John Another point which might resolve some of the confusion regarding detector distances (or add to the confusion) " 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" This is because the detector was x-ray film with a very good spatial resolution. For a detector with poorer resolution, the angle subtended by one resolution element at short distances can be greater then the angular divergence of the x-ray beam (due to incident beam divergence or crystal imperfections). Colin
________________________________ From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of John R Helliwell Sent: 27 November 2009 09:49 To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] decrease of background with distance? 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 -- -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom
