Potassium-40

Potassium-40 (K-40 or ^40K) is a naturally occurring isotope of potassium, which is a vital nutrient for various biological and geological processes. It is one of the three stable isotopes of potassium, alongside Potassium-39 (^39K) and Potassium-41 (^41K). K-40 is notable because it is radioactive and has a long half-life of about 1.248 billion years, which means it decays very slowly.