The meaning of the word neutron. The meaning of the word neutron The role of the neutron in the fission of the uranium nucleus

What is a neutron? This question most often arises among people who are not involved in nuclear physics, because the neutron in it is understood as an elementary particle that has no electric charge and has a mass that is 1838.4 times greater than the electronic one. Together with the proton, whose mass is slightly less than the mass of the neutron, it is the "brick" of the atomic nucleus. In elementary particle physics, the neutron and the proton are considered to be two different forms of one particle - the nucleon.

The neutron is present in the composition of the nuclei of atoms for each chemical element, the only exception is the hydrogen atom, the nucleus of which is one proton. What is a neutron, what structure does it have? Although it is called the elementary "brick" of the kernel, it still has its own internal structure. In particular, it belongs to the family of baryons and consists of three quarks, two of which are down-type quarks, and one is up-type. All quarks have a fractional electric charge: the top one is positively charged (+2/3 of the electron charge), and the bottom one is negatively charged (-1/3 of the electron charge). That is why the neutron does not have an electric charge, because it is simply compensated for by the quarks that make it up. However, the neutron's magnetic moment is not zero.

In the composition of the neutron, the definition of which was given above, each quark is connected to the others with the help of a gluon field. The gluon is the particle responsible for the formation of nuclear forces.

In addition to the mass in kilograms and atomic mass units, in nuclear physics, the mass of a particle is also described in GeV (gigaelectronvolts). This became possible after Einstein's discovery of his famous equation E=mc 2 , which relates energy to mass. What is a neutron in GeV? This is a value of 0.0009396, which is slightly larger than that of the proton (0.0009383).

Stability of the neutron and atomic nuclei

The presence of neutrons in atomic nuclei is very important for their stability and the possibility of the existence of the atomic structure itself and matter in general. The fact is that protons, which also make up the atomic nucleus, have a positive charge. And their approach to close distances requires the expenditure of huge energies due to the Coulomb electric repulsion. The nuclear forces acting between neutrons and protons are 2-3 orders of magnitude stronger than the Coulomb ones. Therefore, they are able to keep positively charged particles at close distances. Nuclear interactions are short-range and manifest themselves only within the size of the nucleus.

The neutron formula is used to find their number in the nucleus. It looks like this: the number of neutrons = the atomic mass of the element - the atomic number in the periodic table.

A free neutron is an unstable particle. Its average lifetime is 15 minutes, after which it decays into three particles:

• electron;
• proton;
• antineutrino.

Prerequisites for the discovery of the neutron

The theoretical existence of the neutron in physics was proposed as early as 1920 by Ernest Rutherford, who tried to explain in this way why atomic nuclei do not fall apart due to the electromagnetic repulsion of protons.

Even earlier, in 1909 in Germany, Bothe and Becker established that if light elements, such as beryllium, boron or lithium, are irradiated with high-energy alpha particles from polonium, then radiation is formed that passes through any thickness of various materials. They assumed that it was gamma radiation, but no such radiation known at that time had such a great penetrating power. The experiments of Bothe and Becker have not been properly interpreted.

Discovery of the neutron

The existence of the neutron was discovered by the English physicist James Chadwick in 1932. He studied the radioactive radiation of beryllium, conducted a series of experiments, obtaining results that did not coincide with those predicted by physical formulas: the energy of radioactive radiation far exceeded theoretical values, and the law of conservation of momentum was also violated. Therefore, it was necessary to accept one of the hypotheses:

1. Or angular momentum is not conserved in nuclear processes.
2. Or radioactive radiation consists of particles.

The scientist rejected the first assumption, since it contradicts the fundamental physical laws, so he accepted the second hypothesis. Chadwick showed that the radiation in his experiments was formed by particles with zero charge, which have a strong penetrating power. In addition, he was able to measure the mass of these particles, establishing that it is slightly larger than that of a proton.

Slow and fast neutrons

Depending on the energy that a neutron has, it is called slow (of the order of 0.01 MeV) or fast (of the order of 1 MeV). Such a classification is important, since some of its properties depend on the speed of the neutron. In particular, fast neutrons are well captured by nuclei, leading to the formation of their isotopes, and causing their fission. Slow neutrons are poorly captured by the nuclei of almost all materials, so they can easily pass through thick layers of matter.

The role of the neutron in the fission of the uranium nucleus

If you ask yourself what a neutron is in nuclear energy, then we can say with confidence that this is a means of inducing the process of fission of the uranium nucleus, accompanied by the release of large energy. This fission reaction also produces neutrons of various speeds. In turn, the generated neutrons induce the decay of other uranium nuclei, and the reaction proceeds in a chain manner.

If the uranium fission reaction is uncontrolled, then this will lead to an explosion of the reaction volume. This effect is used in nuclear bombs. The controlled fission reaction of uranium is the source of energy in nuclear power plants.

