Cesium atomic clocks are not radioactive, nor are they based on radioactive decay. The isotope used for cesium beam atomic clocks is natural and stable Cs-133. The clock is based on an ultra-precise energy transition near 9.192 GHz.

You may be thinking of the nasty radioactive isotope Cs-137, which is unnatural and often a byproduct of nuclear tests or accidents.

It's not unlike Carbon or Potassium; Carbon 12 is the safe stuff, the rarer Carbon 14 is radioactive. Potassium 39 is the safe one and rare Potassium 40 is radioactive. Which is why bananas are slightly radioactive:

https://en.wikipedia.org/wiki/Banana_equivalent_dose

You can make a cesium clock radioactive by placing a banana on top of it. See page 1, 36, 37 of:

http://leapsecond.com/ptti2020/2020-PTTI-tvb-Atomic-Timekeep...

What is the banana equivalent dose of working 8 hours a day in an old office building with granite stone floors, and granite wall paneling? I've read that all granite is slightly radioactive.

Roughly 0.18 to 3 mSv per annum [0]. (EU regulations limit it to 3 mSv/a maximum [0]).

One banana is 0.1 µSv [1].

The granite is therefore 0.18 mSv / 0.1 µSv = 1800 bananas.

This is compared to 4 mSv normal yearly background dose [1]. The granite increases background radiation by 0.18 / 4 * 100 = 4.5%. The yearly dose from natural potassium in the body is 0.39 mSv, so I wouldn't worry about the extra 0.18 mSv if I were you.

You're at much greater risk if you live in a house which used reinforced concrete with steel from the Ciudad Juárez accident - after that accident, 109 houses were demolished. [2] [3]

[0] https://www.nature.com/articles/jes200944

[1] https://xkcd.com/radiation/

[2] https://en.wikipedia.org/wiki/List_of_civilian_radiation_acc...

[3] https://news.ycombinator.com/item?id=24891947

> which is unnatural

There's nothing unnatural about Cs-137. Fission occurs naturally in U-235.

Cs 133 isn't radioactive. Tritium is, but at least until recently, it was still commonly used in watches for illumination. Low-level beta source, expensive but not especially hazardous.

>at least until recently

Has something changed? I know tritium sources are on the decline, but it always tickled my mind that radioactive lume was available to consumers for reasonable prices.

Haven't heard of it being used recently, except perhaps in gunsights, but a quick Google search brings up quite a few tritium-illuminated watches at moderate prices. I would've guessed that button-activated LED backlighting would have replaced it by now.

Edit: interestingly, Wikipedia says that radium was still used in watches until 1970.