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uncharged elementary particleelementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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 of slightly greater mass than the protonproton,
elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
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. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A nucleus with an excess of neutrons is radioactive; the extra neutrons convert to protons by beta decay (see radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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). In a nucleus the neutron can be stable, but a free neutron decays with a half-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 of about 17 min (1,013 sec), into a proton, an electron, and an antineutrino. The fact that the neutron possesses a magnetic moment suggests that it has an internal structure of electric charge, although the net charge is zero. The electron-scattering experiments of Robert Hofstadter indicate that the neutron, like the proton, is surrounded by a cloud of pionspion
or pi meson,
lightest of the meson family of elementary particles. The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together.
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; protons and neutrons are bound together in nuclei by the exchange of virtual pions. The neutron and the proton are regarded by physicists as two aspects or states of a single entity, the nucleon. The antineutron, the neutron's antiparticleantiparticle,
elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
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, was discovered in 1956. The neutron, like other particles, also possesses certain wave properties, as explained by the quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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. The field of neutron optics is concerned with such topics as the diffractiondiffraction,
bending of waves around the edge of an obstacle. When light strikes an opaque body, for instance, a shadow forms on the side of the body that is shielded from the light source.
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 and polarization of beams of neutrons. The formation of images using the techniques of neutron optics is known as neutrography.


See D. J. Hughes, Neutron Story (1959); K. H. Beckurts and K. Wirtz, Neutron Physics (tr. 1964); P. Schofield, The Neutron and Its Applications (1983).


An elementary particle having approximately the same mass as the proton, but lacking a net electric charge. It is indispensable in the structure of the elements, and in the free state it is an important reactant in nuclear research and the propagating agent of fission chain reactions. Neutrons, in the form of highly condensed matter, constitute the substance of neutron stars.

Neutrons and protons are the constituents of atomic nuclei. The number of protons in the nucleus determines the chemical nature of an atom, but without neutrons it would be impossible for two or more protons to exist stably together within nuclear dimensions, which are of the order of 10-13 cm. The protons, being positively charged, repel one another by virtue of their electrostatic interactions. The presence of neutrons weakens the electrostatic repulsion, without weakening the nuclear forces of cohesion. In light nuclei the resulting balanced, stable configurations contain protons and neutrons in almost equal numbers, but in heavier elements the neutrons outnumber the protons; in 238U, for example, 146 neutrons are joined with 92 protons. Only one nucleus, 1H, contains no neutrons. For a given number of protons, neutrons in several different numbers within a restricted range often yield nuclear stability—and hence the isotopes of an element. See Isotope, Nuclear structure, Proton

Free neutrons have to be generated from nuclei, and since they are bound therein by cohesive forces, an amount of energy equal to the binding energy must be expended to get them out. Nuclear machines, such as cyclotrons and electrostatic generators, induce many nuclear reactions when their ion beams strike target material. Some of these reactions release neutrons, and these machines are sources of high neutron flux. Neutrons are released in the act of fission, and nuclear reactors are unexcelled as intense neutron sources. See Nuclear binding energy, Nuclear fission

Neutrons occur in cosmic rays, being liberated from atomic nuclei in the atmosphere by collisions of the high-energy primary or secondary charged particles. They do not themselves come from outer space.

Having no electric charge, neutrons interact so slightly with atomic electrons in matter that energy loss by ionization and atomic excitation is essentially absent. Consequently they are vastly more penetrating than charged particles of the same energy. The main energy-loss mechanism occurs when they strike nuclei. The most efficient slowing-down occurs when the bodies that are struck in an elastic collision have the same mass as the moving bodies; hence the most efficient neutron moderator is hydrogen, followed by other light elements: deuterium, beryllium, and carbon. The great penetrating power of neutrons imposes severe shielding problems for reactors and other nuclear machines, and it is necessary to provide walls, usually of concrete, several feet in thickness to protect personnel. The currently accepted health tolerance levels for an 8-h day correspond for fast neutrons to a flux of 20 neutrons/(cm2)(s) or 130 neutrons/(in.2)(s); for slow neutrons, 700/(cm2)(s) or 4500/(in.2)(s). On the other hand, fast neutrons are useful in some kinds of cancer therapy.

