Analogy with the recoil experienced by a firearm:
The kinematics of an alpha decay are quite similar to those of a firearm where a light bullet takes away most of the energy of the explosion. The momenta (products of mass times velocity) of the recoiling nucleus and the alpha particle are exactly equal. The respective speeds and energies are inversely proportional to the masses, and as a result are highly unequal. In the case of a radium nucleus, the alpha particle takes away 98.3% of the available energy. These alpha particles are always emitted with the same energy values.
IN2P3