Potassium-40 () is a primordial radioactive isotope of potassium which has a very long half-life of years. It occurs naturally in all potassium-bearing minerals and is a major contributor to the natural background radiation dose received by humans and other organisms.
Natural potassium contains approximately 0.0117% of the isotope. Because potassium is an essential element for life and is found in high concentrations in many minerals (such as feldspar and mica), is ubiquitous in the environment. It is the largest source of natural radioactivity within the human body, primarily due to the ingestion of common foods like bananas, beans, and nuts.
Potassium-40 is unique among naturally occurring radionuclides because it decays via multiple pathways:
In the context of NORM, is frequently encountered in the fertilizer industry (potash) and in building materials like granite and concrete. While its radiological hazard is generally lower than that of the uranium and thorium series, it must be accounted for in total activity assessments.
In science, the decay of to is the basis for Potassium-Argon (K-Ar) dating. Because argon is a gas that escapes from molten rock but is trapped upon solidification, this method allows geologists to date the crystallization of volcanic rocks over millions of years.
The 1.46 MeV gamma emission from is a significant component of the external dose from the ground (terrestrial radiation). In health physics, it is used as a reference point for calibrating gamma spectroscopy equipment due to its clear and consistent energy peak.