[time-nuts] Rb lamp lifetime...

[WB6BNQ]

Hi Yuri,

Unless you need the longer term stability of an undisciplined Rb source, I think
you would be better served to get one of the “Timenuts group” Trimble Thunderbolt
GPS units to use as a home standard.  It is available via this page:

http://www.tapr.org/kits_thunder.html

and info about the unit is available at this URL:

http://www.leapsecond.com/tbolt-faq.htm

Add an external very high quality crystal oscillator locked to the Trimble and
you will have the same stability of a good Rb source.  Most would use a high
quality Rb source, undisciplined, as a tool for comparing or generating
specifications of crystal oscillators.

It seems that the Rb standards last longer turned off.  My understanding (subject
to disagreement) is the Rubidium in the lamp ends up coating the walls of the
lamp and reducing the transmission through the glass, i.e., the signal gets more
and more noisy as time goes on.  Also, it seems that the Rubidium may permeate
through the walls of the glass housing and thus reduce the amount of Rb over
time.  I have not heard of anyone reversing the condition.  Of course, other
things can go wrong with the Rb source besides the lamp.

A very high quality crystal oscillator would most likely last longer with fewer
problems.  I have several +40 year old hp counters with decent oscillators still
operating.

Bill....WB6BNQ



Yuri Ostry wrote:

> Hello,
>
> Readed list archives and googled a lot, and seen two opposing points of
> view - one is that there is nothing to age in Rb lamp, another is that
> Rb lamp is degrading when in operation (without details, how and why).
>
> I'm planning to get some Efratom Rb oscillator to use it in a
> constantly running home lab frequency reference. Trying to understand
> for myself, is it worth to get spare unit (or even two) of the same
> model just to have replacement lamp on hand.
>
> One more question - does someone seen Rb standard that is
> malfunctioned due to degraded lamp, that, at same time, does have
> good vacuum. If so, which model, and how the failed lamp looks like?
> Was there any attempts to "rejuvenate" the lamp? (for example, heating
> to remove glass darkening, if any, or similar experiments).
>
> By the way, I'm very curious about physical process that may cause
> lamp degradation.
>
> Rubidium-87 that is included in the lamp (according to
> manuals) is a radionuclide, beta emitter. It decays very slowly (4.7
> billion years half life) to stable Strontium-87, emitting 282.62keV
> electrons and (anti?)neutrino. I don't think that 87Rb decay may
> interfere with unit operation after tens of years, unless 87Sr is
> "poisonous" so much so it can interfere with lamp operation even in
> tiniest amounts.
>
> The only reference I located to date, is following article, that is
> not available to general public. It is hard to understand for me just
> from this abstract, does it specifically related to space environment
> factors, or it is something that may cause degradation of Rb clocks
> that is operated in average lab on Earth. ;)
>
> > A Mechanism of Rubidium Atomic Clock Degradation: Ring-Mode to
> > Red-Mode Transition in rf-Discharge Lamps
> > Camparo, J.   Mackay, R.
> > Aerosp. Corp., El Segundo;
> >
> >
> > This paper appears in: Frequency Control Symposium, 2007 Joint with
> > the 21st European Frequency and Time Forum. IEEE International
> > Publication Date: May 29 2007-June 1 2007
> > On page(s): 45-48
> > Location: Geneva,
> > ISSN: 1075-6787
> > ISBN: 978-1-4244-0647-0
> > INSPEC Accession Number: 9805223
> > Digital Object Identifier: 10.1109/FREQ.2007.4319027
> > Current Version Published: 2007-10-01
> >
> >
> >
> >
> >
> > Abstract
> > In the vapor-cell atomic clock, long-term stability can be
> > influenced by slow variations in the discharge lamp's output via the
> > light-shift effect. Additionally, over a multi-year mission lifetime
> > the lamp's aging can degrade its optical pumping efficiency.
> > Understanding the mechanism(s) that drives these changes is
> > particularly important for spacecraft devices, where the atomic
> > clocks are called upon to function continuously and reliably for
> > many years. Here, we consider the two well-known, but little
> > studied, modes associated with alkali rf-discharge lamp operation:
> > the ring mode and the red mode. Consistent with previous research,
> > we find that the ring mode is best for optical pumping, and that the
> > clock-signal amplitude degrades significantly when the lamp operates
> > in the red mode. Examining the emission spectrum as the lamp
> > transitions between these two modes, we show that the ring-mode to
> > red-mode transition is driven by radiation trapping within the lamp.
>
> --
> Sincerely,
>  Yuri UA3ATQ/KI7XJ                     mailto:yuri at ostry.ru
>
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