Neutron
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225585091
2008-07-14T12:44:52Z
Vsmith
84417
/* Discovery */ rmv prior claim - ref link doesn't work
{{otheruses4|the subatomic particle|other uses|Neutron (disambiguation)}}
:''This article is a discussion of neutrons in general. For the specific case of a neutron found outside the nucleus, see [[free neutron]].''
{{Infobox Particle
| bgcolour =
| classification = [[baryon]]
| name = Neutron
| image = [[Image:Quark structure neutron.svg|250px]]
| caption = The quark structure of the neutron.
| num_types =
| composition = one up, two down
| family = [[Fermion]]
| group = [[Quark]]
| generation =
| interaction = [[Gravity]], [[Electromagnetic interaction|Electromagnetic]], [[Weak interaction|Weak]], [[Strong interaction|Strong]]
| antiparticle = [[Antineutron]]
| theorized =
| discovered = [[James Chadwick]]<ref>[http://nobelprize.org/nobel_prizes/physics/laureates/1935/ 1935 Nobel Prize in Physics]</ref> (1932)
| symbol = {{SubatomicParticle|Neutron}}, {{SubatomicParticle|Neutron0}}, {{SubatomicParticle|Nucleon0}}
| mass = {{val|1.67492729|(28)|e=-27|u=kg}}<br/>{{val|939.565560|(81)|u=MeV/c2}}<br/>{{val|1.0086649156|(6)|u=u}}<ref name="RPP"/>
| mean_lifetime = 885.7(8) s ([[Free neutron|free]])
| electric_charge = {{val|0|u=C}}
| electric_dipole_moment = <{{val|2.9||e=-26|u=e cm}}
| electric_polarizability = {{val|1.16|(15)|e=-3|u=fm<sup>3</sup>}}
| magnetic_moment = [[Neutron magnetic moment|-1.9130427(5)]] [[Nuclear magneton|μ<sub>N</sub>]]
| magnetic_polarizability = {{val|3.7|(20)|e=-4|u=fm<sup>3</sup>}}
| spin = ½
| isospin = ½
| parity = +1
| condensed_symmetries = I(J<sup>P</sup>) = ½(½<sup>+</sup>)
}}
[[Image:Beta Negative Decay.svg|thumb|200px|The [[Feynman diagram]] of the neutron [[beta decay]] process]]
In [[physics]], the '''neutron''' is a [[subatomic particle]] with no net [[electric charge]] and a [[mass]] slightly larger than that of a [[proton]].
The [[atomic nucleus|nuclei]] of all [[atom]]s consist of protons and neutrons, which are therefore collectively referred to as [[nucleon]]s. The number of protons in a nucleus is the [[atomic number]] and defines the type of [[chemical element|element]] the atom forms. The number of neutrons determines the [[isotope]] of an element. For example, the [[carbon-12]] isotope has 6 protons and 6 neutrons, while the [[carbon-14]] isotope has 6 protons and 8 neutrons.
== Neutron stability and beta decay ==
Outside the nucleus, [[free neutron]]s are unstable and have a [[mean lifetime]] of {{val|885.7|0.8|u=s}} (about 15 minutes), decaying by emission of a negative [[electron]] and [[antineutrino]] to become a proton:<ref>[http://pdg.lbl.gov/2007/tables/bxxx.pdf Particle Data Group Summary Data Table on Baryons]</ref>
:{{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron antineutrino}}
This decay mode, known as [[beta decay]], can also transform the character of neutrons within unstable nuclei.
Inside of a bound nucleus, protons can also transform via beta decay into neutrons. In this case, the transformation may occur by emission of a [[positron]] (antielectron) and [[neutrino]] (instead of an antineutrino):
:{{SubatomicParticle|Proton+}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}}
The transformation of a proton to a neutron inside of a nucleus is also possible through [[electron capture]]:
:{{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Electron neutrino}}
Positron capture by neutrons in nuclei that contain an excess of neutrons is also possible, but is hindered due to the fact positrons are repelled by the nucleus, and furthermore, quickly [[annihilation|annihilate]] when they encounter negative electrons.
