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Medicine

XAS

X-ray Absorption Spectroscopy at NSLS-II XFP

The CSB is currently engaged in development of an X-ray Absorption Spectroscopy (XAS) endstation at our XFP beamline, following completion of the construction project and successful beamline commissioning. The XAS program will split time approximately equally with the footprinting program, subject to prioritization of relative user demand, with changeovers between the two programs planned for maintenance periods in order to maximize beamline usage.

The XFP beamline currently delivers X-ray photons over a 4.5 – 16 keV energy range, with the upper energy cutoff defined by the Rh-coated toroidal focusing mirror in the front end. For XAS, a channel-cut Si(111) monochromator of the NSLS Siddons design will be permanently installed in the downstream half of the XFP experimental hutch. In order to improve energy resolution, horizontal acceptance from the three-pole wiggler source onto the toroidal mirror will be reduced to ca. 1 mrad, in order to minimize the energy resolution degradation imparted by this optic. We estimate that monochromatic X-ray flux at 10 keV will be ca. 1012 photons/sec in a spot size of approximately 4 mm x 6 mm (VxH) at the sample position. High precision slits both before and after the monochromator will define the beam and further improve energy resolution as required.

The endstation will consist of an experimental table with motorized alignment motions in all three axes (x, y, and z). The vacuum cryostat previously employed at NSLS X3A / X3B will be upgraded with a new Displex cryocooler system fabricated by ColdEdge Technologies, Inc. which will deliver improved cooling power in the critical 10-20K regime. A Canberra 31 element solid state germanium fluorescence detector with digital DXP-XMAP electronics from XIA will be standard equipment for measurements of dilute low-concentration samples. For higher concentration samples, a non-energy discriminating PIPS detector will be available for fluorescence, while ion chambers will be used for transmission measurements. The endstation equipment will be rounded with a set of attenuators for refining X-ray flux on the sample and access to a gas manifold for controlling delivery of inert gases and oxygen to the sample. The sample stage will also readily accommodate various user provided apparatus such as electrochemical cells. A nearby partitioned space with workbenches will be available for sample preparation activities that do not require laboratory access.

CSB scientific and technical personnel, with assistance from NSLS-II, are currently heavily engaged in XAS development activities. Construction activities are expected to take place over the next 6-8 months, with first light planned for early in 2018, followed by intensive technical commissioning and characterization of beamline performance for XAS. We anticipate early scientific measurements will take place by late spring of 2018. Contact Erik Farquhar with questions about XAS at XFP, or to discuss your planned experiments.