Potassium-40

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Potassium-40 (⁴⁰K) is a radioactive isotope of potassium which has a very long half-life of 1.251×10⁹ years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature.

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 (⁴⁰Ca) with emission of a beta particle (?^-, an electron) with a maximum energy of 1.31 MeV and an antineutrino. About 10.72% of the time it decays to argon-40 (⁴⁰Ar) 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 ³⁶Ar. Very rarely (0.001% of the time) it will decay to ⁴⁰Ar by emitting a positron (?⁺) and a neutrino.

Video Potassium-40

Potassium-argon dating

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.

Maps Potassium-40

Contribution to natural radioactivity

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.

⁴⁰K is the largest source of natural radioactivity in animals including humans. A 70 kg human body contains about 140 grams of potassium, hence about 0.000117 × 140 = 0.0164 grams of ⁴⁰K; whose decay produces about 4,300 disintegrations per second (becquerels) continuously throughout the life of the body.

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See also

• Isotopes of potassium
• Banana equivalent dose
• background radiation

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Notes and references

• Table of radioactive isotopes, K-40, The Lund/LBNL Nuclear Data Search

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External links

• Potassium-40 Section, Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas

Source of the article : Wikipedia