> The source for the X-ray background are points along the air path
> post-collimator including the sample with loop and cryoprotecdant (or
> capillary and mother liquor). So the 1/r^2 falloff is
> noticable going from
> 100 mm to 200 mm. The same counts in a 2x2 pixel area is now
> seen in a 4x4
> pixel area.
Hi Jim,
I think it may be a bit more complicated than this because the
background contribution from the crystalline scattering consists of
non-Bragg elastic ('diffuse') scattering, plus inelastic ('Compton')
scattering, though the latter is probably small & can be ignored. DS
consists of a number of contributions, notably the 'optic' component due
to short range correlated displacements (e.g. of secondary structure
elements), and the 'acoustic' component due to longer range correlated
displacements of whole molecules in adjacent unit cells (i.e. scattering
by lattice phonons). Now the 'optic' component can be regarded as
attached to the reciprocal lattice, so does scale exactly in the way you
describe. However the acoustic component probably represents the
biggest contributor to the X-ray background under normal conditions and
is responsible for the 'tails' under the Bragg spots; in fact the
acoustic DS peaks right under the Bragg spots & there's no practical way
of separating them, because AFAIK (though I could be wrong) the acoustic
peaks scale with the Bragg spots. I don't think it's possible (though
admittedly I've never tried) to separate the acoustic DS tails from the
spots merely by moving the detector further away as you seem to be
implying! I'm by no means an expert on dynamical scattering theory so I
could be talking nonsense!
> The source for Bragg reflections at a synchtrotron is
> upstream a couple
> dozen meters. The divergence is not large as well, so the
> spread in the
> spots (for a source ~30 meters upstream) goes from 1/(30.1 *
> 30.1)^2 to
> 1/(30.2 * 30.2)^2 which is really not that noticable.
I'm genuinely confused by this because I thought the whole point of
modern focusing optics (or at least the confocal mirror design) is to
focus the beam onto (or close to) the sample, in which case wouldn't the
photons diverge from the 'virtual source' (actually a real image of the
real source) at the crystal, instead of from the real source? So then
Bragg spots (and therefore also the acoustic DS) should diverge from the
position of this virtual source?
Cheers
-- Ian
Disclaimer
This communication is confidential and may contain privileged information
intended solely for the named addressee(s). It may not be used or disclosed
except for the purpose for which it has been sent. If you are not the intended
recipient you must not review, use, disclose, copy, distribute or take any
action in reliance upon it. If you have received this communication in error,
please notify Astex Therapeutics Ltd by emailing
i.tic...@astex-therapeutics.com and destroy all copies of the message and any
attached documents.
Astex Therapeutics Ltd monitors, controls and protects all its messaging
traffic in compliance with its corporate email policy. The Company accepts no
liability or responsibility for any onward transmission or use of emails and
attachments having left the Astex Therapeutics domain. Unless expressly
stated, opinions in this message are those of the individual sender and not of
Astex Therapeutics Ltd. The recipient should check this email and any
attachments for the presence of computer viruses. Astex Therapeutics Ltd
accepts no liability for damage caused by any virus transmitted by this email.
E-mail is susceptible to data corruption, interception, unauthorized amendment,
and tampering, Astex Therapeutics Ltd only send and receive e-mails on the
basis that the Company is not liable for any such alteration or any
consequences thereof.
Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park,
Cambridge CB4 0QA under number 3751674
