Answer
In alpha decay the energy and momentum are known and two particles are involved, allowing KE to be determined.
Work Step by Step
In alpha decay, assuming the parent nucleus has a known energy and zero momentum, there are just 2 pieces afterward: the daughter nucleus and the alpha particle. By applying energy and momentum conservation, E and p values of each are determined, and fall in a very narrow range.
In contrast, in beta decay, there are three particles present after the decay (see section 30-5). The energy and momentum conservation equations are not enough to determine the individual energy/momentum of each particle, and a spectrum of energies is the result.
