Neutron Scattering

To study the morphology of surfaces or interfaces of thin layer systems such as polymer
films on silicon substrates or magnetic multilayer systems some specific kinds of small angle
neutron (or x-ray) scattering experiments can bc performed. Especially for buried interfaces
these surface sensitive methods are the only way to investigate the film properties without
destroying the sample. Therefore, they are very frequently applied and have an enormous
impact on solid states and soit condensed matter physics in general .
The so-called specular reflectivity is a scan with a wave vector transfer Q perpendicular
to the sample surface which is defined as the z-direction in this section. Because of the missing
Qz - and Q,-component the reflectivity is only sensitive to the thickness, the potential and the
roughness of euch film . In-plane properties of the interfaces such as lateral correlation lengths
are accessible with different kinds of diffuse scattering experiments where at least one of the
components Qz or Q,, are not vanishing .
In the following, the specular reflectivity and the diffuse scattering are explained in more
detail. The usual experimental setup will bc shown and the basic theory of specular and diffuse
scattering will bc presented with some examples .
6.2 Experimental Principals of Surface Sensitive Neutron Scattering
A sketch of a typical neutron surface scattering experiment is displayed in Figure 6 .3, a
more detailed description is given in other sections . The direction of the primary beam is
defined by some slits. Before the primary beam bits the sample the flux is usually monitored.
The incident angle 6 which is determined with respect to the Sample surface is set by rotating
the Sample in the beam . The scattered beam is detected at an angle 6' (also with respect to the
surface) which is determined by 6 and the scattering angle 0=e'+6 . In the literature 0 is
sometimes called 26 (which is actually inaccurate because ~ is not necessarily equal to 2-6) .
Figure 6 .3 : Sketch ofa typical
surface sensitive neutron
scattering experiment. The
incident angle is denoted by
6, the outgoing angle with
respect to the surface by 6' .
The scattering angle is called
6 .4