The Nucleus and the Strong Interaction 233
cloud chamber has been replaced by the bubble chamber, which operates
on a similar principle. In the cloud chamber the charged particle leaves
tracks by condensing droplets of water in the vapor, whereas in the
bubble chamber tracks are left because the charged particles vaporize
gas bubbles in the superheated liquid of the bubble chamber. The
development of accelerators and particle detectors has allowed the
nuclear physicist to peer into the nucleus to discover its secrets.
When I was a graduate student one of my professors likened the study
of the nucleus through scattering experiments to the situation of a man
trying to discover the nature of an avocado in the following bizarre
fashion. Somebody places an avocado in a pitch-black room. The
physicist enters the room blindfolded with a machine gun and fires a hail
of bullets in the direction of the avocado. The physicist leaves the room
and the avocado is removed. The physicist is allowed to reenter the room
and turn on the lights. By studying the pieces of avocado on the wall he
or she is then expected to determine the nature of the fruit they fired at,
the size of the fruit, the size of the pit, the thickness of the skin, etc. This
is the task, which faces nuclear physicists. They fire into the dark, never
seeing what they are aiming at. Hopefully, by detecting bits of matter
that emerge from the interaction of the projectile and the nucleus, they
will discover the nature of the nucleus and the nuclear force.
The simplest bombardment process of the nucleus is scattering in
which the incoming particle is deflected by the nuclear or electric force
of the nucleus. If the incoming particle is an electron, it will only be
deflected by the electric force since the electron has an electric charge
but no nuclear charge. The electron is unaffected by the nuclear force.
Electron scattering experiments have revealed the charge distribution of
the nucleus. Scattering experiments in which the proton and the neutron
are the incoming projectiles have yielded information on the nature of
the nuclear force such as its dependence on the position of the particles
and the alignment of their spins. These scattering experiments have also
helped to determine the size and shape of individual nuclei.
In most scattering experiments the energy of the projectile before and
after the collision is the same as is the energy of the nucleus. In some
cases the nucleus is left in an excited state as a result of the collision. The
incoming particle suffers an energy loss just equal to the energy gained
by the nucleus. The nucleus does not remain in its excited state for very
long but quickly decays into its ground state by emitting gamma rays.
From the study of the inelastic scattering just described one is able to