# Potassium-40
**Potassium-40** (⁴⁰K) is the long-lived radioactive isotope of potassium, making up 0.0117% of natural potassium, with a half-life of about 1.25 billion years and a branched decay to calcium-40 and argon-40.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) It is the only naturally occurring radioactive isotope of potassium, whose two other natural isotopes, ³⁹K (93.2581%) and ⁴¹K (6.7302%), are stable.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) Its decay underlies the potassium–argon and argon–argon dating methods, contributes roughly a fifth of Earth's present radiogenic heat, and emits the 1460.8 keV gamma ray familiar as the dominant natural gamma background in potassium-bearing materials.[2](https://www.bipm.org/documents/d/guest/k-40_report)
| Key fact | Detail |
|---|---|
| Natural abundance | 0.011668(8)% of natural potassium (about 117 ppm)[2](https://www.bipm.org/documents/d/guest/k-40_report) |
| Half-life | 1.2522(27) × 10⁹ years (2025 DDEP evaluation)[2](https://www.bipm.org/documents/d/guest/k-40_report) |
| Decay branches | β⁻ to ⁴⁰Ca 89.56(7)%; electron capture to the 1460-keV level of ⁴⁰Ar 10.34(7)%; electron capture to the ⁴⁰Ar ground state 0.098(25)%; β⁺ 0.00103(13)%[2](https://www.bipm.org/documents/d/guest/k-40_report) |
| Gamma emission | 1460.822(6) keV, absolute intensity 10.34(7)% per decay[2](https://www.bipm.org/documents/d/guest/k-40_report) |
| Specific activity | 28.23(6) β⁻ decays s⁻¹ g⁻¹ and 3.259(22) gamma photons s⁻¹ g⁻¹ of natural potassium[2](https://www.bipm.org/documents/d/guest/k-40_report) |
| Geochronology | Basis of K–Ar and ⁴⁰Ar/³⁹Ar dating, applicable from about 2,000 years to the age of the Solar System (4.5 billion years)[3](https://www.usgs.gov/usgs-triga-reactor/neutron-irradiations-argon-isotopic-dating) |
| Radiogenic heat | Contributes about 4 TW of geoneutrino-linked heat and nearly 20% of Earth's total radiogenic heat production[4](https://www.awa.tohoku.ac.jp/KamGeo/Contact_%26_Links_files/NatGeosci_Gando_2011.pdf) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) |
## Discovery and origin
The stable potassium isotopes ³⁹K and ⁴¹K were identified in 1921 by F. W. Aston at the [Cavendish Laboratory](https://www.edgepedia.org/cavendish-laboratory), but ⁴⁰K, at only 0.0117% of natural potassium, was not found until 1935, when Alfred Nier of the [University of Minnesota](https://www.edgepedia.org/university-of-minnesota) used an improved double-focusing mass spectrometer.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) Nier's later 1950 redetermination of potassium's isotopic abundances gave 0.0119 ± 0.0001 percent for ⁴⁰K, a figure he described as of special interest in geophysical problems.[5](https://journals.aps.org/pr/abstract/10.1103/PhysRev.77.789)
[Stellar nucleosynthesis](https://www.edgepedia.org/stellar-nucleosynthesis) produces ⁴⁰K by two routes: oxygen burning in massive stellar explosions, the same process that makes the stable isotopes, and the slow neutron-capture (s-process) reaction ³⁹K(n,γ)⁴⁰K, which can occur whenever free neutrons are available in stars.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/)
## Decay scheme
⁴⁰K decays by a branched scheme. The dominant branch is beta-minus emission to the ground state of ⁴⁰Ca, with a transition energy of 1310.91(6) keV, accounting for 89.56(7)% of decays.[2](https://www.bipm.org/documents/d/guest/k-40_report) The argon branch proceeds mainly by electron capture to the first excited state of ⁴⁰Ar at 1460.851(6) keV, a first-forbidden-unique transition with probability 10.34(7)%, followed by emission of the 1460.822(6) keV gamma ray.[2](https://www.bipm.org/documents/d/guest/k-40_report) Two much rarer modes complete the scheme: electron capture directly to the ⁴⁰Ar ground state at 0.098(25)%, and beta-plus emission at 0.00103(13)%.[2](https://www.bipm.org/documents/d/guest/k-40_report)
