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
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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