Macromolecular Crystallography PX Stability Requirements for NSLSII Lonny

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

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)

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).

Angular Stability Angular instabilities can affect PX experiments in at least a couple of ways: (1) 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. (2) 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. (3) 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)

Wavelength Stability Wavelength resolution is determined by the beamline optical system, and wavelength stability is usually driven by angular stability. At 12 ke. V, a vertical angular deviation of 1 µrad gives rise, in a Si(111) crystal monochromator, to a photon energy deviation of 12000 e. V × cot(9. 5°) × 1 µrad = 0. 072 e. V 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 e. V), which is reasonable to aim for. Should not exceed 5% of the absorption edge width.