Neutron

The neutron [ˈnɔɪ̯trɔn] (plural neutrons [nɔɪ̯ˈtroːnən]) is an electrically neutral baryon with the formula symbol . Along with the proton, it is a component of almost all atomic nuclei and thus of the matter with which we are familiar. Neutron and proton, together called nucleons, belong as baryons to the fermions and the hadrons.

If a neutron is not bound in an atomic nucleus - it is then also called "free" - it is unstable, but with a comparatively long half-life of about 10 minutes. It transforms into a proton, an electron and an electron antineutrino by beta decay. Free neutrons are used in the form of neutron radiation. They are critically important in nuclear reactors.

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

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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