|Dimensions||1.008 664 915 95 (49) u
1.674 927 498 04 (95) · 10 −27 kg
1838.683 661 73 (89) m e
|Resting energy||939.565 420 52 (54) MeV
|Compton wavelength||1.319 590 905 81 (75) · 10 −15 m|
|magnetic moment||−9.662 3651 (23) 10 −27 J / T
−1.913 042 73 (45) μ N
|g factor||−3.826 085 45 (90)|
|1.832 471 71 (43) 10 8 1 / ( s T )|
|Spin parity||1/2 +|
|Isospin||1/2 (z component −1/2)|
|average lifespan||880.2 ± 1.0 s|
|1 up, 2 down|
The neutron [ ˈnɔɪ̯trɔn ] ( plural neutrons [ nɔɪ̯ˈtroːnən ]) is an electrically neutral baryon with the symbol . In addition to the proton, it is part of almost all atomic nuclei and thus of the matter we are familiar with . Neutrons and protons, collectively called nucleons , belong to the fermions and hadrons as baryons .
If a neutron is not bound in an atomic nucleus - it is also called "free" - it is unstable, but with a comparatively long half-life of around 10 minutes. It transforms into a proton, an electron and an electron antineutrino through beta decay . Free neutrons are used in the form of neutron radiation . They are critically important in nuclear reactors .
The neutron has no electrical charge (hence the name), but has a magnetic moment of −1.91 nuclear magnetons . Its mass is around 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 approx. 1.7 · 10 −15 m.
A short-lived, observable, but not bound system of two neutrons is the dineutron .
The strong interaction - more precisely the nuclear force, a kind of residual interaction of the strong interaction between the quarks - is responsible for the fact that neutrons are bound in nuclei and also determines the behavior of neutrons when they collide with atomic nuclei.
The neutron is electrically neutral and is therefore not subject to electrostatic attraction or repulsion, but due to its magnetic moment it is nevertheless subject to electromagnetic interaction. This fact and the spatial extent are clear indicators that the neutron is a composite particle.
Decay and lifespan
At 939.6 MeV, the neutron has a rest energy that is 1.3 MeV (0.14%) greater than the proton. If it is not bound in an atomic nucleus, it decays as a beta-minus radiator (β - radiator ) into a proton, an electron and an electron antineutrino:
The mean lifetime of the neutron is 880.2 seconds (just under 15 minutes); this corresponds to a half-life of 610.1 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 (also in air) is usually absorbed again by an atomic nucleus in fractions of a second, so it does not "experience" its decay. Accordingly, the decay is meaningless in practical applications, and the neutron can be regarded as a stable particle for it. In terms of basic physics, however, the decay is interesting. In an early phase of the universe, free neutrons made up a significant part of matter; the formation of the light elements in particular (and their isotope distribution) can be better understood if the lifetime of the neutron is known exactly. In addition, it is hoped that a better understanding of the weak interaction will be gained.
The lifetime of the neutron can be determined with the help of two different methods: with 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) results. With the improvement of the measurement methods, this difference of approx. 1%, which was initially thought to be a measurement error, has become more and more significant and is now a little more than 4 σ. The cause is unknown.
Neutrons as components of atomic nuclei
With the exception of the most common isotope of hydrogen (protium, 1 H), whose atomic nucleus only consists of 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 the 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. In the case of lighter nuclei, the bond is strongest with roughly the same number ( N / Z ≈ 1) (e.g. with a mass number of 40 the most stable nucleus is 40 Ca with 20 protons and 20 neutrons each); with large mass numbers the ratio shifts up to N / Z ≈ 1.5, e.g. B. in 208 Pb, since with increasing Z the electrical repulsion of the protons has an increasingly destabilizing effect. This difference in the binding energy has a stronger effect than the rather small mass difference between proton and neutron, so that nuclei with the same mass number are the most stable.
A nucleus that is too neutron-rich can - like the free neutron - transform itself into a nucleus that has one neutron less and one more proton through β - decay while maintaining the mass number. A neutron has been converted into a proton. On the other hand, a nucleus that is too neutron- poor can transform itself into a nucleus that has one more neutron and one less proton through β + decay. A proton is transformed into a neutron, a process that is not possible with free protons.
The reversal of neutron decay occurs when a proton-rich atomic nucleus reacts with an electron in the atomic shell ( electron capture ) and under the extreme conditions when a neutron star is formed :
There are many different types of neutron sources in which neutrons are released from atomic nuclei.
For the investigation of condensed matter through elastic and inelastic neutron scattering , neutrons from research reactors are mainly used. There the neutrons are released during nuclear fission . These fast neutrons have energies in the range of a few MeV and must first be slowed down to around a millionth of their kinetic energy for material investigations. Spallation sources are a newer alternative to research reactors .
