Ian
Maybe - maybe not. 
Investigations of acoustic and optical components of diffuse scatter
from proteins were carried out in the 80s and 90s including of course
work at Birkbeck (which I am sure you are aware of)

Refs can be found in Glover et. al. Acta Cryst. (1991). B47, 960-968.
This paper includes the statement 
"We have exploited the characteristic fine collimation of synchrotron
radiation in the collection of data in which the acoustic scattering
contributions are minimized
to assess the effect on model refinement"

I think if the acoustic mode is due to correlations extending over 6
cells (say) then the width of the acoustic scatter will reflect this.
The diffuse feature will spread out as the spots separate when the
detector is moved back. However, as you say, separating them from the
diffraction peak could still be a problem. Should this intensity be
regarded as part of the Bragg peak or should it be subtracted from it?
With a poorly collimated beam or close detector distance this problem
does not arise as the acoustic scatter is in any case buried in the rest
of the Bragg peak.

Oh no - something else to argue about!

 Colin



-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
Ian Tickle
Sent: 26 November 2009 11:20
To: [log in to unmask]
Subject: Re: [ccp4bb] decrease of background with distance?

> The source for the X-ray background are points along the air path 
> post-collimator including the sample with loop and cryoprotecdant (or 
> capillary and mother liquor).  So the 1/r^2 falloff is noticable going

> from 100 mm to 200 mm.  The same counts in a 2x2 pixel area is now 
> seen in a 4x4 pixel area.

Hi Jim,

I think it may be a bit more complicated than this because the
background contribution from the crystalline scattering consists of
non-Bragg elastic ('diffuse') scattering, plus inelastic ('Compton')
scattering, though the latter is probably small & can be ignored.  DS
consists of a number of contributions, notably the 'optic' component due
to short range correlated displacements (e.g. of secondary structure
elements), and the 'acoustic' component due to longer range correlated
displacements of whole molecules in adjacent unit cells (i.e. scattering
by lattice phonons).  Now the 'optic' component can be regarded as
attached to the reciprocal lattice, so does scale exactly in the way you
describe.  However the acoustic component probably represents the
biggest contributor to the X-ray background under normal conditions and
is responsible for the 'tails' under the Bragg spots; in fact the
acoustic DS peaks right under the Bragg spots & there's no practical way
of separating them, because AFAIK (though I could be wrong) the acoustic
peaks scale with the Bragg spots.  I don't think it's possible (though
admittedly I've never tried) to separate the acoustic DS tails from the
spots merely by moving the detector further away as you seem to be
implying!  I'm by no means an expert on dynamical scattering theory so I
could be talking nonsense!

> The source for Bragg reflections at a synchtrotron is upstream a 
> couple dozen meters.  The divergence is not large as well, so the 
> spread in the spots (for a source ~30 meters upstream) goes from 
> 1/(30.1 *
> 30.1)^2 to
> 1/(30.2 * 30.2)^2 which is really not that noticable.

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


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