Neutron activation is the process in which

neutron radiation Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new isotopes— ...

induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emitting gamma rays, or particles such as beta particles, alpha particles, fission products, and neutrons (in nuclear fission). Thus, the process of 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. Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. All naturally occurring materials, including air, water, and soil, can be induced (activated) by neutron capture into some amount of radioactivity in varying degrees, as a result of the production of neutron-rich

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s. Some atoms require more than one neutron to become unstable, which makes them harder to activate because the probability of a double or triple capture by a nucleus is below that of single capture. Water, for example, is made up of hydrogen and oxygen.

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

requires a double capture to attain instability as

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

(

hydrogen-3 Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

), while natural

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

( oxygen-16) requires three captures to become unstable oxygen-19. Thus water is relatively difficult to activate, as compared to sodium chloride ( Na Cl), in which both the sodium and chlorine atoms become unstable with a single capture each. These facts were experienced first-hand at the ''

Operation Crossroads Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the ...

'' atomic test series in 1946.

Examples

An example of this kind of a nuclear reaction occurs in the production of cobalt-60 within a

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

: The cobalt-60 then decays by the emission of a beta particle plus gamma rays into

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

-60. This reaction has a half-life of about 5.27 years, and due to the availability of

cobalt-59 Naturally occurring cobalt (Co) consists of a single stable isotope, Co. Twenty-eight radioisotopes have been characterized; the most stable are Co with a half-life of 5.2714 years, Co (271.8 days), Co (77.27 days), and Co (70.86 days). All other ...

(100% of its natural abundance), this neutron bombarded isotope of

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...

is a valuable source of nuclear radiation (namely gamma 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 stable isotope of

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...

lithium-7 Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant on Earth. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon ( for lit ...

, is bombarded with fast neutrons and undergoes the following nuclear reaction: : + → + + + gamma rays + kinetic energy In other words, the capture of a neutron by lithium-7 causes it to split into an energetic

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

nucleus ( alpha particle), a

(

) nucleus and a free neutron. The

Castle Bravo Castle Bravo was the first in a series of high-yield thermonuclear weapon design tests conducted by the United States at Bikini Atoll, Marshall Islands, as part of '' Operation Castle''. Detonated on March 1, 1954, the device was the most powerful ...

accident, in which the thermonuclear bomb test at

Bikini Atoll Bikini Atoll ( or ; Marshallese: , , meaning "coconut place"), sometimes known as Eschscholtz Atoll between the 1800s and 1946 is a coral reef in the Marshall Islands consisting of 23 islands surrounding a central lagoon. After the Seco ...

in 1954 exploded with 2.5 times the expected yield, was caused by the unexpectedly high probability of this reaction. In the areas around a pressurized water reactors or

boiling water reactor A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nu ...

s during normal operation, a significant amount of radiation is produced due to the

fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

activation of coolant water oxygen via a (n,p) reaction. The activated oxygen-16 nucleus emits a proton (hydrogen nucleus), and transmutes to nitrogen-16, which has a very short life (7.13 seconds) before decaying back to oxygen-16 (emitting 6.13 MeV beta particles). : + → + (Decays rapidly) : → + + This activation of the coolant water requires extra biological shielding around the nuclear reactor plant. It is the high energy gamma ray in the second reaction that causes the major concern. This is why water that has recently been inside a nuclear reactor core must be shielded until this radiation subsides. One to two minutes is generally sufficient. In facilities that housed a cyclotron, the reinforced concrete foundation can become radioactive due to neutron activation. Six important long-lived radioactive isotopes (⁵⁴Mn, ⁵⁵Fe, ⁶⁰Co, ⁶⁵Zn, ¹³³Ba, and ¹⁵²Eu) can be found within concrete nuclei affected by neutrons. The residual radioactivity is predominantly due to trace elements present, and thus the amount of radioactivity derived from cyclotron activation is minuscule, i.e., pCi/g or Bq/g. The release limit for facilities with residual radioactivity is 25 mrem/year. An example of ⁵⁵Fe production from iron rebar activation is shown below: : + →

Occurrence

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, in an active nuclear reactor, or in a spallation neutron source. 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 is only just starting to be affected by the explosion within it. The neutron activation of the soon-to-be vaporized metal is responsible for a significant portion of the

nuclear fallout Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...

in nuclear bursts high in the atmosphere. In other types of activation, neutrons may irradiate soil that is dispersed in a mushroom cloud at or near the Earth's surface, resulting in fallout from activation of soil chemical elements.