Free neutrons are radioactive, each transforming spontaneously into a proton, an electron (β- particle), and an antineutrino. This instability is a reflection of the fact that neutrons are slightly heavier than hydrogen atoms. The neutron's rest mass is 1.0086652 atomic mass units on the unified mass scale (1.67495 × 10-24 g), as compared with 1.0078252 atomic mass units for the hydrogen atom.

Neutrons are, individually, small magnets. This property permits the production of beams of polarized neutrons, that is, beams of neutrons whose magnetic dipoles are aligned predominantly parallel to one direction in space. The magnetic moment is -1.913042 nuclear magnetons. See Magneton, Nuclear moments, Nuclear orientation, Spin (quantum mechanics)

Despite its overall neutrality, the neutron does have an internal distribution of electric charge, as has been revealed by scattering experiments. On a still finer scale, the neutron can also be presumed to have a quark structure in analogy of that of the proton. See Quantum chromodynamics, Quarks

When neutrons are completely slowed down in matter, they have a maxwellian distribution in energy that corresponds to the temperature of the moderator with which they are in equilibrium. The de Broglie wavelength of these ultracold neutrons is greater than 50 nm, which is so much larger than interatomic distances in solids that they interact with regions of a surface rather than with individual atoms, and as a result they are reflected from polished surfaces at all angles of incidence. Ultracold neutrons are important in basic physics and have applications in studies of surfaces and of the structure of inhomogeneities and magnetic domains in solids. See Elementary particle, Neutron diffraction


(new -tron) Symbol: n. An elementary particle (a baryon) that is present in the nucleus of all atoms except ordinary hydrogen, 1H. It has zero charge, spin ½, and a rest mass of 1.6749 × 10–24 grams – slightly greater than that of the proton. The absence of charge enables the neutron to penetrate atoms easily since it has a negligible electromagnetic interaction with the constituents of the atom. Unlike free protons, free neutrons are unstable: they beta decay into protons, electrons, plus antineutrinos with a mean life of 914 seconds. The small difference between the neutron and proton mass provides the energy for this decay. The neutron is stable, however, when bound in the nucleus of a nonradioactive atom. The antiparticle of the neutron is the antineutron. See also isotopes; neutron star; nucleosynthesis; quark; r-process; s-process.



a neutral (chargeless) elementary particle with a spin of 1/2 in units of Planck’s constant h and a mass that very slightly exceeds the mass of the proton. All atomic nuclei consist of protons and neutrons. The magnetic moment of the neutron has a magnitude equal to approximately two nuclear magnetons and is negative; that is, the moment is directed opposite to the neutron’s mechanical, or spin, angular momentum. Neutrons belong to the class of strongly interacting particles (hadrons) and are members of the baryon group; that is, neutrons have a special intrinsic characteristic—the baryon charge, equal to +1, the same as for the proton (p).

Neutrons were discovered in 1932 by the British physicist J. Chadwick, who established that the penetrating radiation, detected by the German physicists W. Bothe and H. Becker, that arises when atomic nuclei—beryllium, in particular—are bombarded by alpha particles consists of uncharged particles with a mass close to that of the proton.

