Neutron capture
Encyclopedia
Neutron capture is a kind of nuclear reaction
in which an atomic nucleus
collides with one or more neutron
s and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged proton
s, which are repelled electrostatically
.
Neutron capture plays an important role in the cosmic nucleosynthesis
of heavy elements. In stars it can proceed in two ways - as a rapid process (an r-process
) or a slow process (an s-process
). Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by nuclear fusion
), but can be formed by neutron capture.
, as in a nuclear reactor
, a single neutron is captured by a nucleus. For example, when natural gold
(197Au) is irradiated by neutrons, the isotope
198Au is formed in a highly excited state which then quickly decays to the ground state of 198Au by the emission of γ rays
. In this process, the mass number
(the number of nucleons - both protons and neutrons) increases by one. In terms of a formula, this is written 197Au(n,γ)198Au. If thermal neutrons are used, the process is called thermal capture.
The isotope 198Au is a beta emitter
that decays into the mercury isotope 198Hg (see decay scheme
of this process). In this process, the atomic number
(the number of proton
s in the nucleus) rises by one.
The s-process mentioned above happens in the same way, but inside of stars.
of an isotope of a chemical element
is the effective cross sectional area that an atom of that isotope presents to absorption, and is a measure of the probability of neutron capture. It is usually measured in barns
(b).
Absorption cross section is often highly dependent on neutron energy. Two of the most commonly specified measures are the cross-section for thermal neutron absorption, and resonance integral which considers the contribution of absorption peaks at certain neutron energies specific to a particular nuclide
, usually above the thermal range, but encountered as neutron moderation slows the neutron down from an original high energy.
The thermal energy of the nucleus also has an effect; as temperatures rise, Doppler broadening
increases the chance of catching a resonance peak. In particular, the increase in uranium-238
's ability to absorb neutrons at higher temperatures (and to do so without fissioning) is a negative feedback
mechanism that helps keep nuclear reactors under control.
can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.
. An example of these is xenon-135
(half life about 9.1 hours), which absorbs a neutron to become the stable isotope xenon-136. Xenon-135 is formed in nuclear reactor
s when the splitting of uranium-235
, uranium-233
, or plutonium-239
, in a nuclear chain reaction
commonly leads to the production of some iodine-135. Iodine-135 soon undergoes nuclear decay, by emitting a beta particle
- with a quite short half-life
- to produce xenon-135.
The most important neutron absorber is boron
as B4C in control rod
s, or boric acid
as a coolant water
additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are cadmium
, hafnium
, gadolinium
, cobalt
, samarium
, titanium
, dysprosium
, erbium
, and europium
all of which usually consist of mixtures of various isotopes—some of which are excellent neutron-absorbers. For example the Advanced CANDU Reactor
uses a solution of gadolinium nitrate
to shutdown.
Hafnium
, one of the last stable elements to be discovered, presents an interesting case. Even though hafnium is a heaver element, its electron configuration makes it practically identical with the element zirconium
, and they are always found in the same ores. However, their nuclear properties are different in a profound way. Hafnium absorbs neutrons avidly (Hf absorbs 600 times better than Zr), and it can be used in reactor control rods, whereas natural zirconium is practically transparent to neutrons. Thus, zirconium is very desirable for use in the construction of reactors, including in such parts as the metal cladding of the fuel rods which contain either uranium, plutonium, or an alloy
of the two metals.
Hence, it is quite important to be able to separate the zirconium from the hafnium in their naturally-occurring alloy. This can only be done inexpensively by using modern chemical ion-exchange resin
s. Similar resins are also used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, and sometimes thorium
.
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
in which an atomic nucleus
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...
collides with one or more neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged 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, which are repelled electrostatically
Electrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....
.
Neutron capture plays an important role in the cosmic nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
of heavy elements. In stars it can proceed in two ways - as a rapid process (an r-process
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...
) or a slow process (an s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
). Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by 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...
), but can be formed by neutron capture.
Neutron capture at small neutron flux
At small neutron fluxNeutron 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....
, as in 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...
, a single neutron is captured by a nucleus. For example, when natural gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
(197Au) is irradiated by neutrons, the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
198Au is formed in a highly excited state which then quickly decays to the ground state of 198Au by the emission of γ rays
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...
. In this process, the mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
(the number of nucleons - both protons and neutrons) increases by one. In terms of a formula, this is written 197Au(n,γ)198Au. If thermal neutrons are used, the process is called thermal capture.
The isotope 198Au is a beta emitter
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
that decays into the mercury isotope 198Hg (see decay scheme
Decay scheme
The Decay scheme of a radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships.-Decay schemes of radioactive isotopes:...
of this process). In this process, the atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
(the number of 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 in the nucleus) rises by one.
The s-process mentioned above happens in the same way, but inside of stars.
