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agic even odd halo borromean nuclear stability binding energy p n ratio drip line island of stability valley of stability stable nuclide radioactive decay alpha α beta β 2β 0v β k l capture isomeric gamma γ internal conversion spontaneous fission cluster decay neutron emission proton emission decay energy decay chain decay product radiogenic nuclide nuclear fission spontaneous products pair breaking photofission capturing processes electron 2 neutron s r proton p rp high energy processes spallation by cosmic ray photodisintegration nucleosynthesis and nuclear astrophysics nuclear fusion processes stellar big bang supernova nuclides primordial cosmogenic artificial high energy nuclear physics quark gluon plasma rhic lhc scientists alvarez becquerel bethe blackett a bohr n bohr chadwick cockcroft ir curie fr curie pi curie skłodowska curie davisson fermi hahn jensen lawrence mayer meitner oliphant oppenheimer proca purcell rabi rutherford soddy strassmann świątecki szilárd teller thomson walton wigner physics portal category v t e an atomic nucleus is formed by a number of protons z the atomic number and a number of neutrons n the neutron number bound together by the nuclear force protons and neutrons each have a mass of approximately one dalton the atomic number determines the chemical properties of the atom and the neutron number determines the isotope or nuclide 44 4 the terms isotope and nuclide are often used synonymously but they refer to chemical and nuclear properties respectively 44 4 isotopes are nuclides with the same atomic number but different neutron number nuclides with the same neutron number but different atomic number are called isotones 46 the atomic mass number a is equal to the sum of atomic and neutron numbers nuclides with the same atomic mass number but different atomic and neutron numbers are called isobars 46 the mass of a nucleus is always slightly less than the sum of its proton and neutron masses the difference in mass represents the mass equivalent to nuclear binding energy the energy which would need to be added to take the nucleus apart 47 822 the nucleus of the most common isotope of the hydrogen atom with the chemical symbol 1 h is a lone proton 44 20 the nuclei of the heavy hydrogen isotopes deuterium d or 2 h and tritium t or 3 h contain one proton bound to one and two neutrons respectively 44 20 all other types of atomic nuclei are composed of two or more protons and various numbers of neutrons the most common nuclide of the common chemical element lead 208 pb has 82 protons and 126 neutrons for example 48 the table of nuclides comprises all the known nuclides even though it is not a chemical element the neutron is included in this table 49 nuclear fission caused by absorption of a neutron by uranium 235 the heavy nuclide fragments into lighter components and additional neutrons protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus they are therefore both referred to collectively as nucleons 50 the concept of isospin in which the proton and neutron are viewed as two quantum states of the same particle is used to model the interactions of nucleons by the nuclear or weak forces 51 141 neutrons are a necessary constituent of any atomic nucleus that contains more than one proton as a result of their positive charges interacting protons have a mutual electromagnetic repulsion that is stronger than their attractive nuclear interaction so proton only nuclei are unstable see diproton and neutron proton ratio 52 neutrons bind with protons and one another in the nucleus via the nuclear force effectively moderating the repulsive forces between the protons and stabilizing the nucleus 43 461 heavy nuclei carry a large positive charge hence they require extra neutrons to be stable 43 461 while a free neutron is unstable and a free proton is stable within nuclei neutrons are often stable and protons are sometimes unstable when bound within a nucleus nucleons can decay by the beta decay process the neutrons and protons in a nucleus form a quantum mechanical system according to the nuclear shell model protons and neutrons of a nuclide are organized into discrete hierarchical energy levels with unique quantum numbers nucleon decay within a nucleus can occur if allowed by basic energy conservation and quantum mechanical constraints the decay products that is the emitted particles carry away the energy excess as a nucleon falls from one quantum state to one with less energy while the neutron or proton changes to a proton or neutron for a neutron to decay the resulting proton requires an available state at lower energy than the initial neutron state in stable nuclei the possible lower energy states are all filled meaning each state is occupied by a pair of protons one with spin up another with spin down when all available proton states are filled the pauli exclusion principle disallows the decay of a neutron to a proton 53 3 3 the situation is similar to electrons of an atom where electrons that occupy distinct atomic orbitals are prevented by the exclusion principle from decaying to lower already occupied energy states 53 3 3 the stability of matter is a consequence of these constraints 54 55 56 the decay of a neutron within a nuclide is illustrated by the decay of the carbon isotope carbon 14 which has 6 protons and 8 neutrons with its excess of neutrons this isotope decays by beta decay to nitrogen 14 7 protons 7 neutrons a process with a half life of about 5 730 years 57 nitrogen 14 is stable 58 free neutron edit main article free neutron decay neutrons are tightly bound in atomic nuclei requiring mev sized energies to escape once free neutrons decay in a quarter of an hour on average thus free neutrons are rare compared to other components of atoms electrons are freed by heating a light bulb filament and protons are freed in rapid hydrogen gas combustion moreover once freed in say a nuclear reactor the charge free neutrons are difficult to direct confine or detect 59 the neutron has a mean square radius of about 0 8 10 15 m or 0 8 fm 60 and it is a spin 1 2 fermion 61 the neutron has no measurable electric charge with its positive electric charge the proton is directly influenced by electric fields whereas the neutron is unaffected by electric fields 62 the neutron has a magnetic moment however so it is influenced