Fiber diffraction using the BioCAT undulator beamline at the Advanced Photon Source

The BioCAT undulator-based beamline at the Advanced Photon Source, Argonne IL, USA is designed to be a state-of-the-art instrument for biological non- crystalline diffraction and X-ray absorption spectroscopy. The optics consi st of double crystal monochromators with sagitally focussing second crystal s followed by a vertically focussing mirror which allow independent focussi ng of the beam in the vertical and horizontal directions virtually anywhere along the length of the 12 m experimental enclosure. When configured for a 2 m fiber diffraction camera, a focal spot of less than 40 x 200 mu m (FWH M) has been observed which contained essentially all of the 1.5-2.5 x 10(13 ) ph/s delivered by the cryogenically-cooled Si(1 1 1) double crystal monoc hromator. This combination of highly demagnifying optics and the very low d ivergence of the very small source have yielded excellent quality patterns from various muscle specimens and collagen-containing tissues. Detectors av ailable include a Fuji BAS2500 image plate scanner and a 1kx1k CCD detector optimized for small-angle applications. Future developments will include, in vacuum beam monitoring, longer camera lengths (6-8 m), and optimizations to improve first-order resolution in small-angle applications. (C) 2000 El sevier Science B.V. All rights reserved.