Neutron capture at high neutron flux
The r-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission in between neutron captures. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. Only afterwards, the highly unstable nuclei decay via many β- decays to stable or unstable nuclei of high atomic number.Capture cross section
The absorption neutron cross-sectionNeutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...
of an isotope of a chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
is the effective cross sectional area that an atom of that isotope presents to absorption, and is a measure of the probability of neutron capture. It is usually measured in barns
Barn (unit)
A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
(b).
Absorption cross section is often highly dependent on neutron energy. Two of the most commonly specified measures are the cross-section for thermal neutron absorption, and resonance integral which considers the contribution of absorption peaks at certain neutron energies specific to a particular nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
, usually above the thermal range, but encountered as neutron moderation slows the neutron down from an original high energy.
The thermal energy of the nucleus also has an effect; as temperatures rise, Doppler broadening
Doppler broadening
In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...
increases the chance of catching a resonance peak. In particular, the increase in uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
's ability to absorb neutrons at higher temperatures (and to do so without fissioning) is a negative feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
mechanism that helps keep nuclear reactors under control.
Uses
Neutron activation analysisNeutron 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...
can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.
Neutron absorbers
The most prolific neutron absorbers are the radioactive isotopes of elements that happen to become (nearly) stable by absorbing one neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
. An example of these is xenon-135
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...
(half life about 9.1 hours), which absorbs a neutron to become the stable isotope xenon-136. Xenon-135 is formed in 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...
s when the splitting of uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
, uranium-233
Uranium-233
Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....
, or plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
, in a nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
commonly leads to the production of some iodine-135. Iodine-135 soon undergoes nuclear decay, by emitting 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...
- with a quite short half-life
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...
- to produce xenon-135.
The most important neutron absorber is boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
as B4C in control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s, or boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
as a coolant water
Nuclear reactor coolant
A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment....
additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
, gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, samarium
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, dysprosium
Dysprosium
Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...
, erbium
Erbium
Erbium is a chemical element in the lanthanide series, with the symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements on Earth...
, and europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
all of which usually consist of mixtures of various isotopes—some of which are excellent neutron-absorbers. For example the Advanced CANDU Reactor
Advanced CANDU Reactor
The Advanced CANDU Reactor is a Generation III+ nuclear reactor design and is a further development of existing CANDU reactors designed by Atomic Energy of Canada Limited. The ACR is a light-water-cooled reactor that incorporates features of both Pressurised Heavy Water Reactors and Advanced...
uses a solution of gadolinium nitrate
Gadolinium nitrate
Gadolinium nitrate is an inorganic compound of gadolinium. It is used as a water-soluble neutron poison in nuclear reactors. Gadolinium nitrate, like all nitrates, is an oxidizing agent.-Use:...
to shutdown.
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
, one of the last stable elements to be discovered, presents an interesting case. Even though hafnium is a heaver element, its electron configuration makes it practically identical with the element zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, and they are always found in the same ores. However, their nuclear properties are different in a profound way. Hafnium absorbs neutrons avidly (Hf absorbs 600 times better than Zr), and it can be used in reactor control rods, whereas natural zirconium is practically transparent to neutrons. Thus, zirconium is very desirable for use in the construction of reactors, including in such parts as the metal cladding of the fuel rods which contain either uranium, plutonium, or an alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
of the two metals.
Hence, it is quite important to be able to separate the zirconium from the hafnium in their naturally-occurring alloy. This can only be done inexpensively by using modern chemical ion-exchange resin
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
s. Similar resins are also used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, and sometimes thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
.
See also
- Beta decayBeta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
- Induced radioactivityInduced radioactivityInduced radioactivity occurs when a previously stable material has been made radioactive by exposure to specific radiation. Most radioactivity does not induce other material to become radioactive....
- List of particles
- Neutron emissionNeutron emissionNeutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are helium-5 and beryllium-13...
- Radioactive decayRadioactive decayRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
- Rays: αAlpha particleAlpha 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...
— βBeta particleBeta 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...
— γGamma rayGamma 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...
— δDelta rayA delta ray is sometimes used to describe any recoil particle caused by secondary ionization. The term was coined by J.J. Thomson. It is entirely unrelated to the family of subatomic particles named delta baryon.-Characteristics:...
— εEpsilon radiationEpsilon radiation, coined by J. J. Thomson, is tertiary radiation caused by secondary radiation . Epsilon rays are a form of particle radiation and are composed of electrons. The term is very rarely used today.-See also:* electron... - p-processP-processThe term p-process is used in two ways in the scientific literature concerning the astrophysical origin of the elements . Originally it referred to a proton capture process which is the source of certain, naturally occurring, proton-rich isotopes of the elements from selenium to mercury...
(proton capture)