Neutron

This article is about the subatomic particle. For other uses, seeNeutron (disambiguation).

Not to be confused withNeuronorNeutrino.

Neutron

Thequarkcontent of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated bygluons.
Classification	Baryon
Composition	1up quark,2down quarks
Statistics	Fermionic
Family	Hadron
Interactions	Gravity,weak,strong,electromagnetic
Symbol	n , n0 , N0
Antiparticle	Antineutron
Theorized	Ernest Rutherford(1920)
Discovered	James Chadwick(1932)
Mass	1.67492749804(95)×10−27kg 939.56542052(54)MeV/c2 1.00866491588(49)Da
Mean lifetime	878.4(5) s(free)
Electric charge	0e (−2±8)×10−22e(experimental limits)
Electric dipole moment	<1.8×10−26e⋅cm(experimental upper limit)
Electric polarizability	1.16(15)×10−3fm3
Magnetic moment	−0.96623650(23)×10−26J·T−1 −1.04187563(25)×10−3μB −1.91304273(45)μN
Magnetic polarizability	3.7(20)×10−4fm3
Spin	⁠1/2⁠ħ
Isospin	−⁠1/2⁠
Parity	+1
Condensed	I(JP) =⁠1/2⁠(⁠1/2⁠+)

Theneutronis asubatomic particle,symbol
n
or
n⁰
,which has a neutral (not positive or negative) charge, and amassslightly greater than that of aproton.Protons and neutrons constitute thenucleiofatoms.Since protons and neutrons behave similarly within the nucleus, they are both referred to asnucleons.Nucleons have a mass of approximately one atomic mass unit, ordalton(symbol: Da). Their properties and interactions are described bynuclear physics.Protons and neutrons are notelementary particles;each is composed of threequarks.

Thechemical propertiesof an atom are mostly determined by the configuration ofelectronsthat orbit the atom's heavy nucleus. The electron configuration is determined by the charge of the nucleus, which is determined by the number of protons, oratomic number.The number of neutrons is theneutron number.Neutrons do not affect the electron configuration.

Atoms of achemical elementthat differ only in neutron number are calledisotopes.For example,carbon,with atomic number 6, has an abundant isotopecarbon-12with 6 neutrons and a rare isotopecarbon-13with 7 neutrons. Some elements occur in nature with only onestable isotope,such asfluorine.Other elements occur with many stable isotopes, such astinwith ten stable isotopes, or with no stable isotope, such astechnetium.

The properties of an atomic nucleus depend on both atomic and neutron numbers. With their positive charge, the protons within the nucleus are repelled by the long-rangeelectromagnetic force,but the much stronger, but short-range,nuclear forcebinds the nucleons closely together. Neutrons are required for the stability of nuclei, with the exception of the single-protonhydrogennucleus. Neutrons are produced copiously innuclear fissionandfusion.They are a primary contributor to thenucleosynthesisof chemical elements withinstarsthrough fission, fusion, andneutron captureprocesses.

The neutron is essential to the production of nuclear power. In the decade after theneutron was discoveredbyJames Chadwickin 1932, neutrons were used to induce many different types ofnuclear transmutations.With the discovery ofnuclear fissionin 1938, it was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as anuclear chain reaction.These events and findings led to the first self-sustainingnuclear reactor(Chicago Pile-1,1942) and the firstnuclear weapon(Trinity,1945).

Dedicatedneutron sourceslikeneutron generators,research reactorsandspallation sourcesproduce free neutrons for use inirradiationand inneutron scatteringexperiments. A free neutron spontaneously decays to a proton, anelectron,and anantineutrino,with amean lifetimeof about 15 minutes. Free neutrons do not directly ionize atoms, but they do indirectly causeionizing radiation,so they can be a biological hazard, depending on dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused bycosmic rayshowers,and by the natural radioactivity of spontaneously fissionable elements in theEarth's crust.

Neutrons in an atomic nucleus

Anatomic nucleusis formed by a number of protons,Z(theatomic number), and a number of neutrons,N(theneutron number), bound together by thenuclear force.Protons and neutrons each have a mass of approximately onedalton.The atomic number determines thechemical propertiesof the atom, and the neutron number determines theisotopeornuclide.The terms isotope and nuclide are often usedsynonymously,but they refer to chemical and nuclear properties, respectively.Isotopes are nuclides with the same atomic number, but different neutron number. Nuclides with the same neutron number, but different atomic number, are calledisotones.Theatomic mass number,A,is equal to the sum of atomic and neutron numbers. Nuclides with the same atomic mass number, but different atomic and neutron numbers, are calledisobars.The mass of a nucleus is always slightly less than the sum of its proton and neutron masses: the difference in mass represents themass equivalentto nuclear binding energy, the energy which would need to be added to take the nucleus apart.

The nucleus of the most commonisotopeof thehydrogen atom(with thechemical symbol¹H) is a lone proton.The nuclei of the heavy hydrogen isotopesdeuterium(D or²H) andtritium(T or³H) contain one proton bound to one and two neutrons, respectively.All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons. The most common nuclide of the common chemical elementlead,²⁰⁸Pb, has 82 protons and 126 neutrons, for example.Thetable of nuclidescomprises all the known nuclides. Even though it is not a chemical element, the neutron is included in this table.

Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.

Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. They are therefore both referred to collectively asnucleons.The concept ofisospin,in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces.

