Trichromatic concept at the SPring-8 RIKEN beamline I

At the SPring-8, RIKEN beamline I has been designed and developed for structural biology research by the Institute of Physical and Chemical Research (RIKEN). RIKEN beamline I consists of two experimental stations, protein crystallography (PX) and small-angle X-ray scattering (SAXS). Both experiments can be carried out simultaneously, with dichromatic synchrotron radiation emitted from two coaxial undulators with vertical polarization. The branched beams are generated by a transparent diamond crystal. With synchrotron radiation, the multi-wavelength anomalous diffraction (MAD) method, which gives phases from a single anomalous scatterer, has been developed. Anomalous scattering contributes a small portion of diffraction intensity so that the accuracy of intensity data is definitely important. The PX branch of RIKEN beamline I has been designed based on a 'trichromatic concept' to optimize for the MAD data collection. This concept is that three kinds of intensity data sets with three different wavelengths are taken quasi-simultaneously for the single protein crystal without changing any setting by 'trichromator'. The main feature of this concept is to minimize systematic errors in the measurements of anomalous diffraction for the MAD method. The construction of RIKEN beamline I had been progressed satisfactorily until June 1997. The initial commissioning successfully provided the three different monochromatized undulator beams were successfully observed on the phosphor screen, which located at the near end of the trichromator. (author)