Hi
Both transverse and longitudinal coherence length need to be considered in this. These parameters are detemined by monochromators, focusing optics and the position of the specimen along the path not just the undulator (or x-ray generator).
Matching to the specimen is not necessarily as simple as the dimensions of the mosaic blocks in the specimen. It is the optical path length which is important. One would have to consider the variation in refractive index between mosaic blocks and the surroundings.
Cheers
Colin
-----Original Message-----
From: CCP4 bulletin board on behalf of Ethan Merritt
Sent: Thu 29/01/2009 19:24
To: [log in to unmask]
Subject: Re: [ccp4bb] X-ray photon correlation length
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
<DIV><FONT size="1" color="gray">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
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