The biology spectrometer for small-angle X-ray scattering at the national synchrotron light source

Abstract

A small-angle X-ray scattering (SAXS) spectrometer for structural biology has been installed at the National Synchrotron Light Source. This instrument will be used to study a diverse number of biology samples (e.g., membranes, muscle, protein or DNA complexes, viruses) and has been designed, accordingly, to achieve high small-angle resolution (5000 Å) and high angular resolution (0.1 mrad). The spectrometer incorporates automated motions for cervical and horizontal beam tracking, and raster scan calibration of the detector, in addition to sample and detector positioning axes. The sample-to-detector distance can be adjusted between 50 and 300 cm, and the 2θ axis between −5 and 45°. To provide unobstructed space below and to one side of the sample, the detector mounting arm is catilevered from an offset 2θ axis. The sample mounting position is designed so that a number of different sample-containing devices may be used. Presently, a three-circle goniometer, membrane sample cell and multiple-capillary sample changer are availabe; additional devices including stopped-flow and T-jump cells are planned. Each spectrometer axis is equipped with a stepping motor and absolute position encoder. A general purpose motor control unit [1], connected by an asynchronous serial link to the experiment control computer, translates function commands from the processor into TTL pulse trains output to the selected motor's power supply and reports each axis' status and position back to the processor.