Richard (Rick) Karlquist wrote:
David C. Partridge wrote:
Cough - the rubidium clock or oscillator does have an intrinsic
frequency,
which is the rubidium hyperfine transition of 6 834 682 610.904 324
Hz, it's
just that the frequency generated by the transition in question isn't
used
to DEFINE the second, so by definition, it must be secondary. Only a
Caesium clock is a primary standard, as the second is DEFINED to be
the time
taken for 9,192,631,770 cycles of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the
caesium 133 atom.[1].
Unless of course they changed the rules recently ...
[1] <http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html>
Dave
Well, what you said is true as far as it goes, but not the whole story.
The fact that a clock is based on cesium does not necessarily mean it
is a primary standard. For example the "chip scale atomic clock" uses
cesium and is a secondary standard. OTOH, certain experimental clocks
based on atoms such as rubidium, mercury, etc could be considered
primary standards in spite of the definition of the second.
Indeed.
It's not the type of atom, but the type of clock that is crucial.
"Cesium" usually refers to an atomic beam clock and "Rubidium" usually
refer to a gas cell device. In an atomic beam, the atoms are, on the
average, unperturbed, and will transition at exactly the 9192...
frequency in the definition of the second. Except that they are offset
from this frequency by a known amount due to the C-field. In a gas
cell device, the atoms are perturbed by the buffer gas which results
in a unknown frequency shift from the 6834... frequency. You have
to remove this offset by comparing to a primary standard.
We used to say that in theory you could build a cesium beam standard
from a kit of parts on a desert island having no other clocks, and when
you turned it on, it would be on the correct frequency (within a
tolerance) guaranteed by design/physics. There is no way you
could do this with a rubidium or cesium gas cell standard
to any kind of accuracy associated with atomic clocks (it would only be
in the general neighborhood of 6834...)
That is the difference between primary and secondary standards.
Another difference is that secondary standard have "aging" and
primary standards don't.
It should be pointed out that just because you have a caesium beam
clock, or lately caesium fointain clock, means that you achieve the full
definition of "primary standard" as give above.
A beam standard has many different flaws. Older beam standards will age
since the C-field is not being maintained. Modern "digital" clocks has a
servo-loop to ensure that.
RF-amplitude, phase-difference betweeen the interaction fields,
temperature/average speed of beam provides doppler shifts etc.
The repeatability brings many issues in. Caesium beams have excellent
repeatability compared to rubidium gas-cells.
Any gas-cell standard has wall-shifts, buffer-gas shifts, temperature
shift, excitation signal strength and polarisation etc. etc. etc.
Rubidium cells forms nice gas-cell standards even if the gas cell
technology is limited. Price/performance is usually very good.
So... beyond the atom being used, the clock type and the details of its
operation needs to be overviewed before a well-founded judgement of it's
stability and repeatability... and thus primary standard ability, can
be given.
Cheers,
Magnus
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