Aluminium-26 (26Al)

Aluminium-26 (26Al) is a rare long-lived radio-isotope of Aluminium. Aluminium-26 is called a cosmogenic nuclide, because it is created by energetic cosmic rays impacting matter in a fission reaction, called spallation.
half-life (λ) 730,000y
Protons 13
Neutrons 13