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A '''neutron''' is a subatomic particle that normally is part of the [[nucleus]] of a [[chemical element]]. When free (not bound to a nucleus), a neutron can have important physical, chemical, and biological<ref name=WHOion>{{citation
A '''neutron''' is a subatomic particle that normally is part of the [[nucleus]] of a [[chemical element]]. When free (not bound to a nucleus), a neutron can have important physical, chemical, and biological<ref name=WHOion>{{citation
  | author = World Health Organization
  | author = World Health Organization
  | title = Ionizing Radiation
  | title = Ionizing Radiation
  | url = http://www.who.int/ionizing_radiation/about/what_is_ir/en/index.html}}</ref> effects.
  | url = http://www.who.int/ionizing_radiation/about/what_is_ir/en/index.html}}</ref> effects. Free neutrons are not stable particles, but undergo radioactive decay with a half-life of approximately 10 minutes.


The mass ''m''<sub>n</sub> of a neutron<ref> Value retrieved from [http://physics.nist.gov/cgi-bin/cuu/Value?mn NIST] on October 1, 2008</ref> is close to, but not equal to, the mass of a [[proton]]:
The mass ''m''<sub>n</sub> of a neutron<ref name=NIST1>
:''m''<sub>n</sub> = 1.674&thinsp;927&thinsp;211 &times; 10<sup>&minus;27</sup> kg.
 
{{cite web |title=Neutron mass |work=Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?mn|search_for=neutron+mass |publisher=[[National Institute of Standards and Technology]] |accessdate=2011-03-28}}
 
</ref> is close to, but not equal to, the mass of a [[proton]]:
:''m''<sub>n</sub> = 1.674 927 211 &times; 10<sup>&minus;27</sup> kg.
 
==Structure==
According to the [[standard model]], the neutron consists of three [[quark]]s, one up quark and two down quarks.<ref name=quark>
 
See, for example, {{cite book |title=Nuclear and particle physics |author=Brian Robert Martin |pages=p. 97 |url=http://books.google.com/books?id=WXHG1wSgQDMC&pg=PA97 |isbn=0470742747 |year=2009 |edition=2nd ed |publisher=John Wiley and Sons}} and {{cite book |title=Simulation of W boson production in the PHENIX muon spectrometers |author=Kristin Kiriluk |url=http://books.google.com/books?id=Qpt6qLp5PIIC&pg=PA1 |chapter=Chapter 1: Introduction |pages=pp. 1 ''ff''|isbn=0549402810 |year=2007 |publisher=ProQuest}}
 
</ref> A free neutron shows beta decay,  breaking down into a proton, an [[electron]], and an [[antineutrino]] with a lifetime of about 15 minutes.  Because it  disintegrates, the free neutron does not exist in nature.  Neutrons do not carry electric charge: they pass unhindered through the electrical fields within liquids and solids.
 
The neutron [[g-factor|''g''-factor]] is:<ref name=NIST2>
 
{{cite web |title=Neutron g factor |work=Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?eqgnn|search_for=Neutron+g-factor |publisher=NIST |accessdate=2011-03-28}}
 
</ref>
 
:<math>g_{\rm p}  = \mathrm{-3.826 085 45 } \ , </math>
corresponding to a nuclear [[magnetic moment]] of:<ref name=NIST5>
 
{{cite web |title=Neutron magnetic moment |work=Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?munn|search_for=neutron+magnetic+moment |publisher=NIST |accessdate=2011-03-28}}
 
</ref>
 
:''&mu;<sub>n</sub>'' = −0.966 236 41 × 10<sup>−26</sup> J/T,
 
or about −1.913 nuclear magnetons (''&mu;<sub>N</sub>''):<ref name=NIST3>
 
{{cite web |title=Nuclear magneton |work=Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?eqmun|search_for=nuclear+magneton |publisher=NIST |accessdate=2011-03-28}}
 
</ref>
 
:<math>\mu_N = \frac{e \hbar}{2m_p} =\mathrm {5.050\ 783\ 24\ \times\ 10^{-27}\ J/T}\ . </math>
 
So far, a theoretical calculation of the magnetic moment of the neutron in terms of quarks exchanging [[gluon]]s is a work in progress, with the present estimate as −1.82 nuclear magnetons.<ref name=gluon>
 
See, for example, {{cite book |chapter=Table 3.5 |url=http://books.google.com/books?id=ws8QZ2M5OR8C&pg=PA103 |pages=p. 104  |title=Nuclear and Particle Physics: An Introduction |author=Brian Martin |isbn=0470742747 |year=2009 |edition=2nd ed|publisher=Wiley}}
 
</ref>


The neutron consists of three [[quarks]]. A free neutron shows beta decay,  breaking down into a proton, an [[electron]], and an [[antineutrino]].  Because it  disintegrates, the free neutron does not exist in nature.  Neutrons do not carry electric charge, they pass unhindered through the electrical fields within liquids and solids.
==History==
==History==
The existence of the neutron was discovered, in 1932, by Sir [[James Chadwick]], who received the 1935 [[Nobel Prize]] in Physics for his work. A repeatable experimental demonstration of the existence of the neutron solved a number of then-outstanding problems in physics, although the applications and significance of neutrons were in their infancy.<ref name=PhysLabNeutron>{{citation
The existence of the neutron was discovered, in 1932, by [http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick.html Sir James Chadwick], who received the 1935 [[Nobel Prize]] in Physics for his work. A repeatable experimental demonstration of the existence of the neutron solved a number of then-outstanding problems in physics, although the applications and significance of neutrons were in their infancy.<ref name=PhysLabNeutron>{{citation
  | title = Famous Experiments: The Discovery of the Neutron
  | title = Famous Experiments: The Discovery of the Neutron
  | url = http://dev.physicslab.org/Document.aspx?doctype=3&filename=AtomicNuclear_ChadwickNeutron.xml
  | url = http://dev.physicslab.org/Document.aspx?doctype=3&filename=AtomicNuclear_ChadwickNeutron.xml
Line 20: Line 59:


==Health effects==
==Health effects==
From the biological standpoint, neutrons are indirectly ionizing. <ref name=WHOion />  See [[acute radiation syndrome]]; a given dosage by particles may have greater biological effect than the same dosage from X-rays or gamma rays.
From the biological standpoint, neutrons are indirectly ionizing.<ref name=WHOion />  A given dosage by particles may have greater biological effect than the same dosage from X-rays or gamma rays (see [[Acute radiation syndrome]]).
 
==Applications==
==Applications==
Applications involve a [[neutron generator]] to provide the neutrons, a means of directing them at a target, and an application-specific means for assessing their effects.
===Biological===
===Biological===
===Analytical===
===Analytical===
[[Neutron activation analysis]], which has several variants, is a widely used technique for measuring the proportions of elements in a sample, including trace elements. A basic analyzer has a neutron source, preferably a [[#Neutron generation from nuclear reactors|appropriate nuclear reactor]], detectors for [[gamma ray]]s emitted by the target, and an extensive database and technical skills to interpret the interaction and its results. Portable analyzers are restricted to radioisotope and accelerator sources.
====Neutron capture method====
Also called the (n,gamma) reaction, it depends on non-elastic collisions of neutrons with nuclei in the target. The nucleus struck becomes excited, with the excitation energy dependent on the particular element. The period of excitation is usually quite short, and, as the nucleus loses exciting energy, it will emit one or more gamma rays, at characteristic times and energy levels. Half-lives of excited nuclei can range from less than a second to months, and it is these times and energies that are analyzed.<ref name=WPI>{{citation
| url = http://www.me.wpi.edu/Nuclear/Reactor/Labs/R-naa.html#neu
| title = Neutron Activation Analysis
| author = Worcester Polytechnic Institute's Nuclear Engineering}}</ref>
===Imaging===
===Imaging===
There are various industrial applications of neutrons for such purposes as inspecting the quality of [[welding|welds]], which are relatively straightforward with a generator on one side and an imaging detector on the other. Newer applications, such as for baggage and cargo screening in transportation safety, are both more complex and more powerful.
There are various industrial applications of neutrons for such purposes as inspecting the quality of [[welding|welds]], which are relatively straightforward with a generator on one side and an imaging detector on the other. Newer applications, such as for baggage and cargo screening in transportation safety, are both more complex and more powerful.