Effects on materials over time

In any location with high

neutron flux The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travellin ...

es, 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 (see International Fusion Materials Irradiation Facility). For example, Chromium-51 will form by neutron activation in chrome steel (which contains Cr-50) that is exposed to a typical reactor neutron flux. Carbon-14, most frequently but not solely, generated by the neutron activation of atmospheric nitrogen-14 with a thermal neutron, is (together with its dominant natural production pathway from cosmic ray-air interactions and historical production from

atmospheric nuclear testing Nuclear weapons tests are experiments carried out to determine nuclear weapons' effectiveness, yield, and explosive capability. Testing nuclear weapons offers practical information about how the weapons function, how detonations are affected by ...

) also generated in comparatively minute amounts inside many designs of nuclear reactors which contain nitrogen gas impurities in their fuel cladding, coolant water and by neutron activation of the oxygen contained in the water itself.

Fast breeder reactor A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile mater ...

s (FBR) produce about an order of magnitude less C-14 than the most common reactor type, the pressurized water reactor, as FBRs do not use water as a primary coolant.

Uses

Radiation safety

For physicians and radiation safety officers, activation of sodium in the human body to sodium-24, and phosphorus to phosphorus-32, can give a good immediate estimate of acute accidental neutron exposure.

Neutron detection

One way to demonstrate that

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

has occurred inside a fusor device is to use a Geiger counter to measure the gamma ray radioactivity that is produced from a sheet of aluminium foil. In the ICF fusion approach, the fusion yield of the experiment (directly proportional to neutron production) is usually determined by measuring the gamma-ray emissions of aluminium or copper neutron activation targets. Aluminium can capture a neutron and generate radioactive sodium-24, which has a half life of 15 hours and a beta decay energy of 5.514 MeV. The activation of a number of test target elements such as sulfur, copper,

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

, and gold have been used to determine the yield of both pure fission and thermonuclear weapons.

Materials analysis

Neutron activation analysis is one of the most sensitive and precise 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. Neutron activation analysis can be done in situ. For example, aluminium (Al-27) can be activated by capturing relatively low-energy neutrons to produce the isotope Al-28, which decays with a half-life of 2.3 minutes with a decay energy of 4.642 MeV. This activated isotope is used in oil drilling to determine the

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

content (clay is generally an alumino-silicate) of the underground area under exploration. Historians can use accidental neutron activation to authenticate atomic artifacts and materials subjected to neutron fluxes from fission incidents. For example, one of the fairly unique isotopes found in

trinitite Trinitite, also known as atomsite or Alamogordo glass, is the glassy residue left on the desert floor after the plutonium-based Trinity nuclear bomb test on July 16, 1945, near Alamogordo, New Mexico. The glass is primarily composed of arkosic sa ...

, and therefore with its absence likely signifying a fake sample of the mineral, is a barium neutron activation product, the barium in the Trinity device coming from the slow explosive

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

employed in the device, known as

Baratol Baratol is an explosive made of a mixture of TNT and barium nitrate, with a small quantity (about 1%) of paraffin wax used as a phlegmatizing agent. TNT typically makes up 25% to 33% of the mixture. Because of the high density of barium nitrate, ...

Semiconductor production

Neutron irradiation may be used for float-zone silicon slices (

wafers A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...

) to trigger fractional transmutation of Si atoms into phosphorus (P) and therefore doping it into n-type silicon :

\ce + neutron \xrightarrow \ce + \gamma-ray\xrightarrow\ce + \beta-ray

References

External links

Neutron Activation Analysis webHandbook on Nuclear Activation Cross-Sections, IAEA, 1974
* ttps://web.archive.org/web/20150924072849/http://www.physics.rutgers.edu/~abragg/110/lecture16.html Neutron capture as it relates to nucleosynthesisbr>Neutron capture and the Chart of the nuclidesThe chart of the NuclidesDiscovery of the Chromium isotopes, Chromium-55 by Cr-54 neutron captureORILL
: 1D transmutation, fuel depletion, and radiological protection code