[time-nuts] Rb lamp lifetime...

[Yuri Ostry]

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