ABSTRACT
In the early 1930s Wolfgang Pauli suggested that a low-mass neutral particle, the neutrino, was also emitted in β decay. This solved the problem of the continuous energy spectrum because in a three-body decay (neutron - proton - electron - neutrino) the energy of the electron was no longer required to be unique. This chapter discusses how physicists found that one of the types of particles emitted, the rays, was an electron. In 1900 Max Planck introduced quantization into physics, the idea that physical quantities can have only certain discrete values. Despite the evidence provided by both Kaufmann and Becquerel, the physics community did not at this time (the first decade of the twentieth century) accept that the energy spectrum of electrons emitted in β decay was continuous. The particles emitted in a decay have a unique energy, and physicists believed that the electrons emitted in β decay should also have a unique energy.
