Beta-particle energies

By now it was known that alpha particles are emitted with several different discrete energy levels. Chadwick discovered that beta particles came off in a continuous range of energies, from zero to a determined maximum. This puzzle would remain unexplained for a number of years.
 James Chadwick England 1914 Beta-particle energies

By now it was known that alpha particles are emitted with several different discrete energy levels. Chadwick discovered that beta particles came off in a continuous range of energies, from zero to a determined maximum. This puzzle would remain unexplained for a number of years.