According to the Undisputed Source of All Human Knowledge (wikipedia):
"Micro is an English prefix of Greek origin that refers to an object as
being smaller than an object or scale of focus, in contrast with macro."
So perhaps "smaller than the regular beam" really is the best definition
of "microbeam", albeit not very useful as a global standard.
However, since it sounds like you are looking for a criterion for
admission into the "microbeam club", I imagine you are looking for a
size of the beam that crosses some threshold where an interesting aspect
of the experiment changes. I would propose that this should be about 10
microns, since this is the size of the first Fresnel zone for 1 A
radiation with the detector/source at 1 meter distance. That is, beams
smaller than this can potentially change the "correlation length". Ten
microns is also about the smallest lysozyme crystal that can yield a
full data set due to radiation damage limits.
Personally, I think the terms "minibeam" and "microbeam" are about as
useful as the terms "minicomputer" and "microcomputer". From what I've
heard, the term "minicomputer" was introduced when computers became
smaller than a house, and "microcomputer" when they became smaller than
a desk. Clearly, this was not three orders of magnitude change in size
(as one would expect form analogy to milli- vs micro-), but it was three
orders of magnitude in volume.
I propose that a much more relevant metric for x-ray beams is the volume
of a crystal than can be probed with it. After all, scattering power is
proportional to volume, not linear dimensions. Yes, crystals can have
odd shapes, but the part that stays in the beam as the crystal rotates
is a "round" volume defined by the size of the beam. The units are
actually convenient. For example:
a 1 millimeter cube has a volume of one microliter (uL)
a 100 micron cube is one nanoliter (nL)
a 10 micron cube is a picoliter (pL)
one cubic micron is a femtoliter (fL)
So, why not talk about beams in terms of pL and nL? In this way, the
"sphere of confusion" for the goniometer is implicitly incorporated.
For example, I can say that most beamlines at ALS have 1.0 nL assay
volumes, and can be apertured down to ~30 pL with a 30 micron pinhole,
or to 1 pL with a 10 micron pinhole (although the latter is not popular).
Since damage is proportional to photons/area, there is no radiation
damage advantage to shooting a given crystal volume one tiny bit at a
time. On the other hand, there is also clearly no point in illuminating
cryoprotectant, nylon or other non-crystalline crud unnecessarily, so
using a beam that is bigger than the crystal will only give you
increased background (unless the crystal is naked). Therefore, in
general, it is best practice to match beam size to crystal size. Yes,
some crystals are bent, wrinkled or otherwise malformed, and shooting
just one bit of them can be an advantage (as Tassos pointed out), but in
my mind I simply view this small "good volume" as "the crystal". After
all, anything you don't want to shoot is (by definition) not your
sample. But perhaps I am alone in this view.
-James Holton
MAD Scientist
Richard Gillilan wrote:
Just an interesting question of semantics that annoyingly comes up
from time to time when people are comparing x-ray beam diameters.
What counts as "microbeam?"
Of course "micro" has the precise meaning in SI as being a factor of
10^-6.
The problem is that the prefix "micro" just means "extremely small" in
common usage.
The term is used very confusingly everywhere. Take microwaves.
Microwaves have wavelengths from 1 millimeter to 1 meter. Go figure.
They're just "extremely small" radio waves.
Now I believe that it is more widely accepted that "nanofabrication"
is making objects that are measured in nanometers.
So shouldn't microbeams rightly be x-ray beams with diameters measured
in microns (i.e. < 1 mm and >= 1 micron). Of course this makes all
crystallography beams microbeams and everything smaller than 1 micron
a nanobeam. That won't be popular.
I've always called anything smaller than 50 microns microbeam because
that's about as small of an aperture-based collimator as we could
make. So a user should ask for "microbeam" if regular collimator is
too large.
I was always puzzled at the APS habit of calling this "minibeam", but
it's starting to sound better all the time.
But in practice, I think "microbeam" sometimes means "smaller beam
than yours." So microbeam used to be 30 microns, 10 or 5, now maybe 1
micron. Pretty soon no microbeam at all.
I think maybe I'll stick with "small", "smaller than usual", and
someday "extremely small."
I'd love to hear people's opinion on the topic.
Richard Gillilan
MacCHESS
