<Tr> <Td> γ EC </Td> <Td> 1.5049 MeV </Td> </Tr> <P> 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 . </P> <P> Potassium - 40 is a rare example of an isotope that undergoes both 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 large abundance of argon (nearly 1%) in the earth's atmosphere, as well as its abundance compared to Ar . Very rarely (0.001% of the time) it will decay to Ar by emitting a positron (β) and a neutrino . </P> <P> 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 K 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 K and Ar atoms contained in it . </P>

When radioactive potassium-40 decays the stable daughter isotope