Neutron Scattering

select a special wavelengths band (;, ± Ak/;,) out of the "white" beam according to the Bragg
condition for its scattering plane (hkl)
2dhk1' SinO yk1 = ;~ ,
with the interplanar spacing dhkl and the monochromator scattering angle 26hk1 = 20m. The
width ofthe wavelengths band which is important for the Q-resolution, depends on the
divergences of the beam before and after the monochromator (collimations al and a2), on the
mosaic spread of the monochromator crystal AM, and on the monochromator angle 20m. In
order to increase the intensity of the monochiomatoc beam at the sample position the
monochromator cystal is often bent in vertical direction perpendicular to the diffraction plane
of the experiment. In this way the vertical beam divergence is increased leading to a loss of
resolution in the reciprocal space . Thc diffracted intensity from the sample is measured as a
function of the scattering angle 26 and the sample orientation (especially in case of a single
cystal) :
for a single crystal ---> I(Q), and for a polycrystalline sample -> I( I Q 1 ) .
29 is defmed by the collimators a2 and a 3 . As there is no analysis of the energy of the
Detector
Fig . 1 . Schematic representation of a 2-axes diffractometer .
7-2