ABSTRACT
Radioactivity was discovered in 1896 by Becquerel and it became clear within a few years that decaying nuclei could emit three types of radiation, called α, β and γ rays. An outstanding puzzle was related to the beta-decay process. The continuous energy distribution of beta decay electrons was a confusing experimental result in the 1920s. An example of such a beta spectrum is shown in figure 5.1. The energy distribution extends from zero to an upper limit (the endpoint energy) which is equal to the energy difference between the quantized initial and final nuclear states. A second, equally serious puzzle arose a few years later when it was realized that no electrons are present inside the nucleus. Where, then, do the electrons come from?
