Potassium-40
| General | |
|---|---|
| Symbol | 40K |
| Names | potassium-40 |
| Protons (Z) | 19 |
| Neutrons (N) | 21 |
| Nuclide data | |
| Natural abundance | 0.0117(1)% |
| Half-life (t1/2) | 1.251(3)×109 y |
| Isotope mass | 39.96399848(21) Da |
| Spin | 4− |
| Excess energy | −33505 keV |
| Binding energy | 341523 keV |
| Parent isotopes | Primordial |
| Decay products | 40Ca (β−) 40Ar (EC, γ; β+) |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| β− | 1.31109 |
| EC, γ | 1.5049 |
| Isotopes of potassium Complete table of nuclides | |
Potassium-40 (40K) is a long lived and the main naturally occurring radioactive isotope of potassium. Its half-life is 1.25 billion years. It makes up about 0.012% (120 ppm) of natural potassium.
Potassium-40 undergoes four different types of radioactive decay, including all three main types of beta decay:
- Electron emission (β−) to 40Ca with a decay energy of 1.31 MeV at 89.6% probability
- Positron emission (β+) to 40Ar at 0.001% probability
- Electron capture (EC) to 40Ar* 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 40Ar at 0.1%.
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 EC decay of 40K explains the large abundance of argon (nearly 1%) in the Earth's atmosphere, as well as prevalence of 40Ar over other isotopes.