The Nucleus and the Strong Interaction 231
radium, thorium and uranium. Beryllium-8, however, also undergoes
alpha decay, decaying into two alphas. When a nucleus of A nucleons
and Z protons emits an alpha particle, it becomes a nucleus with A – 4
nucleons and Z – 2 protons. The mass of the alpha particle and
the daughter nucleus is less than the original mass of the parent nucleus.
The mass which is lost in this transition is converted into the kinetic
energy of the daughter nucleus and the alpha particle according to
Einstein formulae, E = mc^2. The kinetic energy of the alpha particles is
usually about 5 or 6 MeV.
Beta decay or the emission of an electron occurs more frequently
among the lighter radioactive nuclei. In this process a neutron in
the nucleus changes into a proton so the parent nucleus of Z protons and
N neutrons changes into a daughter nucleus of Z + 1 protons and N – 1
neutrons. This decay involves a basic force of nature, which we have not
yet encountered and which is referred to as the weak interaction. The
three other basic forces of nature that we have encountered are the
gravitational, electromagnetic and nuclear interactions. The weak decay
of a nucleus actually involves the emission of a second particle in
addition to the electron. This particle, known as the neutrino, is very
unusual. It has very little mass and no charge but carries away energy
and momentum and in addition carries a half unit of spin. We shall study
this process in greater detail in the next chapter, which deals with the
interactions of elementary particles. Not only do the neutrons in unstable
nuclei experience beta decay, but also the free neutron decays in this
manner. The average lifetime of a neutron is 1000 seconds after which it
decays spontaneously into a proton, an electron and a neutrino.
Neutron → proton + electron + neutrino (n → p + e + ν)
We shall leave the discussion of this decay for the next chapter.
Closely related to the process of electron emission is positron emission,
which also involves the weak interaction. In this process a proton in an
unstable nucleus changes into a neutron plus a positron (an antielectron)
plus a neutrino. This means that a nucleus with Z protons and N
neutrons as a result of positron emission is converted into a nucleus with
Z – 1 protons and N + 1 neutrons. A free proton does not undergo
positron emission because the mass of the neutron, the positron and the
neutrino are greater than that of the proton and hence the proton does not
have the energy to make this transition. For all the radioactive nuclei,
which emit electrons or positrons, the mass of the parent nucleus is