[time-nuts] Rb lamp lifetime...

WB6BNQ wb6bnq at cox.net
Sun Nov 2 05:28:26 UTC 2008


Yuri,

Sorry about that, but I guess the Trimble Thunderbolts are out of stock all ready.

Bill....WB6BNQ


WB6BNQ wrote:

> 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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