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. 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
27Co + n → 60
27Co
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 stable isotope of lithium, lithium-7, is bombarded with fast neutrons and undergoes the following nuclear reaction:
- 7
3Li + n → 4
2He + 3
1H + n + gamma rays + kinetic energy
In other words, the capture of a neutron by lithium-7 causes it to split into an energetic helium nuclei (alpha particle), a hydrogen-3 (tritium) nucleus and a free neutron. The Castle Bravo accident, in which the thermonuclear bomb test at Enewetak Atoll in 1954 exploded with 2.5 times the expected yield, was caused by the unexpectedly high probability of this reaction.
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.
Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. Neutrons are only free in quantity in the microseconds of a nuclear weapon's explosion and in an active nuclear reactor. Neutrons, like all forms of radioactivity, are shielded by any material, even plain air, in proportion to its thickness.
In an atomic weapon, neutrons are only generated for from 1 to 50 microseconds, but in huge numbers. Most are absorbed by the metallic bomb casing, which has only just started to be affected by the explosion within. The neutron activation of the soon-to-be vaporized metal is responsible for a good proportion of the nuclear fallout in the mushroom cloud.
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, but still 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 thereby difficult to achieve. Since water cannot be easily activated, it may only be made radioactive by materials dissolved or mechanically mixed with it (and for steam, only the mixing possibility applies). Distillation is very effective in decontaminating such water; evaporating off water safely reduces the bulk of aqueous radioactive wastes. Finally, the largest part of life chemistry is also immune from neutron activation (which does not mean that a human is thereby protected from neutron radiation).
In many cases of neutron activation of such stubborn elements, as for example with the eventual emission of radioactive hydrogen (tritium) from water in the reactor cooling inner loop, frequent changes of the inner loop water serves to avoid the problem by not allowing the buildup of deuterium, and therefore avoiding the irradiation of a one-step activation feedstock.
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.