Neutron

Neutron
classification
Elementary particle
fermion
hadron
baryon
nucleon
characteristics
Quark composition 2 down, 1 UP
charge 0 C
proper mass

1.008 664,915 78 (55) u =
1,674 927 16 (13) ×10 -27 kg =
1838,683 6550 (40) × m e

rest energy

939.565 330 (38) MeV =
1,505 349 46 (12) ×10 -10 J

diameter 1,5·10,-15 m
magnetic moment -0.966 236 40 (23) ×10 -26 J T -1
spin 1/2
middle life span freely 886+ 2 s

the neutron is a relatively long-lived, electrically neutral hadron with the symbol n.It is, like the proton, a nucleon.

Table of contents

physical description

the neutron and is thereby a fermion has the spin 1/2. In addition it belongs to the baryons.Neutrons consist for their part of two D quarks and a u quark (formula udd).

Like its components also the neutron is subject both to the strong one and the weak reciprocal effect.

It is remarkable that the neutron - although it an electrically neutralParticle actual a magnetic moment has and concomitantly the electromagnetic reciprocal effect is subject. The explanation of this magnetic moment is a very difficult problem of theoretical physics.

The proper mass of the neutron is around approximately 0,14% (1.293 MeV) more largely thanthose of the proton. The diameter of the neutron amounts to about 1.6 <math> \ /math <cdot> 10,-15 M.

neutrons as components of atomic nuclei

the neutrons in the atomic nucleus contribute to the atomic total mass and determine thereby the isotope of the element. The chemical behavior however essentially remains the same, since this is determined by the physical characteristics of the atomic shell, for their number of electrons because of the electrical neutrality of the neutron of the neutron number is independent.

The atomic nucleus of nearly all elements consists of protonsand neutrons. The exception is the most frequently arising hydrogen isotope, whose atomic nucleus consists only of an individual proton.


weak reciprocal effect

neutrons are subject to the weak reciprocal effect. The thereby caused beta decay provides for the fragmentation of a neutron ina proton, an electron and an antineutrino. The free neutron is unstable and disintegrates with one life span of approximately <math> 886.7 \ pm1,9< /math> Seconds (scarcely 15 minutes):

<math> \ mathrm {n} \ rightarrow \ mathrm {p} + \ mathrm {e} ^-+ \ bar {\ nu} _e + 0.78 \, \ mathrm {MeV} </math>

The reverse reaction is theoretically possible, but statistically extremely rare,there three particles with an exactly co-ordinated energy to the reaction to be brought would have:

<math> \ mathrm {p} + \ mathrm {e} ^-+ \ bar {\ nu} _e + 0.78 \, \ mathrm {MeV} \ rightarrow \ mathrm {n} </math>


The radioactive half-life of the free neutron is not however very precisely well-known. The reason is the difficult measurement: Free neutrons leave themselveswith neutron sources, nuclear reactions or nuclear fission or by means of the core photo effect win. They are caught however in shortest time by subject, before the decay takes place. For scientific computations the lifetime of free neutrons is however an elementary constant, the one substantial influenceon the development of the cosmos had. In an early phase of the universe free neutrons constituted an important part of the subject. So one could particularly reconstruct the emergence the light elements (and their isotopic distribution) better, if the disintegration constant of theOf neutron exactly admits would be. In addition one expects a better understanding of the weak reciprocal effect, which is responsible for the radioactive beta decay.

A group at the cock Meitner institute (HMI) in Berlin works to measure the decay time of the free neutron more precisely. Neutrons becomein a three-dimensional magnetic trap enclosed. The reciprocal effect of the neutron with the magnetic field strengths of the cage is made by the weak magnetic dipole of the neutron. This causes a particularly sophisticated organization of the field in the cage. The neutrons, those from a research reactor inthe trap arrive, by super+liquid helium in the chamber are braked and caught. The high-energy electron originating from the decay serves as proof in the chamber. It ionizes several helium atoms, those on its flight path over molecule processes (Excimere) a measurable light signalsend. Neutrons do not leave a trace, D in a bubble chamber. h. they do not work ionizing.


strong ones and electromagnetic reciprocal effect

neutrons are subject to the strong reciprocal effect, not however the electrostatic repulsion. Therefore they participate stabilizing on atomic nucleimany protons. While the positively charged protons experience among themselves both attractive (strong reciprocal effect) and repulsive forces (electromagnetic reciprocal effect), no electrostatic repulsion arises between neutrons among themselves and between neutrons and protons.

