Neutron Scattering

7 . Diffractometer
G .
Heger
7 .1 Introduction
For pure elastic scattering the scattering function S(Q,w) is reduced to the special case
without energy transfer (Eo = E l and h is = E o - E l = 0) and equal length of the wave vectors
of the incident and scattered beams ( ko _ k ). S((Q,c)L = 0) and hence the scattering
intensity is only depending on the scattering vector Q = ko - k 1 . The coherent elastic neutron
scattering (- neutron diffraction) yields information on the positions (distribution) of the
atomic nuclei and the arrangement of the localised magnetic spins in crystalline solids, the
pair correlation function of liquids and glasses, and the conformation of polymer chains .
Depending on the scientific problem to be investigated adequate diffraction methods may be
quite différent . For fluids and glasses diffraction data ofhigh statistical relevance over a very
large 1 Q I range are required. The direction of thé scattering vector Q is not defmed for these
non-crystalline states and a good resolution I AQ I / I _QI is of no importance . Besides of thé
pure elastic scattering also inelastic contributions are involved .
Completely différent are thé needs for a diffraction study of crystalline solids . The diffraction
at thé crystal lattice gives rise to pure elastic scattering localised at thé nodes of thé so-called
reciprocal lattice. The scattering vectors Q for thé différent "Bragg-reflections" are well
defined . For thé séparation of reflections with similar Q values a good resolution AQ/Q is
very important. A measured data-set of Bragg-intensities (integrated intensities of Braggreflections)
as complete as possible over a large 1 Q I range is required . An experimental
stability and accuracy leading to a precision of thé intensity data of about 2% is desired and
may be achieved.
Diffraction measurements on polycrystalline samples depend only on thé length of thé
scattering vector 1 Q I . Very small line widths (according to an excellent resolution 1 AQ 1 / 1 Q I )
combined with well defmed reflection profiles are prerequisites for a quantitative line-profile
analysis . The complete powder diagrams resulting from overlapping reflections are described
and analysed by means ofthé Rietveld method .
For all diffraction methods flrstly thé energy of thé incident neutron beam (expressed in
another way as its wavelength or velocity) must be specified . In thé case of angular dispersive
diffraction, thé 2-axes diffractometer (see Fig . 1) is equipped with a crystal monochromator to