Neutron emission is a mode of

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

in which one or more

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s are ejected from a

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

. It occurs in the most neutron-rich/proton-deficient

nuclides A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...

, and also from excited states of other nuclides as in photoneutron emission and beta-delayed neutron emission. As only a neutron is lost by this process the number of

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s remains unchanged, and an atom does not become an atom of a different element, but a different

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

of the same element. Neutrons are also produced in the spontaneous and induced fission of certain heavy nuclides.

Spontaneous neutron emission

As a consequence of the

Pauli exclusion principle In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulated ...

, nuclei with an excess of protons or neutrons have a higher average energy per nucleon. Nuclei with a sufficient excess of neutrons have a greater energy than the combination of a free neutron and a nucleus with one less neutron, and therefore can decay by neutron emission. Nuclei which can decay by this process are described as lying beyond the

neutron drip line The nuclear drip line is the boundary beyond which atomic nuclei decay by the emission of a proton or neutron. An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus. One can think of moving up and/or t ...

. Two examples of isotopes that emit neutrons are beryllium-13 (decaying to beryllium-12 with a mean life ) and

helium-5 Although there are nine known isotopes of helium (2He) ( standard atomic weight: ), only helium-3 () and helium-4 () are stable. All radioisotopes are short-lived, the longest-lived being with a half-life of . The least stable is , with a half- ...

(

helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consis ...

, ). In tables of nuclear decay modes, neutron emission is commonly denoted by the abbreviation ''n''. :

Double neutron emission

Some neutron-rich isotopes decay by the emission of two or more neutrons. For example hydrogen-5 and helium-10 decay by the emission of two neutrons, hydrogen-6 by the emission of 3 or 4 neutrons, and hydrogen-7 by emission of 4 neutrons.

Photoneutron emission

Some nuclides can be induced to eject a neutron by

gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...

. One such nuclide is ⁹Be; its photodisintegration is significant in nuclear astrophysics, pertaining to the abundance of beryllium and the consequences of the instability of ⁸Be. This also makes this isotope useful as a neutron source in nuclear reactors. Another nuclide, ¹⁸¹Ta, is also known to be readily capable of photodisintegration; this process is thought to be responsible for the creation of ^180mTa, the only primordial

nuclear isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state, higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited ...

and the rarest primordial nuclide.

Beta-delayed neutron emission

Neutron emission usually happens from nuclei that are in an excited state, such as the excited ¹⁷O* produced from the beta decay of ¹⁷N. The neutron emission process itself is controlled by the

nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nucle ...

and therefore is extremely fast, sometimes referred to as "nearly instantaneous". This process allows unstable atoms to become more stable. The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons.

Delayed neutrons in reactor control

Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as fission products. These neutrons are sometimes emitted with a delay, giving them the term

delayed neutron In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event, by one of the fission products (or actually, a fission product daughter after beta decay), any time from a few milliseconds to a few minutes after the ...

s, but the actual delay in their production is a delay waiting for the

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay, which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds. Nevertheless, the delayed neutrons emitted by neutron-rich

fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

s aid control of

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 from nu ...

s by making reactivity change far more slowly than it would if it were controlled by prompt neutrons alone. About 0.65% of neutrons are released in a

nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...

in a delayed way due to the mechanism of neutron emission, and it is this fraction of neutrons that allows a nuclear reactor to be controlled on human reaction time-scales, without proceeding to a

prompt critical In nuclear engineering, prompt criticality describes a nuclear fission event in which criticality (the threshold for an exponentially growing nuclear fission chain reaction) is achieved with prompt neutrons alone (neutrons that are released immed ...

state, and runaway melt down.

Neutron emission in fission

Induced fission

A synonym for such neutron emission is "

prompt neutron In nuclear engineering, a prompt neutron is a neutron immediately emitted (neutron emission) by a nuclear fission event, as opposed to a delayed neutron decay which can occur within the same context, emitted after beta decay of one of the fissi ...

" production, of the type that is best known to occur simultaneously with induced

nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...

. Induced fission happens only when a nucleus is bombarded with neutrons, gamma rays, or other carriers of energy. Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process,

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

Spontaneous fission

Spontaneous fission happens when a nucleus splits into two (occasionally

three 3 is a number, numeral, and glyph. 3, three, or III may also refer to: * AD 3, the third year of the AD era * 3 BC, the third year before the AD era * March, the third month Books * '' Three of Them'' (Russian: ', literally, "three"), a 1901 ...

) smaller nuclei and generally one or more neutrons.

References

External links

"Why Are Some Atoms Radioactive?"
EPA. Environmental Protection Agency, n.d. Web. 31 Oct. 2014
The LIVEChart of Nuclides - IAEA
with filter on delayed neutron emission decay
Nuclear Structure and Decay Data - IAEA
with query on Neutron Separation Energy {{Nuclear processes Emission Nuclear physics Radioactivity