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Macromolecular Crystallography (PX) Stability Requirements for NSLS-II

This presentation outlines the stability requirements for PX experiments at NSLS-II, including sample dimensions, position stability, angular stability, and wavelength stability.

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Macromolecular Crystallography (PX) Stability Requirements for NSLS-II

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  1. Macromolecular Crystallography (PX) Stability Requirements for NSLS-II Lonny Berman and Vivian Stojanoff NSLS, BNL Presentation given at NSLS-II Beam Stability Workshop April 18, 2007

  2. PX Stability Requirements for NSLS-II • Sample dimensions steer position resolution • Unit cell size and rocking curve width steer angular resolution • Anomalous diffraction contrast steers wavelength resolution • Intensity should remain stable in time from exposure frame to exposure frame (as fast as few milliseconds per frame) and through the course of collection of a data set (as long as one hour); intensity variation striking the sample must be 5% or less, preferably close to 1% (intensity variations larger than this scale can affect the quality of the data reduction)

  3. Position Stability PX beamlines are focused, so the beam at the sample is a demagnified image of the source. Everything else remaining stable, a change in source position results in a change in focus position, weighted by the demagnification factor. It is anticipated that PX beamlines will demagnify somewhat, anywhere from 1:1 to 10:1. In addition, samples are getting smaller, and small samples (~5-20 µm) will be the paradigm at NSLS-II. It is reasonable to anticipate that a position stability of ~1 µm or better, at the sample, will be necessary (definitely not more than 5% of the sample size). Note that focused beam sizes will be comparable to sample sizes (refer to beamline ray tracing calculations in CDR).

  4. Angular Stability • Angular instabilities can affect PX experiments in at least a couple of ways: • Beam can be steered out of the acceptance aperture for the beamline, changing the intensity. This may be 1.5 mm (2.4 mm) at 30 m distance from the source (this is the vertical [horizontal] dimension of the undulator central cone) which is 50 (80) µrad, and we’d want to keep instabilities within 5% of this. • Angular resolution provided by the optical system may broaden. Adjacent Bragg reflections in a large unit cell crystal may be separated in angle by 1 mrad or less. A decent crystal may have a rocking curve which is 100 µrad wide. Angular instabilities should be kept within 5% of this. • Given that demagnifying optical systems increase the angular divergence, it is reasonable to aim for an angular stability of ~2-5 µrad on the basis of these considerations. However … (next slide)

  5. Wavelength Stability Wavelength resolution is determined by the beamline optical system, and wavelength stability is usually driven by angular stability. At 12 keV, a vertical angular deviation of 1 µrad gives rise, in a Si(111) crystal monochromator, to a photon energy deviation of 12000 eV × cot(9.5°) × 1 µrad = 0.072 eV For anomalous diffraction data collection, this is 4% of the absorption edge width for very sharp x-ray absorption edges such as for selenium (edge width is 1.8 eV), which is reasonable to aim for. Should not exceed 5% of the absorption edge width.

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