The history of beta decay began in 1896 with the discovery of radioactivity by Henri Becquerel. In the years 1899 and 1900 he identified b radiation as one component of radioactivity, and demonstrated that b rays are composed of electrons.
Observation of discrete energy lines in electron spectra from the decay of isotopes in the radium and thorium series was made in Berlin in the years 1910 and 1911 by von Bayer, Hahn, and Meitner. This was around the same time as the experimental evidence for the atomic nucleus by Rutherford.
The continuous b spectrum was discovered in the decay of RaB (214Pb) by Chadwick in 1914, using a magnetic spectrometer. Thus it was known that the electron spectra had monoenergetic lines and a continuous component, in strong contrast with alpha and gamma ray spectra, which were known to consist of monoenergetic lines only. Chadwick further demonstrated that the majority of events were part of the continuous component, the rest being monoenergetic (internal conversion) electrons.
Interpretation of the continuous electron spectrum was the subject of considerable debate. Rutherford (1914) was of the opinion that the b electrons were all emitted from the nucleus with the same energy, but lost different fractions of this energy to the surrounding atoms, depending on the source thickness traveled. This opinion was shared by von Bayer, Hahn, and Meitner (1915). The point was made by Meitner (1922) that a quantized nucleus should not emit electrons of continuously varying energy. The known features of a and g spectra were correctly interpreted as due to transitions of nuclei from one quantum state to another. Thus the continuous electron spectrum was a unique feature of b decay.
A crucial experiment was performed by Ellis and Wooster in 1927, in which they measured the total energy released in the disintegration of a RaE (210Bi) source inside a calorimeter thick enough to stop all the emitted electrons. The endpoint was known to be Eo = 1.05 MeV, and the mean energy,`E, of the b electrons was known to be 390 keV. The calorimeter should have measured a total energy of 1.05 MeV if the above picture was correct. In fact they observed`E = 344 ± 34 keV, which corresponded very well with the mean energy of the emitted electrons. The experiment was repeated in Berlin with an improved calorimeter by Meitner and Orthman in 1930 and the result was 337 ± 20 keV.
These results were very conclusive, but difficult to interpret at the time. Niels Bohr believed that energy conservation was violated in individual decays, although perhaps not statistically. Not only was energy apparently not conserved, but neither were momentum and angular momentum. In order to save the situation, Pauli (in a an open letter to Geiger and Meitner at a physics meeting in Tübingen, in December 1930) proposed the idea of a very penetrating neutral particle of small mass and spin ½, emitted with the electron in b decay. This proposal was made before Chadwick's discovery of the neutron in 1932. Pauli openly proposed his hypothesis at the Solvay Congress in Brussels in 1933. Fermi was present, and proposed the name "neutrino" to distinguish it from the neutron. Soon afterwards, Fermi developed his famous theory of beta decay.
The distinction between "allowed" and "forbidden" decays was first made by Sargent in 1933 on an empirical basis. The so-called Sargent diagram was a log plot of the decay rate vs. the endpoint energy. The decays would line up in distinct bands, depending on their degree of forbiddenness.
During the 1940's, high specific activity b sources were developed, and used in newly designed iron-free magnetic spectrometers with high transmission and moderate resolution. Measurements on allowed spectra led to a wide acceptance of Fermi's theory.
The mass of the electron neutrino continues to be an open question and is being actively investigated in measurements of the beta spectra of tritium and 187Re. One of those experiments is being performed here at Queen's. Our technique is to measure the energy of individual beta decays by the temperature rise they produce in a small cryogenic calorimeter. We note with interest the similarity between our technique and the calorimetric measurement performed in 1927 by Ellis and Wooster, which provided a crucial piece of evidence for the existence of the neutrino. A significant difference between the two experiments is that the calorimeter of Ellis and Wooster measured only the average energy per decay, whereas ours measures the energy deposited in each individual decay.