Nuclear energy

Because of the strength of the nuclear force at short distances, the nuclearenergy bindingnucleons is many orders of magnitude greater than the electromagnetic energy binding electrons in atoms.Innuclear fission,the absorption of a neutron by some heavy nuclides (such asuranium-235) can cause the nuclide to become unstable and break into lighter nuclides and additional neutrons.The positively charged light nuclides, or "fission fragments", then repel, releasing electromagneticpotential energy.If this reaction occurs within a mass offissile material,the additional neutrons cause additional fission events, inducing a cascade known as anuclear chain reaction.For a given mass of fissile material, suchnuclear reactionsrelease energy that is approximately ten million times that from an equivalent mass of a conventional chemicalexplosive.Ultimately, the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible; most of the energy released from fission is the kinetic energy of the fission fragments.

Aschematicof thenucleus of an atomindicating
β⁻
radiation, the emission of a fast electron from the nucleus. The decay also creates an antineutrino (omitted) and converts a neutron to a proton within the nucleus.
Theinsetshows beta decay of a free neutron; an electron and antineutrino are created in this process.

Beta decay

Main article:Beta decay

Neutrons and protons within a nucleus behave similarly and can exchange their identities by similar reactions. These reactions are a form ofradioactive decayknown asbeta decay.Beta decay, in which neutrons decay to protons, or vice versa, is governed by theweak force,and it requires the emission or absorption of electrons and neutrinos, or their antiparticles.The neutron and proton decay reactions are:

n⁰
→
p⁺
+
e⁻
+
ν
_e

where
p⁺
,
e⁻
,and
ν
_edenote the proton, electron and electron anti-neutrinodecay products,and

p⁺
→
n⁰
+
e⁺
+
ν
_e

where
n⁰
,
e⁺
,and
ν
_edenote the neutron, positron and electron neutrino decay products.

The electron and positron produced in these reactions are historically known asbeta particles,denoted β⁻or β⁺respectively, lending the name to the decay process.In these reactions, the original particle is notcomposedof the product particles; rather, the product particles arecreatedat the instant of the reaction.

The "free" neutron

Main article:Free neutron decay

"Free" neutrons or protons are nucleons that exist independently, free of any nucleus.

The free neutron has a mass of939565413.3eV/c²,or939.5654133MeV/c².This mass is equal to1.674927471×10⁻²⁷kg,or1.00866491588Da.The neutron has a mean squareradiusof about0.8×10⁻¹⁵m,or0.8fm,and it is aspin-½fermion.The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced byelectric fields,whereas the neutron is unaffected by electric fields.But the neutron has amagnetic moment,so the neutron is influenced bymagnetic fields.The specific properties of the neutron are described below in theIntrinsic properties section.

Outside the nucleus, free neutrons undergo beta decay with amean lifetimeof about 14 minutes, 38 seconds,corresponding to ahalf-lifeof about 10 minutes, 11 s. The mass of the neutron is greater than that of the proton by1.29332MeV/c²,hence the neutron's mass provides energy sufficient for the creation of the proton, electron, and anti-neutrino. In the decay process, the proton, electron, and electron anti-neutrino conserve the energy, charge, andlepton numberof the neutron.The electron can acquire a kinetic energy up to0.782±0.013 MeV.

Still unexplained, different experimental methods for measuring the neutron's lifetime produce different values for it, the "bottle" and "beam" methods.The "bottle" method employs "cold" neutrons trapped in a bottle, while the "beam" method employs energetic neutrons in a particle beam. The measurements by the two methods have not been converging with time. The lifetime from the bottle method is presently 877.75 swhich is 10 seconds below the value from the beam method of 887.7 s

A small fraction (about one per thousand) of free neutrons decay with the same products, but add an extra particle in the form of an emitted gamma ray:

n⁰
→
p⁺
+
e⁻
+
ν
_e+
γ

Called a "radiative decay mode" of the neutron, the gamma ray may be thought of as resulting from an "internalbremsstrahlung"that arises from the electromagnetic interaction of the emitted beta particle with the proton.

A smaller fraction (about four per million) of free neutrons decay in so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but the electron fails to gain the13.6eVnecessary energy to escape the proton (theionization energyofhydrogen), and therefore simply remains bound to it, forming a neutralhydrogen atom(one of the "two bodies" ). In this type of free neutron decay, almost all of the neutrondecay energyis carried off by the antineutrino (the other "body" ). (The hydrogen atom recoils with a speed of only about (decay energy)/(hydrogen rest energy) times the speed of light, or250km/s.)

Neutrons and protons bound in a nucleus

Main articles:Atomic nucleusandNuclear physics

See also:Valley of stability,Beta-decay stable isobars,andNeutron emission

Neutrons are a necessary constituent of any atomic nucleus that contains more than one proton. As a result of their positive charges, interacting protons have a mutualelectromagnetic repulsionthat is stronger than their attractivenuclear interaction,so proton-only nuclei are unstable (seediprotonandneutron–proton ratio).Neutrons bind with protons and one another in the nucleus via thenuclear force,effectively moderating the repulsive forces between the protons and stabilizing the nucleus.Heavy nuclei carry a large positive charge, hence they require "extra" neutrons to be stable.

While a free neutron is unstable and a free proton is stable, within nuclei neutrons are often stable and protons are sometimes unstable. When bound within a nucleus, nucleons candecayby the beta decay process. The neutrons and protons in a nucleus form aquantum mechanical systemaccording to thenuclear shell model.Protons and neutrons of anuclideare organized into discrete hierarchicalenergy levelswith uniquequantum numbers.Nucleon decay within a nucleus can occur if allowed by basic energy conservation and quantum mechanical constraints. The decay products, that is, the emitted particles, carry away the energy excess as a nucleon falls from one quantum state to one with less energy, while the neutron (or proton) changes to a proton (or neutron).

