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Major instrumental developments and upgrades for small-angle scattering include a dedicated small-angle single-crystal diffraction instrument, a second vacuum flight path for recording scattering at intermediate angles, new signal processing modules for the linear detector system, and characterization/acquisition of a CCD detector system.
It is expected that single-crystal diffraction studies at very small angles, i.e. in the resolution range of 1000-10 Å, will become very important in virus crystallography, and for phase determination in macromolecular crystallography in general. Based on our experience last year in recording low-index reflections from a few different macromolecular crystal systems (see the 1996 Newsletter), we are building a dedicated instrument for this purpose on BL4-2 as part of the Biotechnology SAXS camera system. The instrument will be available for users in January 1998. A crystal flash cooler for single-crystal diffraction, based on the design developed by the protein crystallography group at SSRL, was made available on BL4-2 during 1997.
A second vacuum flight path was constructed and commissioned successfully. The flight path provides approximately 0.9 m of sample-to-detector distance, providing access to higher angles up to approximately 5 Å in Bragg spacing. This second scattering system, for intermediate angles, will be mounted on a horizontal sliding mechanism together with the primary small-angle scattering instrument, so that users can cover a wider range of angles during a short run.
We purchased several new signal processing modules to implement coincident event rejection capability with our linear detector system. We also adopted the modules and the software suite developed by the EMBL Hamburg Outstation. With the help of M. Koch (EMBL), we successfully commissioned the new data acquisition system in August. The new system is expected to widen the dynamic range of the detector system at high count rates by eliminating a very small amount of background signal due to coincident events. A new software suite, which is versatile and user-friendly, was also tested. We will keep the current data acquisition system during the FY98 user run, and at the same time we will provide a way to convert the new data file format to be compatible with the old format.
We have been collaborating with the group led by Y. Amemiya (U. Tokyo) to evaluate CCD X-ray detectors that they have developed for non-crystalline X-ray scattering experiments. A version of the CCD detector, that incorporates an X-ray image intensifier lens coupled to a full-frame rate CCD camera head, was found to have a very low noise level that is comparable to single photon counting detectors. This 2-D detector system allows circular-averaging of isotropic scattering from non-crystalline samples, thus improving data statistics. The detector system can handle very high count rates and, therefore, allows us to take full advantage of the high flux obtained with the multi-layer monochromator installed on BL4-2. In addition to static measurements and fibre diffraction experiments, we also conducted "snap-shot" time-resolved solution X-ray scattering experiments using this system. For example, we initiated an enzyme reaction with a stopped-flow mixer, and turned on the CCD detector system for a short time (e.g. 20 ms) after a certain time (e.g. 5 ms) had passed after the beginning of the reaction. Encouraged by the high data quality of this detector system, we successfully obtained funding from the DOE OBER at the end of FY97 to purchase a comparable detector system. We have just placed a purchase order for an improved version of the same detector system which is expected to arrive at SSRL in May 1998.
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