NEUTRON
Neutron

Neutron is a neutral particle belonging to the class of baryons. Together with the proton, the neutron forms atomic nuclei. Neutron mass mn = 938.57 MeV/c 2 ≈ 1.675 10 -24 g. The neutron, like the proton, has a spin of 1/2ћ and is a fermion.. It also has a magnetic moment μ n = - 1.91μ N , where μ N = e ћ /2m r s is the nuclear magneton (m r is the mass of the proton, the Gaussian system of units is used). The size of a neutron is about 10 -13 cm. It consists of three quarks: one u-quark and two d-quarks, i.e. its quark structure is udd.
The neutron, being a baryon, has the baryon number B = +1. The neutron is unstable in the free state. Since it is somewhat heavier than a proton (by 0.14%), it undergoes decay with the formation of a proton in the final state. In this case, the law of conservation of the baryon number is not violated, since the baryon number of the proton is also +1. As a result of this decay, an electron e - and an electron antineutrino e are also formed. The decay occurs due to the weak interaction.

Decay scheme n → p + e - + e.

The lifetime of a free neutron is τ n ≈ 890 sec. In the composition of the atomic nucleus, the neutron can be as stable as the proton.
The neutron, being a hadron, participates in the strong interaction.
The neutron was discovered in 1932 by J. Chadwick.

Explanatory dictionary of the Russian language. D.N. Ushakov

neutron

neutron, m. (from Latin neutrum, lit. neither one nor the other) (physical. new). A material particle entering the nucleus of an atom, devoid of an electric charge, is electrically neutral.

Explanatory dictionary of the Russian language. S.I. Ozhegov, N.Yu. Shvedova.

neutron

A, m. (special). An electrically neutral elementary particle with a mass almost equal to that of a proton.

adj. neutron, th, th.

New explanatory and derivational dictionary of the Russian language, T. F. Efremova.

neutron

m. Electrically neutral elementary particle.

Encyclopedic Dictionary, 1998

neutron

NEUTRON (eng. neutron, from lat. neuter - neither one nor the other) (n) a neutral elementary particle with a spin of 1/2 and a mass exceeding the mass of a proton by 2.5 electron masses; refers to baryons. In the free state, the neutron is unstable and has a lifetime of approx. 16 min. Together with protons, the neutron forms atomic nuclei; the neutron is stable in nuclei.

Neutron

(eng. neutron, from lat. neuter ≈ neither one nor the other; symbol n), neutral (not possessing an electric charge) elementary particle with spin 1/2 (in units of the Planck constant) and a mass slightly exceeding the mass of a proton. All atomic nuclei are built from protons and nitrogen. The magnetic moment of N. is approximately equal to two nuclear magnetons and is negative, that is, it is directed opposite to the mechanical, spin, angular momentum. N. belong to the class of strongly interacting particles (hadrons) and are included in the group of baryons, that is, they have a special internal characteristic - baryon charge, equal, like that of a proton (p), +

N. were discovered in 1932 by the English physicist J. Chadwick, who established that the penetrating radiation discovered by the German physicists W. Bothe and G. Becker, which occurs when atomic nuclei (in particular, beryllium) are bombarded with a-particles, consists of uncharged particles with a mass close to the proton mass.

N. are stable only as part of stable atomic nuclei. Svobodny N. ≈ unstable particle decaying into a proton, an electron (e-) and an electron antineutrino:

mean lifetime of H. t » 16 min. In matter, free neutrons exist even less (in dense substances, units ≈ hundreds of microseconds) due to their strong absorption by nuclei. Therefore free N. arise in the nature or turn out in laboratory only as a result of nuclear reactions (see. Neutron sources ). In turn, free nitrogen is capable of interacting with atomic nuclei, up to the heaviest; disappearing, nitrogen causes one or another nuclear reaction, of which the fission of heavy nuclei is of particular importance, as well as the radiation capture of nitrogen, which in a number of cases leads to the formation of radioactive isotopes. The great efficiency of neutrons in the implementation of nuclear reactions, the uniqueness of the interaction of very slow neutrons with matter (resonant effects, diffraction scattering in crystals, etc.) make neutrons an exceptionally important research tool in nuclear physics and solid state physics. In practical applications, neutrons play a key role in nuclear power engineering, the production of transuranic elements and radioactive isotopes (artificial radioactivity), and are also widely used in chemical analysis (activation analysis) and geological exploration (neutron logging).

Depending on the energy of N., their conditional classification is accepted: ultracold N. (up to 10-7 eV), very cold (10-7≈10-4 eV), cold (10-4≈5 × 10-3 eV), thermal (5 ×10-3≈0.5 eV), resonant (0.5≈104 eV), intermediate (104≈105 eV), fast (105≈108 eV), high-energy (108≈1010 eV) and relativistic (³ 1010 eV); All neutrons with energies up to 105 eV are united by the common name slow neutrons.

══On the methods of registering neutrons, see Neutron detectors.

Main characteristics of neutrons

Weight. The most precisely determined quantity is the mass difference between the neutron and the proton: mn ≈ mр= (1.29344 ╠ 0.00007) MeV, measured by the energy balance of various nuclear reactions. From a comparison of this quantity with the proton mass, it turns out (in energy units)

mn = (939.5527 ╠ 0.0052) MeV;

this corresponds to mn» 1.6╥10-24g, or mn» 1840 me, where me ≈ the electron mass.

