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