Potassium-40

Potassium-40, ⁴⁰K

General
Symbol	40K
Names	potassium-40, 40K, K-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 radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% (120 ppm) of the total amount of potassium found in nature.

Potassium-40 undergoes three types of radioactive decay. In about 89.28% of events, it decays to calcium-40 (⁴⁰Ca) with emission of a beta particle (β⁻, an electron) with a maximum energy of 1.31 MeV and an antineutrino. In about 10.72% of events, it decays to argon-40 (⁴⁰Ar) by electron capture (EC), with the emission of a neutrino and then a 1.460 MeV gamma ray.^{[Note 1]} The radioactive decay of this particular isotope explains the large abundance of argon (nearly 1%) in the Earth's atmosphere, as well as prevalence of ⁴⁰Ar over other isotopes. Very rarely (0.001% of events), it decays to ⁴⁰Ar by emitting a positron (β⁺) and a neutrino.^[1]

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