Spin and statistics. The value of 1/2 for the spin N. is confirmed by a large body of facts. The spin was directly measured in experiments on the splitting of a beam of very slow neutrons in a nonuniform magnetic field. In the general case, the beam should split into 2J+ 1 individual beams, where J ≈ spin H. In the experiment, splitting into 2 beams was observed, which implies that J = 1/

As a particle with a half-integer spin, N. obeys Fermi ≈ Dirac statistics (it is a fermion); independently, this was established on the basis of experimental data on the structure of atomic nuclei (see Nuclear shells).

The electric charge of the neutron Q = 0. Direct measurements of Q from the deflection of the H beam in a strong electric field show that at least Q< 10-17e, где е ≈ элементарный электрический заряд, а косвенные измерения (по электрической нейтральности макроскопических объёмов газа) дают оценку Q < 2╥10-22е.

Other neutron quantum numbers. In its properties, N. is very close to the proton: n and p have almost equal masses, the same spin, and are able to mutually transform into each other, for example, in the processes of beta decay; they manifest themselves in the same way in processes caused by strong interaction, in particular, the nuclear forces acting between the pairs p≈p, n≈p and n≈n are the same (if the particles are respectively in the same states). Such a deep similarity allows us to consider N. and the proton as one particle ≈ nucleon, which can be in two different states, differing in electric charge Q. A nucleon in a state with Q \u003d + 1 is a proton, with Q \u003d 0 ≈ N. Accordingly, the nucleon is attributed (by analogy with the usual spin) some internal characteristic ≈ isotonic spin I, equal to 1/2, whose “projection” can take (according to the general rules of quantum mechanics) 2I + 1 = 2 values: + 1/2 and ≈1/2. Thus, n and p form an isotopic doublet (see Isotopic invariance): the nucleon in the state with the projection of the isotopic spin onto the quantization axis + 1/2 is a proton, and with the projection ≈1/2 ≈ H. As components of the isotopic doublet, N. and proton, according to modern systematics of elementary particles, have the same quantum numbers: baryon charge B = + 1, lepton charge L = 0, strangeness S = 0 and positive internal parity. The isotopic doublet of nucleons is a part of a wider group of "similar" particles ≈ the so-called baryon octet with J = 1/2, B = 1 and positive intrinsic parity; in addition to n and p, this group includes L-, S╠-, S0-, X
--, X0 - hyperons, which differ from n and p in strangeness (see Elementary particles).

The magnetic dipole moment of the neutron, determined from nuclear magnetic resonance experiments is:

mn = ≈ (1.91315 ╠ 0.00007) me,

where mn=5.05×10-24erg/gs ≈ nuclear magneton. A particle with spin 1/2, described by the Dirac equation, must have a magnetic moment equal to one magneton if it is charged, and zero if it is not charged. The presence of a magnetic moment in N., as well as the anomalous value of the magnetic moment of the proton (mp = 2.79mya), indicates that these particles have a complex internal structure, that is, there are electric currents inside them that create an additional “anomalous” the magnetic moment of the proton is 1.79my and approximately equal in magnitude and opposite in sign to the magnetic moment H. (≈1.9my) (see below).

Electric dipole moment. From a theoretical point of view, the electric dipole moment d of any elementary particle must be equal to zero if the interactions of elementary particles are invariant with respect to time reversal (T-invariance). The search for an electric dipole moment in elementary particles is one of the tests of this fundamental position of the theory, and of all elementary particles, N. is the most convenient particle for such searches. Experiments using the method of magnetic resonance on a beam of cold N. showed that dn< 10-23см╥e. Это означает, что сильное, электромагнитное и слабое взаимодействия с большой точностью Т-инвариантны.

Neutron interactions

N. participate in all known interactions of elementary particles - strong, electromagnetic, weak and gravitational.

Strong interaction of neutrons. N. and the proton participate in strong interactions as components of a single isotopic doublet of nucleons. The isotopic invariance of strong interactions leads to a certain relationship between the characteristics of various processes involving neutrons and protons, for example, the effective cross sections for p+
--mesons on N. are equal, since the systems p + p and pn have the same isotopic spin I = 3/2 and differ only in the values ​​of the projection of the isotopic spin I3 (I3 = + 3/2 in the first and I3 = ≈ 3/2 in the second cases), the scattering cross sections for K+ on a proton and K╟ on H are the same, and so on. The validity of such relationships has been experimentally verified in a large number of experiments on high-energy accelerators. [In view of the absence of targets consisting of N., data on the interaction of various unstable particles with N. are obtained mainly from experiments on the scattering of these particles by the deuteron (d) ≈ ​​the simplest nucleus containing N.]

At low energies, the actual interactions of neutrons and protons with charged particles and atomic nuclei differ greatly due to the presence of an electric charge on the proton, which determines the existence of long-range Coulomb forces between the proton and other charged particles at such distances at which short-range nuclear forces are practically absent. If the collision energy of a proton with a proton or an atomic nucleus is below the height of the Coulomb barrier (which for heavy nuclei is about 15 MeV), the scattering of the proton occurs mainly due to the forces of electrostatic repulsion, which do not allow particles to approach up to distances of the order of the radius of action of nuclear forces. N.'s lack of an electric charge allows it to penetrate the electron shells of atoms and freely approach atomic nuclei. This is precisely what determines the unique ability of neutrons of relatively low energies to induce various nuclear reactions, including the fission reaction of heavy nuclei. For methods and results of investigations into the interaction of neutrons with nuclei, see the articles Slow neutrons, Neutron spectroscopy, Nuclei of atomic fission, Scattering of slow neutrons by protons at energies up to 15 MeV is spherically symmetric in the system of the center of inertia. This indicates that the scattering is determined by the interaction n ≈ p in a state of relative motion with an orbital angular momentum l = 0 (the so-called S-wave). Scattering in the S-state is a specifically quantum-mechanical phenomenon that has no analogue in classical mechanics. It prevails over scattering in other states, when the de Broglie wavelength H.

of the order of or greater than the radius of action of nuclear forces (≈ Planck's constant, v ≈ N.'s velocity). Since at an energy of 10 MeV the wavelength H.

This feature of neutron scattering by protons at such energies directly provides information on the order of magnitude of the radius of action of nuclear forces. A theoretical consideration shows that scattering in the S-state weakly depends on the detailed form of the interaction potential and is described with good accuracy by two parameters: the effective potential radius r and the so-called scattering length a. In fact, to describe the scattering n ≈ p, the number of parameters is twice as large, since the system np can be in two states with different values ​​of the total spin: J = 1 (triplet state) and J = 0 (singlet state). Experience shows that the lengths of N. scattering by a proton and the effective radii of interaction in the singlet and triplet states are different, i.e., the nuclear forces depend on the total spin of the particles. It also follows from experiments that the bound state of the system np (deuterium nucleus) can exist only when the total spin is 1, while in the singlet state the magnitude of the nuclear forces is insufficient for the formation of a bound state H. ≈ proton. The length of nuclear scattering in the singlet state, determined from experiments on the scattering of protons by protons (two protons in the S-state, according to the Pauli principle, can only be in a state with zero total spin), is equal to the scattering length n≈p in the singlet state. This is consistent with the isotopic invariance of strong interactions. The absence of a bound system pr in the singlet state and the isotopic invariance of nuclear forces lead to the conclusion that there cannot exist a bound system of two neutrons ≈ the so-called bineutron (similarly to protons, two neutrons in the S state must have a total spin equal to zero). Direct experiments on scattering n≈n were not carried out due to the absence of neutron targets, however, indirect data (properties of nuclei) and more direct ≈ the study of reactions 3H + 3H ╝ 4He + 2n, p- + d ╝ 2n + g ≈ are consistent with the hypothesis of isotopic invariance nuclear forces and the absence of a bineutron. [If there were a bineutron, then in these reactions peaks would be observed at well-defined energies in the energy distributions of a-particles (4He nuclei) and g-quanta, respectively.] Although the nuclear interaction in the singlet state is not strong enough to form a bineutron, this is not eliminates the possibility of the formation of a bound system consisting of a large number of neutron nuclei alone. This issue requires further theoretical and experimental study. Attempts to discover experimentally nuclei of three or four nuclei, as well as the nuclei 4H, 5H, and 6H, have so far not yielded a positive result. Despite the absence of a consistent theory of strong interactions, on the basis of a number of existing ideas, it is possible to qualitatively understand some regularities of strong interactions and the structure of neutrons. According to these ideas, the strong interaction between N. and other hadrons (for example, the proton) is carried out by the exchange of virtual hadrons (see Virtual particles) ≈ p-mesons, r-mesons, etc. Such a pattern of interaction explains the short-range nature of nuclear forces, the radius which is determined by the Compton wavelength of the lightest hadron ≈ p-meson (equal to 1.4 × 10-13cm). At the same time, it points to the possibility of a virtual transformation of N. into other hadrons, for example, the process of emission and absorption of a p-meson: n ╝ p + p- ╝ n. The intensity of strong interactions known from experience is such that N. must spend most of his time in such "dissociated" states, being, as it were, in a "cloud" of virtual p-mesons and other hadrons. This leads to a spatial distribution of the electric charge and magnetic moment inside the N., the physical dimensions of which are determined by the dimensions of the "cloud" of virtual particles (see also Form factor). In particular, it turns out to be possible to qualitatively interpret the above-mentioned approximate equality in absolute value of the anomalous magnetic moments of the neutron and the proton, if we assume that the magnetic moment of the neutron is created by the orbital motion of charged p
--mesons emitted virtually in the process n ╝ p + p- ╝ n, and the anomalous magnetic moment of the proton ≈ by the orbital motion of the virtual cloud of p+-mesons created by the process p ╝ n + p+ ╝ p.

Electromagnetic interactions of the neutron. The electromagnetic properties of N. are determined by the presence of a magnetic moment in it, as well as by the distribution of positive and negative charges and currents existing inside N.. All these characteristics, as follows from the previous one, are associated with N.'s participation in a strong interaction that determines its structure. The magnetic moment of N. determines the behavior of N. in external electromagnetic fields: the splitting of the N. beam in an inhomogeneous magnetic field, the precession of the N. spin. quanta (photoproduction of mesons). The electromagnetic interactions of neutrons with the electron shells of atoms and atomic nuclei lead to a number of phenomena that are important for studying the structure of matter. The interaction of the magnetic moment of N. with the magnetic moments of the electron shells of atoms is manifested significantly for N., the wavelength of which is of the order of or greater than atomic dimensions (energy E< 10 эв), и широко используется для исследования магнитной структуры и элементарных возбуждений (спиновых волн) магнитоупорядоченных кристаллов (см. Нейтронография). Интерференция с ядерным рассеянием позволяет получать пучки поляризованных медленных Н. (см. Поляризованные нейтроны).

