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   On Saturday, November 17, 2018 at 10:50:12 AM UTC-6, Michael Cole wrote:   
   > Hi all.  I hope you all celebrated the new kilogram last night.  The   
   > Champaign corks were flying last night!  Let us celebrate our liberation   
   > from French tyranny.   
   >   
   > Can anybody recommend a good paper that explains the workings of atomic   
   > clocks?  I am more interested in physical principles than engineering   
   > details.  What exactly is it about the Cesium 133 atom that makes it so   
   > convenient for keeping time?  I assume they considered other atoms, but   
   > settled on Cesium as most convenient.  Is there a good simple reason for   
   > this?   
      
   First, I'd recommend reading the Wikipedia article, that would be   
   a good start.   
      
   All atomic clocks use an atomic transition as their frequency   
   standard.  Almost any transition could conceivably be used, but   
   there are practical matters that make some more useful than others:   
      
   -A narrow transition (one with a long emission time) are better   
   because the frequency is more precise   
   -A higher frequency is better as it allows a finer resolution of time   
   -Too high a frequency (e.g. light wavelengths) are too difficult   
   to count.  Also, the photon absorbtion to excite the atom may result   
   in the atom moving which causes a doppler shift with again distorts   
   the frequency observed.   
   -An atom that can be worked with as a gas tends to be better as the   
   frequency from an isolated atom is less likely to be disturbed by   
   collisions or the close proximity of other atoms.   
   -A heavier atom will be moving more slowly at any given temperature,   
   so that again is preferred.   
      
   Cesium 137 is a heavy atom with a narrow emission at 9.192631770   
   GHz which is about the highest frequency that it is easy to count   
   individual cycles and it can be worked with (excited and emissions   
   observed) as a gas.   
      
   Rich L.   
      
   --- SoupGate-Win32 v1.05   
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