Traces of K-40 are found in all potassium, and it is the most common radioisotope in the human body. K-40 is a radioactive isotope of potassium which has a very long half-life of 1.251×109 years and undergoes both types of beta decay. From this point of view, also a human body can be considered as a source of antimatter.
- About 89.28% of the time (10.72% is by electron capture), it decays to calcium-40 with emission of a beta particle (β−, an electron) with a maximum energy of 1.33 MeV and an antineutrino, which is an antiparticle to the neutrino.
- Very rarely (0.001% of the time) it will decay to Ar-40 by emitting a positron (β+) and a neutrino.
The potassium concentration in the human body is strictly based on the homeostatic principle. Potassium is more or less distributed in the body (especially in soft tissues) following intake in foods. A 70-kg man contains about 126 g of potassium (0.18%), most of that is located in muscles. The daily consumption of potassium is approximately 2.5 gram. Hence the concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv.