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:[log in to unmask]] On Behalf Of Nave,
> C (Colin)
> Sent: Thursday, January 29, 2009 10:14 AM
> To: [log in to unmask]
> 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:[log in to unmask]] On Behalf Of
> Bernhard Rupp
> Sent: 29 January 2009 17:51
> To: [log in to unmask]
> 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
> 
>  
> 
>  
> 
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-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742