On Thursday 29 January 2009 10:59:23 Bernhard Rupp wrote: > Ok, following seems to be correct: > > > > a) interaction length = mean free path : relevant for absorption > > b) correlation length = time correlation between photons : relevant for > multi-photon scattering > > c) coherence length = longitudinal coherence length : relevant for > single photon scattering. > > > > It follows from Heisenberg for a Lorentzian source (anode) with natural > emisson line width per > > formula on p 5007 of Colin's ref > > > > Lc=(2/pi)lambda**2/delLambda > > > > Using 8084 eV and 2.1 eV respectively for Cu, I obtain ~3800 A coherence > length for a Cu (anode) X-ray photon > > > > The pre-factor is different for other source types like synchrotron.
The coherence length for an undulator source is the relativistically contracted length of the undulator. Ref: http://xdb.lbl.gov/Section2/Sec_2-1.html > In any case I would accept the vague term of 'a few 1000 A' or 'several > 1000 A' as a general statement for > > coherence length in materials where the interaction length is larger > (practically always). > > > > Does this sound reasonable? My impression is that the coherence length from synchrotron sources is generally larger than the x-ray path through a protein crystal. But I have not gone through the exercise of plugging in specific storage ring energies and undulator parameters to confirm this impression. Perhaps James Holton will chime in again? Ethan > > > From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Nave, > C (Colin) > Sent: Thursday, January 29, 2009 10:14 AM > To: CCP4BB@JISCMAIL.AC.UK > Subject: Re: [ccp4bb] X-ray photon correlation length > > > > Bernard > > I guess this came from > > "Aren't detwinning methods appropriate only in the case of true twin domains > which are larger than the X-ray photon correlation length in order for the > assumption to be valid that |F|^2 from each domain can be summed? This > wouldn't give rise to the apparent 'diffuse scatter' phenomenon." > > > > I think this is normally called coherence length. Probably best not to think > of photons at all but waves (though there is an equivalent quantum > mechanical treatment based, as V Nagarajan says, on the uncertainty > principle). I don't think the domains have to be larger then the correlation > (sorry coherence) length of the incident x-rays in any case. They have to be > large enough to give an intensity which can be integrated. If smaller > domains are present, the intensity just spread out a bit more.When the > domains are very large, the size of the spots would be determined by the > incident beam properties. > > > > The article cited some years ago on CCP4BB gives a primer on all this > > J. Phys.: Condens. Matter 16 (2004) 5003-5030 PII: S0953-8984(04)75896-8. > Coherent x-ray scattering Friso van der Veen1,2 and Franz Pfeiffer1 > > http://www.iop.org/EJ/article/0953-8984/16/28/020/cm4_28_020.pdf?request-id= > 8848d3f0-5a4b-4ffe-8ea4-c1eabfaf1657 > > > > Cheers > > Colin > > > > _____ > > From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of > Bernhard Rupp > Sent: 29 January 2009 17:51 > To: CCP4BB@JISCMAIL.AC.UK > Subject: [ccp4bb] X-ray photon correlation length > > I always wondered - how is the X-ray photon correlation length defined > > and where do I find it? This is not the interaction length, I assume. > > > > So, to the physicists: How large is the 'X-ray photon correlation length' > > for a given wavelength in a given material? > > > > I had the impression that the term photon correlation refers > > to the time correlation of the scattering such as in photon correlation > spectroscopy. > > > > Best regards, BR > > > > > > 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 > > > > _____ > > > Scanned by iCritical. > > > > -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742