Ph.D. thesis (pdf) - dirac

8.1. Time of flight 143
length of 5 Å yielding a resolution of FMHW=103 µeV. The resolution function,
which is nearly Gaussian, was in all cases measured with the sample at 2 K.
The range in angle (2θ) goes from 14.5 ◦ to 132.5 ◦ with each detector covering approximately
1 ◦ . This corresponds to a elastic Q values of 0.4 Å −1 to 2.2 Å −1 , when
the incoming neutron has λ=5 Å.
The PIB3580 sample (see appendix A for details) was studied in a pressure range
from atmospheric pressure to 1.4 GPa and at temperatures ranging from 140 K
to 430 K. The experiment was performed using a clamp pressure cell and in situ
compression. The sample and the cell were kept in the beam during the entire
experiment, which means that all the measurements on PIB3580 were obtained under
the exact same conditions with regard to position in the beam and the amount of
cell in the beam. The volume of the sample in the beam was also kept constant.
However, the number of sample scattering centers depends on the density.
The compression of PIB3580 was performed at 430 K and the sample was subsequently
cooled along the isobar. This procedure ensured that the compression was
performed in the melt even when going to the highest pressure.
Cumene was studied at atmospheric pressure and at 1.2 GPa in a temperature range
of 100 K to 300 K. The experiment at atmospheric pressure was performed using
a standard aluminum cell while the pressure experiment was performed using the
clamp cell with in situ pressurization. The pressure was applied at 300 K, which
means that the liquid was well below P g when compressed.
We moreover studied a set of liquids with very different isobaric and isochoric
fragility: PB, sorbitol, m-toluidine, m-fluoroaniline and DBP (see appendix A for details
on the samples). These experiments were performed at ambient pressure using
aluminum cells in a temperature range from 100 K to T g (in most cases T g ∼ 200 K).
The clamp and the aluminum cells used are exactly the same as those used in the
back scattering experiments. The description of the cells is given in section 7.1.1.
However the sample thickness used for the high pressure experiment was only 0.1 mm
in this case. The transmission was ∼95% for the experiments in the aluminum cell
and ∼88% for the high pressure experiments.
8.1.2 Corrections
The data underwent a standard correction procedure using the ILL program INX.
INX converts the data from the measured intensity as a function of angle and time
of flight to intensity as a function of angle and energy transfer, I(2θ, ω) corrected for