Potassium-40 (
40K) is a
radioactive isotope of
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
which has a long
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...
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
radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
. In about 89.28% of events, it decays to
calcium-40
Calcium (20Ca) has 26 known isotopes, ranging from 35Ca to 60Ca. There are five stable isotopes (40Ca, 42Ca, 43Ca, 44Ca and 46Ca), plus one isotope ( 48Ca) with such a long half-life that for all practical purposes it can be considered stable. T ...
(
40Ca) with emission of a
beta particle
A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, � ...
(β
−, an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
) with a maximum energy of 1.31
MeV and an
antineutrino. In about 10.72% of events, it decays to
argon-40
Argon (18Ar) has 26 known isotopes, from 29Ar to 54Ar and 1 isomer (32mAr), of which three are stable (36Ar, 38Ar, and 40Ar). On the Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of ...
(
40Ar) by
electron capture
Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. ...
(EC), with the emission of a
neutrino and then a 1.460 MeV
gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
. The radioactive decay of this particular isotope explains the large abundance of argon (nearly 1%) in the
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
, as well as prevalence of
40Ar over
other isotopes. Very rarely (0.001% of events), it decays to
40Ar by emitting a
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collide ...
(β
+) and a neutrino.
[
]
Potassium–argon 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
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
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 radioactive decay of
40K in the Earth's mantle ranks third, after
232Th and
238U, as the source of
radiogenic heat
A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive (a radionuclide) or stable (a stable nuclide).
Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some o ...
. 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.
[
][
]
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 3,850 to 4,300 disintegrations per second (
becquerel) continuously throughout the life of the body.
[
]
Banana equivalent dose
Potassium-40 is famous for its usage in the
banana equivalent dose
Banana equivalent dose (BED) is an informal unit of measurement of ionizing radiation exposure, intended as a general educational example to compare a dose of radioactivity to the dose one is exposed to by eating one average-sized banana. Banana ...
, an informal unit of measurement, primarily used in generalized educational settings, to compare radioactive dosages to the amount received by consuming one
banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", disting ...
. 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.
See also
*
Background radiation
Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.
Background radiation originates from a variety of source ...
*
Isotopes of potassium
Notes
References
Table of radioactive isotopes, K-40The Lund/LBNL Nuclear Data Search
External links
* Potassium-40 Section
Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas
{{isotope
, element=potassium
, lighter=
potassium-39
, heavier=
potassium-41
, before=—
, after=
argon-40
Argon (18Ar) has 26 known isotopes, from 29Ar to 54Ar and 1 isomer (32mAr), of which three are stable (36Ar, 38Ar, and 40Ar). On the Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of ...
,
calcium-40
Calcium (20Ca) has 26 known isotopes, ranging from 35Ca to 60Ca. There are five stable isotopes (40Ca, 42Ca, 43Ca, 44Ca and 46Ca), plus one isotope ( 48Ca) with such a long half-life that for all practical purposes it can be considered stable. T ...
, Stable
Isotopes of potassium
Element toxicology
Positron emitters
Radionuclides used in radiometric dating