Neutron

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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) μN 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:

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.