Neutrons (n) are stable only within stable atomic nuclei. A free neutron is an unstable particle that decays into a proton, an electron (e-), and an electronic antineutrino (̄e);

n → p + e- + ̄e

with an average lifetime T ≈ 16 min. Free neutrons exist for a much shorter period in matter (in dense substances from several to hundreds of microseconds) because of the strong absorption of free neutrons by nuclei. Free neutrons therefore arise in nature or are produced in the laboratory only as a result of nuclear reactions. A free neutron, in turn, is able to interact with all atomic nuclei, even the heaviest; the neutron can induce a given nuclear reaction, disappearing in the process; of particular importance are the fission of heavy nuclei and the radiative capture of neutrons; the latter, in many cases, leads to the formation of radioactive isotopes. The high efficiency of neutrons in inducing nuclear reactions and the unusual interactions of quite slow neutrons with matter (such as resonance effects and diffraction scattering in crystals) make the neutron an exceptionally important tool for research in nuclear physics and solid-state physics. Neutrons play a key role in practical applications in nuclear power engineering and in the production of the transuranic elements and radioactive isotopes (artificial radioactivity); neutrons are also extensively used in chemical analysis (activation analysis) and in geological prospecting (neutron logging).

An arbitrary classification has been adopted for neutrons based on the energy of the neutron: ultracold (up to 10 -7 electron volts [eV]), very cold (10 -7-10-4 eV), cold (10-4-5×10-3 eV), thermal (5×103-0.5 eV), resonance (0.5–104 eV), intermediate (104-105 eV), fast (105-108 eV), high-energy (108-1010 eV), and relativistic (≥ 1010 eV). All neutrons with an energy up to 105 eV are called slow neutrons.

Mass. The difference between the mass of the neutron and the mass of the proton has been determined with the greatest accuracy. In energy units, we have mnmp = (1.29344 ± 0.00007) MeV; this value was obtained from the measurement of the energy balance of various nuclear reactions. By comparing this quantity with the mass of the proton, we obtain

mn = (939.5527 ± 0.0052) MeV

This corresponds to mn ≈ 1.6×10-24 g or mn ≈ 1,840 me, where me is the mass of the electron.

Spin and statistics. The value 1/2 for the spin of the neutron is confirmed by various facts. The spin has been measured directly in experiments on the splitting of a beam of very slow neutrons in an inhomogeneous magnetic field. In the general case, the beam should split into (2 J + 1) separate beams, where J is the spin of the neutron. The splitting into two beams, which was observed in the experiment, implies that J = 1/2. Since the neutron is a particle with half-integral spin, it obeys Fermi-Dirac statistics, that is, it is a fermion. This fact was established independently on the basis of experimental data on the structure of atomic nuclei.

Electric charge of neutron, Q = 0. Direct measurements of Q from the deflection of a neutron beam in a strong electric field show that Q must be less than 10-17e, where e is the elementary electric charge, and indirect measurements (from the electric neutrality of macroscopic volumes of gas) give the estimate Q < 2×10-22e.

Other quantum numbers of neutron. The neutron is very similar to the proton in its properties: n and p have nearly equal masses and the same spin; they can be transmuted into each other, as, for example, in the process of beta decay; they manifest themselves identically in processes caused by the strong interaction and, in particular, the nuclear forces that act between the pairs p-p, n-p, and n-n are identical if the particles are in identical states. This striking similarity makes it possible to consider the neutron and the proton as a single particle (the nucleon) that can exist in two different states differing in electric charge Q. In the state with Q = +1, the nucleon is a proton; with Q = 0, the nucleon is a neutron. Correspondingly, we attribute to the nucleon (by analogy with ordinary spin) a certain intrinsic characteristic (the isotopic spin I), equal to 1/2, whose “projection” may, according to the general rules of quantum mechanics, assume 2 I + 1 = 2 values: + ½ and —½. Thus, the neutron and proton form an isotopic doublet (seeISOTOPIC INVARIANCE). The nucleon in a state in which the component of the isotopic spin on the quantization axis equals + ½ is a proton, whereas when the component of the spin equals —½, the nucleon is a neutron.

