Potassium-40

Potassium–argon dating

Main article: K–Ar 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 traces of potassium, then decay of the K 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 K and Ar atoms contained in it.

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

Contribution to natural radioactivity

The decay of K in Earth's mantle ranks third, after Th and U, in the list of sources of radiogenic heat. Less is known about the amount of radiogenic sources in Earth's outer and inner core, which lie below the mantle. It has been proposed, though, that significant core radioactivity (1–2 TW) may be caused by high levels of U, Th and K.

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 of K; whose decay produces about 3850 to 4300 disintegrations per second (becquerel) continuously throughout the life of an adult person (and proportionally less in young children).The number of radioactive decays per second in a given mass of K is the number of atoms in that mass, divided by the average lifetime of a K atom in seconds. The number of atoms in one gram of K is the Avogadro constant divided by the atomic weight of potassium-40 (39.96 g/mol): about per gram. As in any exponential decay, the average lifetime is the half-life divided by the natural logarithm of 2, or about seconds.

Banana equivalent dose

Potassium-40 is famous for its usage in the banana equivalent dose, an informal unit of measure, primarily used in general educational settings, to compare radioactive dosages to the amount received by eating one banana. The radioactive dosage from eating one banana is around 10 sievert, or 0.1 microsievert, under the assumptions that all of the radiation produced by potassium-40 is absorbed in the body (mostly true, as most of the radiation is beta-minus radiation, which has a short range) and that the biological half-life of potassium-40 is around 30 days (likely too large an estimate, as the body controls potassium levels closely and emits excess potassium quickly through urine). At the estimated 0.1 μSv, one banana equivalent dose is around 1% of the average American's daily exposure to radiation.

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References

References

1. {{AME2020 II
2. {{NUBASE2020
3. {{NNDC
4. (2024). "Rare 40K Decay with Implications for Fundamental Physics and Geochronology". Physical Review Letters.
5. (2024). "Evidence for ground-state electron capture of 40K". Physical Review C.
6. (2024-05-08). "Physicists Observe Rare Nuclear Decay of Potassium Isotope".
7. "Radioactive Human Body".
8. Connor, Nick. "What is Potassium-40 – Characteristics – Half-life – Definition".
9. "The Radioactivity of the Normal Adult Body". rerowland.com.
10. Nick Connor. (14 December 2019). "What is Banana Equivalent Dose – BED – Definition".