Concerning the bathing, I think there is at least this point, which I do not
think has been mentioned, to wit:
Considering that radiation damage is now present, I would guess, in
virtually all synchrotron-collected datasets, and considering that probably
this information could be used in virtually all cases to help phase the data
or at least be corrected for, it really makes sense to bathe the crystal
completely, as this way all of the crystal is exposed evenly (assuming
radiation damage is not direction-specific, and assuming the beam flux is
evenly distributed across the beam cross-section). This way, the damage
should be a simple function of time, not confounded with various parts of
the crystal entering or exiting the beam. Therefore, if one wants either to
use or correct for the radiation damage, it seems obvious to me that,
concerning this aspect, bathing is superior.
Enjoying the debate,
Jacob Keller
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Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.467.4049
cel: 773.608.9185
email: [EMAIL PROTECTED]
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----- Original Message -----
From: "Richard Gillilan" <[EMAIL PROTECTED]>
To: <CCP4BB@JISCMAIL.AC.UK>
Sent: Monday, November 26, 2007 12:02 PM
Subject: Re: [ccp4bb] To bathe or not to bathe.
In our current helium box, there is a total of about 28 mm of beam
exposed. 10 mm from the aperture of the optic and 18 mm from sample to
beamstop. The 10 mm side working distance is very tight for hand mounting
(little room for tongs) and falls just outside the shield stream for
cryo. Could probably decrease the 28 mm some, but not below 10 mm I
think. I think our standard setup places the collimator ion chamber
about 20 mm from sample ... so 30-50 mm may not be far off the mark for
typical stations. Usually, it is the virus crystallographers who are
fussy about getting the smallest beamstop. Next time, I'll have to whip
out a ruler and see what distance makes them happy.
It would be interesting to know typical distances for other facilities.
Anyone else made these measurements?
Richard Gillilan
MacCHESS
Note the density of air is approximately 1000 times less than a protein
crystal. The total scatter for a beam going through a 50 micron thick
crystal will be similar to that from 50mm air. Most beamlines will have
a path length less than this but nevertheless the air scatter will be
significant with small crystals.
In principle, with smaller beams one can have smaller beamstops nearer
the sample thus reducing the path length through the air.
Colin
