Potassium-40

Potassium-40 (⁴⁰K) is a radioactive isotope of potassium which has a very long half-life of 1.251×10⁹ 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 (⁴⁰Ca) 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 (⁴⁰Ar) 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 ⁴⁰Ar by emitting a positron (β⁺) and a neutrino.

Potassium–argon dating

Potassium-40 is especially important in potassium–argon (K–Ar) dating. Argon is a gas that does not ordinarily combine with other elements. So, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid. However, if the mineral contains any potassium, then decay of the ⁴⁰K isotope present will create fresh argon-40, that will remain locked up in the mineral. Since the rate at which this conversion occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of ⁴⁰K and ⁴⁰Ar atoms contained in it.