Interaction of Neutrons with Matter
Neutrons are heavy-neutral particles, and they interact with
electrons weakly due to the magnetic moment present in both electrons
and neutrons. Collisions between neutrons and atomic nuclei are rare
events, because both are tiny compared to atoms. Collisions can be
elastic, or inelastic. Other interactions are (n, g),
(n, a), (n, p), and (n, 2n) nuclear reactions.
In collisions with heavy nuclides, neutrons lose little energy. In
collisions with light nuclides such as H, D, He, and C, neutrons
lose a major portion of their energies. A neutron can lose all its
kinetic energy in a single collision with a proton. Thus, light nuclides
are effective moderators, but not heavy nuclides.
Captures of neutron by proton and deuterium lead to the formation of
deuterium and tritium respectively. The cross sections are small,
and many neutrons decay if not captured.
Neutrons are detected indirectly by observing the protons knocked
loose by them or by nuclear reactions induced by them. For example,
the alpha particles are easily detected in the following reactions.
14N + n ® 11B + a
10B + n ® 7Li + a
6Li + n ® 7Li + a
Slow neutrons are measured by a
proportional counter, which is
filled with BF3 gas. The products, 7Li and
a ionize the gas in the proportional counter
and the signals are detected.
Fission reactions induced by neutrons can also serve for neutron detection.
Neutrons whose kinetic energy is above about 1 keV are generally
classified as fast neutrons. They give large portions of
their kinetic energy to the protons. Energetic protons ionize, and
they are detected as such.