[Ifeffit] Effect of gradual thickness variation in beam

[Bruce Ravel]

On 01/27/2016 07:05 PM, Jason Gaudet wrote:
> 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.


Jason,

There may be some distortion to the data due to the varying thickness.
On the plus side, liquids tend to be very homogeneous.

Probably the best solution to this situation, if it's available at the
beamline, is to focus the beam in the vertical, for example, using a
flat mirror on a bender.  Focusing to a spot would also address the
thickness situation, but you need to be mindful that the intense beam
could generate radicals from the liquid, causing sample damage.

If the sample is sufficiently concentrated, then you could simply slit
the beam down to 100 or 200 microns and still have enough for a
transmission measurement.  In fact, that would be the way to decide if
you are seeing a big problem from the shape of the tube.  Compare the
mu(E) with the slits at 500 to mu(E) with the slits at 100 (carefully
aligning the sample both times, of course).  If they are the same,
then, then you're golden.

B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
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