Chlorine-36 (36Cl)

Chlorine-36 (36Cl) is a rare long-lived radio-isotope of Chlorine. Chlorine-36 is called a cosmogenic nuclide, because it is created by energetic cosmic rays impacting matter in a fission reaction, called spallation.
half-life (λ) 301,100y
Protons 17
Neutrons 19