Potassium-40 (\(^{40}\)K) is a naturally radioactive isotope of potassium, meaning its atomic nucleus is unstable and decays over time. This isotope exists as a small but constant fraction of all potassium found in nature, including in rocks, soil, and biological organisms. The concept of half-life describes the time required for half of the radioactive atoms in any given sample to undergo this decay process. Potassium-40 possesses an extremely long half-life, making it a useful tool for science and a pervasive, though minor, component of the natural world. Its presence makes it a constant source of internal radiation for all living things.
The Half-Life Value and Decay Process
The half-life of Potassium-40 is approximately 1.251 billion years, a duration nearly a third of the age of the universe. This immense time scale explains why a portion of the original \(^{40}\)K present when Earth formed is still abundant today. The nucleus of a \(^{40}\)K atom can transform into a stable atom through two primary decay pathways.
The most frequent decay path (about 89.6% of cases) is beta decay, where a neutron converts into a proton, emitting an electron (beta particle) and an antineutrino. This process transforms the parent isotope, Potassium-40, into the stable daughter isotope, Calcium-40 (\(^{40}\)Ca). The second significant pathway is electron capture, which happens approximately 10.3% of the time.
In electron capture, the \(^{40}\)K nucleus absorbs one of its own inner orbital electrons, causing a proton to convert into a neutron. This transformation results in the formation of the stable noble gas Argon-40 (\(^{40}\)Ar). This decay often leaves the resulting \(^{40}\)Ar atom in an excited state, which then emits a gamma ray as it settles into its stable form.
Dating the Age of Rocks and Fossils
The decay of Potassium-40 to Argon-40 is the foundation of the Potassium-Argon (K-Ar) dating method, a technique used to determine the age of geological formations. Because of its extremely long half-life, this method is effective for dating rocks that are millions or even billions of years old, providing a timescale for Earth’s history. The technique relies on potassium being a common element in many minerals, while argon is an inert gas.
When volcanic rock or magma is in a molten state, any Argon-40 produced by decay escapes easily into the atmosphere. As the rock cools and solidifies, its crystalline structure locks in all the potassium atoms, including the radioactive \(^{40}\)K. It simultaneously traps any newly formed Argon-40 atoms. This cooling event essentially “sets the clock” to zero, as there is no Argon-40 present at that moment.
Scientists can then measure the ratio of the remaining parent isotope, \(^{40}\)K, to the accumulated daughter isotope, \(^{40}\)Ar, in a sample. Knowing the precise half-life of \(^{40}\)K allows researchers to calculate the time elapsed since the rock solidified. This method is particularly valuable for dating volcanic layers that sandwich sedimentary strata containing ancient fossils or archaeological sites. By dating the volcanic rock layers above and below a fossil-bearing layer, scientists can determine a minimum and maximum age for the fossils themselves.
Natural Radioactivity in the Human Body
Potassium is an essential electrolyte required for nerve signaling, muscle contraction, and maintaining fluid balance in the body. Since Potassium-40 is an unavoidable fraction of all natural potassium, every living organism contains this radioactive isotope. For an average adult, the body continuously maintains a mass of potassium, containing a small but constant amount of Potassium-40.
This internal Potassium-40 is a persistent source of natural background radiation, undergoing between 4,000 and 5,000 decay events every second in an adult male. We constantly ingest this radioisotope through our diet, as potassium-rich foods like potatoes, nuts, and bananas contain trace amounts of \(^{40}\)K. The concept of the “Banana Equivalent Dose” is an informal educational tool that illustrates this internal exposure, correlating to about 0.1 microsieverts of radiation.
The radiation dose from ingested \(^{40}\)K is not cumulative because the human body strictly regulates its potassium levels in a process called homeostasis. Any excess potassium absorbed from food is quickly excreted, ensuring that the total amount of radioactive \(^{40}\)K in the body remains constant. While it contributes to our overall radiation exposure, the presence of Potassium-40 within us is a natural component of life, stemming from the same long-lived radioisotope used to date the ancient Earth.