Neutron Scattering

For the Nickel film one gets a thickness of 838 A and a roughness of 15 A . The open dots in
Fig . 8 .13 show the reflection from glass alone . One clearly observes a smaller angle of total
reflection and a decrease equivalent to the Darwin cuive in Eq. (8 .9) . At relatively small
intensities of 10-3 scattering from background becomes visible .
One can also study heterogeneities within the surface which are f i .
formed by phase
separation. In those cases one also observes nonspecular reflection in direction parallel to the
surface.
8 .5. Important Elements of Small Angle Scattering and Reflection
The most important elements of small angle scattering and reflection are (1) the neutron
guides for an effective transport of neutrons, (2) the velocity selector and the perfect single
ciystals for monochromatization ofthe neutrons, and (3) the local sensitive detector for a fast
detennination of the scattering angle .
8 .5.1. Neutron Guide
The phenomenon of total reflection is demonstrated in Fig .8 .13 on a Nickel film . This effect
is used to transport neutrons through neutron guides over long distances without much loses .
The angle of total reflection is given as
o c = ;~ (P / 7L) o .s (8 .11)
with the coherent scattering length density p . Natural Nickel is a good choice for coating
material in order to have a large angle of total reflection (see Figure 8 .13) ; an even better
choice is the isotope Nickel 58 because of its large coherent scattering length. So, for natural
Nickel one gets a total angular of reflection of Oc = 6' . A, [A] and for the isotope 58 a
O c =7.1' .k[A] .
About 30 years ago neutron guides were invented at the research reactor in
München ; Neutron guides lead to a much broader use ofneutron scattering wich instruments
posed far from the source and with much better conditions .
Neutron guides coated with a so-called super minor transport neutrons with even
larger angles of divergence. In addition to Nickel those minors consist of altemating layers of
two different metals (their coherent scattering length density must be sufficiently different)
wich an ingenious sequence of different thickness, in order to excite Bragg scattering in a
continuous range of Q perpendicular to the surface . In this way the angle of reflection of
Nickel can bc increased by more than a factor of two ; however, the best achieved reflectivity
of super minors is still slightly less than one so that one gets appreciably losses of neutrons if
transported over long distances . .
8-12