Noun
English Wikipedia has an article on:Isotopes of potassiumWikipedia
potassium-40 (uncountable)
(physics) A naturally-occurring radioactive isotope of potassium, 19K, having nineteen protons and twenty-one neutrons, and a half-life of 1.3 billion years; it is the chief source of natural radioactivity in living tissue
The atmosphere's supply of argon-40 is due mostly to the radioactive decay of potassium-40 in the time since the formation of the Earth. Source: Internet
Thus, the present activity on earth from uranium-238 is only half as much as it originally was because of its 4.5 billion year half-life, and potassium-40 (half-life 1.25 billion years) is only at about 8% of original activity. Source: Internet