Hi Jason,

You are saying that the outer diameter is 1000 um and that the center of the beam will pass through 1000 micron of the sample... How possible? The tube is not infinitely thin...

The sensitivity of transmission XAS to the non-uniformity of the sample will be different at different energies.

It is important to know how absorbing is the sample (total mu) and what is the delta mu for the X-ray energy range you are interested in.

Anatoly

From: Ifeffit [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of Jason Gaudet [jason.r.gaudet@gmail.com]
Sent: Wednesday, January 27, 2016 7:05 PM
To: XAFS Analysis using Ifeffit
Subject: [Ifeffit] Effect of gradual thickness variation in beam

Sample cell

Hi all,

I'm planning on some transmission-mode XAS with smaller than usual sample tubes. I'm realizing I might be in danger of creating excessive non-uniformity in my samples by having the beam size on the same order of magnitude as the sample tube radius.

For example, let's say I want to measure liquid in a sample tube with a 1000 micron outer diameter, with a beam 500 microns wide and centered on the sample tube. If the tube is orthogonal to the ring plane, the entire vertical portion of the beam will pass through the same length of liquid. But in the horizontal plane, the center of the beam will pass through 1000 microns of sample while the edges of the beam will pass through 866 microns of sample, due to the curvature of the sample cell across the horizontal plane.

Most of what I know about the statistics of thickness effects are about leakage and pinholes - nonlinearity caused by a few spots having very low or negligible sample thickness. But I don't know how significant a "mild" thickness distribution might be. If this sort of thickness distribution is going to be an issue it would be great to know that beforehand and either go with larger samples, smaller beam size, or more creative orientation.

Thanks,

Jason