Bernhard

Yes I stand corrected, 'coherence length' it is.  I think I did mean
'coherence' and not 'correlation', it's just that I had 'correlation' on
my mind from something else I was doing.  What I need is a 'concept
checker' in addition to a spelling & grammar checker!

-- Ian

> -----Original Message-----
> From: [log in to unmask] 
> [mailto:[log in to unmask]] On Behalf Of Bernhard Rupp
> Sent: 29 January 2009 18:59
> To: 'Nave, C (Colin)'; [log in to unmask]
> Subject: RE: [ccp4bb] X-ray photon correlation length
> 
> 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.
> 
>  
> 
> 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?
> 
>  
> 
> BR
> 
>     
> 
>  
> 
> 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.p
> df?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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