The 2025 evaluation by the Decay Data Evaluation Project (DDEP), an international collaboration publishing under the auspices of the Bureau International des Poids et Mesures, recommends a total half-life of 1.2522(27) × 10⁹ years and a total decay constant of 5.535(12) × 10⁻¹⁰ a⁻¹, with partial decay constants of 4.958(11) × 10⁻¹⁰ a⁻¹ for beta decay and 5.778(13) × 10⁻¹¹ a⁻¹ for the electron-capture branch to the excited state.[2](https://www.bipm.org/documents/d/guest/k-40_report) Earlier evaluations had given half-lives of 1.2504(30) × 10⁹ years (2009) and 1.2480(30) × 10⁹ years (2017), and the branching to the ⁴⁰Ar ground state was halved relative to the 2009 value of 0.20(10)%.[2](https://www.bipm.org/documents/d/guest/k-40_report)
## Geochronology
Because potassium constitutes about 3 wt.% of [Earth's crust](https://www.edgepedia.org/earths-crust) and is present in most rock-forming minerals, and because argon diffusion in minerals is temperature-dependent, the decay of ⁴⁰K to ⁴⁰Ar supports both K–Ar and ⁴⁰Ar/³⁹Ar dating, which can date most rocks and also reveal their thermal history.[6](https://doi.org/10.1093/nsr/nwaf277) The conventional K–Ar method measures ⁴⁰K and radiogenic ⁴⁰Ar separately, and requires two assumptions: that no argon was left in the material when it formed, and that the system has remained closed since.[7](https://www.sciencedirect.com/topics/earth-and-planetary-sciences/potassium-argon-dating) In volcanic rocks the clock starts at eruption, when the lava cools enough for argon to be trapped in minerals; at high temperatures argon escapes from magma.[8](https://www.usgs.gov/observatories/yvo/news/going-going-argon-determining-volcanic-eruption-ages-argon-geochronology)
The ⁴⁰Ar/³⁹Ar variant, developed as an alternative, irradiates potassium-bearing samples with fast neutrons in a research reactor, converting a fraction of ³⁹K to ³⁹Ar, which then serves as a proxy for potassium.[3](https://www.usgs.gov/usgs-triga-reactor/neutron-irradiations-argon-isotopic-dating) A sample's geologic age comes from measuring its ⁴⁰Ar/³⁹Ar ratio against that of a standard whose age is already known; the technique serves to determine ages of rocks and minerals on Earth, along with meteorites and lunar material, spanning roughly 2,000 years up to the [Solar System](https://www.edgepedia.org/solar-system)'s age.[3](https://www.usgs.gov/usgs-triga-reactor/neutron-irradiations-argon-isotopic-dating) Modern mass spectrometers analyze material from a single mineral grain to a few millimeters with analytical errors below 0.25% absolute.[3](https://www.usgs.gov/usgs-triga-reactor/neutron-irradiations-argon-isotopic-dating)
**Decay constants.** The total ⁴⁰K decay constant directly affects calculated K–Ar and ⁴⁰Ar/³⁹Ar ages, and numerous values have been proposed over several decades.[6](https://doi.org/10.1093/nsr/nwaf277) Following Steiger and Jäger (1977), the IUGS Subcommission on [Geochronology](https://www.edgepedia.org/geochronology) adopted a total decay constant of 5.543 × 10⁻¹⁰ a⁻¹ with a branching ratio of 0.1171 for the electron-capture fraction, without considering the uncertainties in these values; ages calibrated with these constants are limited in accuracy to no better than 2%, mainly from systematic errors in the decay constants and in the ⁴⁰K–⁴⁰Ar data for standards.[9](https://www.sciencedirect.com/science/article/pii/S0016703710003571) Later constants recommended by Min et al. and Renne et al. have been widely adopted, though each has limitations: Min et al. omitted the electron-capture decay to the ground state, and ages computed with the Renne et al. constant differ significantly from astronomically tuned ages and are generally older.[6](https://doi.org/10.1093/nsr/nwaf277)
**Excess argon.** K–Ar and ⁴⁰Ar/³⁹Ar techniques occasionally produce anomalously old ages attributed to excess argon, argon not produced by decay within the dated mineral.[10](https://www.sciencedirect.com/science/article/pii/S0009254102000645) The first reports appeared soon after [K–Ar dating](https://www.edgepedia.org/k-ar-dating) was established in the 1950s.