Chemical Natural K 40 symbol & mass number abundance (atom %) 0.0117 1.28E9 a Half life, annum - Decay mode and Β 1.33, ε energies γ 1460.8 + β vw σ 30 σ 4.4 σ .42 α γ p Beta decay energy (MeV) E- 1.3111 E+ 1.5049 Naturally Occurring Radionuclide 43