The interaction of the magnetic moment of N. with the electric field of the nucleus causes a specific scattering of N., which was first indicated by the American physicist Yu. Schwinger and therefore called “Schwinger”. The total cross section for this scattering is small, but at small angles (~ 3╟) it becomes comparable with the cross section for nuclear scattering; N. scattered at such angles are highly polarized.

The interaction of N. ≈ electron (n≈e), not related to the intrinsic or orbital moment of the electron, is reduced mainly to the interaction of the magnetic moment of N. with the electric field of the electron. Another, apparently smaller, contribution to the (n≈e) interaction may be due to the distribution of electric charges and currents inside H. Although the (n≈e) interaction is very small, it has been observed in several experiments.

Weak neutron interaction manifests itself in processes such as the decay of N.:

capture of an electron antineutrino by a proton:

and muon neutrino (nm) by neutron: nm + n ╝ p + m-, nuclear capture of muons: m- + p ╝ n + nm, decays of strange particles, for example L ╝ p╟ + n, etc.

Gravitational interaction of a neutron. N. is the only elementary particle with a rest mass for which gravitational interaction has been directly observed, i.e., the curvature of the trajectory of a well-collimated beam of cold N. in the gravitational field of the earth. The measured gravitational acceleration of N., within the limits of experimental accuracy, coincides with the gravitational acceleration of macroscopic bodies.

Neutrons in the Universe and Near-Earth Space

The question of the amount of neutrons in the universe in the early stages of its expansion plays an important role in cosmology. According to the model of the hot Universe (see Cosmology), a significant part of the initially existing free neutrons has time to decay during expansion. The portion of neutron that is captured by protons should eventually lead to approximately 30% content of He nuclei and 70% content of protons. The experimental determination of the percentage composition of He in the Universe is one of the critical tests of the hot Universe model.

The evolution of stars in some cases leads to the formation of neutron stars, which include, in particular, the so-called pulsars.

In the primary component of cosmic rays, neutrons are absent due to their instability. However, the interactions of cosmic ray particles with the nuclei of atoms in the earth's atmosphere lead to the generation of neutrons in the atmosphere. The 14N(n, p)14C reaction caused by these N. is the main source of the radioactive carbon isotope 14C in the atmosphere, from where it enters living organisms; the radiocarbon method of geochronology is based on determining the content of 14C in organic remains. The decay of slow neutrons diffusing from the atmosphere into near-Earth outer space is one of the main sources of electrons that fill the inner region of the Earth's radiation belt.

Bombardment of uranium nuclei neutrons beryllium rod took much more energy than it was released during the primary fission.

Therefore, for the operation of the reactor, it was necessary that each atom split neutrons

Therefore, for the operation of the reactor, it was necessary that each atom split neutrons beryllium rod, in turn caused the splitting of other atoms.

good source neutrons was affordable even for a poor laboratory: a little radium and a few grams of beryllium powder.

The same amount could be obtained in a cyclotron in two days if one used neutrons, knocked out by accelerated deuterons from a beryllium target.

Then it was possible to show that beryllium radiation actually consists of gamma rays and a flux neutrons.

You see, the original flow neutrons will be a simple spherical expansion from the primary explosion, but beryllium will capture it, ”Fromm explained, standing next to Quati.

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To a boron carbide rod, highly absorbent neutrons, hung a graphite displacer 4.5 m long.

Replacement of these pillars with a graphite displacer, which absorbs less neutrons, and creates a local reactor.

Minimum size The minimum size of a living inert natural body of a natural body is determined by the dispersion is determined by respiration, matter-energy - an atom, mainly a gas electron, corpuscle, biogenic migration of atoms neutron etc.

The idea of ​​a long-lived compound nucleus allowed Bohr to foresee that even very slow neutrons.

The structural difference between them is reduced to the number of protons included in them, neutrons, mesons and electrons, but each new addition to the system of a pair of proton-electron sharply changes the functional properties of the entire aggregate unit as a whole and this is a clear confirmation of the regulation of the number of fnl.

The RBMK-1000 reactor is a channel-type reactor, moderator neutrons- graphite, coolant - ordinary water.

Neutron (English neutron, from Latin neuter - neither one nor the other; symbol n)

neutral (without electric charge) elementary particle with spin 1/2 (in units of Planck's constant ħ ) and a mass slightly greater than the mass of a proton. All atomic nuclei are built from protons and nitrogen. The magnetic moment of N. is approximately equal to two nuclear magnetons and is negative, that is, it is directed opposite to the mechanical, spin, angular momentum. N. belong to the class of strongly interacting particles (hadrons) and are included in the group of baryons, that is, they have a special internal characteristic - a baryon charge (See baryon charge) , equal, like that of the proton (p), + 1. N. were discovered in 1932 by the English physicist J. Chadwick , who established that the penetrating radiation discovered by the German physicists W. Bothe and G. Becker, arising from the bombardment of atomic nuclei (in particular, beryllium) with α-particles, consists of uncharged particles with a mass close to that of a proton.