The electromagnetic reciprocal effect is weaker than thoseStrong reciprocal effect, works however contrary to this even over larger distances, since it behaves in reverse proportionally to the square of the distance, while the strong reciprocal effect, which one also as an expression form between the quarks and gluons, outthose the nucleons exist, working color intensity to interpret knows, only very short distance affects and at larger distance fast against zero strives. The stability of an atomic nucleus knows approach as the equilibrium between the attractive strong one and the repulsive electrical Kraftare regarded.

Although they cannot be diverted by static electrical fields, neutrons are subject nevertheless also to the electromagnetic reciprocal effect, since they have over a spin and a magnetic moment.


typical nuclear reactions with neutrons

free neutrons, in particular thermal, i.e. slow neutrons are absorbed by many atomic nuclei. The new atomic nucleus developing thereby, an isotope of the original core, is often radioactive.

Some few nuclides split after catching a neutron. In nuclear reactors this process becomes as nuclear chain reactionto the power production used.

effects of neutron irradiation on subject

(S. also radiation damage)

the material properties of metals and other materials by neutron irradiation are worsened. This limits the life span of components in e.g. Nuclear reactors. In nuclear fusion reactors with their higherEnergy of the neutrons arises this problem strengthened.

The effect on living subject is likewise usually harmful. It been based with fast neutrons to a large extent on of these knocked against protons, which correspond to a strongly ionizing radiation. The use of fast neutrons inthe radiotherapy is limited to few special cases. Thermal neutrons produce 1 H (n , gamma) by the capture nuclear reaction 2 H at hydrogen gamma radiation, which ionizes for its part.

history of the discovery and research

the first steps for discovery of theNeutron from roll ago Bothe and its student harsh ore Becker were done. They described an unusual type of radiation, for which developed in the year 1930, if them beryllium with polonium - alpha particles fired at. A goal of the attempts was it, a theory Ernest Rutherfords tooconfirm, after which with this procedure radiation very high-energy should be emitted. They initially falsely regarded the penetrating radiation, which they could determine with these attempts by electrical counting methods, corresponding as gamma radiation. The same attempts made italso with lithium and boron, and it came finally to the result that the observed „gamma-rays possessed “more energy than the alpha particles, with which they had fired at the atoms. During the irradiation of beryllium with alpha particles non developed like beforeexpected - boron, but carbon.

<math> {} ^ {9} _4 \ mathrm + {} ^ {4} _2 \ mathrm He^ {2+} \ tons {} ^ {12} _6 \ mathrm C + {} ^ {1} _0 \ mathrm n </math>

At the same time thereby the observed, radiation very high-energy, which had a large penetrating power by subject, developed however otherwise infor gamma radiation very unusual behavior showed. The jets were for example able to shift light atoms in fast motion. A more exact analysis showed that the energy of these would have had to be „gamma radiation “so largely that it everything up to thenAcquaintance would have far exceeded. Thus it arose more and more doubt whether it really concerned with the observed radiation gamma-rays. According to the accomplished attempt one called the radiation in the meantime „beryllium radiation “.

Later, Irène Joliot curies placed one year to 1931and its married man Frédéric Joliot curie with experiments with the beryllium radiation the following fact firmly: If one lets „the beryllium radiation meet “into an ionization chamber, then indicates this no considerable river. One brings a wasserstoffhaltige material layer however before the ionization chamber (for example paraffin),then the river in the chamber rises strongly. As a cause for the current rise in the ionization chamber the married couple assumed Joliot curies that from the wasserstoffhaltigen paraffin protons are extracted by „the beryllium radiation “, which then in the ionization chamber the necessary ionizationcause. They could occupy their assumption even by the proof of such recoil protons in the Wilson cloud chamber. As mechanism Joliot curies assumed the Compton-effect relatives procedure. The hard gamma radiation should transfer the protons the necessary impulse. Estimations showed however that toProduction of a recoil proton, whose trace length in the cloud chamber amounted to about 26 cm, a gamma energy of approximately 50 would be MeV necessary, which appeared rather unrealistic.

