AN INCIDENT X-RAY BEAM MONITOR FOR USE WITH PROTEIN CRYSTALLOGRAPHY AT A SYNCHROTRON SOURCE. F. J. Rotella and R. W. Alkire, Center for Mechanistic Biology and Biotechnology, Argonne National Laboratory, Argonne, IL 60439 USA.

A compact incident beam monitor for use with protein crystallography has been developed at Argonne's Structural Biology Center at the National Synchrotron Light Source (beamline X8C). Monitoring the incident x-ray beam at a synchrotron source is critical if accurate normalization factors are to be recorded, and a beam monitor must be capable of operating through a wide dynamic range. In the present design, incident beam intensity is monitored by measuring radiation scattered from a thin polymer film into a PIN diode. The response of the diode is linear over a range of 10 orders of magnitude. For improved statistical accuracy, the scattering film can be replaced with a thin metal foil. Results from measurements with Cr, Mn, Fe and Co foils indicate that fluorescent radiation emitted by a metal foil can increase the beam monitoring signal up to 50 times that produced from scattering alone. With this detector design, the direction of the forward scattered radiation is restricted to an area not larger than the beam stop, minimizing excessive background radiation in diffraction measurements. Support materials have been optimized so that no unwanted absorption edge effects are present between 6-19.5 keV. This design makes the detector useful for monitoring incident beam intensities over a wide range of absorption edges often associated with multiwavelength anomalous diffraction (MAD) experiments. Accurate incident beam monitoring also simplifies optimization of the x-ray beam through the diffractometer collimation after each new electron orbit of the synchrotron is established. This device would be particularly well-suited for operations on third-generation synchrotron source beamlines.

This work was supported by the U. S. Department of Energy under Contract W-31-109-ENG-38.