N. are stable only as part of stable atomic nuclei. Free N. - an unstable particle that decays into a proton, an electron (e -) and an electron antineutrino

mean lifetime of H. τ ≈ 16 min. In matter, free N. exist even less (in dense substances, units - hundreds microsec) due to their strong absorption by nuclei. Therefore, free N. arise in nature or are obtained in the laboratory only as a result of nuclear reactions (see) . In turn, free nitrogen is capable of interacting with atomic nuclei, up to the heaviest; disappearing, nitrogen causes one or another nuclear reaction, of which the fission of heavy nuclei is of particular importance, as well as the radiation capture of nitrogen, which in a number of cases leads to the formation of radioactive isotopes. The great efficiency of neutrons in the implementation of nuclear reactions, the uniqueness of the interaction of very slow neutrons with matter (resonant effects, diffraction scattering in crystals, etc.) make neutrons an exceptionally important research tool in nuclear physics and solid state physics. In practical applications, neutrons play a key role in the production of transuranium elements and radioactive isotopes (artificial radioactivity), and are also widely used in chemical analysis (activation analysis) and geological exploration (neutron logging).

Depending on the energy of N., their conditional classification is accepted: ultracold N. (up to 10 -7 ev), very cold (10 -7 -10 -4 eV), cold (10 -4 -5․10 -3 ev), thermal (5․10 -3 -0.5 eV), resonant (0.5-10 4 ev), intermediate (10 4 -10 5 ev), fast (10 5 -10 8 ev), high-energy (10 8 -10 10 ev) and relativistic (≥ 10 10 eV); all N. with energy up to 10 5 ev are united under the common name Slow neutrons.

Main characteristics of neutrons

Weight. The most precisely determined quantity is the mass difference between the neutron and the proton: m n - m p= (1.29344 ± 0.00007) mev, measured by the energy balance of various nuclear reactions. From a comparison of this quantity with the proton mass, it turns out (in energy units)

m n= (939.5527±0.0052) Mev;

it corresponds m n≈ 1.6 10 -24 G, or m n≈ 1840 m e, where m e - the mass of an electron.

Spin and statistics. The value of 1 / 2 for the spin N. is confirmed by a large set of facts. The spin was directly measured in experiments on the splitting of a beam of very slow neutrons in a nonuniform magnetic field. In the general case, the beam must split into 2 J+ 1 separate bundles, where J- spin H. In the experiment, splitting into 2 beams was observed, from which it follows that J= 1 / 2 . As a particle with a half-integer spin, N. obeys Fermi-Dirac statistics (see Fermi-Dirac statistics) (it is a fermion); independently, this was established on the basis of experimental data on the structure of atomic nuclei (see Nuclear shells).

The electric charge of the neutron Q= 0. Direct measurements Q by the deflection of the N. beam in a strong electric field show that, at least, Q e, where e - elementary electric charge, and indirect measurements (according to the electrical neutrality of macroscopic volumes of gas) give an estimate Q e.

Other neutron quantum numbers. In its properties, N. is very close to the proton: n and p have almost equal masses, the same spin, are capable of mutually transforming into each other, for example, in the processes of beta decay and ; they manifest themselves in the same way in processes caused by strong interactions (See Strong interactions), in particular Nuclear forces , acting between pairs of p-p, n-p and n-n are the same (if the particles are respectively in the same states). Such a deep similarity allows us to consider N. and the proton as one particle - the nucleon, which can be in two different states that differ in electric charge Q. Nucleon in state with Q= + 1 is a proton, with Q = 0 - N. Accordingly, the nucleon is assigned (by analogy with the usual spin) some internal characteristic - isotonic spin I, equal to 1 / 2 , whose "projection" can take (according to the general rules of quantum mechanics) 2 I+ 1 = 2 values: + 1 / 2 and - 1 / 2 . Thus, n and p form an isotopic doublet (see Isotopic invariance) : The nucleon in the state with the projection of the isotopic spin on the quantization axis + 1/2 is a proton, and with the projection - 1/2 - N. As components of the isotopic doublet, N. and the proton, according to the modern systematics of elementary particles, have the same quantum numbers: the baryon charge V=+ 1, Lepton charge L = 0, Weirdness S= 0 and positive intrinsic Parity . The isotopic doublet of nucleons is part of a wider group of "similar" particles - the so-called baryon octet with J = 1 / 2 ,V= 1 and positive intrinsic parity; in addition to n and p, this group includes Λ - , Σ ± -, Σ 0 -, Ξ - -, Ξ 0 - Hyperons , differing from n and p in strangeness (see Elementary particles).

The magnetic dipole moment of the neutron, determined from nuclear magnetic resonance experiments is:

μ n \u003d - (1.91315 ± 0.00007) μ i,

where μ i \u003d 5.05․10 -24 erg/gs - nuclear magneton. A particle with spin 1/2, described by the Dirac equation m , must have a magnetic moment equal to one magneton if it is charged, and zero if it is not charged. The presence of a magnetic moment in N., as well as the anomalous value of the magnetic moment of the proton (μ p \u003d 2.79μ i), indicates that these particles have a complex internal structure, that is, there are electric currents inside them that create an additional “ the anomalous "magnetic moment of the proton is 1.79μ" and approximately equal in magnitude and opposite in sign to the magnetic moment H. (-1.9μ") (see below).

