IMPROVEMENT OF X-RAY BEAM POSITION STABILITY FOR THE BL4-2 SAXS FACILITY

Stanford University, Stanford CA

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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. We continue to mitigate the beam positional instability issue, which were found last year at the new BL4-2 location. Earlier this year, we identified the major cause of the instability, whose magnitude is often as much as 1.5 times the full width half maximum vertical beam size in 24 hours, to be most likely the gradual thermal drifts of a few pieces of thick concrete floor moving at different rates in response to diurnal temperature fluctuation. We began adopting a mirror pitch feedback system very similar to those operating successfully on other SSRL insertion device stations. The first version involved a straight copy of the standard beam position monitor with beryllium signal blades coated with a thin layer of Ti working under helium atmosphere. This approach resulted in an improvement in beam stability, achieving ten micron stability over a 16 hour period, down from typical 0.2 mm overall drifts over the same period. Its use is, however, limited to shorter sample-to-detector distances due to the high back ground level given by the Ti coating at small scattering angles. We have recently modified this design to minimize scattering background. A new vacuum compatible housing has been built and different blade materials have been tested. It was found that un-plated, polished beryllium blades give an appropriate level of signal for the feedback system to work while keeping unwanted parasitic scattering at a tolerable level. The new blades are now being fabricated and finial version of the device will be installed and tested as the new blades become available.