Nuclear physics is the branch
of physics that studies the properties, structure, and reactions
of atomic nuclei. Nuclear physics began around 1900 with
the discovery of radioactivity and the nucleus. Since then,
the development of increasingly powerful and precise tools
has enabled physicists to study nuclei in ever greater detail.
The nucleus contains over 99.9 per cent of an atom's mass.
It consists of two kinds of particles, neutrons and protons,
with nearly identical masses. Protons have a positive electrical
charge, and neutrons have none. The number of protons in
a nucleus determines what chemical element the atom belongs
to, while the neutron number determines what isotope of
that element it represents.
A powerful force called the strong interaction or nuclear
force binds the neutrons and protons in the nucleus and
packs them closely together. Thus, all nuclei have the same
extremely high density. If the earth were compressed until
it had the density of an atomic nucleus, its diameter would
be only about 0.5 kilometre.
Nuclei with different numbers of protons and neutrons can
have remarkably different properties. Some nuclei are shaped
like spheres, while others are slightly elongated or slightly
flattened. Some nuclei are rigid, while others are more
flexible. Some are stable, while others are radioactive
and spontaneously release radiation called alpha particles,
beta particles, and gamma rays in order to take a more stable
form. Physicists explain these properties using sophisticated
theories that allow them to predict the behaviour of new
kinds of nuclei.
Much of the information about atomic nuclei comes from
studies of nuclear reactions. Typically, a particle accelerator
is used to create tiny, high-velocity beams of protons,
neutrons, or other particles. A nuclear reaction occurs
when one of these particles strikes a nucleus. Physicists
then use high-precision tools to analyse the radiations
emitted during the reaction.
Nuclear reactions are used in both nuclear weapons and
devices developed for peaceful purposes. The fission (splitting)
of very heavy nuclei and the fusion (combining) of two very
light nuclei both release large amounts of energy. Usually,
this energy is released in a slow, controlled way. Fission
energy is used to generate electricity and to power ships,
and researchers are working on devices to turn fusion energy
into electricity. In nuclear weapons, very different conditions
force a large number of fission or fusion reactions to occur
nearly simultaneously, producing an atomic bomb or hydrogen
Research in nuclear physics has led to new techniques for
diagnosing and treating disease, sterilizing and preserving
food, and exploring for oil. Often, these techniques make
use of radioactive nuclei called radioisotopes. Nuclear
research is also useful in other branches of physics and
in such fields as astrophysics, biochemistry, and chemistry.