Neutron

This article is about the subatomic particle. For other uses, see 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.

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 1.008665 u Radius: about 1 × 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 stable isotopes 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 consists of two down quarks and one up quark, thus classifying it as a baryon.

Stability

The Feynmann diagram of the neutron beta decay process

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 beta decay or electron capture. Both beta decay and electron capture are types of radioactive decay.

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 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

Main article: neutron 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 behavior 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 aluminum 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.

Sources

Due to the fact that free neutrons are unstable, they (neutron radiation) 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 sources by neutron transport. For access to intense neutron sources, researchers must go to specialist facilities, such as the ISIS facility in the UK, which is currently the world's most intense pulsed neutron and 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 almost no effect on neutrons (there is a small effect of a magnetic field on the free neutron because of its magnetic moment).

Discovery

In 1930 Walther Bothe and H. Becker in Germany found that if the very energetic alpha particles 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 (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

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

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.

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. The normal expectations of radiation protection apply—avoid exposure, but if you do need to be exposed keep as far from the source as you can and keep exposure time to the minimum. Some thought must however be given to how to protect oneselves from such exposure. For "normal" radiation, e.g. alpha particles, beta particles, or gamma rays, then material of a high atomic number and with high density may be used as shielding, and frequently lead is used for this. This approach will not work with neutrons, since the absorption of neutrons is not monotonic with atomic number as it is with alpha, beta, or gamma. 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 since ordinary hydrogen scatters neutrons, so this often means simple concrete blocks, or paraffin loaded plastic blocks may be the best protection. Paradoxically to most people, deuterium does not have anything like the same scattering ability for neutrons as ordinary hydrogen—something used to advantage in heavy water reactors like the CANDU nuclear reactor arising from Canada.

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, and therefore not exist.

See also

Fields concerning neutrons

• particle physics
• quark model
• chemistry
• Neutron Detection
• Neutron Scattering

Types of neutrons

• nucleon
• fast neutron
• free neutron
• thermal neutron
• neutron radiation and the Sievert radiation scale
• neutron temperature is used to classify neutron types

Objects containing neutrons

• nucleus
• dineutron
• tetraneutron
• neutronium
• neutron star

Neutron sources

• Neutron sources
• Neutron generator
• Neutron Sources around the world

Processes involving neutrons

• neutron transport
• neutron diffraction
• neutron bomb