For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state. In stable nuclei the possible lower energy states are all filled, meaning each state is occupied by a pair of protons, one withspinup, another with spin down. When all available proton states are filled, thePauli exclusion principledisallows the decay of a neutron to a proton.The situation is similar to electrons of an atom, where electrons that occupy distinctatomic orbitalsare prevented by the exclusion principle from decaying to lower, already-occupied, energy states.Thestability of matteris a consequences of these constraints.

One example of the decay of a neutron within a nuclide is thecarbonisotopecarbon-14,which has 6 protons and 8 neutrons. With its excess of neutrons, this isotope decays by beta decay tonitrogen-14(7 protons, 7 neutrons), a process with a half-life of about5,730 years.Nitrogen-14 is stable.

"Beta decay" reactions can also occur by the capture of aleptonby the nucleon. The transformation of a proton to a neutron inside of a nucleus is possible throughelectron capture:

p⁺
+
e⁻
→
n⁰
+
ν
_e

A rarer reaction,inverse beta decay,involves the capture of a neutrino by a nucleon. Rarer still, positron capture by neutrons can occur in the high-temperature environment of stars.

Competition of beta decay types

Three types of beta decay in competition are illustrated by the single isotopecopper-64(29 protons, 35 neutrons), which has a half-life of about 12.7 hours.This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay.This particular nuclide is almost equally likely to undergo proton decay (bypositron emission,18% or byelectron capture,43%; both forming⁶⁴
Ni
) or neutron decay (by electron emission, 39%; forming⁶⁴
Zn
).

The neutron in elementary particle physics - the Standard Model

The principalFeynman diagramfor
β⁻
decay of a neutron into a proton, electron, andelectron antineutrinovia an intermediate heavy
W⁻
boson

The principal Feynman diagram for
β⁺
decay of a proton into a neutron, positron, andelectron neutrinovia an intermediate heavy
W⁺
boson

Main article:Standard Model

Within the theoretical framework of the Standard Model for particle physics, a neutron comprises twodown quarkswith charge−⁠1/3⁠eand oneup quarkwith charge+⁠2/3⁠e.The neutron is therefore acomposite particleclassified as ahadron.The neutron is also classified as abaryon,because it is composed of threevalence quarks.The finite size of the neutron and its magnetic moment both indicate that the neutron is acomposite,rather thanelementary,particle.

The quarks of the neutron are held together by thestrong force,mediated bygluons.The nuclear force results fromsecondary effects of the more fundamental strong force.

The only possible decay mode for the neutron thatconservesbaryon numberis for one of the neutron's quarks tochangeflavourvia theweak interaction.The decay of one of the neutron's down quarks into a lighter up quark can be achieved by the emission of aW boson.By this process, the Standard Model description of beta decay, the neutron decays into a proton (which contains one down and two up quarks), an electron, and anelectron antineutrino.

The decay of the proton to a neutron occurs similarly through the weak force. The decay of one of the proton's up quarks into a down quark can be achieved by the emission of a W boson. The proton decays into a neutron, a positron, and an electron neutrino. This reaction can only occur within an atomic nucleus which has a quantum state at lower energy available for the created neutron.

Discovery

Main article:Discovery of the neutron

The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. In the 1911Rutherford model,the atom consisted of a small positively charged massive nucleus surrounded by a much larger cloud of negatively charged electrons. In 1920,Ernest Rutherfordsuggested that the nucleus consisted of positive protons and neutrally charged particles, suggested to be a proton and an electron bound in some way.Electrons were assumed to reside within the nucleus because it was known thatbeta radiationconsisted of electrons emitted from the nucleus.About the time Rutherford suggested the neutral proton-electron composite, several other publications appeared making similar suggestions, and in 1921 the American chemistW. D. Harkinsfirst named the hypothetical particle a "neutron".The name derives from theLatinroot forneutralis(neuter) and theGreeksuffix-on(a suffix used in the names of subatomic particles, i.e.electronandproton).References to the wordneutronin connection with the atom can be found in the literature as early as 1899, however.

Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons",but this raised obvious problems. It was difficult to reconcile the proton–electron model of the nucleus with theHeisenberg uncertainty relationof quantum mechanics.TheKlein paradox,discovered byOskar Kleinin 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus.Observed properties of atoms and molecules were inconsistent with the nuclear spin expected from the proton–electron hypothesis. Both protons and electrons carry an intrinsic spin of⁠1/2⁠ħ.Isotopes of the same species (i.e. having the same number of protons) can have both integer or fractional spin, i.e. the neutron spin must be also fractional (⁠1/2⁠ħ). But there is no way to arrange the spins of an electron and a proton (supposed to bond to form a neutron) to get the fractional spin of a neutron.

In 1931,Walther BotheandHerbert Beckerfound that ifAlpha particleradiation frompoloniumfell onberyllium,boron,orlithium,an unusually penetrating radiation was produced. The radiation was not influenced by an electric field, so Bothe and Becker assumed it wasgamma radiation.The following yearIrène Joliot-CurieandFrédéric Joliot-Curiein Paris showed that if this "gamma" radiation fell onparaffin,or any otherhydrogen-containing compound, it ejected protons of very high energy.Neither Rutherford norJames Chadwickat theCavendish LaboratoryinCambridgewere convinced by the gamma ray interpretation.Chadwick quickly performed a series of experiments that showed that the new radiation consisted of uncharged particles with about the same mass as the proton.These properties matched Rutherford's hypothesized neutron. Chadwick won the 1935Nobel Prize in Physicsfor this discovery.

