[time-nuts] a link to a explanation of Rb vs Cs?

[Magnus Danielson]

Hi,

On 04/12/2017 12:15 AM, Richard (Rick) Karlquist wrote:
> The "magic" of Rb in a gas cell standard is that you
> can make an optical filter cell out of radioactive
> Rb87 isotope that allows you to selectively optically pump
> to the quantum level you need.  It is just "luck"
> that the absorption line falls where you need it.
> And the RF pumping is at a doable 6.8 GHz.

Indeed. Rubidium was more suitable for optical pumping, so that is why 
it is suitable to build gas-cells, as it is relatively easy to do.

Rb-85 and Rb-87 has interesting properties in their D1 and D2 lines, so 
by matching temperature you can filter out one of the D-lines with the 
other isotope and that way get pumping action into one of two states. 
The better pumping, the better S/N.

> I think the CSAC uses lasers so all of this doesn't
> apply.

Correct.

> Cesium of course is part of the definition of the second,
> so it's good to use for that reason.
>
> It is a different discussion as to why Cs was chosen
> to define the second, but the line being at 9.2 GHz might have had
> something to do with it.  That's a doable frequency in terms
> of technology 60 years ago.

For a gas-cell the Cs being a reference is however moot, since the wall 
pull and gas pull makes such systematics effects that it is not 
suitable. For a small gas-cell the use of Cesium or Rubidium is more 
about practicality. Several other projects work with rubidiums.

Thallium was competing with cesium for some time, but the higher 
frequency made it impractical, and also considering that ionization was 
harder was an issue back then, giving lower signal to noise. The 
frequency of Thallium is not an issue today, and with modern laser 
technology ionization could be replaced by optical detection. I haven't 
checked the optical frequencies for Thallium and it's suitability with 
modern lasers, but the selection is much wider now.

Turns out that in the laser-cooled fountains, rubidium respond better 
than cesium due to the smaller cross-section, making likelihood for 
collisions and loss of state less, causing less frequency pull due to 
this effect.

Cheers,
Magnus