An incident-beam monitor for use in protein crystallography at a synchrotron source

A compact incident-beam monitor has been developed for use in synchrotron protein crystallography at the National Synchrotron Light Source beamline X8C. Incident-beam intensity is monitored by measurement of the radiation scattered from a thin polymer film into a PIN diode. For improved statistical accuracy, the scattering film can be replaced by a thin metal foil. Fluorescent radiation emitted by the metal foil increases the beam-monitoring signal up to 50 times relative to that produced by scattering alone. With this design, the forward-scattered radiation is restricted to an area not larger than the beam stop, minimizing excessive background radiation. Support materials have been optimized so that no unwanted edge effects are present between 6 and 19.5 keV, making the detector useful for monitoring incident-beam intensities over the 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 is established in the synchrotron.