Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom
nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an
alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they
are also produced from the decay of radioactive elements. Because of its size and lack of charge,
the neutron is fairly difficult to stop, and has a relatively high penetrating power.
Neutrons may collide with nuclei causing one of the following reactions: inelastic scattering,
elastic scattering, radiative capture, or fission.
Inelastic scattering causes some of the neutrons kinetic energy to be transferred to the target
nucleus in the form of kinetic energy and some internal energy. This transfer of energy slows
the neutron, but leaves the nucleus in an excited state. The excitation energy is emitted as a
gamma ray photon. The interaction between the neutron and the nucleus is best described by the
compound nucleus mode; the neutron is captured, then re-emitted from the nucleus along with
a gamma ray photon. This re-emission is considered the threshold phenomenon. The neutron
threshold energy varies from infinity for hydrogen, (inelastic scatter cannot occur) to about 6
MeV for oxygen, to less than 1 MeV for uranium.
Elastic scattering is the most likely interaction between fast neutrons and low atomic mass
number absorbers. The interaction is sometimes referred to as the "billiard ball effect." The
neutron shares its kinetic energy with the target nucleus without exciting the nucleus.
Radiative capture (n, g) takes place when a neutron is absorbed to produce an excited nucleus.
The excited nucleus regains stability by emitting a gamma ray.
The fission process for uranium (U235 or U238) is a nuclear reaction whereby a neutron is absorbed
by the uranium nucleus to form the intermediate (compound) uranium nucleus (U236 or U239). The
compound nucleus fissions into two nuclei (fission fragments) with the simultaneous emission
of one to several neutrons. The fission fragments produced have a combined kinetic energy of
about 168 MeV for U235 and 200 MeV for U238, which is dissipated, causing ionization. The
fission reaction can occur with either fast or thermal neutrons.
The distance that a fast neutron will travel, between its introduction into the slowing-down
medium (moderator) and thermalization, is dependent on the number of collisions and the
distance between collisions. Though the actual path of the neutron slowing down is tortuous
because of collisions, the average straight-line distance can be determined; this distance is called
the fast diffusion length or slowing-down length. The distance traveled, once thermalized, until
the neutron is absorbed, is called the thermal diffusion length.