MICRODIFFRACTION ON BEAMLINE X25

Brookhaven Science Assoc-Brookhaven Lab, Upton NY

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objectives [unreadable]The objectives stated in the recent renewal proposal are these: We will commission the new microdiffractometer, develop routine use of the automounters for microcrystals, install a Bilderback focusing capillary to provide the cleanest possible beam, make plans to employ 266nm front-lighting for crystal visualization, develop novel methods to deliver microcrystals to an x-ray beam. We have dropped the 266nm-illumination project because it is very expensive, and the ESRF, who had developed the method, found it to be not worth the trouble. We have added a new objective, and this is to characterize and exploit the use of slits to provide a micro beam. Results [unreadable]Commissioning the new microdiffractometer: The instrument is in routine use. We've made minor mechanical modifications during the course of the work. The major improvement has been to improve the visualization of the sample by our installation of a digital camera with a larger field of view. This is a great success with the users. Routine use of automounters: See the automounter section;we are building an instrument. Install a Bilderback focusing capillary: Don Bilderback's group created a focusing capillary for us that matched the characteristics of our existing focusing system. It was intended to produce a 25[unreadable]m beam, with a convergence of 2mrad, that would have an increased intensity of about 2.5X over a slitted beam that size. We acquired a focusing mechanism that fit in the position of the current beam-limiting slits and took some pains to characterize the capillary. Indeed, we could get a very bright beam at the specimen position, and it seemed to be possibly smaller than predicted. Because of difficulties in getting an absolute intensity for a beam this size, we didn't get a firm number for the gain. We also were dismayed to discover the reality of having a 2mrad-converging beam. To resolve large unit cells, one would like the beam divergence to be less than maybe one quarter of the angle subtended by the most closely spaced reciprocal lattice points. That is, to use this system well the largest unit-cell spacing would need to be 1/a >0.004[unreadable]-1 or a >250[unreadable]. Too many of the specimens we encounter have at least one axis that large. (This has implications for both the rocking curve width and the size of reflections on the detector.) Probably we won't pursue the focusing capillaries further. However this stimulated our new objective Characterize and exploit the use of slits to provide a micro beam: The x-ray beam at the X25 diffractometer converges by about 1mrad horizontally and is about parallel vertically. Our experience with the capillary suggested that, in order to preserve the sharp peak from a small crystal, we'd like the beam at the detector to be no wider than about twice its width at the specimen. A typical specimen-to-detector distance used with the Q315 is 300mm. This gives a resolution of about 2[unreadable] at the edge with 1[unreadable] radiation. For the beam to increase from about 20 to 40[unreadable]m over this 300mm distance, the divergence would be 0.07mrad. A set of slits lies 17m upstream from the x-ray hutch. We discovered empirically that the divergence will be cut to about 0.07mrad when the horizontal slit is 0.7mm. The whole beam from the undulator gets through at 4mm;a 0.7mm setting reduces the total beam intensity to about [unreadable] the maximum value. We feel this is an appropriate price to pay to get high signal-to-noise from a tiny crystal. Operational issues: The previously-reported intermittent problem of low frequency fluctuations of the focused x-ray beam was addressed through re-construction of the second crystal assembly of the monochromator, done in a way to permit the full capabilities of aligning and tuning the monochromator to be undertaken. Hoping to isolate other sources of low-frequency noise that we continue to notice intermittently, and resolve them, we continue to monitor the stability of the x-ray beam closely, and to coordinate with the Light Source Control Room, analyzing any fluctuations seen in the data. In fact, such fluctuations are observed intermittently at the other PXRR beamlines too, which leads us to suspect that they are of global origin. Some difficulties encountered in the beamline motor control system were resolved through replacement of some of the motor controls modules. Plans [unreadable]Continued development of the capability to handle microcrystals at X25 was an important part of our renewal proposal, submitted in Sep 07 and reviewed in Mar 08. This next year will be filled with balancing routine data collection with exploring the use of our new micro-crystal diffractometer. Objectives will be to install a digital video on the periscope and to bring use of the slitted small beam into normal operations. We imagine that X25 will continue to be the site for very difficult and perhaps adventuresome projects, complementing the high-throughput emphasis at X29. Significance [unreadable]The new upgrades to the optics and micro-diffractometer will allow us to push further into the use of tiny crystals. The brightness of X25, which surpasses that of X29 in certain photon energy ranges, also will provide us with experience that will be valuable as we work to plan development of beamlines for NSLS-II. Publication [unreadable]T. Shaftan, S. Hulbert, and L. Berman, "Comparison of calculated brightness and flux of radiation from a long-period wiggler and a short-period undulator", J. Synch. Rad. 15, 335-340 (2008).