When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus.
Beta decay and electron capture are types of [[radioactive decay]] and are both governed by the [[weak interaction]].
== Interactions ==
The neutron interacts through all four [[fundamental interaction]]s: the [[electromagnetic interaction|electromagnetic]], [[weak interaction|weak nuclear]], [[strong interaction|strong nuclear]] and [[gravitation]]al interactions.
Although the neutron has zero net charge, it may interact electromagnetically in two ways: first, the neutron has a [[magnetic moment]] of the same order as the [[proton]] (see [[neutron magnetic moment]]);<ref name="RPP"/> second, it is composed of electrically charged [[quark]]s. Thus, the electromagnetic interaction is primarily important to the neutron in [[deep inelastic scattering]] and in [[magnetism|magnetic]] interactions.
The neutron experiences the weak interaction through [[beta decay]] into a proton, [[electron]] and [[neutrino|electron antineutrino]]. It experiences the gravitational force as does any energetic body; however, gravity is so weak that it may be neglected in [[particle physics]] experiments.
The most important force to neutrons is the strong interaction. This interaction is responsible for the binding of the neutron's three [[quark]]s into a single particle. The [[residual strong force]] is responsible for the binding of neutrons and protons together into [[atomic nucleus|nuclei]]. This nuclear force plays the leading role when neutrons pass through matter. Unlike charged particles or photons, the neutron cannot lose energy by [[ionization|ionizing]] atoms. Rather, the neutron goes on its way unchecked until it makes a collision with an atomic nucleus. For this reason, [[neutron radiation]] is extremely penetrating.
== Detection ==
{{main|neutron detection}}
The common means of detecting a [[electric charge|charged]] [[elementary particle|particle]] by looking for a track of ionization (such as in a [[cloud 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.
A common method for detecting neutrons involves converting the energy released from such reactions into electrical signals. The nuclides {{SimpleNuclide|Helium|3}}, {{SimpleNuclide|Lithium|6}}, {{SimpleNuclide|Boron|10}}, {{SimpleNuclide|Uranium|233}}, {{SimpleNuclide|Uranium|235}}, {{SimpleNuclide|Neptunium|237}} and {{SimpleNuclide|Plutonium|239}} are useful for this purpose. A good discussion on neutron detection is found in chapter 14 of the book ''Radiation Detection and Measurement'' by Glenn F. Knoll (John Wiley & Sons, 1979).
== Uses ==
The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in [[neutron activation]], inducing [[radioactivity]]. In particular, knowledge of neutrons and their behavior has been important in the development of [[nuclear reactor]]s and [[nuclear weapon]]s. The [[Nuclear fission|fissioning]] of elements like [[uranium-235]] and [[plutonium-239]] is caused by their absorption of neutrons.
[[neutron temperature|''Cold'', ''thermal'' and ''hot'']] [[neutron radiation]] is commonly employed in [[neutron scattering]] facilities, where the radiation is used in a similar way one uses [[X-ray]]s for the analysis of [[condensed matter]]. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering [[Cross section (physics)|cross section]]s; 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.<ref>{{cite journal |last=Kumakhov |first=M. A. |authorlink= |coauthors=Sharov, V. A. |year=1992 |month= |title=A neutron lens |journal=Nature |volume=357 |issue= |pages=390–391 |doi=10.1038/357390a0 |url= |accessdate= |quote= }}</ref><ref>[http://www.physorg.com/news599.html Physorg.com, "New Way of 'Seeing': A 'Neutron Microscope'"]</ref><ref>[http://www.nasa.gov/vision/earth/technologies/nuggets.html NASA.gov: "NASA Develops a Nugget to Search for Life in Space"]</ref>
One use of neutron emitters is the detection of light nuclei, particularly 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, a [[neutron probe]] may determine the water content in soil.
== Sources ==
Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from [[neutron source]]s by [[neutron transport]]. For access to intense neutron sources, researchers must go to specialist facilities, such as the [[ISIS neutron source|ISIS facility]] in the [[United Kingdom|UK]], which is currently the world's most intense pulsed neutron and [[muon]] source.{{Fact|date=June 2008}}
Neutrons' lack of total electric charge prevents engineers or experimentalists from being able to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by [[electricity|electric]] or [[magnetic field]]s. However, these methods have no effect on neutrons except for a small effect of a magnetic field because of the neutron's [[magnetic moment]].
==Discovery==
In 1930 [[Walther Bothe]] and H. Becker in [[Germany]] found that if the very energetic [[alpha particle]]s emitted from [[polonium]] fell on certain light elements, specifically [[beryllium]], [[boron]], or [[lithium]], an unusually penetrating radiation was produced. At first this radiation was thought to be [[gamma radiation]], although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis. The next important contribution was reported in 1932 by [[Irène Joliot-Curie]] and [[Frédéric Joliot]] in [[Paris]]. They showed that if this unknown radiation fell on [[paraffin]] or any other [[hydrogen]]-containing compound it ejected protons of very high energy. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis. Finally, in 1932 the physicist [[James Chadwick]] in [[England]] performed a series of experiments showing that the gamma ray hypothesis was untenable. He suggested that in fact the new radiation consisted of uncharged particles of approximately the mass of the [[proton]], and he performed a series of experiments verifying his suggestion.<ref>{{cite journal |last=Chadwick |first=James |authorlink= |coauthors= |year=1932 |month= |title=Possible Existence of a Neutron |journal=Nature |volume=129 |issue= |pages=312 |doi=10.1038/129312a0 |url= |accessdate= |quote= }}</ref>
These uncharged particles were called ''neutrons'', apparently from the [[Latin]] root for ''neutral'' and the [[Greek language|Greek]] ending ''-on'' (by imitation of ''[[electron]]'' and ''[[proton]]'').
==Anti-neutron==
{{main|antineutron}}
The antineutron is the [[antiparticle]] of the neutron. It was discovered by [[Bruce Cork]] in the year [[1956]], a year after the [[antiproton]] was discovered.
[[CPT-symmetry]] puts strong constraints on the relative properties of particles and [[antiparticles]] and, therefore, is open to stringent tests. The fractional difference in the masses of the neutron and antineutron is {{val|9|5|e=-5}}<!-- What unit of mass?-->. Since the difference is only about 2 standard deviations away from zero, this does not give any convincing evidence of CPT-violation.<ref name="RPP">[http://pdg.lbl.gov/2006/tables/bxxx.pdf Particle Data Group's Review of Particle Physics 2006]</ref>
== Current developments ==
=== Electric dipole moment ===
An experiment at the [[Institut Laue-Langevin]] has attempted to measure an electric dipole, or separation of charges, within the neutron, and is consistent with an [[electric dipole moment]] of zero. These results are important in developing theories that go beyond the [[Standard Model]], but are inconsistent with it due to the lack of explanation of the fundamental interactions.<ref>[http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=5F4&style=feature&fdmode=text FRONTIERS, "Particle physics chills out"], retrieved [[25 November]] [[2007]]</ref><ref>[http://www.neutronedm.org/ Neutron Electric Dipole Moment experiment's web page], retrieved [[25 November]] [[2007]]</ref>
=== Tetraneutrons ===
{{main|tetraneutron}}
The existence of stable clusters of four neutrons, or tetraneutrons, has been hypothesised by a team led by Francisco-Miguel Marqués at the CNRS Laboratory for Nuclear Physics based on observations of the disintegration of [[beryllium]]-14 nuclei. This is particularly interesting because current theory suggests that these clusters should not be stable.
==Protection==
Exposure to neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to [[molecules]] and [[atoms]], and can also cause reactions which give rise to other forms of [[radiation]] (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. Some particular thought must be given to how to protect from neutron exposure, however. For other types of radiation, e.g. [[alpha particles]], [[beta rays|beta particles]], or [[gamma ray]]s, material of a high atomic number and with high density make for good shielding; frequently [[lead]] is 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 which has high affinity for slow neutrons without causing secondary capture-radiation, such as lithium-6.
Hydrogen-rich [[water|ordinary water]] effects neutron absorption in [[nuclear fission]] reactors: usually neutrons are so strongly absorbed by normal water that fuel-enrichement with fissionable isotope, is required. The [[deuterium]] in [[heavy water]] has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is therefore used in [[CANDU]]-type reactors, in order to slow ("moderate") neutron velocity, so that they are more effective at causing [[nuclear fission]], without capturing them.
==See also==
===Neutron capture nucleosynthesis===
{|
|
*[[R-process]]
|
*[[S-process]]
|}
===Fields concerning neutrons===
{|
|
*[[Particle physics]]
*[[Quark model]]
|
*[[Chemistry]]
*[[Neutron Detection]]
|
*[[Neutron scattering|Neutron Scattering]]
.
|}
===Types of neutrons===
{|
|
*[[Fast neutron]]
*[[Free neutron]]
|
*[[Neutron radiation]] and the [[Sievert|Sievert radiation scale]]
*[[Neutron temperature]], used to classify neutron types
|
*[[Thermal neutron]]
*[[Dineutron]]
|
*[[Tetraneutron]]
.
|}
===Objects containing neutrons===
{|
|
*The nuclei of atoms with the exception of [[protium]], the most common isotope of hydrogen <br>(and as a result, all ordinary matter with the exception of [[hydrogen]])
|
*[[Dineutron]]
*[[Neutronium]]
|
*[[Neutron star]]
*[[Tetraneutron]]
|}
===Neutron sources===
*[[Neutron source]]s
*[[Neutron generator]]
===Processes involving neutrons===
{|
|
*[[Neutron bomb]]
*[[Neutron diffraction]]
|
*[[Neutron flux]]
*[[Neutron transport]]
|}
{{commonscat}}
==References==
{{reflist}}
<BR>
{{particles}}
[[Category:Neutron| ]]
[[Category:Fundamental physics concepts]]
{{Link FA|lmo}}
[[af:Neutron]]
[[ar:نيوترون]]
[[ast:Neutrón]]
[[az:Neytron]]
[[bn:নিউট্রন]]
[[zh-min-nan:Tiong-chú]]
[[be-x-old:Нэўтрон]]
[[bs:Neutron]]
[[br:Neutron]]
[[bg:Неутрон]]
[[ca:Neutró]]
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[[eu:Neutroi]]
[[fa:نوترون]]
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[[ga:Neodrón]]
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[[ko:중성자]]
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[[it:Neutrone]]
[[he:נייטרון]]
[[kn:ನ್ಯೂಟ್ರಾನ್]]
[[sw:Nyutroni]]
[[ku:Nutron]]
[[la:Neutron]]
[[lv:Neitrons]]
[[lt:Neutronas]]
[[ln:Netron]]
[[lmo:Neütrún]]
[[hu:Neutron]]
[[mk:Неутрон]]
[[ml:ന്യൂട്രോണ്]]
[[ms:Neutron]]
[[mn:Нейтрон]]
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[[ja:中性子]]
[[no:Nøytron]]
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[[nov:Neutrone]]
[[oc:Neutron]]
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[[ru:Нейтрон]]
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[[scn:Neutroni]]
[[simple:Neutron]]
[[sd:نِپُوسيو]]
[[sk:Neutrón]]
[[sl:Nevtron]]
[[sr:Неутрон]]
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[[uk:Нейтрон]]
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[[zh-yue:中子]]
[[bat-smg:Neutruons]]
[[zh:中子]]