Models depicting the nucleus and electron energy levels in hydrogen, helium, lithium, and neon atoms. In reality, the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.

Models for an atomic nucleus consisting of protons and neutrons were quickly developed byWerner Heisenbergand others.The proton–neutron model explained the puzzle of nuclear spins. The origins of beta radiation were explained byEnrico Fermiin 1934 by theprocess of beta decay,in which the neutron decays to a proton bycreatingan electron and a (at the time undiscovered) neutrino.In 1935, Chadwick and his doctoral studentMaurice Goldhaberreported the first accurate measurement of the mass of the neutron.

By 1934, Fermi had bombarded heavier elements with neutrons to induce radioactivity in elements of high atomic number. In 1938, Fermi received the Nobel Prize in Physics "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery ofnuclear reactionsbrought about by slow neutrons ".In December 1938Otto Hahn,Lise Meitner,andFritz Strassmanndiscoverednuclear fission,or the fractionation of uranium nuclei into lighter elements, induced by neutron bombardment.In 1945 Hahn received the 1944Nobel Prize in Chemistry"for his discovery of the fission of heavy atomic nuclei".

The discovery of nuclear fission would lead to the development of nuclear power and the atomic bomb by the end of World War II. It was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as a nuclear chain reaction.These events and findings led Fermi to construct theChicago Pile-1at the University of Chicago in 1942, the first self-sustainingnuclear reactor.Just three years later theManhattan Projectwas able to test the firstatomic bomb,theTrinity nuclear testin July 1945.

Properties

Mass

The mass of a neutron cannot be directly determined bymass spectrometrysince it has no electric charge. But since the masses of a proton and of adeuteroncan be measured with a mass spectrometer, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus thebinding energyof deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy ($B_{d}$) of the single2.224 MeVgamma photon emitted when a deuteron is formed by a proton capturing a neutron (this is exothermic and happens with zero-energy neutrons). The small recoil kinetic energy ($E_{rd}$) of the deuteron (about 0.06% of the total energy) must also be accounted for.

$m_{n}=m_{d}-m_{p}+B_{d}-E_{rd}$

The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, et al.These give a neutron mass of:

m_neutron=1.008644904(14)Da

The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion ofDato MeV/c²:

m_neutron=939.56563(28)MeV/c².

Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured.

Spin

The neutron is a spin⁠1/2⁠particle, that is, it is afermionwith intrinsic angular momentum equal to⁠1/2⁠ħ,whereħis thereduced Planck constant.For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin⁠1/2⁠Dirac particle,the possibility that the neutron was a spin⁠3/2⁠particle lingered. The interactions of the neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron.In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin⁠1/2⁠.In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in aStern–Gerlach experimentthat used a magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin⁠1/2⁠particle.

As a fermion, the neutron is subject to thePauli exclusion principle;two neutrons cannot have the same quantum numbers. This is the source of thedegeneracy pressurewhich counteracts gravity inneutron starsand prevents them from forming black holes.

See also:Delta baryon

Magnetic moment

Main article:Nucleon magnetic moment

Even though the neutron is a neutral particle, the magnetic moment of a neutron is not zero. The neutron is not affected by electric fields, but it is affected by magnetic fields. The value for the neutron's magnetic moment was first directly measured byLuis AlvarezandFelix BlochatBerkeley, California,in 1940.Alvarez and Bloch determined the magnetic moment of the neutron to beμ_n=−1.93(2)μ_N,whereμ_Nis thenuclear magneton.The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin.

The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution.In thequark modelforhadrons,the neutron is composed of one up quark (charge +2/3e) and two down quarks (charge −1/3e).The magnetic moment of the neutron can be modeled as a sum of the magnetic moments of the constituent quarks.The calculation assumes that the quarks behave like pointlike Dirac particles, each having their own magnetic moment. Simplistically, the magnetic moment of the neutron can be viewed as resulting from the vector sum of the three quark magnetic moments, plus the orbital magnetic moments caused by the movement of the three charged quarks within the neutron.

In one of the early successes of the Standard Model, in 1964 Mirza A.B. Beg,Benjamin W. Lee,andAbraham Paiscalculated the ratio of proton to neutron magnetic moments to be −3/2 (or a ratio of −1.5), which agrees with the experimental value to within 3%.The measured value for this ratio is−1.45989805(34).

The above treatment compares neutrons with protons, allowing the complex behavior of quarks to be subtracted out between models, and merely exploring what the effects would be of differing quark charges (or quark type). Such calculations are enough to show that the interior of neutrons is very much like that of protons, save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton.

The neutron magnetic moment can be roughly computed by assuming a simplenonrelativistic,quantum mechanicalwavefunctionforbaryonscomposed of three quarks. A straightforward calculation gives fairly accurate estimates for the magnetic moments of neutrons, protons, and other baryons.For a neutron, the result of this calculation is that the magnetic moment of the neutron is given byμ_n= 4/3μ_d− 1/3μ_u,whereμ_dandμ_uare the magnetic moments for the down and up quarks, respectively. This result combines the intrinsic magnetic moments of the quarks with their orbital magnetic moments, and assumes the three quarks are in a particular, dominant quantum state.

