Atomic and Nuclear Physics
INTERACTION OF RADIATION WITH MATTER
orbiting electron in an atom. Although more penetrating than the alpha, the beta is relatively
easy to stop and has a low power of penetration. Even the most energetic beta radiation can be
stopped by a few millimeters of metal.
Positively charged electrons are called positrons. Except for the positive charge, they are
identical to beta-minus particles and interact with matter in a similar manner. Positrons are very
short-lived, however, and quickly are annihilated by interaction with a negatively charged
electron, producing two gammas with a combined energy (calculated below) equal to the rest
mass of the positive and negative electrons.
Neutrons have no electrical charge. They have nearly the same mass as a proton (a hydrogen
atom nucleus). A neutron has hundreds of times more mass than an electron, but 1/4 the mass
of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but
they may also be produced from the decay of radioactive nuclides. Because of its lack of
charge, the neutron is difficult to stop and has a high penetrating power.
Neutrons are attenuated (reduced in energy and numbers) by three major interactions, elastic
scatter, inelastic scatter, and absorption. In elastic scatter, a neutron collides with a nucleus and
bounces off. This reaction transmits some of the kinetic energy of the neutron to the nucleus
of the atom, resulting in the neutron being slowed, and the atom receives some kinetic energy
(motion). This process is sometimes referred to as "the billiard ball effect."
As the mass of the nucleus approaches the mass of the neutron, this reaction becomes more
effective in slowing the neutron. Hydrogenous material attenuates neutrons most effectively.
In the inelastic scatter reaction, the same neutron/nucleus collision occurs as in elastic scatter.
However, in this reaction, the nucleus receives some internal energy as well as kinetic energy.
This slows the neutron, but leaves the nucleus in an excited state. When the nucleus decays to
its original energy level, it normally emits a gamma ray.
In the absorption reaction, the neutron is actually absorbed into the nucleus of an atom. The
neutron is captured, but the atom is left in an excited state. If the nucleus emits one or more
gamma rays to reach a stable level, the process is called radiative capture. This reaction occurs
at most neutron energy levels, but is more probable at lower energy levels.