225Chapter 23The Nucleus and the Strong
Interaction
So far in our study of the atom we have basically examined the nature
of the electromagnetic force and the interactions of electrons and
antielectrons with photons. The electromagnetic force, however, cannot
explain how the protons and neutrons that make up the nucleus of the
atom can be confined to a space of the order of only 10-13 cm. Because
the electric force between the protons is repulsive there must be another
force that binds the nucleons, i.e. the protons and neutrons, together in
the nucleus of atoms. This force is the nuclear force or the strong
interaction, which is approximately 100 times stronger than the electric
force.
The atomic nucleus was first discovered in 1912 by Rutherford and
his co-workers in the experiment described in Chapter 19 in which atoms
were bombarded by α particles. These experiments revealed that the
nucleus is less than 10-12 cm in radius. Later experiments show that this
number was closer to 2 or 3 × 10-13 cm (or 2 or 3 fermi). The fermi (fm)
is a unit of length equal to 10-13 cm used frequently in nuclear physics
because just about all distances in this field range from 1 to 10 fermis.
Because the electron is such a light particle, the nucleus carries almost all
the weight of the atom. The volume the nucleus occupies, on the other
hand, is only 10-15 the total volume of the atom. The density of matter in
the nucleus is therefore quite considerable. It is in fact approximately
1014 times the density of water.
The charges and masses of individual nuclei were discovered
basically by the chemists. From this information it became immediately
evident that the proton was not the sole constituent of the nucleus. The
helium nucleus for example has a mass approximately four times the