE/E ~ 3.3x10-4accuracy. Finally, Si K-edge was found experimentally to be positioned at 6.7175 A.
To measure X-ray spectra in a wide wavelength range 4 - 9 A we used a set of FSSR spectrometers equipped bya-quartz and mica bent crystals with different curvature radius.
Mica crystals compare to SiO2 crystals are able to effectively reflect X-ray radiation in different orders of reflection due to complicated polycrystalline structure (K(Mg, Fe)3AlSi3O10(OH,F)2). Consequently the detailed reflectivity properties of mica crystals, particularly bent ones, are not exactly known. By the way, we implemented the way to calibrate the mica spectral reflectivity comparingthe spectra measured by mica and pre-calibrated SiO2 crystals in overlapping wavelength range 6.72 - 8.7 A.
The obtained spectral calibration curve for bent mica crystal demonstrates remarkable difference to that one calculated for flat mica crystal given in Henke tables and has to be applied for further correction of measured X-ray spectra.It is suggested the most reliable explanation for the discrepancy between theoretically evaluated and experimentally calibrated curves is in the presenceof Al atoms in mica chemical composition with the absorption k-edge lying at 7.94 A.
References
[1]Li D.,Bancroft G. M., Fleet M. E., Feng, X. H., Phys Chem. Minerals, 1995, 22,115-122.
[2] Mo, S. D., Ching, W. Y., Appl. Phys. Let.,2001,78, 3809.
