Neutron (subatomic particle)
A neutral baryon found in atomic nuclei, made of quarks. Discovered in 1932, neutrons determine isotopes, enable nuclear reactions and scattering probes, and play key roles in astrophysics.
Neutrons are electrically neutral subatomic particles that, together with protons and electrons, form the structure of ordinary matter. They occupy the nucleus of an atom alongside protons and help determine the mass and stability of atomic nuclei. A neutron's lack of net electric charge distinguishes it from the positively charged proton and the negatively charged electron; this neutrality allows neutrons to penetrate atomic electron clouds and to mediate nuclear forces without Coulomb repulsion. See basic atomic structure: atoms, and the concept of electric charge, and the notion of a neutral particle.
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Physical characteristics and internal structure
Neutrons have a rest mass slightly greater than that of protons and far larger than electrons. In free form (outside a nucleus) a neutron is unstable and undergoes beta decay with a mean lifetime on the order of minutes, transforming into a proton while emitting an electron and an antineutrino. At the sub‑nucleon level neutrons are composite particles classified as baryons and, more broadly, as hadrons. Each neutron consists of three valence quarks: one up quark and two down quarks. The fractional electric charges of these quarks (+2/3 for an up quark, −1/3 for each down quark) sum to zero, which explains the neutron's overall neutral electric charge. The quarks are bound together by the strong interaction, which is carried by force carriers called gluons, and their behavior is described by quantum chromodynamics (quarks and the strong force).
Role in nuclei and isotopes
Within atomic nuclei neutrons contribute to nuclear binding through the residual strong force and help offset the electrostatic repulsion between protons. The number of neutrons relative to protons determines an element's isotopes: atoms with the same proton count but different neutron numbers exhibit distinct nuclear properties and may be stable or radioactive. Nuclear stability depends on shell structures and neutron-to-proton ratios; heavy nuclei generally require a higher proportion of neutrons to remain bound. Neutrons themselves do not experience Coulomb repulsion, which makes them especially important in forming tight nuclear configurations and in processes such as neutron capture and neutron-induced fission.
Discovery and historical context
The existence of a neutral nuclear constituent had been anticipated by theorists before it was observed experimentally. Early 20th‑century research into nuclear structure led Ernest Rutherford to suggest the possibility of a neutral particle in the nucleus (Ernest Rutherford). The neutron was discovered in 1932 by James Chadwick, who interpreted penetrating radiation emitted when alpha particles struck a thin foil of beryllium and other light elements; his experiments showed that the radiation could eject protons from paraffin, and he concluded the new radiation consisted of neutral particles with mass similar to the proton (James Chadwick).
Uses, applications and importance
Free and bound neutrons play central roles across science and technology. In nuclear reactors and weapons, neutrons initiate and sustain chain reactions by inducing fission in certain heavy isotopes. In experimental physics, neutron scattering and diffraction are powerful tools for probing atomic and magnetic structures because neutrons interact with nuclei and magnetic moments rather than electric charge, enabling studies of crystals, liquids and biological macromolecules. In astrophysics, neutrons dominate the composition of neutron stars—compact remnants of massive stellar collapse—where matter exists at densities far beyond terrestrial experience.
Notable properties and distinctions
- Decay: a free neutron beta-decays into a proton, an electron and an antineutrino, so ‘‘free’’ neutrons are unstable whereas many bound neutrons in nuclei are stable.
- Magnetic moment and spin: despite lacking net charge, neutrons possess a magnetic moment and intrinsic spin, reflecting their internal charged constituents and quantum structure.
- Interactions: neutrons interact via the strong nuclear force and the weak force; their lack of electric charge gives them unique penetrating power useful in imaging and materials analysis.
Because neutrons bridge particle physics, nuclear physics and astrophysics, they remain a subject of active research. Studies of neutron behavior test theories of the strong and weak interactions, inform nuclear engineering, and deepen our understanding of the universe's densest objects.
Physical description
Elementary properties
The neutron carries no electric charge (hence the name), but a magnetic moment of -1.91 nuclear magnetons. Its mass is about 1.675 - 10-27 kg (1.008 665 u). As a baryon, it is composed of three quarks - one up quark and two down quarks (formula udd). The neutron has the spin 1/2 and is therefore a fermion. As a composite particle it is spatially extended with a diameter of about 1.7 - 10-15 m.
