Neutron irradiation

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. Thus 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
27
Co
+ n → 60
27
Co

The cobalt-60 then 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 due to the availability of cobalt-59 (100% of its natural abundance) this neutron bombarded isotope of cobalt is a valuable source of nuclear radiation (namely gamma radiation) for radiotherapy.[1]

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
4
Be
+ n → 2(4
2
He
) + 2n + energy

In other words, the capture of a neutron by beryllium-9 causes it to split into two energetic helium-4 nuclei (alpha particles) plus two free neutrons. Similarly, any nuclear reaction or decay that produces beryllium-8 causes its immediate splitting into two helium-4 nuclei because a 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; periodically the lining 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 this problem and the risk of a meltdown. 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 from a sheet of aluminum foil.

In a nuclear disaster such as the radioactive fallout from a meltdown or from a fissile nuclear weapon it can be seen from the above that ongoing nuclear reactions can continue as a result of the neutron activation of the bomb itself, all surrounding materials and the produced fission products.

Some elements are very difficult to activate, because the capture of a neutron by the most common isotopes of those elements converts the atom into another, heavier stable isotope. The primary elements this is true for are hydrogen, helium, carbon, nitrogen, oxygen, neon, silicon, sulfur, titanium, chromium, iron and platinum, while magnesium, krypton and mercury yield either only long half-life isotopes or only activate 10% or less of the common atoms. These materials can only be activated by sufficient neutron flux to cause more than one neutron to be captured per nucleus; in some cases, like oxygen, three must be captured. The implications of this are significant because activation of by far the largest segments of the hydrosphere and the atmosphere, and a significant fraction of the lithosphere, is very difficult to achieve. Steam and water may be radioactive only because of dissolved or mechanically mixed contaminants, not because of activation of the water itself. Distillation is very effective in decontaminating such water. Finally, the largest part of life chemistry is also immune from neutron activation (which is not to say that a human is protected from neutron radiation).

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.

See also

References

External links

  • Handbook on Nuclear Activation Cross-Sections, IAEA, 1974
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