Neutron Scattering - JUWEL - Forschungszentrum Jülich

12 H. Morhenn, S. Busch, G. Simeoni and T. Unruh
4.3 The experiment itself
We might either produce a sample together or fill an existing sample into the aluminium hollow
cylindrical container or a flat aluminium container. This sample is then measured at TOFTOF
with a vanadium standard and the empty aluminum container. The length and number of measurements
will have to be adjusted to the available time, it will be necessary to use some measurements
of the preceding groups.
You will do all sample changes in the presence of a tutor who explains the procedure in detail.
4.4 Data reduction
The instrument saves the number of counts as a function of scattering angle and time-of-flight,
N(2θ, tof). The next step is the data reduction which applies several corrections and transforms
to get rid of many instrument-specific properties of the data and convert them to a scattering
function S(Q, ω).
Data reduction (and later on also data evaluation) is done with the program FRIDA [6]. Inside
an X Terminal window, start FRIDA by entering ida. The question “Load file ?” has to be
answered by simply pressing “Enter”. You can get an overview of the available commands in
FRIDA by entering hc or looking into the online manual [3].
As you can see there, the routine to load TOFTOF data is started with rtof. The values in
square brackets are default values proposed by the program, they can be accepted by pressing
“Enter”. You can find all the commands you have to enter in the online manual – they make the
program do the following:
While reading the raw data N(2θ, tof), all the given runs will be added up (which means that
you will have to do this procedure for one sample temperature at a time) and normalized to the
monitor counts. This is followed by the calculation of the energy transfer from the time-of-flight
so that one obtains S(2θ, ω). The time-of-flight when the neutrons arrive which were scattered
elastically by the vanadium is set as energy transfer zero for each detector independently.
Now that the x-axis is energy transfer, FRIDA can correct for the neutron energy dependent
efficiency of the detectors which was given by the manufacturer. Also the differences in sensitivity
between the detectors will be corrected. To do that, all scattered intensities are normalized
to the elastically scattered intensity of the vanadium standard. As vanadium is an incoherent
scatterer, it should scatter the same intensity in all directions. The only effect which causes
deviations from an isotropic scattering is the Debye-Waller-factor (DWF) which is well-known
and can be corrected by the program as well.
4.5 Data evaluation
To get an impression how much coherent and incoherent scattering the sample produces and
to check the existence of Bragg peaks, diffraction patterns have to be produced. To make
diffraction patterns, stop here and extract the intensity at ω =0. Should you find Bragg peaks,
delete all spectra which contain intensity from these peaks before continuing.