• Tags: alpha, decay, tunneling
• Difficulty: Moderate

In alpha decay, a nucleus emits a He-4 particle (2 protons + 2 neutrons): _$Z^A$ X -> _{Z-2}^{A-4} Y + _2^4 He. Alpha decay occurs primarily in heavy nuclei (A > 200) where reducing the nuclear size increases stability. The Q-value (energy released) = [$M_{parent}$ - $M_{daughter}$ - $M_{alpha}$] x 931.5 MeV. This energy is shared between the alpha particle and the daughter nucleus as kinetic energy. By momentum conservation (both initially at rest): $KE_{alpha}$ = Q x $\frac{A-4}{A}$ and $KE_{daughter}$ = Q x 4/A. The alpha particle carries most of the energy. Alpha particles are monoenergetic for a given decay (discrete spectrum). Quantum tunneling explains alpha decay: the alpha particle has insufficient energy to classically overcome the Coulomb barrier but tunnels through it. The Geiger-Nuttall law relates half-life to alpha energy: higher energy alphas correspond to shorter half-lives.