Neutron Scattering

Equation (6 .13) also shows that the diffuse scattering can be detected in the whole Q-
space even at Q,e=0 which is actually the position of the specularily reflected signal .
This means
that reflectivity measurements (Q, scans at QX=O) always contain both the specular reflectivity
and the diffuse scattering at Q,e=0 .
To extract the specular reflectivity from the reflectivity
measurement the diffuse scattering has to be subtracted. This is usually donc by performing a
QZscan with a small offset 4Q~ so that the specular condition is not exactly matched. This socalled
longitudinal diffuse scan is subtracted from the measured reflectivity to get the true
specular reflectivity .
6
5
15
^®^ specular reflectivity
diff . int . a t 0,=0
c 2
ô
1
0
0
-6 -4 -2 0 2 4 6 0.00 0.05 0.10 0.15 0.20
% [0.001Ä-'] 0 . [Ä - ' l
Figure 6 .8 : Examplcs of diffuse scattering experiments. Left: Q x-scan atfixed Qz from a single
rough surface . The solid curve corresponds to a sample with an in-plane correlation length of
~p =5000A the dashed line to ~p=IOOOA. The peaks at the condition Qx=O (where 0=0') are
not diffuse scattering but caused by the specular reflectivity . Right: Specular reflectivity
(symbols) and diffuse scattering Qz scans at QX=0 (solid lines) of a monolayer system with
d=300Â, b2pz=4, 62=5A, b i p 1=3 and 6 1 =5A . The diffuse scans show the effect of crosscorrelations
. The measured reflectivity is detennined by the sum of the specular and the
diffuse scan .
Unfortunately, the intensity of the diffuse scattering is usually orders of magnitude
smaller than the specularily reflected signal . To get gond statistics and reliable data a very high
primary flux is necessary. This can easily be achieved with synchrotron radiation x-ray sources
but is hardly possibly for neutron sources (sec section 6 .5) . Therefore, it is extremely difficult
to extract quantitative information from the neutron diffuse scattering data .
6 . 1 1