Neutron capture

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Chart of nuclides showing thermal neutron capture cross section values Chart of Nuclides - Thermal neutron capture cross sections.png
Chart of nuclides showing thermal neutron capture cross section values

Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. [1] Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically. [1]

Nuclear reaction process in which two nuclei collide to produce one or more nuclides

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 one or more nuclides that are different from the nuclide(s) that began the process. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.

Atomic nucleus core of the atom; composed of bound nucleons (protons and neutrons)

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. An atom is composed of a positively-charged nucleus, with a cloud of negatively-charged electrons surrounding it, bound together by electrostatic force. Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force.

Neutron nucleon (constituent of the nucleus of the atom) that has neutral electric charge (no charge); symbol n

The neutron is a subatomic particle, symbol
n
or
n0
, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics.

Contents

Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways: as a rapid (r-process) or a slow process (s-process). [1] Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by nuclear fusion), but can be formed by neutron capture. [1] Neutron capture on protons yields a line at 2.223 MeV predicted [2] and commonly observed [3] in solar flares.

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons, primarily protons and neutrons. The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis. Seventeen minutes later the universe had cooled to a point at which these processes ended, so only the fastest and simplest reactions occurred, leaving our universe containing about 75% hydrogen, 24% helium, and traces of other elements such as lithium and the hydrogen isotope deuterium. The universe still has approximately the same composition today.

<i>r</i>-process

The rapid neutron-capture process, or so-called r-process, is a set of nuclear reactions that in nuclear astrophysics is responsible for the creation (nucleosynthesis) of approximately half the abundances of the atomic nuclei heavier than iron, usually synthesizing the entire abundance of the two most neutron-rich stable isotopes of each heavy element. Chemical elements heavier than iron typically are enabled by the force between nucleons to be capable of six to ten stable isotopic forms having the same nuclear charge Z but differing in neutron number N, each of whose natural abundances contribute to the natural abundance of the chemical element. Each isotope is characterized by the number of neutrons that it contains. The r-process typically synthesizes new nuclei of the heaviest four isotopes of any heavy element, being totally responsible for the abundances of its two heaviest isotopes, which are referred to as r-only nuclei. The most abundant of these contribute to the r-process abundance peaks near atomic weights A = 82, A = 130 and A = 196.

The slow neutron-capture process or s-process is a series of reactions in nuclear astrophysics that occur in stars, particularly AGB stars. The s-process is responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron.

Neutron capture at small neutron flux

Decay scheme of Au Au-198 Decay Scheme.svg
Decay scheme of Au

At small neutron flux, 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, and quickly decays to the ground state of 198Au by the emission of γ rays. In this process, the mass number increases by one. This is written as a formula in the form 197Au+n → 198Au+γ, or in short form 197Au(n,γ)198Au. If thermal neutrons are used, the process is called thermal capture.

Neutron flux

The neutron flux 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 travelling through a small sphere of radius in a time interval, divided by and by the time interval. The usual unit is cm−2s−1.

Nuclear reactor device to initiate and control a sustained nuclear chain reaction

A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research. As of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world.

Gold Chemical element with atomic number 79

Gold is a chemical element with symbol Au and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the native element silver and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium.

The isotope 198Au is a beta emitter that decays into the mercury isotope 198Hg. In this process the atomic number rises by one.

Beta decay decay where electrons (β-, beta minus) or positrons (β+, positron emission) are emitted

In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta ray is emitted from an atomic nucleus. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino, or conversely a proton is converted into a neutron by the emission of a positron with a neutrino, thus changing the nuclide type. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. For either electron or positron emission to be energetically possible, the energy release or Q value must be positive.

Atomic number number of protons found in the nucleus of an atom

The atomic number or proton number of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.

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-section 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).

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have exactly 8 protons.

A barn is a unit of area equal to 10−28 m2 (100 fm2). Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also 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 probability of interaction between small particles. A barn is approximately the cross-sectional area of a uranium nucleus. The barn is also the unit of area used in nuclear quadrupole resonance and nuclear magnetic resonance to quantify the interaction of a nucleus with an electric field gradient. While the barn is not an SI unit, the SI standards body acknowledges its existence due to its continued use in particle physics.

Absorption cross section is often highly dependent on neutron energy. As a generality, the likelihood of absorption is proportional to the time the neutron is in the vicinity of the nucleus. The time spent in the vicinity of the nucleus is inversely proportional to the relative velocity between the neutron and nucleus. Other more specific issues modify this general principle. 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.

Thermochemical significance

Neutron capture is involved in the formation of isotopes of chemical elements. As a consequence of this fact the energy of neutron capture intervenes in the standard enthalpy of formation of isotopes.

Uses

Neutron activation analysis 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

Neutron cross section of boron (top curve is for B and bottom curve for B) Neutroncrosssectionboron.png
Neutron cross section of boron (top curve is for B and bottom curve for B)

The most important neutron absorber is 10 B as 10B4C in control rods, or boric acid as a coolant water additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are xenon, cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, molybdenum and ytterbium; [4] all of which usually consist of mixtures of various isotopes—some of which are excellent neutron-absorbers. These also occur in combinations such as Mo2B5, hafnium diboride, titanium diboride, dysprosium titanate and gadolinium titanate.

