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Stability Requirements

This document outlines the environmental stability requirements critical for conducting EXAFS and XANES experiments at NSLS-II. It highlights how even minor variations in signal (0.1%) can significantly affect data quality, particularly for non-uniform or low-concentration samples. Key parameters such as sample uniformity, beam motion, and optical stability are discussed, along with the necessary conditions for achieving optimal energy stability and repeatability in measurements across the 1000 eV scan range. Recommendations for setup and design considerations to meet these stability requirements are provided.

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Stability Requirements

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  1. XAS at NSLS-II Stability Requirements Paul Northrup April 19, 2007 Environmental Research and Technology Division

  2. EXAFS data requirements 1000 eV scan range Data can be influenced by 0.1% change in signal Ravel

  3. EXAFS experiment Ravel • Ideal experiment not sensitive to stability • Uniform sample, energy calibration reference • Challenging real-world experiment sensitive to stability • Non-uniform sample, no transmitted beam, etc.

  4. Non-uniform samples Low-concentration heterogeneous samples: • Beam motion changes signal due to variations in • composition • matrix • particle distribution • surface irregularities

  5. Maximum tolerable beam motion? For random distribution of particles: • Function of particle size, • Number of particles in beam spot for a given concentration • Probability that a certain degree of motion will result in a 0.1% change in signal • For 1x1mm beam spot at low concentration: 5 microns

  6. Positional stability requires… 1) Aperture close to sample, overfilled by 10% 2) Source stability within 10% of source size and divergence 3) Monochromator (and optics) output position and angle must be stable over 1000 eV scan

  7. Energy stability (repeatability) requirements XANES Data: • High-resolution XANES requires stability within 0.05-0.1 eV • K edges: P, Cl, Cr, Mn, As, Se… • L edges: Cd, Sb, Mo… • M edges: Yb, Au, Hg, U… P K-edge XANES

  8. Energy stability/repeatability Is a function of: • Source angular stability (vertical) incident to mono • requires 1 microrad • Monochromator repeatability and angular stability • fixed exit position and angle Can be achieved by: • Beamline monochromator design • Appropriate for each source type (Damping Wiggler, 3-pole, Soft Bend) • Beamline feedback system (vertical angle/position) • For example, NSLS X15B achieves 0.1 eV, 10 micron stability on less-stable source

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