Electric dipole moment. From a theoretical point of view, the electric dipole moment d of any elementary particle must be equal to zero if the interactions of elementary particles are invariant under time reversal (See Time reversal) (T-invariance). The search for an electric dipole moment in elementary particles is one of the tests of this fundamental position of the theory, and of all elementary particles, N. is the most convenient particle for such searches. Experiments using the method of magnetic resonance on a beam of cold N. showed that d n see e. This means that strong, electromagnetic and weak interactions with high accuracy T-invariant.

Neutron interactions

N. participate in all known interactions of elementary particles - strong, electromagnetic, weak and gravitational.

Strong interaction of neutrons. N. and the proton participate in strong interactions as components of a single isotopic doublet of nucleons. The isotopic invariance of strong interactions leads to a certain relationship between the characteristics of various processes involving N. and the proton, for example, the effective cross sections for the scattering of a π + meson on a proton and a π - meson on N. are equal, since the systems π + p and π - n have same isotopic spin I= 3 / 2 and differ only in the values ​​of the projection of the isotopic spin I 3 (I 3 = + 3 / 2 in the first and I 3 = - 3 / 2 in the second case), the scattering cross sections for K + on a proton and K ° on H are the same, etc. The validity of such relationships has been experimentally verified in a large number of experiments on high-energy accelerators. [In view of the absence of targets consisting of N., data on the interaction of various unstable particles with N. are obtained mainly from experiments on the scattering of these particles by the deuteron (d), the simplest nucleus containing N.]

At low energies, the actual interactions of neutrons and protons with charged particles and atomic nuclei differ greatly due to the presence of an electric charge on the proton, which determines the existence of long-range Coulomb forces between the proton and other charged particles at such distances at which short-range nuclear forces are practically absent. If the collision energy of a proton with a proton or an atomic nucleus is below the height of the Coulomb barrier (which for heavy nuclei is about 15 mev), proton scattering occurs mainly due to the forces of electrostatic repulsion, which do not allow particles to approach each other up to distances of the order of the radius of action of nuclear forces. N.'s lack of an electric charge allows it to penetrate the electron shells of atoms and freely approach atomic nuclei. This is precisely what determines the unique ability of neutrons of relatively low energies to induce various nuclear reactions, including the fission reaction of heavy nuclei. For methods and results of studies of the interaction of neutrons with nuclei, see the articles Slow neutrons, Neutron spectroscopy, Atomic fission nuclei , Scattering of slow neutrons by protons at energies up to 15 mev spherically symmetrical in the system of the center of inertia. This indicates that scattering is determined by the interaction of n - p in a state of relative motion with the orbital angular momentum l= 0 (the so-called S-wave). Scattering in S-state is a specifically quantum-mechanical phenomenon that has no analogue in classical mechanics. It prevails over scattering in other states, when the de Broglie wavelength H.

of the order of or greater than the radius of action of nuclear forces ( ħ is Planck's constant, v- N speed). Since at an energy of 10 mev wavelength N.

This feature of neutron scattering by protons at such energies directly provides information on the order of magnitude of the radius of action of nuclear forces. Theoretical consideration shows that the scattering in S-state weakly depends on the detailed form of the interaction potential and is described with good accuracy by two parameters: the effective radius of the potential r and the so-called scattering length a. In fact, to describe n - p scattering, the number of parameters is twice as large, since the np system can be in two states with different values ​​of the total spin: J= 1 (triplet state) and J= 0 (singlet state). Experience shows that the lengths of N. scattering by a proton and the effective radii of interaction in the singlet and triplet states are different, i.e., the nuclear forces depend on the total spin of the particles. It also follows from experiments that the bound state of the system np (deuterium nucleus) can exist only when the total spin is 1, while in the singlet state the magnitude of the nuclear forces is insufficient for the formation of the bound state H. - proton. The nuclear scattering length in the singlet state, determined from experiments on the scattering of protons by protons (two protons in S-state, according to the Pauli principle y , can only be in a state with zero total spin) is equal to the scattering length n-p in the singlet state. This is consistent with the isotopic invariance of strong interactions. The absence of a bound system np in the singlet state and the isotopic invariance of nuclear forces lead to the conclusion that a bound system of two neutrons cannot exist - the so-called bineutron (similar to protons, two neutrons in S-states must have a total spin equal to zero). Direct experiments on the scattering of nn were not carried out due to the absence of neutron targets, however, indirect data (properties of nuclei) and more direct ones - the study of reactions 3 H + 3 H → 4 He + 2n, π - + d → 2n + γ - are consistent with the isotopic hypothesis invariance of nuclear forces and the absence of a bineutron. [If there were a bineutron, then in these reactions peaks would be observed at well-defined energies in the energy distributions of α-particles (4He nuclei) and γ-quanta, respectively.] Although the nuclear interaction in the singlet state is not strong enough to form a bineutron, this does not exclude the possibility of the formation of a bound system consisting of a large number of neutron nuclei alone - neutron nuclei. This issue requires further theoretical and experimental study. Attempts to discover experimentally nuclei of three or four nuclei, as well as nuclei 4 H, 5 H, and 6 H, have not yet yielded a positive result. structures of neutrons. According to these ideas, strong interaction between neutrons and other hadrons (for example, the proton) occurs through the exchange of virtual hadrons (see Virtual particles) - π-mesons, ρ-mesons, etc. Such a pattern of interaction explains the short-range nature of nuclear forces, the radius of which is determined by the Compton wavelength (See Compton wavelength) of the lightest hadron - the π-meson (equal to 1.4․10 -13 cm). At the same time, it points to the possibility of a virtual transformation of neutrons into other hadrons, for example, the process of emission and absorption of a π meson: n → p + π - → n. The intensity of strong interactions known from experience is such that N. must spend most of his time in this kind of "dissociated" states, being, as it were, in a "cloud" of virtual π-mesons and other hadrons. This leads to a spatial distribution of the electric charge and magnetic moment inside the N., the physical dimensions of which are determined by the dimensions of the "cloud" of virtual particles (see also Form factor). In particular, it turns out to be possible to qualitatively interpret the above-mentioned approximate equality in absolute value of the anomalous magnetic moments of N. and proton, if we assume that the magnetic moment of N. is created by the orbital motion of charged π - mesons emitted virtually in the process n → p + π - → n, and the anomalous magnetic moment of the proton - by the orbital motion of the virtual cloud of π + -mesons created by the process p → n + π + → r.

