Dear All,
let's get some straight facts about neutron resolution. The 'important'
resolution is not the 2theta FWHM, but the deltad/d FWHM. Now, everybody
knows this formula:
deltad/d=[(cot(theta)dtheta)^2+dlambda/lambda^2]
This, combined with Bragg's law, yields:
deltad/d=[dtheta^2*(2d^2/lambda^2-1)+dlambda/lambda^2]
In other words, everything being equal, the resolution gets better at longer
wavelengths and at shorter d-spacings. This is the key to understand why
the resolution of (high-res) neutron diffractometers is comparable to that
of synchrotron x-ray diffractometers, and much better than lab x-ray
machines (sorry Armel). Here is why:
1) Neutrons can generally measure at much longer wavelengths (up to 4-5 A
with thermal machines, in excess of 10 A for cold diffractometers). X-rays
are already in trouble above 1.5 A.
2) Neutrons can measure at shorter d-spacings, because of the flat form
factor.
Time-of-Flight and Constant-Wavelength diffractometers exploit these
advantages in different ways, but the crux of the matter is that the best
exemplaries of either type always have sample-limited profiles, and that's a
fact.
As far as structural solution from powder data is concerned, there are
various reasons why neutrons have not made an impact there. One, as Armel
correctly points out, is that neutrons usually see too much. Another is
that neither TOF nor CW machines are really optimised for structural
solution. Basically this is because an optimised machine would require a
wavelength (or wavelength band) tunability that has not yet been
implemented, although it could be and will be with some of the new machines.
However, we are out of the game anyway for organic molecules, because
deuteration is a hassle and the high scattering from deuterons just
complicates the solution. What is left is small fry and we will gladly
leave it to x-rays, since we are so much better at virtually everything
else.
Paolo
