neutron

neutron, uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A nucleus with an excess of neutrons is radioactive; the extra neutrons convert to protons by beta decay (see radioactivity). In a nucleus the neutron can be stable, but a free neutron decays with a half-life of about 17 min (1,013 sec), into a proton, an electron, and an antineutrino. The fact that the neutron possesses a magnetic moment suggests that it has an internal structure of electric charge, although the net charge is zero. The electron-scattering experiments of Robert Hofstadter indicate that the neutron, like the proton, is surrounded by a cloud of pions; protons and neutrons are bound together in nuclei by the exchange of virtual pions. The neutron and the proton are regarded by physicists as two aspects or states of a single entity, the nucleon. The antineutron, the neutron's antiparticle, was discovered in 1956. The neutron, like other particles, also possesses certain wave properties, as explained by the quantum theory. The field of neutron optics is concerned with such topics as the diffraction and polarization of beams of neutrons. The formation of images using the techniques of neutron optics is known as neutrography.

Any of a class of extremely dense, compact stars thought to be composed mainly of neutrons with a thin outer atmosphere of primarily iron atoms and electrons and protons. Though typically about 12 mi (20 km) in diameter, they have a mass roughly twice the Sun's and thus extremely high densities (about 100 trillion times that of water). Neutron stars have very strong magnetic fields. A solid surface differentiates them from black holes. Below the surface, the pressure is much too high for individual atoms to exist; protons and electrons are compacted together into neutrons. The discovery of pulsars in 1967 provided the first evidence of the existence of neutron stars, predicted in the early 1930s and believed by most investigators to be formed in supernova explosions. Seealso white dwarf star.

Learn more about neutron star with a free trial on Britannica.com.

or enhanced radiation warhead

Small thermonuclear weapon that produces minimal blast and heat but releases large amounts of lethal radiation. The blast and heat are confined to a radius of only a few hundred yards; within a somewhat larger area, the bomb throws off a massive wave of neutron and gamma radiation, which is extremely destructive to living tissue. Such a bomb could be used with deadly efficiency against tank and infantry formations on the battlefield without endangering towns or cities only a few miles away. It can be carried in a missile or delivered by a howitzer or even an attack aircraft.

Learn more about neutron bomb with a free trial on Britannica.com.

One of the constituent particles of every atomic nucleus except ordinary hydrogen. Discovered in 1932 by James Chadwick (1891–1974), it has no electric charge and has nearly 1,840 times the mass of the electron. Free neutrons undergo beta decay with a half-life of about 10 minutes. Thus, they are not readily found in nature, except in cosmic rays. They are a penetrating form of radiation. When bombarded with neutrons, various elements undergo nuclear fission and release more free neutrons. If enough free neutrons are produced, a chain reaction can be sustained. This process led to the development of nuclear power as well as the atomic bomb. Neutron beams produced in cyclotrons and nuclear reactors are important probes of matter, revealing details of structure in both organic and inorganic susbtances.

Learn more about neutron with a free trial on Britannica.com.

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

The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.

The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a nucleus is the atomic number and defines the type of element the atom forms. 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.

Neutron stability and beta decay

Outside the nucleus, free neutrons are unstable and have a mean lifetime of (about 15 minutes), decaying by emission of a negative electron and antineutrino to become a proton:

→ + +

This decay mode, known as beta decay, can also transform the character of neutrons within unstable nuclei.

Bound inside a nucleus, protons can also transform via inverse beta decay into neutrons. In this case, the transformation occurs by emission of a positron (antielectron) and a neutrino (instead of an antineutrino):

→ + +

The transformation of a proton to a neutron inside of a nucleus is also possible through electron capture:

+ → +

Positron capture by neutrons in nuclei that contain an excess of neutrons is also possible, but is hindered because positrons are repelled by the nucleus, and 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.

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 , , , , , and 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. The fissioning of elements like uranium-235 and plutonium-239 is caused by their absorption of neutrons.

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

Because 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 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 no effect on neutrons except for a small effect of a magnetic field because of the neutron's 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, 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.

These uncharged particles were called neutrons, apparently from the Latin root for neutral and the Greek ending -on (by imitation of electron and proton).

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 . Since the difference is only about 2 standard deviations away from zero, this does not give any convincing evidence of CPT-violation.

Current developments

Electric dipole moment

The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent electric dipole moment. The predicted value is, however, well below the current sensitivity of experiments. From several unsolved puzzles in particle physics, it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories going beyond the Standard Model generally lead to much larger predictions for the electric dipole moment of the neutron. Currently, there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment.

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.

Protection

Exposure to free 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 (such as protons). The normal precautions of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. Some particular thought must be given to how to protect from neutron exposure, however. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density make for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. 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 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 water effects neutron absorption in nuclear fission reactors: usually neutrons are so strongly absorbed by normal water that fuel-enrichement with fissionable isotope, is required. The deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is therefore used in CANDU-type reactors, in order to slow ("moderate") neutron velocity, so that they are more effective at causing nuclear fission, without capturing them.

See also

Neutron capture nucleosynthesis

R-process

S-process

Fields concerning neutrons

Particle physics Quark model

Chemistry Neutron Detection

Neutron Scattering .

Types of neutrons

Fast neutron Free neutron

Neutron radiation and the Sievert radiation scale Neutron temperature, used to classify neutron types

Thermal neutron Dineutron

Tetraneutron .

Objects containing neutrons

The nuclei of atoms with the exception of protium, the most common isotope of hydrogen (and as a result, all ordinary matter with the exception of hydrogen)

Dineutron Neutronium

Neutron star Tetraneutron

Neutron sources

• Neutron sources
• Neutron generator

Processes involving neutrons

Neutron bomb Neutron diffraction

Neutron flux Neutron transport

References