"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.

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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