Dear Ian and James, Here I learned something new. I assumed that coherence length would be limited by crystal quality, e.g. mosaicity and microdomains etc. which apparently is not the case. For me, one of the characteristics of twinning is that there is no interference between the twin domains. If the twin domains get so small that there is interference, these phenomenon are usually referred to as lattice translocation disorder, but that might be my personal interpretation. So maybe I focused too narrowly on the word twinning and should have mentioned the lattice translocation disorder. However, in this case the usual twinning options in refinement programs cannot be used, since these assume summation of intensities.
Cheers, Herman Von: Ian Tickle [mailto:ianj...@gmail.com] Gesendet: Freitag, 25. April 2014 01:01 An: Schreuder, Herman R&D/DE Cc: CCP4BB@JISCMAIL.AC.UK Betreff: Re: [ccp4bb] AW: [ccp4bb] Twinning VS. Disorder Dear Herman On 24 April 2014 22:32, <herman.schreu...@sanofi.com<mailto:herman.schreu...@sanofi.com>> wrote: The X-ray coherent length is depending on the crystal, not the synchrotron and my gut feeling is that it is at least several hundred unit cells, but here other experts may correct me. I assume you meant that the coherence length is a property of the beam (e.g. for a Cu target source it's related to the lifetime of the excited Cu K-alpha state), not the crystal, e,g, see http://www.aps.anl.gov/Users/Meeting/2010/Presentations/WK2talk_Vartaniants.pdf (slides 8-11). The relevant property of the crystal is the size of the microdomains. You don't get interference because coherence length << domain size, i.e. the beam is not coherent over more than 1 domain. This is true for in-house sources & synchrotrons, I guess for FELs it's different, i.e. much greater coherence length? This relates to a question I asked on the BB some time ago: if the coherence length is long enough would you start to see the effects of interference in twinned crystals, i.e. would the summation of intensities break down? I defer to the experts on synchrotrons & FELs! Cheers -- Ian
