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

[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.


> 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)

I have not seen any measurement of Rb absorption into glass
and its effect, so I cannot comment on that.


			Attila Kinali
-- 
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always 
wonder about. And that's why we do research.
		-- Kobayashi Makoto