Experiments with Supersonic Beams as a Source of Cold Atoms

of an MCP, and thus will not be detected. However, the RGA used to detect helium
in chapter 3 can also be used to detect molecular oxygen. The oxygen is ionized by
electron impact and accelerated into a quadrupole which provides mass filtering. The
ions are then detected with an electron multiplier. The RGA is mounted on the same
translation stage as the MCP, which again permits measurement of the velocity of
the beam. However, the ionization region of the RGA has some depth, as compared
to the flat MCPm, which creates inherent uncertainty in the position of an oxygen
molecule when it is ionized and subsequently detected. The distance from the end
of the coilgun to the ionizer with the translation stage in the retracted(extended)
position is 100 mm(150.8mm).
4.6.3 Oxygen Data and Results
The results of the oxygen slowing are now presented. Using the oxygen-krypton
mixture described above, and with the nozzle cooled to 150 K, the oxygen beam
without any slowing has a velocity of 389 ± 5 m/s with a standard deviation of
28 m/s. This corresponds to a temperature of 3.0 K. This is significantly hotter that
the previous beams described here. Clustering in the beam is the likely explanation
of the increased beam temperature, as the valve is operated close to the condensation
temperature of the krypton carrier gas.
Time-of-flight profiles of the oxygen beam are presented in figure 4.33. The
coilgun is operated at constant phase, where the phases used are between 47.8 ◦ and
63.2 ◦ . This corresponds to slowing of the beam to 242 m/s at the lowest phase, and
to 83 m/s at the highest phase. For the 83 m/s beam, over 95% of the initial kinetic
energy is removed. A full summary of the slowing results can be found in table 4.3.
The time-of-flight profile in the graph on the right in figure 4.33 shows the
entire beam, including the perturbed fast beam. As is clear from the figure, the
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