David had developed an empirical theory to model the air, solvent,
Compton & acoustic contributions and correct the integrated data for
these, without background correction of course since the optic DS
background was ultimately to be our data!
...
Hi Ian, did David publish this theory somewhere? I'd love to get a
reference.
I'm genuinely confused by this because I thought the whole point of
modern focusing optics (or at least the confocal mirror design) is to
focus the beam onto (or close to) the sample, in which case
wouldn't the
photons diverge from the 'virtual source' (actually a real
image of the
real source) at the crystal, instead of from the real source? So
then
Bragg spots (and therefore also the acoustic DS) should
diverge from the
position of this virtual source?
Cheers
-- Ian
This does seem confusing now. Here at CHESS, focus is either at the
collimator (just upstream of the sample), or, in the case of
capillaries, on the sample. In rare cases, on the detector. True,
the focal spot would be a virtual source and, if the spot is
reflected by an ideal crystal, that wouldn't change anything. But it
is a very well collimated source even though it is slightly divergent
(by a few mrad at most) ... this is in contrast to background scatter
where photons are emitted in the whole q range (assuming it is "that
kind" of scatter). So maybe it is more accurate to say Bragg
reflections and background scatter are two rather different kinds of
sources located at the crystal.
Pardon my ignorance, but how can lattice phonons be a significant
effect at low temperature? I presume the correlated displacements you
refer to must be phonon modes frozen in place to create static
disorder or something like that ... or perhaps stuff is moving more
than I think at 100 K.
Richard
