[time-nuts] Re: Rb standards (was: Project Great)

[Magnus Danielson]

Hi Attila,

Den 2021-12-02 kl. 00:10, skrev Attila Kinali:
> On Mon, 29 Nov 2021 13:23:33 +0000
> "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
>
>> 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.
> Modern, laser based Rb standards can measure doppler-free
> by selecting only those atoms that are in rest or only move
> laterally to the laser beam. But, as far as I know, there
> is no commercial product available that does that.
>
There exists at least two commercial products for that, but availability 
remain scarse so far. SDI and uClock. So, commericial products, but 
commercially available may remain the issue. Hope that resolves itself soon.

With cold rubidium you have that cold relatively unperturbed small ball 
of rubidium, not very diffferent from that of a cesium fountain, but 
more compact.

>> 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.
> Largest sources of uncertainty for an Rb are:
> * Temperature (affects Rb density and buffer gas pressure)
> * Atmospheric pressure (affects pressure inside the glass cell)
> * Light and microwave intensity variation (shifts the electron energies)
> * Helium absorption (changes buffer gas compostion)
Wall shift vs. buffer gas shift. Helium disappation is part of that.
> I have not seen any measurement of Rb absorption into glass
> and its effect, so I cannot comment on that.

For gas cells, contamination onto the glass provides both reduced pump 
light shift as well as worse S/N. That at least is known and understood.

Cheers,
Magnus