X-ray and neutron diffraction by nematic liquid crystals

An expression for the neutron (or X-ray) diffraction cross section for a nematic liquid crystal is derived. From this expression it is deduced that such diffraction experiments may be used to measure the nematic order parameter. It is further shown that scattering recorded at temperature close to the nematic-isotropic transition may show weak pretransitional effects even at finite values of the scattering wave vector.