Ian Surely the Bragg peaks, acoustic and optic components of the diffuse scatter all occupy a finite volume in reciprocal space. In x-ray scattering/diffraction the area they occupy on a detector therefore expands as the detector is moved further away. However, one will not always see this. If the beam divergence is large (or the detector resolution poor) the size of the feature on the detector is instead dominated by these instrument parameters. By improving these instrument parameters one will eventually see the true shape of the Bragg peaks (the sharpest feature) and be able to distinguish them optimally from the various types of diffuse scatter (originally called background!). After this, moving the detector further away will cause the size of the diffraction spots to increase just like the diffuse scatter components and no further benefit would be obtained.
However, the acoustic mode will still be centred on the Bragg peak and it is not clear (to me) how one should treat it if one wants to have the best integrated intensities. You imply that it should be included within the peak. I think this is right but it is not clear that the profile fitting programs will always do this. In the majority of cases it probably doesn't matter but there might be some case where it does. As you say, the acoustic scatter has little interest from the structure function point of view. I think I had a discussion about this with James Holton some time ago but can't remember the conclusions! Colin -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Ian Tickle Sent: 26 November 2009 14:00 To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] decrease of background with distance? Hi Colin Yes I know, I worked with David Moss at Birkbeck for many years to develop software to process the DS data. I think the point of using finely collimated SRS X-rays (from the Daresbury SRS of course!) was to scale down the spot size of both the Bragg & acoustic peaks by the same factor, so that the peak integration included both contributions in the 'peak' region of the integration box in Mosflm, in order to prevent the tail of the acoustic peak from straying into the 'background' region of the adjacent spots. 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! It was only the Bragg & optic DS components that were interesting from a structure/function point of view. I don't recall any discussion that we could get a handle on separating the Bragg and acoustic components by changing the divergence of the beam or the distance. Cheers -- Ian > -----Original Message----- > From: owner-ccp...@jiscmail.ac.uk > [mailto:owner-ccp...@jiscmail.ac.uk] On Behalf Of > colin.n...@diamond.ac.uk > Sent: 26 November 2009 11:54 > To: Ian Tickle; CCP4BB@JISCMAIL.AC.UK > Subject: RE: [ccp4bb] decrease of background with distance? > > > 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:ccp...@jiscmail.ac.uk] On Behalf Of > Ian Tickle > Sent: 26 November 2009 11:20 > To: CCP4BB@JISCMAIL.AC.UK > 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 > > > Disclaimer > This communication is confidential and may contain privileged > information intended solely for the named addressee(s). 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