Cesium-137 is made in nuclear reactors, but some radioactive isotopes with shorter half lives (well, short compared to the age of the Earth) exist in nature bec

Cesium-137 is made in nuclear reactors, but some radioactive isotopes with shorter half lives (well, short compared to the age of the Earth) exist in nature because they are a product of the natural radioactive decay of other, long-lived materials like Uranium.