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Neutron

2007 Schools Wikipedia Selection. Related subjects: General Physics

                                                      Neutron
                                                   Classification


                      Subatomic particle
                           Fermion
                            Hadron
                            Baryon
                           Nucleon
                           Neutron

                                                    Properties


                               Mass:          1.674 927 29(28) × 10^−27 kg
                                                 939.565 560(81) MeV/c²
                              Radius:             about 0.8 × 10^−15 m
                         Electric charge:                 0 C
                               Spin:                       ½
                      Magnetic dipole moment:     -1.91304273(45) μ[N]
                        Quark composition:            2 Down, 1 Up

   In physics, the neutron is a subatomic particle with no net electric
   charge and a mass of 939.573 MeV/c² (1.6749 × 10^-27 kg, slightly more
   than a proton). Its spin is ½. Its antiparticle is called the
   antineutron. The neutron, along with the proton, is a nucleon.

   The nucleus of most atoms (all except the most common isotope of
   hydrogen, protium, which consists of a single proton only) consists of
   protons and neutrons. 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.)
   Isotopes are atoms of the same element that have the same atomic number
   but different masses due to a different number of neutrons.

   A neutron is classified as a baryon, and consists of two down quarks
   and one up quark.

Stability

   Outside the nucleus, free neutrons are unstable and have a mean
   lifetime of 885.7±0.8 seconds (about 15 minutes), decaying by emitting
   an electron and antineutrino to become a proton:

          \hbox{n}\to\hbox{p}+\hbox{e}^-+\overline{\nu}_{\mathrm{e}}

   This decay mode, known as beta decay, can also occur within certain
   unstable nuclei. Protons can also transform into neutrons through the
   process of electron capture, sometimes called Inverse Beta Decay. Both
   beta decay and electron capture are types of radioactive decay.

   Particles inside the nucleus are typically resonances between neutrons
   and protons, which transform into one another by the emission and
   absorption of pions.

Interactions

   The neutron interacts through all four fundamental interactions: the
   electromagnetism, 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
   moment of the same order as the proton; second, it is composed of
   electrically charged quarks. Thus, the electromagnetic interaction is
   primarily important to the neutron in deep inelastic scattering and in
   magnetic interactions.

   The neutron experiences the weak interaction through beta decay into a
   proton, electron and electron antineutrino. It experiences the
   gravitational force as does any energetic body; however, gravity is so
   weak that it may be neglected in most 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
   quarks (one up quark, two down quarks) into a single particle. The
   residual strong force is also responsible for the binding of nuclei:
   the nuclear force. The nuclear force plays the leading role when
   neutrons pass through matter. Unlike charged particles or photons, the
   neutron cannot lose energy by ionizing atoms. Rather, the neutron goes
   on its way unchecked until it makes a head-on collision with an atomic
   nucleus. For this reason, neutron radiation is extremely penetrating
   and dangerous.

Detection

   The common means of detecting a charged 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
   ^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 neutron activation, inducing
   radioactivity. In particular, knowledge of neutrons and their behaviour
   has been important in the development of nuclear reactors and nuclear
   weapons.

   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-rays for the analysis of condensed matter. Neutrons are
   complementary to the latter in terms of atomic contrasts by different
   scattering 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
   aluminium plates has driven ongoing research into neutron microscopy
   and neutron/gamma ray tomography.

   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.

Discovery

   In 1930 Walther Bothe and H. Becker in Germany found that if the very
   energetic alpha particles emitted from polonium fell on certain of the
   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 (later 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. Such
   uncharged particles were eventually called neutrons, apparently from
   the Latin root for neutral and the Greek ending -on (by imitation of
   electron and proton).

Current developments

   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 such clusters should
   not be stable, and therefore should not exist.

   An experiment at the Institut Laue-Langevin (ILL) 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. See FRONTIERS article, and the experiment's web page.

Anti-Neutron

   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.

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