Pion

A pion, in particle physics, consists of a quark and an antiquark. They are therefore mesons. Pions are the lightest mesons and more generally the lightest hadrons. Charged pions often decay into muons and muon neutrinos. Neutral pions generally decay into gamma rays. 

Pions are not produced in radioactive decay. However, they commonly appear in  high energy collisions between hadrons. Pions can also result from matter-antimatter annihilation events.