A micro-XAS and confocal micro-XANES set-up at DUBBLE

The current optics layout of the XAS line of the DUBBLE station at the ESRF allows standard XAS measurements with X-ray beams of the order of mm. A focusing system based on a polycapillary half-lens for transmission and fluorescence µ-XAS at DUBBLE is presented. Polycapillaries consist of a bundle of hundreds of thousand of small bent glass fibres all oriented towards the same focal point. Due to repeated total reflections the incoming X-rays are guided towards that focal point, resulting in an achromatic optic. Glass polycapillary optics are shown to be easy to use focusing optics for bending magnet XAS stations with acceptances of several mm horizontally and vertically, while the angular acceptance can be matched to the source divergence by the design of the polycapillary optic. The polycapillary lens is placed between the first ionisation chamber and the sample. As demonstrated, the vertical shift of the incoming X-ray during an EXAFS scan due to the use of a non-fixed exit Si(111) monochromator is essentially cancelled by the focusing effect of the polycapillary. Beam sizes of 10-20 µm, transmission efficiencies of 25-45% and intensity gain factors of about 2000 were obtained in the 7-14 keV energy range1. Although the polycapillary optic has a smoothly changing energy dependent transmission efficiency, the amplitude and shape of the EXAFS oscillations are not influenced by this. An EXAFS spectrum on a probed mass of 3 pg of Zn was achieved on a biological sample in fluorescence mode. In addition, by mounting a second polycapillary half-lens in front of an energy dispersive detector, a confocal set-up is obtained, which restricts the part of the sample seen by the detector to a volume of about 15×20×20 µm3 at the Fe K absorption edge. This detection mode allows to measure a microscopic sub-volume within a larger object. The applicability of the polycapillary µ-XAS set-up and the confocal µ-XANES is illustrated within the field of environmental science and geology