James Chadwick - a pupil Rutherfords - did not believe like its instructor in one „Compton-effect withProton “and assumed that „the beryllium radiation “had to consist of particles. When Irene and Frederic Joliot curie published their test results, in which they showed that Bothes was „beryllium radiation “able to drive out from paraffin protons with high energy were for Chadwickclearly that it could concern not gamma radiation, but only around particles with the proton comparable measures. In the numerous attempts it repeated the experiments of the married couple Joliot curie and confirmed the Joliot Curie core centrifuge effect. Further it could do 1932 experimentallyprove that it acted with Bothes „beryllium radiation “not around gamma-rays, but rather around a projectile rain out of fast moved particles, which possess approximately the mass of the proton however is electrically neutral. It recognized that the characteristics of this type radiation ratherwith those one already twelve years before of Ernest Rutherford as core component of assumed neutral particle in agreement to bring were. Since the particles discovered now did not carry an electrical charge, it called them neutrons.

With this discovery the description could of the Atomic structure to be completed for the time being: The atomic nucleus, consisting of protons and neutrons is surrounded by a covering from electrons. With an electrically neutral atom the number of negatively charged electrons in the atomic shell corresponds always exactly to those of the positively charged protonsin the atomic nucleus, whereas the number of neutrons in the core can vary.

In the same year 1932 Heisenberg set up his nucleon theory.

Around 1940 one assumed that the neutron represents a connection from proton and electron. So one would have all atomsto these 2 components to attribute can. Only with the further development of quantum mechanics and nuclear physics it became clear that there cannot be electrons as durable components of the core.

production and proof of free neutrons

it give manydifferent kinds of neutron sources. In the applied research (investigation of the structure and dynamics of subject by flexible and inelastic neutron dispersion) above all neutrons from research reactors are used. There the neutrons become during the nuclear fission freely. These fission neutrons have howeveran energy within the range of some MeV and are to be used therefore for the most experiments and applications not directly. By impacts with atomic nuclei these fast neutrons can transfer their energy gradually to these. This happens in that the coresurrounding water tank (light or heavy water as moderator). The water usually possesses a temperature of for instance 300K. After numerous collisions with the cores of the water atoms the neutrons exhibit an appropriate power spectrum. One speaks then of thermalNeutrons (see following table). The power spectrum of the neutrons can be shifted by bringing in additional moderators additionally to higher or lower energies. One calls these additional moderators also secondary neutron sources. To the production of so-called cold neutrons comes frequentlyliquid deuterium with a temperature from for instance 20K to the employment. So-called hot neutrons are usually moderated with graphite moderators with for instance 3000K. The neutrons from the moderator tank or the secondary neutron sources are led by so-called jet pipes to the experiments.However still sufficient many neutrons must remain in the reactor core or reflect there back, in order to keep the nuclear chain reaction upright. Cold, thermal and hot neutrons exhibit in each case a certain energy distribution and thus wavelength distribution. For many experiments however monochromatic becomeor monoenergetic neutrons, thus neutrons of uniform energy, needs. One e.g. receives these. by the employment of a Monochromators. As Monochromatoren perfect of single crystals or mosaic crystals are used (silicon, germanium, copper, Grahpit etc.), whereby by the selection of certain Bragg reflexes and Monochromatorwinkeldifferent wavelengths (energies) from the wavelength distribution to be extracted can.

Since neutrons do not carry an electrical charge, they cannot be proven directly with the detectors which are based on ionization. The proof of neutrons happens by means of neutron detectors.

classification

that Activation cross-section of reactions between neutrons and other particles varies strongly with the kinetic energy of the neutrons. The following classification developed:

kinetic energy wavelength [Å]
cold neutrons < 2 MeV 12,8 - 6,4
thermal neutrons < 100 MeV 6.4 -0.9
epithermal neutrons < 1 eV 0,9 - 0,28
intermediate neutrons 0,5 eV to 10 keV
snaps neutrons 10 keV relativistic to 20
neutrons MeV > 20 MeV

literature

  • Dirk Dubbers, pure hard Scherm: Neutron research at Institut Laue Langevin. Physics in our time 34 (3), S. , ISSN 0031-9252 Arno, Helmut Schober was called 108
  • - 111 (2003): Neutron spectroscopy at solids: With neutrons on the trace of the electrons. Physics in our time 34 (3), S. 112 - 118 (2003), ISSN 0031-9252
  • Torsten mercenary: Neutrons in particle physics: The neutron, the cosmos and the forces. Physics in our time 34 (3), S. 127 - 132 (2003), ISSN 0031-9252
  • M. Honal, W. Scherer, G. Hitting a corner old: For what do chemists Neutronen need? Message from chemistry51 (11), S. 1133 - 1138 (2003), ISSN 1439-9598

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Wiktionary: neutron - word origin, synonyms and translations
Wiktionary: Neutron - word origin, synonyms and translations
 

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