ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012) P 09
DETECTION OF X-RAY ABASORPTION ANISOTROPY USING FLUORESCENCE RADIATION FOR ATOMIC RESOLVED IMAGING
K.M. D¸abrowski1∗, D.T. Dul1, M. Tolkiehn2, D.V. Novikov2, and P. Korecki1
1Institute of Physics, Jagiellonian University, Reymonta 4, 30–059 Krak´ow, Poland
2DESY, Notkestrasse 85, D–22603 Hamburg, Germany
Keywords: X-ray absorption, atomic structure determination, polycapillary optics
∗e-mail : karol.dabrowski@uj.edu.pl
X-ray absorption anisotropy (XAA) recorded using a white synchrotron beam contains an unique information about local atomic structure. For a crystal, fine-structures in absorption pattern mea- sured for various incident angles of the incident beam can be considered as distorted real-space pro- jections of close packed atomic planes and direc- tions.
Until recently, white beam XAA experiments used total electron yield to probe the absorption.
In [1] it was shown that white-beam XAA can be recorded using x-ray fluorescence. In principle, flu- orescence detection of XAA allows element sensi- tivity and enables experiments in a presence of an electric or magnetic field. Since XAA signal is very weak (0.1% of the background), experiments require high counting rates.
In this work we present results of XAA experi- ments performed for a LiNbO3 (001) wafer using a hard x-ray wiggler on BW5 beamline at DORIS III (DESY/Hamburg). The white beam had a mean energy of 64 keV and FWHM of 36 keV.
Fluorescence detection of XAA was performed in two configurations. In the first geometry, x-ray fluorescence was detected using an avalanche photo diode (APD). The moderate energy resolution of the APD was sufficient to separate x-ray fluorescence signal from Compton and Bragg scattered radiation and to obtain XAA patterns with a quality allowing a direct observation of the projection of the LiNbO3
structure (Figure 1).
The quantitative analysis of XAA was performed using wavelets [2] and reproduced local structure of Nb atoms inside a 10 ˚A sphere. Because of a weak scattering, direct visualization of oxygen atoms was impossible. However, their location could be deter- mined indirectly, by examination of a distortion in the shape of Nb peaks.
In the second experimental geometry, prelimi- nary test of polycapillary optics for recording of XAA patterns were performed. A polycapillary el- ement was used simultaneously as a collimating de- vice and a low-pass energy filter.
While, the quality of the XAA pattern recorded using APD was higher than for the setup with poly- capillary optics, laboratory-based measurements
showed that the later configuration seems to be more promising for future application. For exam- ple, achieving of a two order of magnitude increase in count rate is likely with polycapillary optics op- timized for white beam XAA experiments.
Figure 1 : White beam x-ray absorption anisotropy pat- terns recorded for a LiNbO3 sample using two different experimental geometries. Left: using APD. Right: using polycapillary optics and a PIN diode. Bottom: geomet- rical projection of the local structure of LiNbO3. Acknowledgments: This work was sup- ported by Polish National Science Center (DEC- 2011/01/B/ST3/00506) and by grant No. 226716 founded by European Community’s Seventh Framework Programme (FP7/2007 – 2013).
References
[1] P. Korecki, D.V. Novikov, M. Tolkiehn, “Projections of local atomic structure revealed by wavelet analysis of x-ray absorption anisotropy,” Phys. Rev. B 80 (2009) 014119.
[2] P. Korecki, M. Tolkiehn, K.M. Dabrowski, D.V. Novikov, “Fluorescence detection of white beam x-ray absorption anisotropy: Towards element sensitive projections of atomic structure,”
J. Synchrotron Radiat. 18 (2011) 851.
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