Radioactive alpha decay had been explained in 1928 by Russian physicist George Gamow, which he described as quantum tunneling of alpha-particles (i.e., He-4 nuclei, 2 protons and 2 neutrons) from inside the nucleus, through the strong force barrier (which usually keeps the nucleus together), and finally emerging free. In the case of U-238 transforming to Th-234, the alpha-particle is emitted with about 4.2 MeV of kinetic energy. (This means that they are traveling at about 9.5% of the speed of light, or about 64 million miles per hour. Even though they are traveling fast, they are very light, and therefore are easily stopped - and absorbed - by a thin sheet of paper. In fact, they only travel about 4 cm in air.) Here is one of Ernest Rutherford's early papers - 1899 - describing his experiments on radioactive decay.
Beta decay, on the other hand, was more confusing. The beta particles (now known to be electrons) were emitted with a variety of kinetic energies, prompting Niels Bohr to speculate that the long-held principle of the conservation of energy did not apply to radioactive decay. However, in 1930, Wolfgang Pauli proposed that another particle was emitted along with the electron, which he called a "neutrino," because it was (a) electrically neutral, like the neutron, and (b) very light, like a "little neutron." This particle had not been observed (and would not be directly observed until 1956), but it saved the principle of the conservation of energy.
Then, at Christmas 1933, Enrico Fermi developed a theory of beta decay, based on the quantum theory of the electron developed by Paul Dirac in 1928. As Robert Crease and Charles Mann put it, "The theory was his first and only work on beta decay and was so complete that its description of the phenomenon has remained essentially unchanged to this day. At a stroke, Fermi first accurately described beta decay, first identified the separate nature of its cause, and first predicted most of its essential features."
It wasn't until 1995 that Frederick Reines won the Nobel Prize for his neutrino discovery, and subsequently two other neutrinos were discovered (the mu and tau neutrinos), leading to the 2015 Nobel Prize for the discovery of "neutrino oscillations," in which the different types of neutrinos can change into each other.
Fascinating stuff.
