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 consist of double crystal monochromators with sagitally focussing second crystals followed by a vertically focussing mirror which allow independent focussing 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 40×200 μm (FWHM) has been observed which contained essentially all of the 1.5–2.5×1013 ph/s delivered by the cryogenically-cooled Si(1 1 1) double crystal monochromator. This combination of highly demagnifying optics and the very low divergence of the very small source have yielded excellent quality patterns from various muscle specimens and collagen-containing tissues. Detectors available include a Fuji BAS2500 image plate scanner and a 1 k×1 k 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.