How x-ray diffraction with synchrotron radiation got started

The need to record low angle scattering x-ray fibre diagrams from muscle with milli-second time resolution drove the use of synchrotron radiation as an x-ray light source. The first smudgy diffraction patterns were obtained from a slice of insect flight muscle. Out of this grew the EMBL Outstation at DESY.

The first beam line

In the mean time work went on. Inside Bunker 2 a massive neutron-proof concrete wall separated the operators from the beam. Therefore, all adjustments had to be made by remote control. In collaboration with John Barrington Leigh, Gerd Rosenbaum set about building a fully remotely controlled optical bench (Barrington Leigh & Rosenbaum, 1974; Barrington Leigh & Rosenbaum, 1976) (Fig. 5). A Guinier monochromator was used to focus the fan of radiation from the synchrotron in the horizontal plane and 2 x 20 cm. adjustable bent mirrors were used to focus the much smaller divergence in the vertical plane. The mirrors (fused quartz) were nearest to the synchrotron and were housed in a helium-filled box separated from the machine vacuum by a beryllium window. Beams were accommodated in vacuum tubes fitted with mylar windows. It proved difficult to obtain mirrors polished to the necessary flatness: optical mirror manufacturers had no way of monitoring the micro-flatness necessary for x-ray mirrors. Here we were considerably helped by the pioneering work of Franks (Franks & Breakwell, 1974). Movements were controlled by about 100 small DC motors with potentiometers as position sensors. DC motors were chosen rather than stepping motors because they are light: the whole apparatus was built on a mini budget and the apparatus could not become massive. A SIT-vidicon camera was used to observe in line the image of the direct beam formed on a cesium iodide crystal. Two other steerable TV cameras fitted with zinc sulfide screens were mounted on a parallel optical bench for visual observation. Zinc sulfide screens could be inserted into the beam path by remote control for monitoring the beam e.g. before and after the slits. The monochromator (quartz) was cut at 7° to the surface so as to approximate to the Guinier-condition for the given source distance and the desired focal distance, i.e. the desired demagnification. The deviation from the exact Guinier-geometry resulted in a wavelength inhomogeneity across the converging beam. However, this effect was small for the apertures being used and was not important in small-angle diffraction. The angle of the latter part of the optical bank to the direct beam was fixed for _=1.5Å. Since the optical elements were about 40m from the tangent-point of the synchrotron and focused within 2-3m a demagnification of H 15 was achieved.

The electron beam of DESY was relatively compact so that a focused beam of dimensions 200 x 250 m could be obtained. With a flux of H 10 9 photons/s and excellent optical properties this was a very good beam for low angle scattering. The flux density was two orders of magnitude better than could be achieved with the best rotating anode tubes. Images were registered on film or on one-dimensional single-wire (Gabriel & Dupont, 1972) position sensitive detectors. The beam line was in operation in 1972 and for a couple of years remained a unique facility.

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