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.

Neutron Detection

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.

©cchieh@uwaterloo.ca