Natural alpha decay takes place in heavy nuclei (A;SPMgt;208). Each alpha decay leads to . Since this tends to move nuclei off the line of beta stability to the neutron-rich side, beta (-minus) decays are found in conjunction with alpha decays. There are thus four series (or chains) of alpha decays into which the natural alpha decays can be fitted; these correspond to where n is an integer.
For alpha decay to occur, the energy release Q must be greater than zero.
From the semiempirical mass formula, Q;SPMgt;0 for .
The energy Q is shared between the kinetic energy of the alpha and that of the residual nucleus.
It follows from conservation of energy and momentum that the kinetic energy of the alpha and the residual nucleus has a fixed value. Alpha particles are thus emitted with a sharply peaked spectrum.