The antiparticle of the neutron is the antineutron, which was first detected in 1956 by Bruce Cork at the bevatron in proton-proton collisions.
A short-lived, observable, but unbound system of two neutrons is the dineutron.
Elementary interactions
The neutron is subject to all four interactions known in physics: the gravitational force, the strong interaction, the electromagnetic interaction and the weak interaction.
The strong interaction - or more precisely the nuclear force, a kind of residual interaction of the strong interaction acting between the quarks - is responsible for neutrons being bound in nuclei and also determines the behaviour of neutrons in collisions with atomic nuclei.
Although the neutron is electrically neutral and thus not subject to electrostatic attraction or repulsion, it is nevertheless subject to electromagnetic interaction due to its magnetic moment. This fact as well as the spatial expansion are clear indications that the neutron is a composite particle.
The weak interaction is responsible for the beta decay of the (free, see below) neutron into a proton, an electron and an electron antineutrino.
Decay and lifetime
The neutron has a rest energy of 939.6 MeV, 1.3 MeV (0.14%) greater than the proton. If it is not bound in an atomic nucleus, it decays as a beta-minus emitter (β-emitter) into a proton, an electron and an electron antineutrino:
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The average lifetime of the neutron is about 880 seconds (just under 15 minutes); this corresponds to a half-life of about 610 seconds. This is by far the largest half-life of all unstable hadrons. It is difficult to measure, because a neutron released in a normal material environment (even in air) is usually reabsorbed by an atomic nucleus in a fraction of a second, so it does not "experience" its decay. Accordingly, the decay is meaningless in practical applications, and the neutron can be considered a stable particle for this purpose. However, decay is interesting from a fundamental physics point of view. In an early phase of the universe, free neutrons made up a significant part of matter; one can better understand the formation of especially the light elements (and their isotopic distribution) if the lifetime of the neutron is known precisely. It is also hoped to gain a better understanding of the weak interaction.
The lifetime of the neutron can be determined using two different methods: the beam method, which gives 888.0 ± 2.0 s, and the bottle method, which gives 879.6 ± 0.6 s (according to a more recent (2018) measurement, 877.7 s). As the measurement methods have improved, this difference of about 1%, initially thought to be a measurement error, has become more significant and is now slightly more than 4 σ. The cause is unknown.
Neutrons as constituents of atomic nuclei
With the exception of the most common hydrogen isotope (protium, 1H), whose nucleus consists of only a single proton, all atomic nuclei contain both protons and neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. The particle types proton and neutron are collectively called nucleons (from Latin nucleus, nucleus).
β-- and β+-decay of atomic nuclei
→ Main article: Beta radiation
How strongly an atomic nucleus is bound depends on the number of protons Z and neutrons N, but above all on the ratio of these numbers. For lighter nuclei, the bond is strongest at about the same number (N/Z ≈ 1) (e.g., at mass number 40, the most stable nucleus is 40Ca, with 20 protons and neutrons each); at large mass numbers, the ratio shifts up to N/Z ≈ 1.5, e.g., in 208Pb, because as Z increases, the electrical repulsion of the protons has an increasingly destabilizing effect. This difference in binding energy has a stronger effect than the rather small mass difference of proton and neutron, so that of nuclei with the same mass number, these are the most stable in each case.
A nucleus that is too rich in neutrons can - like the free neutron - transform into a nucleus that has one neutron less and one proton more by β--decay while retaining its mass number. In this process, a neutron has transformed into a proton. In contrast, a nucleus that is too low in neutrons can transform by β+-decay into a nucleus that has one more neutron and one less proton. In this process, a proton transforms into a neutron, a process that is not possible for free protons.
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The reversal of neutron decay occurs when a proton-rich atomic nucleus reacts with an electron of the atomic shell (electron capture) as well as under the extreme conditions of the formation of a neutron star:
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Related articles
Author
AlegsaOnline.com Neutron (subatomic particle) Leandro Alegsa
URL: https://en.alegsaonline.com/art/69388
Sources
- hyperphysics.phy-astr.gsu.edu : "Protons and neutrons"
- chemed.chem.purdue.edu : chemed.chem.purdue.edu/genchem/history/rutherford.html
- helmholtz-berlin.de : "Discovery of Neutrons"
- nobelprize.org : "The Nobel Prize in Physics 1935: James Chadwick"
- britannica.com : "Neutron (subatomic particle)"