Hi Anatoly,

Sorry, I meant to write 1000 um inner diameter.

Energies will be V and Fe, 5465 & 7112 eV. Samples are roughly 1 absorption length thick for the metal (V, Fe) and about 2-2.5 absorption lengths for the entire sample.

Bruce,

I may just flip over the sample holder and have the sample tubes in plane with the storage ring to take advantage of the greater horizontal spread, rather than focusing on the vertical. Cutting the vertical slits down to 100-300 um while keeping the horizontal around 500 um ought to get me the most beam at the least variation in mu(E), especially if I go with 1500 um ID tubes. The samples are nice and concentrated so I can afford to give up a lot of photon flux. Bar-napkin calculations tell me with a 1500 um ID tube and 200 um V 750 um H slits I'll lose an acceptable amount of photon flux and have a <1% thickness variation. I'm fairly confident I've made pressed pellets with as much or more variation from one spot to another, without noticeable mu(E) changes from spot to spot, in the vicinity of this energy range.

I'll go ahead and take your advice, and amend my setup procedure with some measurements of Io intensity and mu(E) reproducibility as a function of slit width. While a bit of a bother, it's still worth it to keep our sample prep simple and inexpensive.

Thanks,

Jason

On Thu, Jan 28, 2016 at 9:12 AM, Bruce Ravel <bravel@bnl.gov> wrote:

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@bnl.gov

National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Building 535A
Upton NY, 11973

Homepage: http://bruceravel.github.io/home/
Software: https://github.com/bruceravel
Demeter: http://bruceravel.github.io/demeter/
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