[time-nuts] Rb resonance

Brian Kirby kirbybq at bellsouth.net
Sat Sep 17 05:36:55 EDT 2005


"The buffer gas used in the (rubidium) gas cell causes the transistion 
linewidth to be quite narrow because it increases the interaction time 
of the atoms with the EM field.  Unfortunately, it also produces a very 
large shift in frequency, just as collisions with the Teflon coating of 
the storage bulb produced a net shift in the H maser.  The uncertainy in 
the determination of the shift due to the buffer gas is quite large, 
and, consequently, the rubidium cell serves only as a secondary 
standard."  -Allan S. Rislet, NBS Tech Note 399, April 1971.



Tom Van Baak wrote:

>>The second is defined in terms of the Cs133 resonance- hence there is 
>>an integer number of Hz in the resonance frequency viz. 9192 631 770 
>>Hz.
>>    
>>
>
>A definition doesn't imply it has to be an integer. An
>inch, for example, is defined to be 25.4 mm.
>
>The cesium 9192631770 value looks like an integer only
>because when the atomic second was calibrated against
>the astronomical second in the 1950's the measurements
>were precise or accurate to only 9 or 10 digits.
>
>The value was reported as 9 192 631 770 +- 20 Hz which
>was the simple average of several measurements made
>between 1954 and 1958. A scan of the raw data is at:
>
>http://www.leapsecond.com/history/phys-rev-aug-1958-et-cs.gif
>
>If you do the math you can see that the average of
>...761, 767, 772, 780 is ...770, the value we still use
>today.
>
>See also the original 1958 Markowitz/Essen paper under:
>http://www.leapsecond.com/history/
>
>  
>
>> But what about the Rb resonance frequency? It is always quoted as 
>>being an integer number of Hz as well- viz 6834 682 608 Hz.   Is there 
>>some aspect of the physics of these transition frequencies that means 
>>the Rb resonance has to be an integer number of Hz, based on the Cs133 
>>definition?
>>Or is the Rb resonance frequency, which is generally quoted as above, 
>>actually rounded to be an integer number of Hz for convenience? 
>>Perhaps a set of environmental conditions need to be specified as 
>>well?
>>
>>
>>Regards,
>>Dave Brown, NZ
>>    
>>
>
>Yes, depending on where you see the value it will be
>rounded. The most accurate measurement that I've
>seen for the Rb resonance is 6 834 682 612.904324 Hz.
>But it is common to see  6.8 GHz, 6 834 MHz, and
>6 834 682 612 Hz as well. Not sure about the ...608
>vs. ...612 thing.
>
>For Hydrogen and Mercury frequencies see the last
>paragraph of:
>http://www.leapsecond.com/pages/unix/
>
>Note that due to a variety of factors atomic frequency
>standards don't actually operate right at their magic
>frequencies. Most Cs, for example, run a few Hz higher
>and the correction is transparently applied internally in
>a combination of hardware or firmware.
>
>/tvb
>
>
>
>
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>  
>




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