Baryon	Magnetic moment of quark model	Computed ($\mu _{\mathrm {N} }$)	Observed ($\mu _{\mathrm {N} }$)
p	4/3μu− 1/3μd	2.79	2.793
n	4/3μd− 1/3μu	−1.86	−1.913

The results of this calculation are encouraging, but the masses of the up or down quarks were assumed to be 1/3 the mass of a nucleon.The masses of the quarks are actually only about 1% that of a nucleon.The discrepancy stems from the complexity of the Standard Model for nucleons, where most of their mass originates in thegluonfields, virtual particles, and their associated energy that are essential aspects of thestrong force.Furthermore, the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment.But the nucleon magnetic moment has been successfully computed numerically fromfirst principles,including all of the effects mentioned and using more realistic values for the quark masses. The calculation gave results that were in fair agreement with measurement, but it required significant computing resources.

Electric charge

The total electric charge of the neutron is0e.This zero value has been tested experimentally, and the present experimental limit for the charge of the neutron is−2(8)×10⁻²²e,or−3(13)×10⁻⁴¹C.This value is consistent with zero, given the experimentaluncertainties(indicated in parentheses). By comparison, the charge of the proton is+1e.

Structure and geometry of charge distribution

An article published in 2007 featuring a model-independent analysis concluded that the neutron has a negatively charged exterior, a positively charged middle, and a negative core.In a simplified classical view, the negative "skin" of the neutron assists it to be attracted to the protons with which it interacts in the nucleus; but the main attraction between neutrons and protons is via thenuclear force,which does not involve electric charge.

The simplified classical view of the neutron's charge distribution also "explains" the fact that the neutron magnetic dipole points in the opposite direction from its spin angular momentum vector (as compared to the proton). This gives the neutron, in effect, a magnetic moment which resembles a negatively charged particle. This can be reconciled classically with a neutral neutron composed of a charge distribution in which the negative sub-parts of the neutron have a larger average radius of distribution, and therefore contribute more to the particle's magnetic dipole moment, than do the positive parts that are, on average, nearer the core.

Electric dipole moment

Main article:Neutron electric dipole moment

The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanentelectric dipole moment.But the predicted value is well below the current sensitivity of experiments. From severalunsolved puzzles in particle physics,it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories goingbeyond the Standard Modelgenerally lead to much larger predictions for the electric dipole moment of the neutron. Currently, there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment, including:

• Cryogenic neutron EDM experimentbeing set up at theInstitut Laue–Langevin
• n2EDM experiment under construction at the UCN source at thePaul Scherrer Institute
• nEDM experiment being envisaged at theSpallation Neutron Source
• nEDM experiment being built at theInstitut Laue–Langevin

Antineutron

Main article:Antineutron

The antineutron is theantiparticleof the neutron. It was discovered byBruce Corkin 1956, a year after theantiprotonwas discovered.CPT-symmetryputs strong constraints on the relative properties of particles and antiparticles, so studying antineutrons provides stringent tests on CPT-symmetry. The fractional difference in the masses of the neutron and antineutron is(9±6)×10⁻⁵.Since the difference is only about twostandard deviationsaway from zero, this does not give any convincing evidence of CPT-violation.

Neutron compounds

Dineutrons and tetraneutrons

Main articles:DineutronandTetraneutron

Thedineutronis considered an unbound isotope with lifetimes around 10^-22seconds. The first evidence for this state was reported by Haddock et al. in 1965.In 2012,Artemis Spyroufrom Michigan State University and coworkers reported that they observed, for the first time, direct dineutron emission in the decay of¹⁶Be. The dineutron character is evidenced by a small emission angle between the two neutrons. The authors measured the two-neutron separation energy to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.

Evidence for unbound clusters of 4 neutrons, ortetraneutronas resonances in the disintegration ofberyllium-14 nuclei,in⁸He-⁸Be interactions,and collisions of⁴He nuclei give an estimated lifetime around 10^-22seconds. These discoveries should deepen our understanding of the nuclear forces.

Neutron stars and neutron matter

Main articles:Neutron matterandNeutron star

At extremely high pressures and temperatures, nucleons and electrons are believed to collapse into bulk neutronic matter, calledneutron matter.This is presumed to happen inneutron stars.

The extreme pressure inside a neutron star may deform the neutrons into a cubic symmetry, allowing tighter packing of neutrons.

Detection

Main article:Neutron detection

The common means of detecting achargedparticleby looking for a track of ionization (such as in acloud chamber) does not work for neutrons directly. Neutrons that elastically scatter off atoms can create an ionization track that is detectable, but the experiments are not as simple to carry out; other means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more commonly used. The commonly used methods to detect neutrons can therefore be categorized according to the nuclear processes relied upon, mainlyneutron captureorelastic scattering.

Neutron detection by neutron capture

A common method for detecting neutrons involves converting the energy released fromneutron capturereactions into electrical signals. Certain nuclides have a high neutron capturecross section,which is the probability of absorbing a neutron. Upon neutron capture, the compound nucleus emits more easily detectable radiation, for example an Alpha particle, which is then detected. The nuclides³
He
,⁶
Li
,¹⁰
B
,²³³
U
,²³⁵
U
,²³⁷
Np
,and²³⁹
Pu
are useful for this purpose.