Electromagnetic interactions of the neutron. The electromagnetic properties of N. are determined by the presence of a magnetic moment in it, as well as by the distribution of positive and negative charges and currents existing inside N.. All these characteristics, as follows from the previous one, are associated with N.'s participation in a strong interaction that determines its structure. The magnetic moment of N. determines the behavior of N. in external electromagnetic fields: splitting of the N. beam in an inhomogeneous magnetic field, N. spin precession. The internal electromagnetic structure of N. manifests itself when high-energy electrons are scattered by N. and in the processes of meson production on N. - quanta (photoproduction of mesons). The electromagnetic interactions of neutrons with the electron shells of atoms and atomic nuclei lead to a number of phenomena that are important for studying the structure of matter. The interaction of the magnetic moment of N. with the magnetic moments of the electron shells of atoms is manifested significantly for N., the wavelength of which is of the order of or greater than atomic dimensions (energy E ev) , and is widely used to study the magnetic structure and elementary excitations (spin waves (See Spin waves)) magnetically ordered crystals (see Neutronography). Interference with nuclear scattering makes it possible to obtain beams of polarized slow neutrons (see Polarized neutrons) .

The interaction of the magnetic moment of N. with the electric field of the nucleus causes a specific scattering of N., which was first indicated by the American physicist Yu. Schwinger and therefore called “Schwinger”. The total cross section for this scattering is small, but at small angles (Neutron 3°) it becomes comparable to the cross section for nuclear scattering; N. scattered at such angles are highly polarized.

The interaction of N. - electron (n-e), not associated with the intrinsic or orbital moment of the electron, is reduced mainly to the interaction of the magnetic moment of N. with the electric field of the electron. Another, apparently smaller, contribution to the (n-e) interaction may be due to the distribution of electric charges and currents inside H. Although the (n-e) interaction is very small, it has been observed in several experiments.

Weak neutron interaction manifests itself in processes such as the decay of N.:

and muon neutrino (ν μ) by neutron: ν μ + n → p + μ - , nuclear capture of muons: μ - + p → n + ν μ , decays of strange particles (See Strange Particles) , for example, Λ → π° + n, etc.

Gravitational interaction of a neutron. N. is the only elementary particle with a rest mass for which gravitational interaction has been directly observed - the curvature of the trajectory of a well-collimated beam of cold N. in the field of Earth's gravity. The measured gravitational acceleration of N., within the accuracy of the experiment, coincides with the gravitational acceleration of macroscopic bodies.

Neutrons in the Universe and Near-Earth Space

The question of the amount of neutrons in the universe in the early stages of its expansion plays an important role in cosmology. According to the hot universe model (see Cosmology) , a significant part of the originally existing free N. has time to disintegrate during expansion. The portion of neutron that is captured by protons should eventually lead to approximately 30% content of He nuclei and 70% content of protons. The experimental determination of the percentage composition of He in the Universe is one of the critical tests of the hot Universe model.

In the primary component of cosmic rays (see Cosmic rays), neutrinos are absent due to their instability. However, the interactions of cosmic ray particles with the nuclei of atoms in the earth's atmosphere lead to the generation of neutrons in the atmosphere. The reaction 14 N (n, p) 14 C caused by these N. is the main source of the radioactive carbon isotope 14 C in the atmosphere, from where it enters living organisms; the radiocarbon method of geochronology is based on the determination of the 14 C content in organic remains. The decay of slow neutrons diffusing from the atmosphere into near-Earth space is one of the main sources of electrons that fill the interior of the Earth's radiation belt.

Lit.: Vlasov N. A., Neutrons, 2nd ed., M., 1971; Gurevich I. I., Tarasov L. V., Physics of Low Energy Neutrons, Moscow, 1965.

F. L. Shapiro, V. I. Lushchikov.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .