Neutron Scattering

ackscattering-z-spectrometer ; for details sec chapter 10 . The second method leads
to the spectrometer types that are the topic of this chapter, namely different variants of
time-of-flight (TOF)-spectrometers . Also the neutron spin-echo spectrometer,
NSE that is described in the next chapter employs -somewhat less obvious- directly the
velocity change of the neutrons to infer the energy transfer . The generic geometry of
a scattering experiment in reciprocal (i .e . velocity, momentum or wavevector) space is
illustrated in Fig . 1 . The scattering triangle consisting of the incoming wavevector k,
Figure 10 .1 : Scattering triangle .
the wavevector of the scattered neutrons k' and the resulting momentum transfer ( . ),
Q, the figure shows the general situation of inelastic scattering (here : energy gain of the
neutron) . 20 is the scattering angle, Qe indicates the momentum transfer for elastic
scattering (i .e . without energy transfer) .
A nuclear research reactor as neutron source basically yields a thermal (Vlaxwellian)
spectrum of neutron velocities, the temperature of the moderator (D 2 0-cooling water
approx . 60°C) determines the temperature of the neutron cloud . Vlany facilities contain
additional small moderators of different temperature that supply single beam tubes with
neutrons of a different spectral distribution (different temperature) . In particular the so
called "cold sources" have to bc mentioned . In Jülich the "cold source", which supplies
the neutron guide hall (ELLA) with long wavelength neutrons, consists of a small volume
filled with liquid H 2 corresponding to a temeparture of 20 K . The following table enables
a quick survey over the average values of the corresponding neutron spectra .
10- 2