[Ifeffit] Transmission EXAFS sample

[Matt Newville]

Dear Jatin,

The idea that the optimum absorption length (mu*t) for transmission
experiments is 2.3 assumes that the errors in the measurement are due
to counting statistics of the x-rays.  For any synchrotron experiment,
the number of x-rays in the transmission chamber is high enough that
the noise from counting statistics is rarely significant.  This means
that using a value of 2.3 is really not that important.

The more important issues are
  a) having a uniform sample.
  b) not having (mu*t) so high that higher-order harmonics dominate
the transmission measurement.

For transmission measurements, it's difficult to overstate the
importance of a uniform sample.  For an ideal thickness, I would say
that the better rules of thumb than mu*t = 2.3 are to aim for an edge
step of 0.1 to 1.0, and a total absorption less than 3.0.

If you only have enough material for an edge step as low as 0.02 (as
you imply), then measuring in fluorescence or electron emission is
probably a better choice.  Such a sample won't be severely affected by
"self-absorption" (or "over absorption" to use the term this mailing
list prefers) in the fluorescence measurement.  I would recommend
simultaneously measuring transmission and florescence for such a
sample.

My concern about a very thin sample is uniformity.  Specifically, is
the grain size really well below mu/0.02 so that a collection of
particles can give a uniform thickness?  Since you didn't give any
details of the system, it's hard to guess.

Is it feasible to pack that material into a smaller area so that the
thickness is increased and use a smaller x-ray beam?

> -- Can my sample be only few percentage of the "actual amount" (i.e.
> calculated based on above fact) required, and still i can perform
> transmission EXAFS ? How would this affect my data ? (I guess, it will be
> heavily dominated by noise)

I would guess that a sample with mu*t of 0.02 would be dominated by pinholes.

> -- What if, i have required amount of sample but since material's density is
> so high that it yields only small volume of powder (for a given weight),
> that it can not be covered up on multiple layers of Kapton tape to ensure
> pinhole-free sample ?

If you cannot get the grain size small enough to have many overlapping
grains in the sample, the sample won't be uniform enough for good
transmission data.  The techniques of using multiple layers of mixing
with a low-Z binder don't solve this problem.  These do help to make a
uniform collection of overlapping grains, but don't make the grains
smaller.

I would recommend trying to increase the thickness at the expense of
cross-sectional area, and/or measuring in both transmission and
fluorescence.

Hope that helps,

--Matt