Neutron capture

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Neutron capture

Neutron capture is a kind of nuclear reaction

Nuclear reaction

In nuclear physics, a nuclear reaction is the process in which two atomic nucleus or subatomic particles collide to produce products different from the initial particles....
in which an atomic nucleus

Atomic nucleus

The nucleus of an atom is the very dense region, consisting of nucleons , at the center of an atom. Although the size of the nucleus varies considerably according to the mass of the atom, the size of the entire atom is comparatively constant....
collides with one or more neutron

Neutron

The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutrons are usually found in atomic nucleus....
s and they merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than charged particles which are repelled by electrostatic

Electrostatics

Electrostatics is the branch of science that deals with the phenomena arising from stationary or slowly moving electric charges.Since classical antiquity it was known that some materials such as amber attract light particles after Triboelectric effect....
repulsion.

Neutron capture plays an important role in the cosmic nucleosynthesis

Nucleosynthesis

Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons . It is thought that the primordial nucleons themselves were formed from the quark-gluon plasma from the Big Bang as it cooled below ten million 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 occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich Atomic nucleus that are Heavy metals....
) 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....
).

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Encyclopedia

Neutron capture is a kind of nuclear reaction

Nuclear reaction

Atomic nucleus

Neutron

Electrostatics

Nucleosynthesis

R-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....
). By neutron capture, nuclei of masses greater than 56 can be formed that could not be formed by thermonuclear reactions, i.e., by nuclear fusion

Nuclear fusion

In nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus....
.

Neutron capture at small neutron flux

At small neutron flux

Neutron flux

Neutron flux is a term referring to the number of neutrons passing through an area over a span of time. It is most commonly measured in neutrons/....
, as in a nuclear reactor

Nuclear reactor

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explosion....
, a single neutron is captured by a nucleus. For example, when natural gold

Gold

Gold is a chemical element with the symbol Au and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history....
(¹⁹⁷Au) is irradiated by neutrons, the isotope

Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
¹⁹⁸Au is formed in a highly excited state which then quickly decays to the ground state of ¹⁹⁸Au by the emission of ? rays

Gamma ray

Gamma rays are a form of electromagnetic radiation produced by atom particle interactions, such as electron-positron annihilation or radioactive decay....
. 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....
increases by one. In terms of a formula, this is written ¹⁹⁷Au(n,?)¹⁹⁸Au. If thermal neutrons are used, this 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. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus ....
that decays into the mercury isotope ¹⁹⁸Hg (see decay scheme

Decay scheme

The Decay scheme of a Radioactive decay substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships....
). In this process, the atomic number

Atomic number

In chemistry and physics, the atomic number is the number of protons found in the atomic nucleus of an atom. It is conventionally represented by the symbol Z....
(the number of proton

Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
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 ß^- decays to stable or unstable nuclei of high atomic number.

Capture cross section

The absorption neutron cross-section

Neutron cross-section

The total neutron cross-section of an isotope of a chemical element is the effective cross section area that an atom of that isotope presents to neutron scattering and neutron absorption....
of an isotope of a chemical element

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
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. While the barn is not an SI unit, it is accepted for use with the SI. Originally used in nuclear physics for expressing the cross section area of nuclei and nuclear reactions, today it is used in all fields of particle physics to express the cross sections of any scattering process....
" (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 a species of atom characterized by the constitution of its Atomic nucleus and hence by the number of protons, the number of neutrons, and the energy content of the nucleus....
, 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 in which the heat movement of atoms or molecules shifts the apparent frequency of each emitter....
increases the chance of catching a resonance peak. In particular, the increase in U-238

U-238

U238 or U-238 may be:* German submarine U-238, a German World War II U-Boat * Uranium-238, the most common isotope of uranium...
'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 the same event/phenomenon in the present or future....
mechanism that helps keep nuclear reactors under control.

Uses

Neutron activation analysis

Neutron Activation Analysis is a nuclear reaction process used for determining certain concentrations of chemical element in a vast amount of materials....
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 capture

Neutron capture at small neutron flux

Neutron capture at high neutron flux

Capture cross section

Uses

See also