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]

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.

The argon found in Earth's atmosphere is 99.6% ⁴⁰Ar; whereas the argon in the Sun – and presumably in the primordial material that condensed into the planets – is mostly ³⁶Ar, with less than 15% of ³⁸Ar. It follows that most of the terrestrial argon derives from potassium-40 that decayed into argon-40, which eventually escaped to the atmosphere.

The radioactive decay of ⁴⁰K in the Earth's mantle ranks third, after ²³²Th and ²³⁸U, as the source of radiogenic heat. The core also likely contains radiogenic sources, although how much is uncertain. It has been proposed that significant core radioactivity (1–2 TW) may be caused by high levels of U, Th, and K.^[2][3]

Potassium-40 is the largest source of natural radioactivity in animals including humans. A 70 kg human body contains about 140 g of potassium, hence about 140g × 0.0117% ≈ 16.4 mg of ⁴⁰K;^[4] whose decay produces about 3850^[5] to 4300 disintegrations per second (becquerel) continuously throughout the life of the person.^{[Note 2][6]}

Potassium-40 is famous for its usage in the banana equivalent dose, an informal unit of measurement, primarily used in general educational settings, to compare radioactive dosages to the amount received by consuming one banana. The radioactive dosage from consuming one banana is generally agreed to be 10⁻⁷ sievert, or 0.1 microsievert, which is 1% of the average American's daily exposure to radiation.^[7]

• Background radiation
• Isotopes of potassium

• Table of radioactive isotopes, K-40
• The Lund/LBNL Nuclear Data Search
• Potassium-40 Section, Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas