Neutron



This article is a discussion of neutrons in general. For the specific case of a neutron found outside the nucleus, see free neutron.
Neutron

The quark structure of the neutron.
Composition: one up, two down
Family: C
Spin: ½

In physics, the neutron is a nucleon.

The carbon-14 isotope has 6 protons and 8 neutrons.

A neutron consists of two down baryon.

Neutron Stability and Beta Decay

  Outside the nucleus, antineutrino to become a proton:[2] \hbox{n}\to\hbox{p}+\hbox{e}^-+\overline{\nu}_{\mathrm{e}}. 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 electron capture: \hbox{p}+\hbox{e}^{-}\to\hbox{n}+{\nu}_{\mathrm{e}} . 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 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 interactions: the electromagnetic, weak nuclear, strong nuclear and gravitational interactions.

Although the neutron has zero net charge, it may interact electromagnetically in two ways: first, the neutron has a magnetic interactions.

The neutron experiences the weak interaction through 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 neutron radiation is extremely penetrating.

Detection

Main article: neutron detection

The common means of detecting 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 3He, 6Li, 10B, 233U, 235U, 237Np and 239Pu 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 radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons.

cross 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.[4][5][6]

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

Due to the fact that 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 muon source.

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 electric or magnetic fields. 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

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Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. (November 2007)

In 1930 Walther Bothe and H. Becker in Germany found that if the very energetic proton).

Anti-Neutron

Main article: 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 (9±5)×10−5. Since the difference is only about 2 standard deviations away from zero, this does not give any convincing evidence of CPT-violation.[3]

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.[7][8]

Tetraneutrons

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. a

Protection

Exposure to neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and 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 ordinary effects neutron absorption in nuclear fission, without capturing them.

See also

Fields concerning neutrons

Types of neutrons

Objects containing neutrons

Neutron sources

Processes involving neutrons

v • d • e
Polaron

References

  1. ^ 1935 Nobel Prize in Physics
  2. ^ Particle Data Group Summary Data Table on Baryons
  3. ^ a b Particle Data Group's Review of Particle Physics 2006
  4. ^ Nature 357, 390-391 (04 June 1992); doi:10.1038/357390a0
  5. ^ Physorg.com, "New Way of 'Seeing': A 'Neutron Microscope'"
  6. ^ NASA.gov: "NASA Develops a Nugget to Search for Life in Space"
  7. ^ FRONTIERS, "Particle physics chills out", retrieved 25 November 2007
  8. ^ Neutron Electric Dipole Moment experiment's web page, retrieved 25 November 2007
 
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