Environmental Sciences Department

Stability Requirements Stability Requirements

XAS at NSLS-IIXAS at NSLS-II

Paul NorthrupApril 19, 2007

Environmental Researchand Technology

Division

Environmental Sciences Department

EXAFS data requirementsEXAFS data requirements

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• 1000 eV scan range• Data can be influenced by 0.1% change in signal

Ravel

Environmental Sciences Department

EXAFS experimentEXAFS experiment

• 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.

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Ravel

Environmental Sciences Department

Non-uniform samplesNon-uniform samples

Low-concentration heterogeneous samples:• Beam motion changes signal due to variations in

• composition• matrix• particle distribution• surface irregularities

Environmental Sciences Department

Maximum tolerable beam motion? 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

Environmental Sciences Department

Positional stability requires… Positional stability requires…

1) Aperture close to sample, overfilled by 10%2) Source stability within 10% of source size and divergence3) Monochromator (and optics) output position and angle must be stable over 1000 eV scan

Environmental Sciences Department

Energy stability (repeatability) requirementsEnergy stability (repeatability) requirementsXANES 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

Environmental Sciences Department

Energy stability/repeatability 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