[10](https://www.sciencedirect.com/science/article/pii/S0009254102000645) Partition coefficients between minerals and hydrous fluids as low as 10⁻⁶ lead fluid inclusions to dominate the radiogenic argon budget, particularly in low-potassium minerals such as amphibole and plagioclase.[10](https://www.sciencedirect.com/science/article/pii/S0009254102000645) In a ⁴⁰Ar versus ⁴⁰K isochron, a positive intercept indicates an approximately constant amount of excess argon, while a negative intercept indicates argon loss.[11](https://www.sciencedirect.com/science/article/pii/0016703774900921) Incremental (step) heating can identify both excess ⁴⁰Ar and ⁴⁰Ar loss using inverse isochron and age-spectrum diagrams.[6](https://doi.org/10.1093/nsr/nwaf277)
## Radiogenic heat and geoneutrinos
Decay of ⁴⁰K, together with ²³²Th and ²³⁸U, provides a significant fraction of Earth's total heat budget.[12](https://escholarship.org/content/qt8t67641k/qt8t67641k_noSplash_4d3e7237476e7d1c980cb30770c41a05.pdf) In one bulk silicate Earth model, ⁴⁰K decay accounts for nearly 20% of the total radiogenic heat production, following ²³²Th and ²³⁸U.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) During Earth's early differentiation, ⁴⁰K was one of the primary radiogenic heat sources after the short-lived ²⁶Al and ⁶⁰Fe.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/)
Geoneutrinos, the electron antineutrinos emitted during beta decay of these nuclides, give a direct measure of the planet's heat-producing elements.[13](https://www.sciencedirect.com/science/article/pii/S0012821X2200320X) The neutrinos emitted from ⁴⁰K decay are below the limits of detection in current experiments, but the decay is known to contribute an estimated 4 TW of radiogenic heating.[4](https://www.awa.tohoku.ac.jp/KamGeo/Contact_%26_Links_files/NatGeosci_Gando_2011.pdf) • [20](https://physicstoday.aip.org/news/neutrinos-from-earths-interior-measure-the-planets-radiogenic-heating) Because ⁴⁰K also emits neutrinos (through electron capture) rather than only antineutrinos, proposed detection schemes exploit this antimatter signature to separate the potassium signal from backgrounds.[14](https://arxiv.org/abs/2308.04154) High-pressure experiments show that potassium enters iron sulfide melts in a strongly temperature-dependent fashion, so ⁴⁰K could serve as a substantial heat source in the cores of Earth and Mars; earlier studies suggested that roughly 250 ppm of potassium in the core could sustain the geodynamo.[15](https://pubmed.ncbi.nlm.nih.gov/12736683/) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/)
## The rare ground-state electron capture
The electron-capture decay of ⁴⁰K directly to the ground state of ⁴⁰Ar had been predicted but never observed, and theoretical predictions of its intensity spanned an order of magnitude.[16](https://arxiv.org/abs/2211.10343) In 2023 the KDK (potassium decay) collaboration reported strong evidence for this rare mode: a blinded analysis found a ratio of ground-state to excited-state electron-capture intensities of 0.0095 ± 0.0022 (statistical) ± 0.0010 (systematic), rejecting the null hypothesis at 4σ.[16](https://arxiv.org/abs/2211.10343) Expressed as a branching ratio, the signal yields 0.098%, roughly half of the commonly used prediction.[17](https://pubs.usgs.gov/publication/70247468) Because the K–Ar age equation depends on the electron-capture decay constant, this nonzero measurement shifts geochronological ages by up to a percent, with implications for Earth and Solar System chronologies.[16](https://arxiv.org/abs/2211.10343) The 2025 DDEP evaluation incorporates this measurement, giving the 0.098(25)% branching listed above.[2](https://www.bipm.org/documents/d/guest/k-40_report)
## Everyday radioactivity
Since ⁴⁰K follows potassium chemically, wherever potassium occurs, ⁴⁰K occurs with it, at about 0.0117% of the total.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/) [Potassium chloride](https://www.edgepedia.org/potassium-chloride) sold for water softeners is a commonly cited accessible gamma source, and the 1460.8 keV gamma line is used as a positive control in gamma-emitting radionuclide detection methods such as the FDA's WEAC method.[18](https://www.fda.gov/media/174923/download) The human body regulates potassium tightly regardless of dietary intake, so the ⁴⁰K activity of a person's tissues stays essentially constant.[19](https://www.cdc.gov/salt/sodium-potassium-health/)
