RIKEN beamline I (BL45XU) is an undulator beamline with two branches. One is for protein crystallography (PX) and the other is for small‐angle x‐ray scattering (SAXS). The beam is split into the two branches by a diamond monochromator so that two experiments can be done simultaneously [Yamamoto et al. (1995) Rev. Sci. Instrum.66, 1833‐1835]. The SAXS branch was designed for studying the weak interaction of proteins or subunits of fibrous or protein solutions especially using hydrostatic pressure. The optics makes use of the good parallelism of the undulator beam in order to reduce parasitic scattering. The beamline consists of a double crystal monochromator and a K‐B type focusing mirror system. In order to cope with the high flux of the beam, an x‐ray image intensifier (Hamamatsu Photonics, V5445P) with a cooled CCD camera (C4880‐82) was used. As a result, decreases in both collection time and sample amount were realized in standard static experiments. These improvements will greatly facilitate SAXS experiments under high pressure.

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CCD detectors are now widely used in many synchrotron small-angle X-ray scattering beamlines. The use of an X-ray image intensi®er with cooled CCD (XR-II + CCD) was studied, especially for use in synchrotron solution X-ray scattering. Two samples, polystyrene latex and apoferritin, were used. These two samples have ®ne structure in the solution scattering pro®le due to symmetry and narrow size distribution. The recorded scattering pro®le, in comparison with that obtained by a positionsensitive proportional counter (PSPC), showed that XR-II + CCD has a much smaller practical dynamic range (100:1) than that of a pixel well (7500:1). This limited dynamic range was overcome by placing various-size masks on the detection plane, thereby eliminating the high-intensity region. The images recorded with various masks were combined, and the reconstituted solution scattering pro®le was submitted to various analyses, including Guinier analysis, power-law analysis, size distribution analysis and calculation of radial density distributions. The results were the same as those obtained with the PSPC. This indicates that spatial distortion as well as shading, a decrease in sensitivity from the centre to the edge of the detecting region [Amemiya, Ito, Yagi, Asano, Wakabayashi, Ueki & Endo (1995). Rev. Sci. Instrum. 66, 2290±2294], have very little effect on the SAXS results. This paper presents a practical protocol for obtaining a reliable solution scattering pro®le given the limitations of XR-II + CCD for synchrotron solution X-ray scattering.

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