Experiments with Supersonic Beams as a Source of Cold Atoms

calculated slow beam velocity is quite sensitive to the rotor position calculated from
the optimal nozzle delay. Thus, a problem with the method of choosing the optimal
delay could result in the difference in slope observed.
The maximum velocity removed by the rotor is 246m/s, resulting in a 265m/s
helium beam. This is not a fundamental limit, but is the result of the 42 Hz rotation
limit imposed by vibrations.
3.4.3 Crystal Lifetime
The reflected intensity of the helium beam is observed to decay over the course
of the experimental runs. The crystal itself appears to be the source of this decay,
since attempts to continue to use a crystal after it had been in the vacuum chamber
overnight resulted in significantly reduced reflected intensity. The crystal surface
should be passive, but it is possible for hydrocarbons to stick to the surface, reducing
the intensity of the reflected beam. A cold trap was inserted into the vacuum chamber
to try to improve the lifetime of the crystal. Another possibility is that something in
the vacuum system is damaging the passivation. Any stray electrons or metastable
atoms would deposit enough energy to remove the hydrogen terminating the silicon
bonds, reducing the passivation and increasing the probability of background gas
sticking to the surface.
A possible source of electrons and metastable atoms in the chamber is the fil-
ament of the RGA used to ionize the atoms for detection. Some of the electrons from
the filament could escape the ionization region and hit the crystal surface. Addition-
ally, atoms passing through the detection region could be excited by the electrons
to a metastable state, creating atoms which will remove the passivating hydrogen if
they strike the surface of the crystal. To investigate whether the RGA filament is the
cause of the crystal decay, a data set is taken with the RGA kept off, and only turned
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