Neutron activation

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha particles. The neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years. An example of this kind of a nuclear reaction occurs in the production of cobalt-60 within a nuclear reactor:

59
27Co + n → 60
27Co

The cobalt-60 decays by the emission of a beta particle plus gamma rays into nickel-60. This reaction has a half-life of about 5.27 years, and cobalt-60 is a valuable source of nuclear radiation for radiotherapy.

In other cases, and depending on the kinetic energy of the neutron, the capture of a neutron can cause nuclear fission - the splitting of the atomic nucleus into two smaller nuclei. If the fission requires an input of energy, that comes from the kinetic energy of the neutron. An example of this kind of fission in a light element can occur when the only stable isotope of beryllium, beryllium-9, is bombarded with fast neutrons and undergoes the following nuclear reaction:

9
4Be + n → 2(4
2He) + 2n + energy

In other words, the capture of the neutron by beryllium-9 causes it to split into two energetic helium-4 nuclear (alpha particles) plus two free neutrons. Actually, any nuclear reaction that produces beryllium-8 causes the immediate splitting into two helium-4 nuclei because the beryllium-8 nucleus is extremely unstable.

In any location with high neutron fluxes, such as within the cores of nuclear reactors, neutron activation contributes to material erosion, and the materials themselves must be disposed of as low-level radioactive waste. Some materials are more subject to neutron activation than others, so a suitably chosen low-activation material can significantly reduce these problems. One way to demonstrate that nuclear fusion has occurred inside a fusor device is to use a Geiger counter to measure the radioactivity that is produced in a sheet of aluminum foil.

The lasting radiation from a nuclear weapon is in part due to the neutron activation of the bomb itself and the surrounding material, in addition to fission products.

Neutron activation also has a practical use. Neutron activation analysis is one of the most sensitive and accurate methods of trace element analysis. It requires no sample preparation or solubilization and can therefore be applied to objects that need to be kept intact such as a valuable piece of art. Although the activation induces radioactivity in the object, its level is typically low and its lifetime may be short, so that its effects soon disappear. In this sense, neutron activation is a non-destructive analysis method.

External links

• Handbook on Nuclear Activation Cross-Sections, IAEA, 1974
• Innocence Network and National Association of Criminal Defense Lawyers Announce Joint Task Force to Review Cases Impacted by Discredited FBI Bullet Analysis, (November 19, 2007)