Neutron Scattering

10 .3 .1 Analysis by filters
The analyzer that is indicated by the grey segment labelled M/F in fig. 10 .12 may either
consist of an array of crystals reflecting scattered neutrons of the selected wavelengtli
A o ente, the associated detectors or of a filter . Especially in the early days of neutron
scattering filters made from polycrystalline blocks of Be , BeO etc . were used . These
filters are transparent only for neutrons wich energies below the "Bragg-edge'" , i .e . for
wavelengths larger than twice the largest lattice spacing . All faster neutrons will be
reflected by some crystallite in the block . By integrated absorbing plates the thus reflected
neutrons are removed . For the filter to be sufficiently transparent below the Braggedge
the used material may only have a very low absorption cross section . In addition
the thermal diffuse scattering by fluctuating lattice deformations (phonons) bas to be
suppressed by cooling (liquid nitrogen) . Below the Bragg-edge these filters transmit all
neutrons from nearly zero energy to the edge energy of a few meV . For the spectroscopy
of high energy excitation this is accceptable since the energy transfer is then determined
by the incident energy of a few 100 meV . High resolution quasielastic scattering bas to
be donc with other instruments . By employing the difference between data obtained with
two different filters (e .g . Be and BeO) the effective window of final energies may be
narrowed, however two measurements are needed and the final signal is obtained from a
small difference of two larger counting signals with the corresponding statistical errors .
10 .4 Resolution and intensity
One important quality of a spectrometer is its resolution in (Q, w)-space . For TOFspectrometers
-unlike for triple-axis instruments- for many applications mainly/only the
energy resolution is important silice the scattering intensity bas to be collected and accumulated
in a large solid angle anyway to yield a sufficient number of counts . The energy
resolution is determined by the accuracy of the TOF-measurement and by the width of
the incoming (or analyzed) wavelength band . The latter is given by the beam divergence
in combination wich the mosaic width of the crystals (sec also chapter on triple-axis spectrometres)
. The TOF-uncertainty is given by the chopper pulse length and the accuracy
of the flight path . The flight path cannot be defined wich arbitrary accuracy, rince finite
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