What Is Neutron Activation?
Neutron activation is a physical phenomenon that occurs when a neutron collides with an atom. Any given atom is made up of mostly empty space between the furthest edge of its electron cloud and its nucleus, which is tiny by comparison; because neutrons have no electrical charge, they cannot interact with electrons and instead can only interact with the nucleus, which itself contains other neutrons.
When a neutron collides with a nucleus, the nucleus can absorb the neutron. When an atom contains an extra neutron, it becomes an isotope with extra energy in a process called “neutron capture.” In many cases, an atom with an extra neutron will become an unstable isotope, or radioisotope. In order to return to a state of stability, it must expel the extra energy the neutron gave it.
Different atoms react with neutrons in different ways depending on their atomic makeup. One way unstable isotopes shed this energy is by releasing a short burst of gamma radiation. Atoms can also expel this energy and return to a more stable state by releasing beta particles, alpha particles, neutrons, or smaller atoms (i.e. fission byproducts). For example, when fissile uranium is bombarded with neutrons, it releases more neutrons along with smaller elements. Specific nuclides, or atoms characterized by the number of protons and neutrons in their nuclei, are especially of interest when it comes to neutron irradiation.
In other words, when you bombard a material with neutrons, some of the atoms comprising the material will absorb some of the neutrons and become an unstable radionuclide. In order to return to a state of stability, the radionuclide must release radiation to expel the excess energy. As a result, the material will be, for a short period, somewhat radioactive. How radioactive it will be, and how long it will stay radioactive, depends on the nature of the radionuclides produced by neutron irradiation. The byproducts of neutron activation can remain radioactive for anywhere from a few fractions of a second to a few years.
Certain materials are more conducive to neutron activation than others. Some substances are very difficult to activate and will not react very much to neutrons. Other materials might react much, much more strongly and readily. How much a material might react to neutron irradiation is an extremely important fact to keep in mind when developing materials for use in high-radiation environments. Materials that are activated can undergo negative transformations due to the effects of radiation and become weaker or less suitable for their roles in critical components, resulting in complications and hazardous situations for personnel or for the environment.