Since neutrons do not carry an electrical charge, they cannot be detected directly with detectors based on ionization . The detection of neutrons is done by means of neutron detectors . At low neutron energies (below about one hundred keV) these are always based on a suitable nuclear reaction , e.g. B. Neutron absorption with subsequent decay:
The interaction of free neutrons with matter is very different depending on their kinetic energy. That is why neutrons are classified according to their energy. The terms are not used consistently. The following table is based on:
Classification kinetic energy speed temperature Slow neutrons up to 100 eV up to 150 km / s up to 800,000 K Ultra cold neutrons (UCN) below 0.05 to 0.23 µeV below 3.2 to 6.8 m / s below 0.4 to 1.8 mK Very cold neutrons (VCN) ~ 10 −4 eV ~ 150 m / s ~ 1 K Cold neutrons below 0.025 eV under 2.2 km / s up to 200 K Thermal neutrons about 0.025 eV about 2.2 km / s about 200 K Epithermal neutrons 0.025 to 1 eV 2.2 to 15 km / s 200 to 8,000 K Resonance neutrons 1 to 100 eV 15 to 150 km / s 8,000 to 800,000 K. Medium-fast neutrons 100 eV to 500 keV 150 to 10,000 km / s 800,000 K to 4 billion K Fast neutrons from 500 keV from 10,000 km / s over K 4 billion
Neutron sources of any kind generate fast neutrons with 2 to 5 MeV. These can be slowed down by moderators to temperatures up to that of the moderator. Depending on the strength of the moderation, medium-speed up to thermal neutrons can be generated. With the help of frozen moderators, cold to very cold neutrons (VCN) can be generated. Neutrons can be cooled even further with the aid of neutron centrifuges.
“Cold” and “hot” neutrons
The energy spectrum of the neutrons can be shifted with additional moderators at high or low temperatures . These additional moderators at research reactors are also known as secondary neutron sources. Liquid deuterium with a temperature of around 20 K is often used to obtain “cold” neutrons . “Hot” neutrons are usually produced at around 3000 K with graphite moderators. Cold, thermal and hot neutrons each have a specific, more or less broad energy distribution and thus wavelength distribution .
The neutrons from a research reactor are guided to the experiments through beam pipes ( neutron guides ) from the moderator tank or the secondary neutron sources. However, enough neutrons still have to remain in the reactor core or be reflected back there in order to maintain the chain reaction.
Ultra-cold neutrons (UCN) have very little kinetic energy and move at less than 5 m / s, so that they can be stored magnetically, mechanically or gravitationally. They are reflected by vessel walls made of beryllium , beryllium oxide , magnesium , aluminum or nickel below a material-dependent limit energy. Storage experiments allow observation times of minutes, much longer than experiments with neutron beams.
For many experiments, monoenergetic neutrons, i.e. neutrons of uniform energy, are required. This is obtained from reactors such. B. by using a monochromator . This is a single crystal or mosaic crystal made of, for example, silicon, germanium, copper or graphite; By using certain Bragg reflections and monochromator angles, different wavelengths (energies) can be selected from the wavelength distribution (see also neutron super mirror ).
Monochromatic neutrons of higher energies can be obtained from accelerators from suitable nuclear reactions.
Effect of neutron beams
Typical processes triggered by neutrons
The scattering can be elastic or inelastic. In the case of inelastic scattering, the atomic nucleus remains in an excited state , which then (mostly) returns to the ground state by emitting gamma radiation . The elastic scattering of fast neutrons by light atomic nuclei ( moderators ) causes them to slow down until they become thermal neutrons .
Thermal neutrons in particular are absorbed by many atomic nuclei. If afterwards only gamma radiation and no particles with mass are emitted, this reaction is called neutron capture . The resulting new atomic nucleus is the isotope of the original nucleus , which is one mass unit heavier, and can be radioactive ( neutron activation ). Nuclides with a particularly large cross-section for the absorption of thermal neutrons are known as neutron absorbers . 113 Cd and 10 B are mostly used technically , for example in neutron shields and to control nuclear reactors .
Some very heavy nuclides can be split by neutron absorption . If the splitting of an atomic nucleus releases several new neutrons, a chain reaction can result with the release of large amounts of energy. This is used both in a controlled manner in nuclear reactors and in an uncontrolled manner in nuclear weapons .
Effects on matter
The material properties of metals and other materials are impaired by neutron irradiation. This limits the life of components in e.g. B. Nuclear reactors. In possible nuclear fusion reactors with their higher energy of neutrons this problem would occur more intensely.
The effect on living tissue is also detrimental. In the case of fast neutrons, it is largely based on protons triggered by them, which correspond to strongly ionizing radiation . This harmful effect has occasionally been tested as radiation therapy to combat cancer cells. Thermal neutrons generate gamma radiation by capturing neutrons in hydrogen, which in turn ionizes.