According to the current systematization of elementary particles, the neutron and proton, as components of an isotopic doublet, have the following identical quantum numbers: the baryon charge B = +1, lepton charge L = 0, strangeness S = 0, and positive intrinsic parity. The isotopic doublet of nucleons belongs to a broader group of “similar” particles—the baryon octet with J = ½, B = 1, and positive intrinsic parity; in addition to n and p, this group includes the hyperons Λ, Σ±, Σ°, Ξ-, and Ξ°, which differ from η and ρ in strangeness.

Magnetic dipole moment of neutron. The magnetic dipole moment of the neutron is determined from experiments based on nuclear magnetic resonance and is equal to

μn = −(1.91315 ± 0.00007) μnucl

where μnucl = 5.05 × 10−24 ergs/gauss is the nuclear magneton. A particle with spin ½, which is described by the Dirac equation, should have a magnetic moment equal to one magneton if charged and equal to zero if uncharged. The existence of a magnetic moment for the neutron, like the anomalous value of the magnetic moment of the proton (μρ = 2.79 μnucl), indicates that the neutron and proton have complicated internal structures; that is, there exist electric currents within the neutron and proton that create the additional and “anomalous” magnetic moment of the proton (1.79 μnucl) and the magnetic moment of the neutron, which is approximately equal to this in magnitude but opposite in sign (—1.9 μnucl). (See below.)

Electric dipole moment. From a theoretical standpoint, the electric dipole moment d of any elementary particle should be equal to zero if the interactions of the elementary particles are invariant with respect to time reversal (T-invariance). A search for electric dipole moments of elementary particles is thus one method of verifying this fundamental theoretical premise, and out of all the elementary particles the neutron is the most convenient for this search. Experiments using magnetic resonance in a cold-neutron beam have showed that dn< 10−23 cm • e. This means that, to a high degree of accuracy, the strong, electromagnetic, and weak interactions are T-invariant.

Neutrons take part in all known interactions of elementary particles—the strong, electromagnetic, weak, and gravitational interactions.

Strong interaction of neutrons. The neutron 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 definite relation between the characteristics of different processes involving the neutron and proton; for example, the effective cross section for the scattering of the π+- meson by the proton and the effective cross section for the scattering of the π- - meson by the neutron are equal, since the systems π+ p and π-n have an identical 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 case, and I3 = —3/2 in the second case); the cross section for the scattering of K+ by a proton and the cross section for the scattering of K 0 by a neutron are identical. The validity of relationships of this kind has been verified in a large number of experiments in high-energy accelerators. Because of the lack of targets consisting of neutrons, data on the interaction between neutrons and various unstable particles are drawn primarily from experiments on the scattering of these particles by the deuteron (d), which is the simplest nucleus that contains a neutron.

At low energies, the actual interactions of neutrons and protons with charged particles and atomic nuclei differ considerably because the proton has an electric charge that accounts for the existence of long-range Coulomb forces between the proton and other charged particles at distances at which short-range nuclear forces are virtually absent. If the energy of the collision of a proton with a proton or with an atomic nucleus is less than the height of the Coulomb barrier (which is of the order of 15 MeV for heavy nuclei), then the scattering of the proton is primarily caused by forces of electrostatic repulsion; these forces prevent the particles from approaching closer than distances of the order of the action range of the nuclear forces. The absence of an electric charge on the neutron enables the neutron to pass through the electron shells of atoms and freely approach atomic nuclei. It is precisely this fact that accounts for the unique ability of neutrons of comparatively low energies to induce various nuclear reactions, including the fission of heavy nuclei.

The scattering of slow neutrons by protons at energies up to 15 MeV is spherically symmetric in the center-of-mass coordinate system. This indicates that the scattering is determined by the n-p interaction in a state of relative motion with orbital angular momentum I = 0 (an S-wave). Scattering in the S-state is a specifically quantum-mechanical phenomenon that has no analogue in classical mechanics. It predominates over scattering in other states when the de Broglie wavelength of the neutron ƛ = ħ/mnv is of the order of or greater than the action range of the nuclear forces, where ħ is Planck’s constant and ν is the velocity of the neutron. Since the wavelength of the neutron is ƛ = 2 × 10−13 cm for an energy of 10 MeV, this distinctive feature of the scattering of neutrons by protons at such energies directly provides data on the order of magnitude of the range of the nuclear forces.

