Incoming links: Caesium
How an atomic clock works, and its use in the global positioning system (GPS) by EngineerGuy (2012)
Source. Shows how conceptually an atomic clock is based on a feedback loop of two hyperfine structure states of caesium atoms (non-radioactive caesium-133 as clarified by the Wikipedia page). Like a quartz clock, it also relies on the piezoelectricity of quartz, but unlike the quartz clock, the quartz is not shaped like a tuning fork, and has a much larger resonating frequency of about 7 MHz. The feedback is completed by producing photons that resonate at the right frequency to excite the caesium.Inside the HP 5061A Cesium Clock by CuriousMarc (2020)
Source. A similar model was used in the Hafele-Keating experiment to test special relativity on two planes flying in opposite directions. Miniaturization was key.
Contains a disposable tube with 6g of Caesium. You boil it, so when it runs out, you change the tube, 40k USD. Their tube is made by Agilent Technologies, so a replacement since that opened in 1999, and the original machine is from the 60s.
Detection is done with an electron multiplier.
youtu.be/eOti3kKWX-c?t=1166 They compare it with their 100 dollar GPS disciplined oscillator, since GPS satellites have atomic clocks in them.
Quick presentation of the atomic clock at the National Physical Laboratory (2010)
Source. Their super accurate setup first does laser cooling on the caesium atoms.Highly radioactive isotope of caesium with half-life of 30.17 y. Produced from the nuclear fission of uranium, TODO exact reaction, not found in nature.
The fucked thing about this byproduct is that it is in the same chemical family as sodium, and therefore forms a salt that looks like regular table salt, and dissolves in water and therefore easily enters your body and sticks to things.
Another problem is that its half-life is long enough that it doesn't lose radioactivity very quickly compared to the life of a human person, although it is short enough to make it highly toxic, making it a terrible pollutant when released.
This is why for example in the goiânia accident a girl ended up ingesting Caesium-137 after eating an egg after touching the Caesium with her hands.
caesium-137 decay scheme
. Source. 
Uses the frequency of the hyperfine structure of caesium-133 ground state, i.e spin up vs spin down of its valence electron , to define the second.
International System of Units definition of the second since 1967, because this is what atomic clocks use.
TODO why does this have more energy than the hyperfine split of the hydrogen line given that it is further from the nucleus?