[time-nuts] Re: Project Great

[Poul-Henning Kamp]

--------
Hal Murray writes:
> 
> jim at luxfamily.com said:
> > And a lot ofsources may have a low flat spot in the curve, but it
> > eventually trends up. Except for primary standards like Cs beam.
>
> What's magic about "primary standard" or "Cs beam" that keeps the ADEV from 
> trending up?

Primarily (no pun intended) that there are no (relevant) mechanisms of
frequency drift.

Cesium beam standards measure free-flying atoms which means the
only relevant external influence is the black-body radiation, which
is constant, thanks to the thermostatic ovens.

Rubidium standards measure atoms which are confined in a glass-bulb,
which means they literally bounce of the walls all the time.

The resonance we measure is not the real one, but one affected by
the doppler-shift of the atoms speed, ie: velocity without sign,
in the direction of interrogation.

This also broadens the resonance.

I belive the primary source of drift in Rb's are adsorption and
absorption of Rb molecules onto and into the glass itself.  This
causes a drop of gas pressure, which changes the collision dynamics
for the remaining Rb gas, which affects their velocity distribution,
which again moves the "appearant resonance" we measure.

One of the things which set the 5065A apart from "telecom" Rb's is that
there is a side-reservoir which is cooled with a TEC to stabilize the
gas pressure and thus velocity.

If you put your Rb atoms in a fountain instead, they work as well, or
even slightly better than Cs atoms.

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