Subatomic Physics

332 The Weak Interaction
Radioactivity was discovered in 1896 by Becquerel, and it became clear within a
few years that the decaying nuclei emitted three types of radiation, called α, β, and
γ rays. The outstanding puzzle was connected with the beta rays. Careful measurements
over more than 20 years indicated that the beta particles were electrons
and that they were not emitted with discrete energies but as a continuum.
An example of such a
beta spectrum is shown in
Fig. 11.1. We have discussed
nuclear energy levels
in chapter 5. The existence
of quantized levels was well
known in 1920, and the first
puzzle posed by the continuous
beta spectrum thus
was: Why is the spectrum
of electrons continuous and
not discrete? A second puzzle
arose a few years later
when it was realized that no
electrons are present inside
nuclei. Where, then, do the
electrons come from?
Figure 11.1: Example of a beta spectrum. [This figure is
taken from one of the classic papers: C.D. Ellis and W.A.
Wooster, Proc. R. Soc. (London) A117, 109 (1927).]
Present experimental techniques yield more accurate energy
spectra, but all essential aspects are already contained
in the curve reprinted here.
The first puzzle was solved by Pauli, who suggested the existence of a new,
very light, uncharged, and penetrating particle, the neutrino. (1) Today, with so
many particles known, proposing a new particle scarcely raises eyebrows. In 1930,
however, it was a revolutionary step. Only two particles were known, the electron
and the proton. Destroying the simplicity of the subatomic world by addition of a
third citizen was considered to be heresy, and very few people took the idea seriously.
One of the ones who did was Fermi; he used Pauli’s neutrino hypothesis to solve
the second puzzle. Fermi assumed with Pauli that a neutrino is emitted together
with the beta particle in every beta decay. Consequently, the simplest nuclear beta
decay, the one of the neutron, is written as
n −→ pe − ¯ν.
Since the neutrino is chargeless, it is not observed in a spectrometer. Electron and
neutrino share the decay energy, and the observed electrons sometimes have very
little of it and sometimes nearly the maximum energy. The spectrum shown in
1 Pauli first suggested the neutrino in a letter addressed to some of his friends who were attending
a physics meeting in Tübingen. He declared that he was unable to be present at the gathering
because he wanted to attend the famous annual ball of the Swiss Federal Institute of Technology.
The letter is reprinted in R. Kronig and V.F. Weisskopf, eds., Collected Scientific Papers by
Wolfgang Pauli, Vol. II, Wiley-Interscience, New York 1964, p. 1316. See also L. M. Brown,
Phys. Today 31, 23 (September 1978).