Neutron Scattering

machine which will be followed by an optimization of the physics involved (one sided in a
certain sense) in the shape of the backscattering spectrometer.
10 .2 Principle of the crystal spectrometer with three variable axes
Fig. 1) shows the principle layout of a triple-axis spectrometer. Neutrons of a defmed wavelength
 are selected from an incident "white" beam by means of a single crystal (monochromator
M, ferst axis) under the associated angle 20, according to eq . (1) . The sample S
under inspection is positioned in the diffracted beam. The secondary spectrometer moves
around the sample thereby selecting neutrons scattered by an angle 20, (second axis) which
are further sorted with regard to their energy via the angular setting 20, of an analyzer crystal
A (third axis) and counted by detector D . The scattering vector 2 as well as the energy transfer
ho) are determined by the angles 20, and 20, for given incident wave vector k (see
also Fig . 2) .
Fig. 1)
schematic layout of a triple-axis
spectrometer
with :
collimators a ,, monochromator
M, sample S,
analyzer A, detector D,
incident (scattered) wave
vector k (k
Thereby, the wave vectors k and k' are connected with the variables Q and ho) by the conservation
of momentum and energy . In the resulting scattering triangle (Fig. 2) the deviation of
the foot F from Y Q determines the energy transfer hco . The sense of this deviation determines
whether the neutron bas gained (k < k, E-E'=hco < 0) or lost energy (k > k, hco > 0)
by the scattering process . In order to influence the divergence of the neutron beam, collimators
a are inserted before and alter the crystals by which neutrons are selected . Detection of neutrons
is by materials with an exceptionally large absorption cross section . The involved nuclear
processes are e.g . 1OB(n,a), 6Li(n,a), and 3He(n,p) . The most common type of detectors
is conceived as proportional counter filled with 3He having an absorption cross section of
5300 barn for neutrons with 25 meV . With the typical active length of about 5 cm and a filling
pressure of about 5 bar, the counting probability reaches >95% for thermal neutrons (the absorption
cross section is reciprocal to the speed of the neutron) .
It is meaningful to insert a neutron monitor at the exit of the primary spectrometer . This device
is principally similar to a counter absorbing and detecting, however, a small fraction of
the incident neutrons, only . The measured spectra may thus be normalized to the number of
incident neutrons which is essential for long measuring periods since the neutron flux may
9- 3