Hafnium, one of the last stable elements to be discovered, presents an interesting case. Even though hafnium is a heavier 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 more than Zr), and it can be used in reactor control rods, whereas natural zirconium is practically transparent to neutrons. So, zirconium is a very desirable construction material for reactor internal parts, including the metallic cladding of the fuel rods which contain either uranium, plutonium, or mixed oxides of the two elements (MOX fuel).

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 resins. [5] Similar resins are also used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, and sometimes thorium.

See also

Related Research Articles

Nuclear fission nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller, lighter nuclei. The fission process often produces free neutrons and gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

Beta particle ionizing radiation

A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β decay and β+ decay, which produce electrons and positrons respectively.

Neutron moderator medium that reduces the speed of fast neutrons, turning them into thermal neutrons that can sustain nuclear chain reactions; e.g. water, graphite, heavy water, beryllium

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide.

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

Control rod

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning. Because these elements have different capture cross sections for neutrons of varying energies, the composition of the control rods must be designed for the reactor's neutron spectrum. Boiling water reactors (BWR), pressurized water reactors (PWR) and heavy water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons.

Neutron activation process in which neutron radiation induces radioactivity in materials

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 gamma rays, or particles such as beta particles, alpha particles, fission products, and neutrons. 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.

Nuclear fuel material that can be used in nuclear fission or fusion to derive nuclear energy

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

Neutron cross section scalar physical quantity

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 a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10−28 m2 or 10−24 cm2. The larger the neutron cross section, the more likely a neutron will react with the nucleus.

Uranium (92U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes that have long half-lives and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The standard atomic weight of natural uranium is 238.02891(3). Other isotopes such as uranium-232 have been produced in breeder reactors.

Naturally occurring zirconium (40Zr) is composed of four stable isotopes (of which one may in the future be found radioactive), and one very long-lived radioisotope (96Zr), a primordial nuclide that decays via double beta decay with an observed half-life of 2.0×1019 years; it can also undergo single beta decay, which is not yet observed, but the theoretically predicted value of t1/2 is 2.4×1020 years. The second most stable radioisotope is 93Zr, which has a half-life of 1.53 million years. Twenty-seven other radioisotopes have been observed. All have half-lives less than a day except for 95Zr (64.02 days), 88Zr (83.4 days), and 89Zr (78.41 hours). The primary decay mode is electron capture for isotopes lighter than 92Zr, and the primary mode for heavier isotopes is beta decay.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states, though none are very stable; all meta states have half-lives of less than one second.

Nuclear binding energy energy required to split a nucleus of an atom into its component parts.

Nuclear binding energy is the minimum energy that would be required to disassemble the nucleus of an atom into its component parts. These component parts are neutrons and protons, which are collectively called nucleons. The binding is always a positive number, as we need to spend energy in moving these nucleons, attracted to each other by the strong nuclear force, away from each other. The mass of an atomic nucleus is less than the sum of the individual masses of the free constituent protons and neutrons, according to Einstein's equation E=mc2. This 'missing mass' is known as the mass defect, and represents the energy that was released when the nucleus was formed.

Neutron temperature

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 a certain temperature. The neutron energy distribution is then adapted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. The momentum and wavelength of the neutron are related through the De Broglie relation. The large wavelength of slow neutrons allows for the large cross section.

A nucleogenic isotope, or nuclide, is one that is produced by a natural terrestrial nuclear reaction, other than a reaction beginning with cosmic rays. The nuclear reaction that produces nucleogenic nuclides is usually interaction with an alpha particle or the capture of fission or thermal neutron. Some nucleogenic isotopes are stable and others are radioactive.

Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.

Nuclear transmutation

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Because any element is defined by its number of protons in its atoms, i.e. in the atomic nucleus, nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus is changed.

References

  1. 1 2 3 4 Ahmad, Ishfaq; Hans Mes; Jacques Hebert (1966). "Progress of theoretical physics: Resonance in the Nucleus". Institute of Physics. 3 (3): 556–600.
  2. Morrison, P. (1958). "On gamma-ray astronomy". Il Nuovo Cimento. 7 (6): 858–865. Bibcode:1958NCim....7..858M. doi:10.1007/BF02745590.
  3. Chupp, E.; et al. (1973). "Solar Gamma Ray and Neutron Observations". NASA Special Publication. 342: 285. Bibcode:1973NASSP.342..285C.
  4. Prompt Gamma-ray Neutron Activation Analysis. International Atomic Energy Agency
  5. D. Franklin; R. B. Adamson (1 January 1984). Zirconium in the Nuclear Industry: Sixth International Symposium. ASTM International. pp. 26–. ISBN   978-0-8031-0270-5 . Retrieved 7 October 2012.