First, by using multiple beams and detectors, three-dimensional views of the contents of a container can be visualized. Second, neutron activation of conventional materials in the container help identify their content, such as nitrogen-rich compounds that might be [[explosives]]. Third, if fissionable materials are present, there will be a net increase of neutrons emitted when the container is irradiated.
First, by using multiple beams and detectors, three-dimensional views of the contents of a container can be visualized. Second, neutron activation of conventional materials in the container help identify their content, such as nitrogen-rich compounds that might be explosives. Third, if fissionable materials are present, there will be a net increase of neutrons emitted when the container is irradiated.
===Nuclear weapons===
===Nuclear weapons===
===Nuclear reactors===
===Nuclear reactors===
In [[nuclear reactor]]s for power and research, neutron generation steadily increases as more and more fissionable material comes into close proximity. The challenge is less to generate them than to control their rate of flow, and the basic mechanism is to have control rods, of neutron-absorbing materials, interspersed with the rods containing the fissionables. Mechanically inserting or withdrawing numbers of control rods is the usual method of fine-tuning the neutron generation rate.
In [[nuclear reactor]]s for power and research, neutron generation steadily increases as more and more fissionable material comes into close proximity. The challenge is less to generate them than to control their rate of flow, and the basic mechanism is to have control rods of neutron-absorbing materials, interspersed with the rods containing the fissionables. Mechanically inserting or withdrawing numbers of [[neutron moderator]] control rods is the usual method of fine-tuning the neutron generation rate.
==Neutron generators==
 
There are three basic ways to generate neutrons: radioisotopes, particle accelerators, and nuclear reactors. Each has advantages and disadvantages for specific applications.
Neutron flux (&phi;) is the total path length covered by all neutrons in one cubic centimeter during one second, expressed as
===Neutron generation from radioisotopes===
 
In the first nuclear weapons, an initiator, at the center of the fissionable material, emitted neutrons. Codenamed the "Urchin", it was a sphere of mixed [[polonium]] (<sub>210</sub>Po) and [[beryllium]]. <ref name=Sublette8.0>{{citation
::&phi; =<math>n</math> <math>v</math>
| contribution = Section 8.0 The First Nuclear Weapons
 
| title = Nuclear Weapons Frequently Asked Questions
where:
| date = Version 2.18: 3 July 2007 
 
| first = Carey | last = Sublette
::&phi; = neutron flux (neutrons/cm<sup>2</sup>-sec)
| url = http://nuclearweaponarchive.org/Nwfaq/Nfaq8.html}}</ref> Without going through its complex mechanical design, the basic material was a hollow beryllium sphere, grooved on the inside, and with a solid beryllium pellet at the center. As the urchin was explosively compressed and vaporized, alpha particles emitted by the Po-210 then struck beryllium atoms, which released neutrons. The bomb also depended on a number of other mechanical aids to generate enough neutrons for the critical mass, such as neutron-reflecting outer shells that redirected neutrons into the core
 
===Neutron generators from particle accelerators===
::<math>n</math> = neutron density (neutrons/cm<sup>3</sup>)
Later neutron sources, based on [[linear particle accelerators]], which is a cylinder with an ion source at one end and an ion target at the other end. The space between them contains deuterium, tritium, or some mixture depending on the specific generator design.  Electrical current supplied to the source causes an electrical arc and generates hydrogen ions, which are then accelerate using electromagnetic force from another, accelerating electrode, which sends the accelerated cloud into the target. Individual neutrons (i.e., not a beam) are generated by the ions hitting the target, which has one or more hydrogen isotopes on its surface. <ref name=Sublette4.1>{{citation
 
| contribution = 4.1.8.2 External Neutron Initiators (ENIs)  
::<math>v</math> = neutron velocity (cm/sec)
| title = Nuclear Weapons Frequently Asked Questions
 
| date = Version 2.18: 3 July 2007 
The term neutron flux in some applications (for example, cross section measurement) is used as parallel beams of neutrons traveling in a single direction.  
| first = Carey | last = Sublette
| url = http://nuclearweaponarchive.org/Nwfaq/Nfaq4.html}}</ref>


The most obvious difference between the unclassified generators used in industry, and the classified detectors used in weapons, is size and ruggedness. Both types do have a superficial resemblance to a household hair dryer, with the ion source at the motor/heater end. The first tube used [[titanium]] hydride targets, but the standard in the indusctry uses [[scandium]] hydride. 
:Intensity (''I'') of a neutron beam = neutron density <math>n</math> times the average neutron velocity <math>v</math>


One unclassified design described by Sublette is the Milli-Second Pulse (MSP) tube developed at Sandia. "It has a scandium tritide target, containing 7 curies of tritium as 5.85 mg of ScT2 deposited on a 9.9 cm<sup>2</sup> [[molybdenum]] backing. A 0.19-0.25 amp deuteron beam current produces about 4-5 x 10<sup>7</sup> neutrons/amp-microsecond in a 1.2 millisecond pulse with accelerator voltages of 130-150 KeV for a total of 1.2 x 10<sup>10</sup> neutrons per pulse. For comparison the classified Sandia model TC-655, which was developed for nuclear weapons, produced a nominal 3 x 10<sup>9</sup> neutron pulse." The neutrons are not produced as a burst, as a stream that triggers successive neutron multiplication cycles in the ultimate target. In the design of an ENI, the critical parameters are the beam intensity, and the speed and shape of the initial ionization pulse.
:Directional beam intensity is equal to the number of neutrons per unit area and time (neutrons/cm<sup>2</sup>-sec) falling on a surface perpendicular to the direction of the beam.


In a weapon, the ENI can be placed wherever mechanically convenient, as long as it is within 1-2 meters of the core and not separated by a neuttron absorber.  Since the neutron generator usually contains [[tritium]], radioactive decay of the tritium means that the generators are components that need periodic replacement. <ref name=Burroughs>{{citation
The neutron flux in a reactor is made up of many neutron beams traveling in various directions. Then, the neutron flux becomes the scalar sum of these directional flux intensities (added as numbers and not vectors), that is,
|  title = Researchers model neutron generator in hostile radiation environment: Reentry vehicle radiation transport simulated in 3-D for first time
::&phi; = ''I<sub>1</sub> + I<sub>2</sub> + I<sub>3</sub> +...I<sub>n</sub>''
  | first = Chris | last = Burroughs
| journal = Sandia Labs News
| Volume = 52
| issue= 2  
| date = January 28, 2000
| url = http://www.sandia.gov/LabNews/LN01-28-00/neutron_story.html}}</ref>


New, more compact and long-lived ENIs are available. Obviously, an ENI for a bomb does not need a long service life once active, but industrial generators have tended to exhaust their ions. New generator designs, however, provide the target with a source of fresh ions.  The lifetime of these new devices may be in the thousands of hours. <ref name=LBL-NG>{{citation
Since the atoms in a reactor do not interact preferentially with neutrons from any particular direction, all of these directional beams contribute to the total rate of reaction.<ref>{{citation
  | title = Compact Neutron Generators
  | url = http://www.hss.doe.gov/nuclearsafety/ns/techstds/standard/hdbk1019/h1019v1.pdf
  | id = IB-1764
  | id = DOE-HDBK-1019/1-93
  | author = Technology Transfer, Lawrence Berkeley Laboratories
  | title = Reactor Theory (Neutron Characteristics)
  | url =http://www.lbl.gov/tt/techs/lbnl1764.html}}</ref>
  | contribution = Neutron Sources}}, page 2</ref>


==References==
==References==
{{reflist|2}}
{{reflist|2}}[[Category:Suggestion Bot Tag]]

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A neutron is a subatomic particle that normally is part of the nucleus of a chemical element. When free (not bound to a nucleus), a neutron can have important physical, chemical, and biological[1] effects. Free neutrons are not stable particles, but undergo radioactive decay with a half-life of approximately 10 minutes.

The mass mn of a neutron[2] is close to, but not equal to, the mass of a proton:

mn = 1.674 927 211 × 10−27 kg.

Structure

According to the standard model, the neutron consists of three quarks, one up quark and two down quarks.[3] A free neutron shows beta decay, breaking down into a proton, an electron, and an antineutrino with a lifetime of about 15 minutes. Because it disintegrates, the free neutron does not exist in nature. Neutrons do not carry electric charge: they pass unhindered through the electrical fields within liquids and solids.

The neutron g-factor is:[4]

corresponding to a nuclear magnetic moment of:[5]

μn = −0.966 236 41 × 10−26 J/T,

or about −1.913 nuclear magnetons (μN):[6]

So far, a theoretical calculation of the magnetic moment of the neutron in terms of quarks exchanging gluons is a work in progress, with the present estimate as −1.82 nuclear magnetons.[7]

History

The existence of the neutron was discovered, in 1932, by Sir James Chadwick, who received the 1935 Nobel Prize in Physics for his work. A repeatable experimental demonstration of the existence of the neutron solved a number of then-outstanding problems in physics, although the applications and significance of neutrons were in their infancy.[8]

Role in the nucleus

Health effects

From the biological standpoint, neutrons are indirectly ionizing.[1] A given dosage by particles may have greater biological effect than the same dosage from X-rays or gamma rays (see Acute radiation syndrome).