In nuclear reactors, nuclear fusion reactors and nuclear weapons, free (thermal to fast) neutrons play a decisive role. The most important physical quantity is the position and time dependent neutron flux . It is treated numerically with the theory of neutron diffusion or on the basis of the Boltzmann equation or the Monte Carlo simulation .
Discovery and Exploration
In 1920, Ernest Rutherford predicted a neutral core building block, which could possibly be a proton-electron combination, he spoke of a "collapsed hydrogen atom". William Draper Harkins called this particle a neutron in 1921.
The first steps towards the discovery of the neutron were taken by Walther Bothe and his student Herbert Becker. In 1930 they described an unusual type of radiation that was produced when they bombarded beryllium with alpha radiation from the radioactive decay of polonium . The aim was to confirm Ernest Rutherford's observations that very high-energy radiation was emitted during this process. Accordingly, they initially mistook the penetrating radiation, which they were able to determine in these experiments with the help of electrical counting methods, to be gamma radiation . They made the same experiments with lithium and boron , and finally came to the conclusion that the “gamma rays” observed had more energy than the alpha particles with which they had bombarded the atoms. When beryllium was irradiated with alpha particles, it was not boron, as expected, that was produced, but carbon . In today's notation, the observed nuclear reaction reads :
or in short
The observed, very high-energy radiation had a great ability to penetrate matter, but otherwise showed a behavior that was unusual for gamma radiation. For example, it was able to set light atoms in rapid motion. A more detailed analysis showed that the energy of this “gamma radiation” should have been so great that it would have exceeded anything known up to then by far. So more and more doubts arose as to whether these were really gamma rays. According to the experiment carried out, the radiation was now called "beryllium radiation".
In 1931 Irène Joliot-Curie and her husband Frédéric Joliot-Curie established the following fact during experiments with beryllium radiation: If the "beryllium radiation" is allowed to hit an ionization chamber , it does not show any significant current. However, if a hydrogenous material layer (for example paraffin) is placed in front of the ionization chamber, the current in the chamber increases sharply. The Joliot-Curie couple suspected that the cause was that the “beryllium radiation” releases protons from the hydrogen-containing paraffin, which then cause ionization in the ionization chamber . They were able to substantiate their assumption by demonstrating such recoil protons in Wilson's cloud chamber . They suspected that the mechanism was a process related to the Compton effect . The hard gamma radiation should give the protons the necessary momentum. Estimates showed, however, that an unrealistically high gamma energy of around 50 MeV would be required to generate a recoil proton whose track length in the cloud chamber was about 26 cm.
James Chadwick - a student of Rutherford who, like him, initially supported the hypothesis of a strongly bound electron-proton state - did not believe, like the latter, in a “Compton effect in the proton” and assumed that the “beryllium radiation” consisted of particles must. When Irène and Frédéric Joliot-Curie published their experimental results, in which they showed that Bothes “beryllium radiation” was able to knock protons out of paraffin with high energy, it was clear to Chadwick that it was not gamma radiation, only could be particles with a mass comparable to that of a proton. In the numerous experiments he repeated the experiments of Joliot-Curie and confirmed their observation. In 1932 he was able to confirm experimentally that the "beryllium radiation" was not gamma rays, but fast moving particles that had roughly the same mass as the proton, but were electrically neutral; the properties of this radiation were more likely to be reconciled with those of a neutral particle that Ernest Rutherford had assumed to be a core component twelve years earlier. Since the particles that had now been discovered had no electrical charge, he called them neutrons. Chadwick published his discovery in 1932. The publication appeared under Letters to the Editor , is just under a page long, and won him the Nobel Prize in Physics in 1935.
The fact that the combination of beryllium as the target and polonium as the source of alpha particles results in a high neutron yield is, according to current knowledge, explained by the fact that the energy gain (Q value) of the reaction on 9 Be is particularly high at 5.7 MeV and that 210 Po with 5.3 MeV provides one of the highest natural alpha energies.
With the discovery of the neutron, the description of the atomic structure was completed for the time being: the atomic nucleus, consisting of protons and neutrons, is surrounded by a shell of electrons . In the case of an electrically neutral atom, the number of negatively charged electrons is the same as that of positively charged protons in the atomic nucleus, whereas the number of neutrons in the nucleus can vary.
In the same year 1932 Werner Heisenberg presented his nucleon theory.
In 1940 it was still assumed that the neutron was a combination of protons and electrons. So one could have traced all atoms back to these two building blocks. Only with the further development of quantum mechanics and nuclear physics did it become clear that there can be no electrons as permanent constituents of the nucleus.
“Neutron” was originally Wolfgang Pauli's name for the occurrence of an (anti-) neutrino during beta decay, which he postulated in 1930 . The term neutrino, proposed by Enrico Fermi , was only established later.
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