Potassium-40

Source: Wikipedia, the free encyclopedia.
Potassium-40, 40K
General
Decay mode
Decay energy (MeV)
β−1.31109
EC, γ1.5049
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

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–argon dating

Decay scheme
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 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.

Contribution to natural radioactivity

The evolution of Earth's mantle radiogenic heat flow over time: contribution from 40K in yellow.

The radioactive decay of 40K in the Earth's mantle ranks third, after

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]

Banana equivalent dose

Potassium-40 is famous for its usage in the banana equivalent dose, an informal unit of measurement, primarily used in generalized 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 radioactive intake.[7]

See also

Notes

  1. gamma rays
    " irrespective of their energy.
  2. atomic weight of potassium-40 (39.96 g/mol), which is about 0.1507×1023 per gram. As in any exponential decay, the average lifetime is the half-life divided by the natural logarithm
    of 2, or about 56.82×1015 seconds.

References

  1. ^ Engelkemeir, D. W.; Flynn, K. F.; Glendenin, L. E. (1962). "Positron Emission in the Decay of K40".
    doi:10.1103/PhysRev.126.1818
    .
  2. ^ Wohlers, A.; Wood, B. J. (2015). "A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd".
    PMID 25877203
    .
  3. ^ Murthy, V. Rama; Van Westrenen, Wim; Fei, Yingwei (2003). "Experimental evidence that potassium is a substantial radioactive heat source in planetary cores". Nature. 423 (6936): 163–5.
    S2CID 4430068
    .
  4. ^ "Radioactive Human Body". Harvard Natural Sciences Lecture Demonstrations.
  5. ^ Connor, Nick. "What is Potassium-40 – Characteristics – Half-life – Definition". Radiation Dosimetry.
  6. ^ Bin Samat, S.; Green, S.; Beddoe, A. H. (1997). "The 40K activity of one gram of potassium".
    S2CID 250778838
    .
  7. ^ Nick Connor (14 December 2019). "What is Banana Equivalent Dose – BED – Definition". Radiation Dosimetry.

External links


Lighter:
potassium-39
 		Potassium-40 is an
isotope of potassium 		Heavier:
potassium-41
Decay product of:
— 		Decay chain
of potassium-40
calcium-40
, Stable
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