Neutron detection by elastic scattering

Neutrons can elastically scatter off nuclei, causing the struck nucleus to recoil. Kinematically, a neutron can transfer more energy to a light nucleus such as hydrogen or helium than to a heavier nucleus. Detectors relying on elastic scattering are called fast neutron detectors. Recoiling nuclei can ionize and excite further atoms through collisions. Charge and/or scintillation light produced in this way can be collected to produce a detected signal. A major challenge in fast neutron detection is discerning such signals from erroneous signals produced by gamma radiation in the same detector. Methods such as pulse shape discrimination can be used in distinguishing neutron signals from gamma-ray signals, although certain inorganic scintillator-based detectors have been developedto selectively detect neutrons in mixed radiation fields inherently without any additional techniques.

Fast neutron detectors have the advantage of not requiring a moderator, and are therefore capable of measuring the neutron's energy, time of arrival, and in certain cases direction of incidence.

Sources and production

Main articles:Neutron source,Neutron generator,andResearch reactor

Free neutrons are unstable, although they have the longest half-life of any unstable subatomic particle by several orders of magnitude. Their half-life is still only about 10 minutes, so they can be obtained only from sources that produce them continuously.

Natural neutron background.A small natural background flux of free neutrons exists everywhere on Earth.In the atmosphere and deep into the ocean, the "neutron background" is caused bymuonsproduced bycosmic rayinteraction with the atmosphere. These high-energy muons are capable of penetration to considerable depths in water and soil. There, in striking atomic nuclei, among other reactions they induce spallation reactions in which a neutron is liberated from the nucleus. Within the Earth's crust a second source is neutrons produced primarily by spontaneous fission of uranium and thorium present in crustal minerals. The neutron background is not strong enough to be a biological hazard, but it is of importance to very high resolution particle detectors that are looking for very rare events, such as (hypothesized) interactions that might be caused by particles ofdark matter.Recent research has shown that even thunderstorms can produce neutrons with energies of up to several tens of MeV.Recent research has shown that the fluence of these neutrons lies between 10⁻⁹and 10⁻¹³per ms and per m²depending on the detection altitude. The energy of most of these neutrons, even with initial energies of 20 MeV, decreases down to the keV range within 1 ms.

Even stronger neutron background radiation is produced at the surface of Mars, where the atmosphere is thick enough to generate neutrons from cosmic ray muon production and neutron-spallation, but not thick enough to provide significant protection from the neutrons produced. These neutrons not only produce a Martian surface neutron radiation hazard from direct downward-going neutron radiation but may also produce a significant hazard from reflection of neutrons from the Martian surface, which will produce reflected neutron radiation penetrating upward into a Martian craft or habitat from the floor.

Sources of neutrons for research.These include certain types ofradioactive decay(spontaneous fissionandneutron emission), and from certainnuclear reactions.Convenient nuclear reactions include tabletop reactions such as natural Alpha and gamma bombardment of certain nuclides, often beryllium or deuterium, and inducednuclear fission,such as occurs in nuclear reactors. In addition, high-energy nuclear reactions (such as occur in cosmic radiation showers or accelerator collisions) also produce neutrons from disintegration of target nuclei. Small (tabletop)particle acceleratorsoptimized to produce free neutrons in this way, are calledneutron generators.

In practice, the most commonly used small laboratory sources of neutrons use radioactive decay to power neutron production. One noted neutron-producingradioisotope,californium-252 decays (half-life 2.65 years) byspontaneous fission3% of the time with production of 3.7 neutrons per fission, and is used alone as a neutron source from this process.Nuclear reactionsources (that involve two materials) powered by radioisotopes use anAlpha decaysource plus a beryllium target, or else a source of high-energy gamma radiation from a source that undergoesbeta decayfollowed bygamma decay,which producesphotoneutronson interaction of the high-energygamma raywith ordinary stable beryllium, or else with thedeuteriuminheavy water.A popularsource of the latter typeis radioactiveantimony-124plus beryllium, a system with a half-life of 60.9 days, which can be constructed from natural antimony (which is 42.8% stable antimony-123) by activating it with neutrons in a nuclear reactor, then transported to where the neutron source is needed.

Institut Laue–Langevin(ILL) in Grenoble, France – a major neutron research facility

Nuclear fission reactorsnaturally produce free neutrons; their role is to sustain the energy-producingchain reaction.The intenseneutron radiationcan also be used to produce various radioisotopes through the process ofneutron activation,which is a type ofneutron capture.

Experimentalnuclear fusion reactorsproduce free neutrons as a waste product. But it is these neutrons that possess most of the energy, and converting that energy to a useful form has proved a difficult engineering challenge. Fusion reactors that generate neutrons are likely to create radioactive waste, but the waste is composed of neutron-activated lighter isotopes, which have relatively short (50–100 years) decay periods as compared to typical half-lives of 10,000 yearsfor fission waste, which is long due primarily to the long half-life of Alpha -emitting transuranic actinides.Somenuclear fusion-fission hybridsare proposed to make use of those neutrons to either maintain asubcritical reactoror to aid innuclear transmutationof harmful long lived nuclear waste to shorter lived or stable nuclides.

Neutron beams and modification of beams after production

Free neutron beams are obtained fromneutron sourcesbyneutron transport.For access to intense neutron sources, researchers must go to a specializedneutron facilitythat operates aresearch reactoror aspallationsource.

