Topic summary
Potassium-40
Potassium-40 (K) is a long lived and the main naturally occurring radioactive isotope of potassium, with a half-life of 1.248 billion years. It makes up about 117 ppm of natural potassium, making that mixture very weakly radioactive. The half-life is much shorter than the age of Earth, meaning this fraction was significantly larger earlier in Earth's history.
Potassium-40 undergoes four different paths of radioactive decay, including all three main types of beta decay:
- Electron emission (β) to Ca with a decay energy of 1.31 MeV at 89.6% probability
- Electron capture (EC) to Ar followed by a gamma decay emitting a photon with an energy of 1.46 MeV at 10.3% probability
- Direct electron capture (EC) to the ground state of Ar at 0.1% probability
- Positron emission (β) to Ar at 0.001% probability
Both forms of the electron capture decay release further photons, when electrons from the outer shells fall into the inner shells to replace the electron taken from there. The total energy for the decay to argon is 1.51 MeV.
The EC decay of K explains the large abundance of argon (nearly 1%) in the Earth's atmosphere, as well as prevalence of Ar over other isotopes.