Neutron activation
Encyclopedia
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 state
s. The excited nucleus often decays immediately by emitting particles such as neutrons, proton
s, or alpha particle
s. 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
:
The cobalt-60 decays by the emission of a beta particle
plus gamma ray
s 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:
In other words, the capture of the neutron by beryllium-9 causes it to split into two energetic helium-4
nuclear (alpha particle
s) 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 flux
es, 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 product
s.
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.
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
induces radioactivity in materials, and occurs when atomic nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
capture free neutrons, becoming heavier and entering excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s. The excited nucleus often decays immediately by emitting particles such as neutrons, proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s, or alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s. The neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
, even after any intermediate decay, often results in the formation of an unstable activation product
Activation product
Activation products are materials made radioactive by neutron activation.Fission products and actinides produced by neutron absorption of nuclear fuel itself are normally referred to by those specific names, and activation product reserved for products of neutron capture by other materials, such as...
. Such radioactive nuclei can exhibit half-lives
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
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
Cobalt-60
Cobalt-60, , is a synthetic radioactive isotope of cobalt. Due to its half-life of 5.27 years, is not found in nature. It is produced artificially by neutron activation of . decays by beta decay to the stable isotope nickel-60...
within a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
:
- + n →
The cobalt-60 decays by the emission of a beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
plus gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s into nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-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
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the neutron, the capture of a neutron can cause nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
- 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
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, beryllium-9, is bombarded with fast neutrons and undergoes the following nuclear reaction:
- + n → 2 + 2n + energy
In other words, the capture of the neutron by beryllium-9 causes it to split into two energetic helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
nuclear (alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s) 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 flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
es, 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
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...
. 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
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
has occurred inside a fusor device
Fusor
The Farnsworth–Hirsch fusor, or simply fusor, is an apparatus designed by Philo T. Farnsworth to create nuclear fusion. It has also been developed in various incarnations by researchers including Elmore, Tuck, and Watson, and more recently by George H. Miley and Robert W. Bussard...
is to use a Geiger counter
Geiger counter
A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a...
to measure the radioactivity that is produced in a sheet of aluminum foil.
The lasting radiation from a nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
is in part due to the neutron activation of the bomb itself and the surrounding material, in addition to fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s.
Neutron activation also has a practical use. Neutron activation analysis
Neutron activation analysis
In chemistry, neutron activation analysis is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on...
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.