Potassium-calcium decay system

Potassium has three isotopes (see Potassium); potassium-40 ( ⁴⁰K) is radioactive and decays to both calcium-40 ( ⁴⁰Ca) and argon-40 ( ⁴⁰Ar). The combined half-life of ⁴⁰K is 1.25 billion years. The branched decay scheme of ⁴⁰K is shown in Figure P33. It decays by β- decay to ⁴⁰Ca and to ⁴⁰Ar by both electron capture and positron decay (see Potassium-argon decay system). Although 89.52% of the decays produce ⁴⁰Ca, the system is of somewhat limited use due to the large natural abundance of ⁴⁰Ca (96.9% of calcium, see  Calcium).