236 The Poetry of Physics and The Physics of Poetry
will take with the release of energy as occurs when 4 protons combine to
form a helium nucleus.
After the maximum at A = 60, the binding energy per nucleus
decreases. If a nucleus with A > 60 were to split into two medium sized
nuclei, then the total binding energy would increase and the total mass
would decrease. The destruction of mass would then result in the release
of energy as occurs with nuclear fission
The first fission reaction was discovered when uranium was
bombarded by neutrons. It was discovered that the uranium nucleus
divided into barium and krypton plus three additional neutrons. The
release of the additional neutrons allows for the possibility of a chain
reaction since the extra neutrons can trigger the fission of other uranium
nuclei which in turn release extra neutrons, etc. the fission reaction
involves the release of 200 MeV per nucleus and requires only 10-14 sec.
Because the chain reaction spreads so quickly, a great deal of energy
is released in a very short time. This is the principle behind the release
of energy in the atomic bomb. It is possible to slow down the rate at
which new fission reactions are triggered by not concentrating the
uranium in one lump and by placing material, which absorbs neutrons
between the various concentrations of uranium. By controlling the
amount of neutron insulating material, the rate of fission is controlled
allowing the amount of atomic energy released at a given time to be
controlled, which is how a nuclear power plant works. The term atomic
energy is really a misnomer since the energy released in fission is nuclear
energy and not atomic energy. Atomic energy is the energy released in
chemical reactions.
Fusion reactions always involve the release of energy, however a
certain amount of kinetic energy is needed at the outset in order for the
reactions to proceed. A neutron can penetrate a heavy nucleus at any
energy because it is neutral and does not have to overcome a repulsive
electric force. If two nuclei try to combine together, they are pushed
apart by their respective positive charges. There is, therefore, a threshold
energy for fusion reactions below which the nuclei will not combine.
Once the threshold is exceeded, however, nuclear energy is readily
released. The fusion of hydrogen into helium is responsible for the
release of energy in the hydrogen bomb. In order to achieve the threshold
energy necessary to make this reaction proceed, an atomic bomb is
exploded in the heart of the hydrogen bomb. The high temperature
generated by this explosion then triggers the fusion reaction.