## References
1. [Geochemistry and Cosmochemistry of Potassium Stable Isotopes (Wang, Huang, Teng et al., 2021), PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8740523/)
2. [Comments on evaluation 40K, X. Mougeot, CEA-LNHB, May 2025, DDEP/BIPM](https://www.bipm.org/documents/d/guest/k-40_report)
3. [Neutron Irradiations for Argon Isotopic Dating, USGS TRIGA Reactor](https://www.usgs.gov/usgs-triga-reactor/neutron-irradiations-argon-isotopic-dating)
4. [Partial radiogenic heat model for Earth revealed by geoneutrino measurements (Gando et al., 2011), Nature Geoscience](https://www.awa.tohoku.ac.jp/KamGeo/Contact_%26_Links_files/NatGeosci_Gando_2011.pdf)
5. [A Redetermination of the Relative Abundances of the Isotopes of Carbon, Nitrogen, Oxygen, Argon, and Potassium (Nier, 1950), Physical Review 77, 789](https://journals.aps.org/pr/abstract/10.1103/PhysRev.77.789)
6. [Argon-based geochronology: advances, limitations and perspectives (Chang & Shi, 2025), National Science Review 12, nwaf277](https://doi.org/10.1093/nsr/nwaf277)
7. [Potassium-Argon Dating: an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/earth-and-planetary-sciences/potassium-argon-dating)
8. [Going, going, argon! Determining volcanic eruption ages with argon geochronology, USGS Yellowstone Volcano Observatory](https://www.usgs.gov/observatories/yvo/news/going-going-argon-determining-volcanic-eruption-ages-argon-geochronology)
9. [Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard (Renne et al., 2010), Geochimica et Cosmochimica Acta](https://www.sciencedirect.com/science/article/pii/S0016703710003571)
10. [Excess argon in K–Ar and Ar–Ar geochronology (Kelley, 2002), Chemical Geology 188, 1–22](https://www.sciencedirect.com/science/article/pii/S0009254102000645)
11. [Evaluation of K–Ar isochron methods (Shafiqullah & Damon, 1974), Geochimica et Cosmochimica Acta](https://www.sciencedirect.com/science/article/pii/0016703774900921)
12. [Heat from the decay of 238U, 235U, 232Th, and 40K in Earth's heat budget, Geophysical Research Letters (2022)](https://escholarship.org/content/qt8t67641k/qt8t67641k_noSplash_4d3e7237476e7d1c980cb30770c41a05.pdf)
13. [Quantifying Earth's radiogenic heat budget (McDonough et al., 2022), Earth and Planetary Science Letters](https://www.sciencedirect.com/science/article/pii/S0012821X2200320X)
14. [Probing Earth's Missing Potassium using the Unique Antimatter Signature of Geoneutrinos (2023), arXiv:2308.04154](https://arxiv.org/abs/2308.04154)
15. [Experimental evidence that potassium is a substantial radioactive heat source in planetary cores (Gessmann, Wood et al., 2003), Nature](https://pubmed.ncbi.nlm.nih.gov/12736683/)
16. [Evidence for ground-state electron capture of 40K (Stukel et al., KDK Collaboration, 2023), arXiv:2211.10343; Physical Review Letters 131, 052503](https://arxiv.org/abs/2211.10343)
17. [Rare 40K decay with implications for fundamental physics and geochronology (Stukel et al., 2023), USGS publication record](https://pubs.usgs.gov/publication/70247468)
18. [Total Diet Study Report: Fiscal Years 2018–2020 Radionuclides Data, U.S. Food and Drug Administration](https://www.fda.gov/media/174923/download)
19. [Effects of Sodium and Potassium, Centers for Disease Control and Prevention](https://www.cdc.gov/salt/sodium-potassium-health/)
20. [Neutrinos from Earth’s interior measure the planet’s radiogenic heating](https://physicstoday.aip.org/news/neutrinos-from-earths-interior-measure-the-planets-radiogenic-heating)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Nuclear physics › Radioactivity and nuclear decay › Decay kinetics and decay chains › Radiometric dating systematics*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license