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