Theoretical examination shows that scattering in the S-state depends weakly on the detailed shape of the interaction potential and can be described with good accuracy by two parameters: the effective radius of the potential r and the scattering length a. The number of parameters used to describe n-p scattering is actually twice as large because the n-p system can exist in two states, which have different values of total spin: J = 1 (the triplet state) and J = 0 (the singlet state). Experiments show that the scattering lengths for the scattering of a neutron by a proton and the effective radii of interaction are different in the singlet and triplet states, that is, the nuclear forces depend on the total spin of the particles. It also follows from experiments that a bound state of the n-p system (the deuterium nucleus) can exist only when the total spin is 1, whereas in the singlet state the magnitude of the nuclear forces is insufficient to form a bound neutron-proton state.

(According to the Pauli principle, two protons in the S-state can exist only in a state with zero total spin.) The nuclear scattering length in this singlet state, as determined from experiments on the scattering of protons by protons, is equal to the n-p scattering length in the singlet state. This is in accord with the isotopic invariance of strong interactions. The absence of a bound n-p system in the singlet state and the isotopic invariance of the nuclear forces lead to the conclusion that a bound system of two neutrons—a dineutron—cannot exist. Like protons, two neutrons in the S-state must have a total spin equal to zero.

No direct experiments on n-n scattering have been conducted because of the lack of neutron targets, but indirect data such as the properties of nuclei and more direct data, for example, from the study of the reactions 3H + 3H 4He + 2n and π - + d → 2n + γ, are in accord with the hypothesis of the isotopic invariance of the nuclear forces and the nonexistence of the dineutron. (If the dineutron existed, then peaks in the energy distributions of the alpha particles [4He nuclei] and gamma quanta in the above reactions would be observed at well-defined values.) Although the nuclear interaction in the singlet state is insufficient to produce a dineutron, this does not exclude the possibility that a bound system consisting of a large number of neutrons alone (neutron nuclei) could be formed. This question requires further theoretical and experimental study. Attempts to detect experimentally nuclei consisting of three or four neutrons, as well as the nuclei 4H, 5H and 6H, have so far been unsuccessful.

Despite the absence of a systematic theory of strong interactions, certain regularities in the strong interactions and in the structure of the neutron can be qualitatively understood on the basis of a number of current concepts. According to these concepts, the strong interaction between the neutron and other hadrons, such as the proton, is brought about by the exchange of virtual hadrons such as pions (π-mesons) and p-mesons. This interpretation of the interaction explains the short-range character of the nuclear forces; the range of these forces is determined by the Compton wavelength of the lightest hadron—the pion (1.4 × 10−13cm). This interpretation also indicates the possibility of the virtual transformation of the neutron into other hadrons—for example, the process of the emission and absorption of a pion: η → ρ + π- —. n. η. The intensity of the strong interactions, as it is known from experiments, is such that the neutron should spend most of its time in such “dissociated states,” as if it were located in a “cloud” of virtual pions and other hadrons. This leads to a spatial distribution of the electric charge and of the magnetic moment inside the neutron; the physical dimensions of these distributions are determined by the dimensions of the cloud of virtual particles. In particular, it proves to be possible to qualitatively interpret the approximate equality (referred to above) in the absolute values of the anomalous magnetic moments of the neutron and the proton; it is assumed that the magnetic moment of the neutron is created by the orbital motion of charged π −-mesons, which are virtually emitted in the n → ρ + π- → n process, and it is also assumed that the anomalous magnetic moment of the proton is created by the orbital motion of the virtual cloud of π+-mesons that is produced by the ρ → η + π+ → ρ process.

