Neutron Scattering - JUWEL - Forschungszentrum Jülich

TOFTOF 13
Fig. 6: Left: Neutron diffraction patterns of solid pentafluortoluene at 100 K (λi =6˚A), taken
from [4]. If the scattering vector is a reciprocal lattice vector, the positive interference of
neutron waves yields a maximum in the scattered intensity. The sharp features in the diffraction
pattern indicate an ordered lattice. Right: The spectra S(Q, ω) of pentafluortoluene (◦) and
vanadium (–) at a momentum transfer of Q =1.1 ˚A −1 , cf. also [4]. The solid sample shows
only an internal motion, can therefore be described by equation 16.
To look at the dynamics, the next step is to calculate the momentum transfer Q from the scattering
angle 2θ and the energy transfer ω using equation (3). During this step, we obtain about
1000 spectra with relatively low statistics each and a varying value of Q as the energy transfer
varies. To get a better statistics and to have spectra which have the same Q for all values of
energy transfer, the 1000 spectra are grouped into about 20 spectra of constant Q in the same
routine.
These spectra S(Q, ω) contain information about sample and sample container. Therefore,
the spectra of the empty sample container are subtracted. To do so, the same data reduction
procedure has to be repeated with the empty container as “sample”.
We want to use the spectra of vanadium as a measure of the instrumental resolution. This
means that the whole procedure has to be done one more time with vanadium as “sample” (and
as “vanadium”).
Fit the spectra with the functions given in section 4.2, a single Lorentzian and a sum of
Lorentzians. As the theoretical functions have to be convolved with the experimental resolution,
the program will ask for the converted and grouped spectra of vanadium.
The numbers shown in brackets are the reduced χ2 which is a measure for the quality of the fit,
the other numbers are the fitted values of the parameters. Judge the fit quality by the reduced
χ2 and by visually inspecting the fits together with the data. Plot the parameters as a function
of Q or Q2 and determine the diffusion coefficient.
If you measured the sample at different temperatures, repeat the procedure for all of them.
5 Questions to be answered before the experiment
1. Do you expect the vanadium sample to be activated by the neutron beam? What about the
aluminum container with the real sample? (2 min)