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3 Example Experiment
The paramagnetic signal in liquid n-hexane is difficult to observe because the
muonium polarisation is found to decay on a microsecond time scale, which is further
reduced by the presence of dissolved oxygen. A series of commissioning experiments
were carried out using n-hexane in order to gauge the ability of the liquid handling rig
to degas the liquid.
This experiment was carried out using a detachable liquid-sample cell.
At the start of the experiment, 100 ml of n-hexane was loaded into Vessel 1 and
frozen to allow the removal of any dissolved gases. An inert atmosphere of helium
gas was introduced and the liquid subjected to four freeze-pump-thaw cycles using the
procedures outlined above. The sample cell was attached to one of the sample
delivery tubes and pumped for 1 hr. Using the vacuum created in the cell, a small
amount of n-hexane, enough to fill the cell, was transferred. To avoid contamination
of the cleaned liquid, excess n-hexane was transferred to Vessel 3.
After filling, the sample cell was sealed and detached from the rig. Muon spin
resonance experiments were performed at 2 G and 20 G followed by reattached to the
rig. At this point, the rig was open to the air and so had to be evacuated and pumped
for 30 minutes. The n-hexane held in the sample cell was transferred to Vessel 2 by
freezing the vessel and allowing the vacuum to draw the n-hexane out of the cell.
Gentle heating ensured that all the n-hexane was recovered.
Two further sets of freeze-pump-thaw cycles resulted in samples for muon study after
7 and 10 cycles had been performed on the clean n-hexane held in Vessel 1.
The signal due to the paramagnetic species in n-hexane after each set freeze-pump-
thaw cycles is shown in Figure 1. The fitted parameters for the diamagnetic,
paramagnetic and missing fraction percentage along with the relaxation rate for the
paramagnetic signal are shown in Table 1.
Table 1 Effect of successive freeze-pump-thaw cycles on the initial asymmetry
attributable to the diamagnetic, paramagnetic and missing fractions and muonium
relaxation rate.
Initial asymmetry (%)
Experiment
Diamagnetic Paramagnetic Missing
Relaxation
Rate (µs
-1
)
4 fpt cycles 12.87 3.41 6.72 7.96
7 fpt cycles 12.41 5.86 4.73 6.66
10 fpt cycles 12.86 4.90 5.24 5.45
6 fpt cycles
sodium dried
12.59 4.39 6.02 5.20
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