I just noticed this thread. I should make a few comments.
We regularly provide microbeam with and without Helium here at
MacCHESS. Yes, there are cases in which microbeam can give you good
diffraction on large crystals when a larger beam cannot. Just last
week we had a user group collecting on column-shaped crystals about
40 microns wide by 200-300 microns long. When exposed using a 150
micron diameter beam, the diffraction patterns were a mess, producing
multiple lattices. When they exposed the crystals using a 20 micron
beam, they were able to find enough "sweet spots" with single lattice
to give good results. I can only offer anecdotes at this time, but,
based on user experiences so far, this is not uncommon at all.
I don't usually think of microbeam alone as a significant reduction
in background except that one avoids hitting solvent (which is a
major source of scatter!). While the beam scatters less air due to
its small size, microbeams also can have much increased flux which
compensates for the smaller size ... so I don't think there will be
less air scatter unless you are just using an aperture with no flux
gain.
Regarding helium: we regularly use helium at 95K combined with a
helium enclosure. The setup is awkward for manual sample mounting,
but quite convenient for automounting. The main advantage here is
that direct beam scattering with air is nearly eliminated.
How important is this effect? It depends upon how much free path
there is between the end of the optic (slits, etc) and the beamstop.
It also depends upon how small your sample is and how strongly it
diffracts. In a carefully-controlled experiment using the identical
crystal and orientation range with both nitrogen and helium, I saw a
signal-to-noise improvement by over a factor of 3 in the 2.5 Angstrom
range and lower. At higher resolution the benefit decreases, but
still looks no worse than a factor of 2. This is with a 50 micron
crystal illuminated with an 18 micron beam.
I am currently working on guidelines for when helium and microbeam
are necessary (based on both simulations and explicit measurements).
At the present time, my feeling is that crystals below 50 micron can
certainly make the extra hassle worthwhile. It really depends upon
how badly you want that extra resolution. In the case above, it
pushed the resolution from above to below the 2.0 Angstrom mark based
on I/SIG.
As for radiation damage, I do think it is important that a beam
intensity profile be as flat at the top as possible (not a sharp
hotspot) .. so defocusing a little may be useful. This is not usually
a factor that regular users have any control over. I don't have any
hard data on this.
Richard Gillilan
MacCHESS
