[time-nuts] How Rubidiums make their frequency

[Magnus Danielson]

From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] How Rubidiums make their frequency
Date: Wed, 19 Apr 2006 22:00:11 +0200
Message-ID: <1454.1145476811 at critter.freebsd.dk>

> In message <ad9f78230604191255m6deb020arf3bb3b583d385782 at mail.gmail.com>, "Matt
>  Ettus" writes:
> 
> >Since we can now make DDS's with arbitrary frequency resolution, could
> >you make an Rb oscillator without the magnetic field adjustment? 
> >Wouldn't that reduce a source of error in frequency?  Then we'd be
> >left with the ideal resonance frequency, right?
> >
> >Are there any other influences on the resonance frequency?  I assume
> >temperature and density don't matter.
> 
> In fact density/pressure does matter and is one of the major reasons
> why rubidiums drift:  Rubidium is absorbed into the glas container
> and as the pressure drops the frequency pulls.  The absorption
> also makes the glas darker and darker, being a major wear-out
> mechanism for Rb units.
> 
> As far as I know, this is why Rb is never classified as a primary
> standard:  A drift-free unit has yet to be constructed.

Well, you could run a Rubidium beam, or even Rubidium fountain. It is just that
Cesium outperforms it. Thallium was infact competing with Cesium early out,
having a higher frequency, but at that time the waveguide issues was a major
concern, so Cesium won that too.

In the same fasion you could run Cesium in a cell, which is infact how they
have done these small Cesium clocks. Neither Cesium or Rubidium is particular
well suited for the active maser that we let Hydrogen do.

When the Rubidium cell was created, the Rb-85 and RB-87 combination for
creating the right optical pumping wavelength made it suitable for such a cell
solution, but these days we can do the same with tuneable laser diodes and for
other atoms than Rubidium. The Rubidium cell is cheap but gives a fair
performance, so it filled the gap up to the big boys early out.

The technical solutions (beam, cell, active maser) and the suitable atoms
(Hydrogen, Rubidium and Cesium) found each other to make feasable solutions for
different needs.

So, the Rubidium Cell would never be a real contender for a primary reference,
but toss me a Rubidium beam or Rubidium fountain and we have a very nice clock
indeed. The primary classification might however be a matter of agreeing it
too would realize the SI second.

Cheers,
Magnus