Potassium-40

Potassium-40 (K) is a radioactive isotope of potassium which has a very long half-life of 1.251×10 years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature.

Potassium-40 is a rare example of an isotope that undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 (Ca) with emission of a beta particle (β, an electron) with a maximum energy of 1.33 MeV and an antineutrino. About 10.72% of the time it decays to argon-40 (Ar) by electron capture, with the emission of a 1.460 MeV gamma ray and a neutrino. The radioactive decay of this particular isotope explains the fact that argon is the cheapest totally noble gas available. Very rarely (0.001% of the time) it will decay to Ar by emitting a positron (β) and a neutrino.