Alpha Decay

When a nucleus decays in this way an alpha particle (a helium nucleus) is ejected from the nucleus.

or

All the emitted alpha particles travelled at the same speed, meaning they had the same amount of energy. The law of conservation of mass-energy is met, the energy of the nucleus before the decay is the same as the energy of the nucleus and alpha particle after the decay.

Alpha decay is NOT due to the weak interaction but Beta decay IS