Neutron Scattering

0.4° . The full dynamical theory can only be deduced for the specular reflectivity. For the diffuse
scattering no exact solution exist right now .
6.6 Important Applications of Neutron Reflectrometry
For many standard scattering experiments on layer systems x-rays are much more suitable
than neutrons because of the higher flux, the better collimation and the less divergence . In
numbers : A standard synchrotron radiation source bas a primary beam intensity of about 10' °
counts/sec at a typical spot size of (0.2x1)mm2 . The beam divergence which determines the Q-
space resolution is less than 1/100 of a degree . For a modern neutron source one gets less than
10 7 counts/sec in an area of (0.5x20)mm2 with a divergence of larger than 0 .02° .
However, for some topics of research neutron scattering is superior . One example is the
investigation of materials which mainly contain hydrogen, carbon, nitrogen or oxygen such as
organic molecules . In this case the electron density [which for x-rays replaces the potentiel
V(r)] is very low and the x-ray contrast becomes very small. In contrast, for neutron V(r)
strongly depends on the isotope of the elements . Therefore, by using deuterated or
hydrogenated organic materials the scattering contrast can easily be tuned without changing the
chemical properties of the semples . Figure 6.11 shows an example of apolymer bilayer.
h-PS
12
10
ö
s
6
rn
4
0
50 100 150 200 250 300
Figure 6 .11 : Comperison
of an x-ray (left column)
and a neutron (right
column) reflectivity of a
polymer bilayer (210Â
polystyrene [PSI on 70Â
deuterated polyvinylpyrridine
[d-P2VP] on a sifcon
z ra1 substrate) . The x-ray
contrast between the two
polymes given by the
electron density lis only
about 5% .
0.10 0,15 020 0 .25 A 30 2 0 .00 0.05 010 015 0.20 0 .25 0.30
0. rß - ' 1 0= rx- ' 1
It can clearly be seen that the x-ray reflectivity only exhibits one signiflcant oscillation
period which is due to the whole fihm thickness . The x-ray contrast at the polymer interface is
6 . 15