Dear Colleagues, Further discussions of this most interesting topic have continued between Colin and myself off-line. Not least we have basically ended up assembling a references list for further reading! One such I need to check and my copy of the relevant book has required me to get a replacement as my copy has disappeared. I certainly did not wish to raise 'anxieties' either about the nature of inelastic background (not acoustic ie rather Compton) nor about the success or otherwise of ever smaller crystals or ever larger complexes and associated S/N of the diffraction data. On this aspect Colin and I diverge between his optimism and my own (not like me!) caution. However the spectrum of structural information between a polypeptide chain trace on the one hand and the detailed structural chemistry of eg protonation states of ionisable groups on the other will no doubt embrace these differing views. No doubt more later..... Seasons' Greetings, John Prof John R Helliwell DSc On Fri, Nov 27, 2009 at 9:48 AM, John R Helliwell <jrhelliw...@gmail.com>wrote:
> > Dear Richard, > A most interesting discussion has ensued! > > The balance of elastic versus inelastic scattering becomes the core point > re benefit of moving back the detector as mentioned by Ian. It should be > easier now ie with much more beamtime available to measure this as a > function of wavelength. Colin I believe has made a start in this direction. > > The acoustic scattering discussion needs to recall from:- > > I.D. Glover, G.W. Harris, J.R. Helliwell and D.S. Moss 'The variety of > X-ray diffuse scattering from macromolecular crystals and its respective > components' Acta Cryst. (1991) *B47*, 960-968. > and page 966 in particular > > that moving the detector back was not the setting required but a small > collimator (0.2mm) and slitting down the divergence to control the spot size > versus the broader halo of acoustic scattering. These days much more readily > accomplished with an undulator. > > These are both important points then for the growing categories > of microcrystals, which I know you have been very usefully surveying, and > ever larger molecular weight complexes ie both of which are challenged by > S/N for the Bragg spots notably at higher resolution. > > Best wishes, > John > Professor John R Helliwell DSc > beam divergence. > > On Mon, Nov 23, 2009 at 10:54 PM, Richard Gillilan <r...@cornell.edu>wrote: > >> It seems to be widely known and observed that diffuse background >> scattering decreases more rapidly with increasing detector-to-sample >> distance than Bragg reflections. For example, Jim Pflugrath, in his 1999 >> paper (Acta Cryst 1999 D55 1718-1725) says "Since the X-ray background falls >> off as the square of the distance, the expectation is that a larger >> crystal-to-detector distance is better for reduction of the x-ray >> background. ..." >> >> Does anyone know of a more rigorous discussion of why background scatter >> fades while Bragg reflections remain collimated with distance? >> >> >> Richard Gillilan >> MacCHESS >> > > > > -- > > > -- Professor John R Helliwell DSc
