Potassium-40 - isotopic data and properties (2024)

Properties and data of the isotope ⁴⁰K.

Potassium-40 - symbol ⁴⁰ K - is a naturally occurring, primordial, radioactive isotope of the chemical element potassium with a half-life of 1.25 billion years; Earth's potassium consists of about 0.0117% (117 ppm) of this isotope, which is responsible for the weak natural radioactivity of the alkali metal.

The atomic nucleus of the nuclei consists of ⁴⁰K 21 neutrons and the element-specific 19 protons.

See also: list of Potassium isotopes.

General data

Radioactive Decay

Potassium-40 is a rare example of an isotope that undergoes both types of beta decay, and these major decay channels are well studied and experimentally verified. However, despite decades of research and great interest in this natural radionuclide, there is uncertainty about the details of ⁴⁰K decay.

• In about 89.28% of the events, the nuclide decays with the emission of a beta particle β^- - an electron with the kinetic energy of 0.56018 ± 17 MeV and a neutrino (0.6494 ± 12 MeV) - into the ground state of the isotope Calcium-40:
  
  4019K → 4020Ca + e^- + v_e.
  
• With a probability of 10.72%, after capture of an orbital electron (EE), the decay occurs into an excited ⁴⁰Ar state, which occurs almost simultaneously (T_1/2 = 1.15 picoseconds, spin = 2+) with emission of gamma rays with an energy of 1.460 MeV and a neutrino (0.00506 MeV) into the ground state of the isotope Argon-40 passes:
  
  4019K + e^- → 4018Ar + v_e.
  
• Extremely rarely - in only 0.001% of events - potassium-40 decays, emitting a neutrino v_e and a positron (β⁺-decay) to ⁴⁰Ar [1, 5]. If the positron e⁺ (anti-electron) hits an electron e^-, the two particles undergo pair annihilation with an annihilation energy of 0.511 MeV:
  
  4019K → 4018Ar + e⁺ + v_e.
  

Another type of decay that has not been confirmed experimentally and is controversial in science:

• Based on theoretical considerations, the nuclide ⁴⁰K could radioactively decay from its ground state directly into the ground state of ⁴⁰Ar after electron capture and with a probability of 0.2% . A recent article [Carter et al., 2020] deals with the latest findings and the experimental difficulties in this regard [5].

The radioactive decay of this potassium isotope explains the large amount of argon of almost one percent in the Earth´s atmosphere as well as the prevalence of ⁴⁰Ar compared to the other argon isotopes; at the same time it is according to 238Th and 238U the third most common source of radiogenic heat in the Earth´s mantle.

Half-life T_½ = 1.248(3) × 10⁹ a respectively 3.9356928 × 10¹⁶ seconds s.

Formation

The ⁴⁰K atom nucleus is one of the primordial radionuclides - this means that the terrestrial deposits were already present when they were created and were not replenished in significant quantities through radioactive decay processes or human influences. The potassium-40 therefore comes from stellar processes - particularly supernova explosions - that took place before the Earth was formed. Recent studies of meteorites also indicate that most of the terrestrial potassium (90%) comes from non-carbonaceous matter, the origin of which lies in the interior of the solar system [6].

Occurrence

Potassium-40 is a natural radionuclide found in potassium - but its proportion is only 0.0117%. Since potassium is a mineral found practically everywhere, the radioactive K-40 it contains is the largest source of natural radioactivity in humans, animals and the environment. A human body weighing 70 kg contains approximately 140 g of potassium, which is approximately 0.000117 × 140 g = 0.0164 g of the isotope ⁴⁰ K; The decay causes approximately 4,300 decays per second (Becquerels) within the body continuously throughout life.

NMR data

Nuclear magnetic properties of the NMR active Nuclide ⁴⁰K

Radiation Protection

Potassium compounds and preparations are ubiquitous in nature, technology and everyday life and are not considered radioactive hazardous substances despite the K-40 content; The radiation emitted by it is too weak for this.

Substances enriched with ⁴⁰K have no practical or technical significance; K-40 is one of the radioactive substances for which even high levels of activity do not lead to classification as a highly radioactive radiation source.

Nuclear Isomers

Nuclear isomers or excited states with the activation energy in keV related to the ground state.

Isotones and Isobars

External data and identifiers

Literature and References

[1] - D. W. Engelkemeir, K. F. Flynn, L. E. Glendenin:
Positron Emission in the Decay of ⁴⁰K.
In: Physical Review, 126,1818, (1962), DOI 10.1103/PhysRev.126.1818.

[2] - H. Leutz, G. Schulz, H. Wenninger:
The decay of potassium-40.
In: Zeitschrift für Physik, (1965), DOI 10.1007/BF01387190.

[3] - A. Ažman, A. Moljk, J. Pahor:
Electron capture in potassium 40.
In: Zeitschrift für Physik A, (1968), DOI 10.1007/BF01379914.

[4] - W. Sahm, A. Schwenk:
³⁹K, ⁴⁰K and ⁴¹K Nuclear Magnetic Resonance Studies.
In: Zeitschrift für Naturforschung A, (2014), DOI 10.1515/zna-1974-1208.

[5] - Jack Carter, Ryan B. Ickert, Darren F. Mark et al.:
Production of 40Ar by an overlooked mode of ⁴⁰K decay with implications for K-Ar geochronology.
In: Geochronology, 2(2), 355–365, (2020), DOI 10.5194/gchron-2-355-2020.

[6] - Nicole X. Nie et al.:
Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae.
In: Science, 379, 6630, 373-376, (2023), DOI 10.1126/science.abn178.

Last update: 2023-12-04

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