Potassium-40

Potassium-40, ⁴⁰K
General
Symbol	40K
Names	potassium-40, K-40
Protons	19
Neutrons	21
Nuclide data
Natural abundance	0.0117(1)%
Half-life	1.251(3)×109 y
Parent isotopes	Primordial
Decay products	40Ca (β−); 40Ar (EC, γ; β+)
Isotope mass	39.96399848(21) u
Spin	4−
Excess energy	−33505 keV
Binding energy	341523 keV
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 1.251×10⁹ 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. 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.[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 will decay to ⁴⁰Ar by emitting a positron (β⁺) and a neutrino.[2]

Potassium–argon dating

Decay scheme

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 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.[3][4]

Potassium-40 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 (becquerel) continuously throughout the life of the body.[5][6]

gollark: But in practice probably yes.

gollark: I don't know if the spec actually guarantees this.

gollark: yes.

gollark: /a

gollark: It's very powerful, if sometimes accursed.

Notes

This photon would be called an x-ray if emitted from an electron. In nuclear physics, it is common to name photons according to their origin rather than their energy, high energy photons produced by electrical transitions are called "x-rays" while those emitted from atomic nuclei are called "gamma rays" irrespective of their energy.
Engelkemeir, D. W.; Flynn, K. F.; Glendenin, L. E. (1962). "Positron Emission in the Decay of K⁴⁰". Physical Review. 126 (5): 1818. Bibcode:1962PhRv..126.1818E. doi:10.1103/PhysRev.126.1818.
Wohlers, A.; Wood, B. J. (2015). "A Mercury-like component of early Earth yields uranium in the core and high mantle ¹⁴²Nd". Nature. 520 (7547): 337–340. Bibcode:2015Natur.520..337W. doi:10.1038/nature14350. PMC 4413371. PMID 25877203.
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. Bibcode:2003Natur.423..163M. doi:10.1038/nature01560. PMID 12736683.
The number of radioactive decays per second in a given mass of ⁴⁰K is the number of atoms in that mass, divided by the average lifetime of a ⁴⁰K atom in seconds. The number of atoms in one gram of ⁴⁰K is Avogadro's number 6.022×10²³ (the number of atoms per mole) divided by the atomic weight of potassium-40 (39.96 grams per mole), 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.
Bin Samat, S.; Green, S.; Beddoe, A. H. (1997). "The ⁴⁰K activity of one gram of potassium". Physics in Medicine and Biology. 42 (2): 407–13. Bibcode:1997PMB....42..407S. doi:10.1088/0031-9155/42/2/012. PMID 9044422.

References

External links

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

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

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[1] This photon would be called an x-ray if emitted from an electron. In nuclear physics, it is common to name photons according to their origin rather than their energy, high energy photons produced by electrical transitions are called "x-rays" while those emitted from atomic nuclei are called "gamma rays" irrespective of their energy.

[Engelkemeir-2] Engelkemeir, D. W.; Flynn, K. F.; Glendenin, L. E. (1962). "Positron Emission in the Decay of K⁴⁰". Physical Review. 126 (5): 1818. Bibcode:1962PhRv..126.1818E. doi:10.1103/PhysRev.126.1818.

[WohlersWood2015-3] Wohlers, A.; Wood, B. J. (2015). "A Mercury-like component of early Earth yields uranium in the core and high mantle ¹⁴²Nd". Nature. 520 (7547): 337–340. Bibcode:2015Natur.520..337W. doi:10.1038/nature14350. PMC 4413371. PMID 25877203.

[MurthyWestrenenFei2003-4] 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. Bibcode:2003Natur.423..163M. doi:10.1038/nature01560. PMID 12736683.

[5] The number of radioactive decays per second in a given mass of ⁴⁰K is the number of atoms in that mass, divided by the average lifetime of a ⁴⁰K atom in seconds. The number of atoms in one gram of ⁴⁰K is Avogadro's number 6.022×10²³ (the number of atoms per mole) divided by the atomic weight of potassium-40 (39.96 grams per mole), 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.

[Samat-6] Bin Samat, S.; Green, S.; Beddoe, A. H. (1997). "The ⁴⁰K activity of one gram of potassium". Physics in Medicine and Biology. 42 (2): 407–13. Bibcode:1997PMB....42..407S. doi:10.1088/0031-9155/42/2/012. PMID 9044422.