Proton radioactivity is a process by which an unstable nucleus spontaneously decays by the emission of a proton. Other common types of radioactivity include beta decay (the emission of electrons or positrons), alpha decay (the emission of a helium-4 nucleus), and gamma decay (the emission of high-energy photons). Proton decay is simpler than other forms of radioactivity since the proton is one of the constituents of the nucleus, and thus provides an opportunity to study details of nuclear structure.