Sample Geometry and Size
Alfred Baron, 2023
For those fortunate enough to be able to choose the sample size and geometry, the following may
be of interest. Please also read the advice for single crystals in any case (if you are measuring asingle crystal).

Transmission/Laue Geometry:A transmission or Laue geometry is usually the most flexible allowing access to the largest
range of accessible momentum transfers. It can be the best geometry if you want very
low momentum transfers. It is also generally less sensitive to surface preparation/quality.
The ideal thickness for transmission is 1 attenuation length. Practically between 1 and 2 isusually OK. Much less than 1 or much more than 2 essentially wastes beam time. It is
usually easiest if the transverse dimensions of the sample are ~1-5 mm. See also the note
about the cryostat below.

Reflection/Bragg Geometry:
A reflection or Bragg geometry (for thick samples) generally can provide higher rates thana transmission geometry, but limits one to total momentum transfers, Q, that are not too
far from the surface normal (n.b., little q, q, is not limited in the same way). If you know
where (what Q) you want to measure it can be a very good choice. Note, however, thatspectra can be sample surface sensitive and you need a larger flat surface - large enoughto comfortably intercept the entire transverse beam size in all Bragg geometries of interest.
Flat surfaces of ~2-5 mm length scale are easiest. Also, lightly etching the surface canremove strain and reduce elastic scattering in the phonon spectra. For samples with smalllattice constants also consider how to access two alignment reflections (primary andsecondary).

Chi & Sample Stability when using a Cryostat/Cryofurnace.
In general the spectrometer has a sphere of confusion of a few 10’s of microns – sample
positions when mounted in the center of the circles will tend to stay stable at that level.
However, if the cryostat or cryofurnace is mounted, these have a center of mass that is
far outside the center of circles and when chi is changed by more than (say, roughly) +-20
degrees, it is very easy for the sample position to shift by 0.1 mm (even 0.5 mm for chi~90 degrees). The position is motorized, and we can correct it, but the correction needs
to be done (usually by hand) and can slow an experiment. And, of course, if for somereason your sample has a small (< 0.1 mm scale) transverse size, this can slow theexperiment a lot if you need to work at larger chi-values.

Determining grazing angles onto the sample surface.
Diffractometer codes usually can provide the x-ray beam grazing angles onto and off of thesample surface. For spec and related codes, set the sample normal vector (in h,k,l) using
the “setaz” command and then alpha is the angle onto the sample surface and beta is theangle out of the sample surface into analyzer 6 (at BL43LXU). If alpha and beta are thesame sign then you have a Bragg/reflection geometry, if they are different signs, aLaue/transmission geometry. Note that having alpha small (say <~3 deg) can makealignment to the incident beam tricky while having beta small (<~3 deg) can mean someanalyzers may be blocked by the sample surface.