The neutron's lack of total electric charge makes it difficult to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected byelectricormagnetic fields.These methods have little effect on neutrons. But some effects may be attained by use of inhomogeneous magnetic fields because of theneutron's magnetic moment.Neutrons can be controlled by methods that includemoderation,reflection,andvelocity selection.Thermal neutronscan be polarized by transmission throughmagneticmaterials in a method analogous to theFaraday effectforphotons.Cold neutrons of wavelengths of 6–7 angstroms can be produced in beams of a high degree of polarization, by use ofmagnetic mirrorsand magnetized interference filters.

Applications

The neutron plays an important role in many nuclear reactions. For example, neutron capture often results inneutron activation,inducingradioactivity.In particular, knowledge of neutrons and their behavior has been important in the development ofnuclear reactorsandnuclear weapons.Thefissioningof elements likeuranium-235andplutonium-239is caused by their absorption of neutrons.

Cold,thermal,andhotneutron radiationis commonly employed inneutron scatteringfacilities forneutron diffraction,small-angle neutron scattering,andneutron reflectometry.Slow neutronmatter wavesexhibit properties similar to geometrical and wave optics of light, including reflection, refraction, diffraction, and interference.Neutrons are complementary toX-raysin terms of atomic contrasts by different scatteringcross sections;sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.

The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.

A major use of neutrons is to excite delayed and promptgamma raysfrom elements in materials. This forms the basis ofneutron activation analysis(NAA) andprompt gamma neutron activation analysis(PGNAA). NAA is most often used to analyze small samples of materials in anuclear reactorwhilst PGNAA is most often used to analyze subterranean rocks aroundbore holesand industrial bulk materials on conveyor belts.

Another use of neutron emitters is the detection of light nuclei, in particular the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, aneutron probemay determine the water content in soil.

Medical therapies

Main articles:Fast neutron therapyandNeutron capture therapy of cancer

Because neutron radiation is both penetrating and ionizing, it can be exploited for medical treatments. However, neutron radiation can have the unfortunate side-effect of leaving the affected area radioactive.Neutron tomographyis therefore not a viable medical application.

Fast neutron therapy uses high-energy neutrons typically greater than 20 MeV to treat cancer.Radiation therapyof cancers is based upon the biological response of cells to ionizing radiation. If radiation is delivered in small sessions to damage cancerous areas, normal tissue will have time to repair itself, while tumor cells often cannot.Neutron radiation can deliver energy to a cancerous region at a rate an order of magnitude larger thangamma radiation.

Beams of low-energy neutrons are used inboron neutron capture therapyto treat cancer. In boron neutron capture therapy, the patient is given a drug that contains boron and that preferentially accumulates in the tumor to be targeted. The tumor is then bombarded with very low-energy neutrons (although often higher than thermal energy) which are captured by theboron-10isotope in the boron, which produces an excited state of boron-11 that then decays to producelithium-7and anAlpha particlethat have sufficient energy to kill the malignant cell, but insufficient range to damage nearby cells. For such a therapy to be applied to the treatment of cancer, a neutron source having an intensity of the order of a thousand million (10⁹) neutrons per second per cm²is preferred. Such fluxes require a research nuclear reactor.

Protection

Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption tomoleculesandatoms,and can also cause reactions that give rise to other forms ofradiation(such as protons).The normal precautions of radiation protection apply: Avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. But particular thought must be given to how to protect from neutron exposure. For other types of radiation, e.g.,Alpha particles,beta particles,orgamma rays,material of a high atomic number and with high density makes for good shielding; frequently,leadis used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with Alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example,hydrogen-rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. This often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. After slowing, neutrons may then be absorbed with an isotope that has high affinity for slow neutrons without causing secondary capture radiation, such as lithium-6.

Hydrogen-richordinary watereffects neutron absorption innuclear fissionreactors: Usually, neutrons are so strongly absorbed by normal water that fuel enrichment with a fissionable isotope is required. (The number of neutrons produced per fission depends primarily on the fission products. The average is roughly 2.5 to 3.0 and at least one, on average, must evade capture in order to sustain thenuclear chain reaction.) Thedeuteriuminheavy waterhas a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is, therefore, used inCANDU-type reactors, in order to slow (moderate) neutron velocity, to increase the probability ofnuclear fissioncompared toneutron capture.

Neutron temperature

Main article:Neutron temperature

Thermal neutrons

Thermal neutronsarefree neutronswhose energies have aMaxwell–Boltzmann distributionwith kT =0.0253eV(4.0×10⁻²¹J) at room temperature. This gives characteristic (not average, or median) speed of 2.2 km/s. The name 'thermal' comes from their energy being that of the room temperature gas or material they are permeating. (seekinetic theoryfor energies and speeds of molecules). After a number of collisions (often in the range of 10–20) with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.

In many substances, thermal neutron reactions show a much larger effective cross-section than reactions involving faster neutrons, and thermal neutrons can therefore be absorbed more readily (i.e., with higher probability) by anyatomic nucleithat they collide with, creating a heavier – and oftenunstable–isotopeof thechemical elementas a result.

Mostfission reactorsuse aneutron moderatorto slow down, orthermalizethe neutrons that are emitted bynuclear fissionso that they are more easily captured, causing further fission. Others, calledfast breederreactors, use fission energy neutrons directly.

Cold neutrons

Cold neutronsare thermal neutrons that have been equilibrated in a very cold substance such as liquiddeuterium.Such acold sourceis placed in the moderator of a research reactor or spallation source. Cold neutrons are particularly valuable forneutron scatteringexperiments.

The use of cold and very cold neutrons (VCN) have been a bit limited compared to the use of thermal neutrons due to the relatively lower flux and lack in optical components. However, Innovative solutions have been proposed to offer more options to the scientific community to promote the use of VCN.