Electromagnetic interactions of neutron. The electromagnetic properties of the neutron are determined by the existence of a magnetic moment and an internal distribution of positive and negative charges and currents for the neutron. As follows from the above discussion, all these characteristics are related to the neutron’s participation in the strong interaction, which accounts for the structure of the neutron. The magnetic moment of the neutron determines the behavior of the neutron in external electromagnetic fields: the splitting of a neutron beam in an inhomogeneous magnetic field and the precession of the neutron’s spin. The internal electromagnetic structure of the neutron is manifested in the scattering of high-energy electrons by neutrons and in the processes of meson production in neutrons by gamma quanta (photomeson production). The electromagnetic interactions of neutrons with the electron shells of atoms and with atomic nuclei lead to a number of phenomena that are of great importance for investigating the structure of matter.

The interaction of the magnetic moment of the neutron with the magnetic moments of the electron shells of atoms is considerable for neutrons whose wavelength is of the order of or greater than atomic dimensions (energy E < 10 eV); hence, this interaction is used extensively (in neutron-diffraction methods) to investigate the magnetic structure and elementary excitations (spin waves) of magnetically ordered crystals. Interference with nuclear scattering makes it possible to produce beams of polarized slow neutrons.

The interaction of the magnetic moment of the neutron with the electric field of the nucleus causes a particular type of neutron scattering, which was first explained by the American physicist J. Schwinger and is thus called Schwinger scattering. The total cross section of this scattering is small, but at small angles (≃3°) it becomes comparable to the cross section of nuclear scattering; neutrons scattered through these angles are highly polarized.

The neutron-electron (n-e) interaction, which is not connected with the spin or orbital angular momentum of the electron, reduces basically to the interaction between the neutron’s magnetic moment and the electron’s electric field. A different, apparently smaller, contribution to the (n-e) interaction may be due to the distribution of the electric charges and currents within the neutron. Although the (n-e) interaction is very weak, it has been observed in a few experiments.

Weak interaction of neutron. The weak interaction of the neutron is manifested in such processes as (1) neutron decay (n → p + e- + ⊽e), (2) the capture of an electronic antineutrino by a proton (⊽e + p → n + e+), (3) the capture of a muonic neutrino (νμ) by a neutron (νμ + n → ρ + μ-), (4) the nuclear capture of muons (μ- + ρ → η + νμ), and (5) decays of strange particles, for example, Λ → πo+n.

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

The problem of the number of neutrons in the universe during the early stages of the universe’s expansion plays an important role in cosmology. According to the primordial fireball model, a large fraction of the free neutrons that existed originally decayed during the expansion. The fraction of neutrons that are captured by protons should lead to a final approximately 30-percent content of He nuclei and a 70-percent content of protons. The experimental determination of the percentage of helium in the universe is one of the critical tests of the primordial fireball model.

In many cases, the evolution of stars leads to the formation of neutron stars; the pulsar is an example of a neutron star.

Because neutrons are unstable, they are absent in the primary component of cosmic rays. However, the interactions of the particles in cosmic rays 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, which is caused by these neutrons, is the primary source of the 14C radioactive isotope of carbon in the atmosphere; it is this atmospheric l4C that is taken up by living organisms. The radiocarbon method of geochronology is based on the determination of the 14C content in the remains of organic matter. The decay of slow neutrons, which diffuse from the atmosphere into near-terrestrial space, is one of the primary sources of the electrons that fill the inner region of the Van Allen radiation belt.


Vlasov, N. A. Neitrony, 2nd ed. Moscow, 1971. Gurevich, I. I., and L. V. Tarasov. Fizika neitronov nizkikh energii. Moscow, 1965.



An elementary particle which has approximately the same mass as the proton but lacks electric charge, and is a constituent of all nuclei having mass number greater than 1.


Physics a neutral elementary particle with a rest mass of 1.674 92716 × 10--27 kilogram and spin ½; classified as a baryon. In the nucleus of an atom it is stable but when free it decays
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