Applications

Applications involve a neutron generator to provide the neutrons, a means of directing them at a target, and an application-specific means for assessing their effects.

Biological

Analytical

Neutron activation analysis, which has several variants, is a widely used technique for measuring the proportions of elements in a sample, including trace elements. A basic analyzer has a neutron source, preferably a appropriate nuclear reactor, detectors for gamma rays emitted by the target, and an extensive database and technical skills to interpret the interaction and its results. Portable analyzers are restricted to radioisotope and accelerator sources.

Neutron capture method

Also called the (n,gamma) reaction, it depends on non-elastic collisions of neutrons with nuclei in the target. The nucleus struck becomes excited, with the excitation energy dependent on the particular element. The period of excitation is usually quite short, and, as the nucleus loses exciting energy, it will emit one or more gamma rays, at characteristic times and energy levels. Half-lives of excited nuclei can range from less than a second to months, and it is these times and energies that are analyzed.[9]

Imaging

There are various industrial applications of neutrons for such purposes as inspecting the quality of welds, which are relatively straightforward with a generator on one side and an imaging detector on the other. Newer applications, such as for baggage and cargo screening in transportation safety, are both more complex and more powerful.

First, by using multiple beams and detectors, three-dimensional views of the contents of a container can be visualized. Second, neutron activation of conventional materials in the container help identify their content, such as nitrogen-rich compounds that might be explosives. Third, if fissionable materials are present, there will be a net increase of neutrons emitted when the container is irradiated.

Nuclear weapons

Nuclear reactors

In nuclear reactors for power and research, neutron generation steadily increases as more and more fissionable material comes into close proximity. The challenge is less to generate them than to control their rate of flow, and the basic mechanism is to have control rods of neutron-absorbing materials, interspersed with the rods containing the fissionables. Mechanically inserting or withdrawing numbers of neutron moderator control rods is the usual method of fine-tuning the neutron generation rate.

Neutron flux (φ) is the total path length covered by all neutrons in one cubic centimeter during one second, expressed as

φ =

where:

φ = neutron flux (neutrons/cm2-sec)
= neutron density (neutrons/cm3)
= neutron velocity (cm/sec)

The term neutron flux in some applications (for example, cross section measurement) is used as parallel beams of neutrons traveling in a single direction.

Intensity (I) of a neutron beam = neutron density times the average neutron velocity
Directional beam intensity is equal to the number of neutrons per unit area and time (neutrons/cm2-sec) falling on a surface perpendicular to the direction of the beam.

The neutron flux in a reactor is made up of many neutron beams traveling in various directions. Then, the neutron flux becomes the scalar sum of these directional flux intensities (added as numbers and not vectors), that is,

φ = I1 + I2 + I3 +...In

Since the atoms in a reactor do not interact preferentially with neutrons from any particular direction, all of these directional beams contribute to the total rate of reaction.[10]

References

  1. 1.0 1.1 World Health Organization, Ionizing Radiation
  2. Neutron mass. Fundamental physical constants. National Institute of Standards and Technology. Retrieved on 2011-03-28.
  3. See, for example, Brian Robert Martin (2009). Nuclear and particle physics, 2nd ed. John Wiley and Sons, p. 97. ISBN 0470742747.  and Kristin Kiriluk (2007). “Chapter 1: Introduction”, Simulation of W boson production in the PHENIX muon spectrometers. ProQuest, pp. 1 ff. ISBN 0549402810. 
  4. Neutron g factor. Fundamental physical constants. NIST. Retrieved on 2011-03-28.
  5. Neutron magnetic moment. Fundamental physical constants. NIST. Retrieved on 2011-03-28.
  6. Nuclear magneton. Fundamental physical constants. NIST. Retrieved on 2011-03-28.
  7. See, for example, Brian Martin (2009). “Table 3.5”, Nuclear and Particle Physics: An Introduction, 2nd ed. Wiley, p. 104. ISBN 0470742747. 
  8. Colwell, Catharine H., Famous Experiments: The Discovery of the Neutron, PhysicsLab
  9. Worcester Polytechnic Institute's Nuclear Engineering, Neutron Activation Analysis
  10. , Neutron Sources, Reactor Theory (Neutron Characteristics), DOE-HDBK-1019/1-93, page 2