Cold neutron source providing neutrons at about the temperature of liquid hydrogen

Ultracold neutrons

Ultracold neutronsare produced by inelastic scattering of cold neutrons in substances with a low neutron absorption cross section at a temperature of a few kelvins, such as soliddeuteriumor superfluidhelium.An alternative production method is the mechanical deceleration of cold neutrons exploiting the Doppler shift.

Fission energy neutrons

Main article:Nuclear fission

Afast neutronis a free neutron with a kinetic energy level close to1MeV(1.6×10⁻¹³J), hence a speed of ~14000km/s(~ 5% of the speed of light). They are namedfission energyorfastneutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such asnuclear fission.Neutrons produced in fission, as noted above, have aMaxwell–Boltzmann distributionof kinetic energies from 0 to ~14 MeV, a mean energy of 2 MeV (for²³⁵U fission neutrons), and amodeof only 0.75 MeV, which means that more than half of them do not qualify as fast (and thus have almost no chance of initiating fission infertile materials,such as²³⁸U and²³²Th).

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with aneutron moderator.In reactors, typicallyheavy water,light water,orgraphiteare used to moderate neutrons.

Fusion neutrons

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The D–T rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

Further information:Nuclear fusion § Criteria and candidates for terrestrial reactions

D–T (deuterium–tritium) fusion is thefusion reactionthat produces the most energetic neutrons, with 14.1MeVofkinetic energyand traveling at 17% of thespeed of light.D–T fusion is also the easiest fusion reaction to ignite, reaching near-peak rates even when the deuterium and tritium nuclei have only a thousandth as much kinetic energy as the 14.1 MeV that will be produced.

14.1 MeV neutrons have about 10 times as much energy as fission neutrons, and are very effective at fissioning even non-fissileheavy nuclei,and these high-energy fissions produce more neutrons on average than fissions by lower-energy neutrons. This makes D–T fusion neutron sources such as proposedtokamakpower reactors useful fortransmutationof transuranic waste. 14.1 MeV neutrons can also produce neutrons byknocking them loose from nuclei.

On the other hand, these very high-energy neutrons are less likely to simplybe captured without causing fission or spallation.For these reasons,nuclear weapon designextensively uses D–T fusion 14.1 MeV neutrons tocause more fission.Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such asdepleted uranium(uranium-238), and these materials have been used in the jackets ofthermonuclear weapons.Fusion neutrons also can cause fission in substances that are unsuitable or difficult to make into primary fission bombs, such asreactor grade plutonium.This physical fact thus causes ordinary non-weapons grade materials to become of concern in certainnuclear proliferationdiscussions and treaties.

Other fusion reactions produce much less energetic neutrons. D–D fusion produces a 2.45 MeV neutron andhelium-3half of the time, and producestritiumand a proton but no neutron the rest of the time. D–³He fusion produces no neutron.

Intermediate-energy neutrons

Transmutation flow inlight water reactor,which is a thermal-spectrum reactor

A fission energy neutron that has slowed down but not yet reached thermal energies is called an epithermal neutron.

Cross sectionsfor bothcaptureandfissionreactions often have multipleresonancepeaks at specific energies in the epithermal energy range. These are of less significance in afast-neutron reactor,where most neutrons are absorbed before slowing down to this range, or in a well-moderatedthermal reactor,where epithermal neutrons interact mostly with moderator nuclei, not with eitherfissileorfertileactinidenuclides. But in a partially moderated reactor with more interactions of epithermal neutrons with heavy metal nuclei, there are greater possibilities fortransientchanges inreactivitythat might make reactor control more difficult.

Ratios of capture reactions to fission reactions are also worse (more captures without fission) in mostnuclear fuelssuch asplutonium-239,making epithermal-spectrum reactors using these fuels less desirable, as captures not only waste the one neutron captured but also usually result in anuclidethat is notfissilewith thermal or epithermal neutrons, though stillfissionablewith fast neutrons. The exception isuranium-233of thethorium cycle,which has good capture-fission ratios at all neutron energies.

High-energy neutrons

High-energy neutrons have much more energy than fission energy neutrons and are generated as secondary particles byparticle acceleratorsor in the atmosphere fromcosmic rays.These high-energy neutrons are extremely efficient ationizationand far more likely to causecelldeath thanX-raysor protons.

See also

• Ionizing radiation
• Isotope
• List of particles
• Neutron radiationand theSievert radiation scale
• Neutronium
• Nuclear reaction
• Nucleosynthesis
  • Neutron capture nucleosynthesis
  • R-process
  • S-process
• Thermal-neutron reactor

Neutron sources

• Neutron generator
• Neutron source

Processes involving neutrons

• Neutron bomb
• Neutron diffraction
• Neutron flux
• Neutron transport
• Cosmogenic radionuclide dating

Further reading

• James Byrne,Neutrons, Nuclei and Matter: An Exploration of the Physics of Slow Neutrons.Mineola, New York: Dover Publications, 2011.ISBN0486482383.
• Abraham Pais,Inward Bound,Oxford: Oxford University Press, 1986.ISBN0198519974.
• Sin-Itiro Tomonaga,The Story of Spin,The University of Chicago Press, 1997
• Herwig Schopper,Weak interactions and nuclear beta decay,Publisher, North-Holland Pub. Co., 1966.
• Annotated bibliography for neutrons from the Alsos Digital Library for Nuclear Issues