by magnetic fields 63 the specific properties of the neutron are described below in the intrinsic properties section dineutrons and tetraneutrons edit main articles dineutron and tetraneutron the dineutron is considered a component in neutron rich 16 be nuclei 64 and an unbound state with lifetimes less than 10 22 seconds the first evidence for this state was reported by haddock et al in 1965 65 275 evidence for unbound clusters of 4 neutrons or tetraneutron as resonances in the disintegration of beryllium 14 nuclei 66 in 8 he 8 be interactions 67 and collisions of 4 he nuclei give an estimated lifetime around 10 22 seconds 68 these discoveries should deepen our understanding of the nuclear forces 69 70 neutron stars and neutron matter edit main articles neutron matter and neutron star at extremely high pressures and temperatures nucleons and electrons are believed to collapse into bulk neutronic matter called neutron matter this is presumed to happen in neutron stars 71 the extreme pressure inside a neutron star may deform the neutrons into a cubic symmetry allowing tighter packing of neutrons 72 composition edit the principal feynman diagram for β decay of a neutron into a proton electron and electron antineutrino via an intermediate heavy w boson the principal feynman diagram for β decay of a proton into a neutron positron and electron neutrino via an intermediate heavy w boson main article standard model within the theoretical framework of the standard model for particle physics a neutron comprises two down quarks with charge 1 3 e and one up quark with charge 2 3 e the neutron is therefore a composite particle classified as a hadron the neutron is also classified as a baryon because it is composed of three valence quarks 73 the finite size of the neutron and its magnetic moment both indicate that the neutron is a composite rather than elementary particle the quarks of the neutron are held together by the strong force mediated by gluons 74 the nuclear force results from secondary effects of the more fundamental strong force the only possible decay mode for the neutron that obeys the conservation law for the baryon number is for one of the neutron s quarks to change flavour through a cabibbo kobayashi maskawa matrix via the weak interaction the decay of one of the neutron s down quarks into a lighter up quark can be achieved by the emission of a w boson by this process the standard model description of beta decay the neutron decays into a proton which contains one down and two up quarks an electron and an electron antineutrino the decay of the proton to a neutron occurs similarly through the weak force the decay of one of the proton s up quarks into a down quark can be achieved by the emission of a w boson the proton decays into a neutron a positron and an electron neutrino this reaction can only occur within an atomic nucleus which has a quantum state at lower energy available for the created neutron a schematic of the nucleus of an atom indicating β radiation the emission of a fast electron from the nucleus the decay also creates an antineutrino omitted and converts a neutron to a proton within the nucleus the inset shows beta decay of a free neutron an electron and antineutrino are created in this process beta decay edit main article beta decay neutrons and protons within a nucleus behave similarly and can exchange their identities by similar reactions these reactions are a form of radioactive decay known as beta decay 75 beta decay in which neutrons decay to protons or vice versa is governed by the weak force and it requires the emission or absorption of electrons and neutrinos or their antiparticles 76 the neutron and proton decay reactions are n 0 p e ν e where p e and ν e denote the proton electron and electron anti neutrino decay products 77 and p n 0 e ν e where n 0 e and ν e denote the neutron positron and electron neutrino decay products the electron and positron produced in these reactions are historically known as beta particles denoted β or β respectively lending the name to the decay process 76 in these reactions the original particle is not composed of the product particles rather the product particles are created at the instant of the reaction 43 369 370 beta decay reactions can also occur by the capture of a lepton by the nucleon the transformation of a proton to a neutron inside of a nucleus is possible through electron capture 78 p e n 0 ν e a rarer reaction inverse beta decay involves the capture of a neutrino by a nucleon 79 rarer still positron capture by neutrons can occur in the high temperature environment of stars 80 properties edit mass edit the mass of a neutron cannot be directly determined by mass spectrometry since it has no electric charge but since the masses of a proton and of a deuteron can be measured with a mass spectrometer the mass of a neutron can be deduced by subtracting proton mass from deuteron mass with the difference being the mass of the neutron plus the binding energy of deuterium expressed as a positive emitted energy the latter can be directly measured by measuring the energy b d displaystyle b_ d of the single 2 224 mev gamma photon emitted when a deuteron is formed by a proton capturing a neutron this is exothermic and happens with zero energy neutrons the small recoil kinetic energy e r d displaystyle e_ rd of the deuteron about 0 06 of the total energy must also be accounted for m n m d m p b d e r d displaystyle m_ n m_ d m_ p b_ d e_ rd the energy of the gamma ray can be measured to high precision by x ray diffraction techniques as was first done by bell and elliot in 1948 the best modern 1986 values for neutron mass by this technique are provided by greene et al 81 these give a neutron mass of m neutron 1 008 664 916 06 40 da 5 the value for the neutron mass in mev is less accurately known due to less accuracy in the known conversion of da to mev c 2 53 18 19 m neutron 939 565 421 94 48 mev c 2 4 another method to determine the mass of a neutron starts from the beta decay of the neutron when the momenta of the resulting proton and electron are measured spin edit the neutron is a spin 1 2 particle that is it is a fermion with intrinsic angular momentum equal to 1 2 ħ where ħ is the reduced planck constant for many years after the discovery of the neutron its exact spin was ambiguous although it was assumed to be a spin 1 2 dirac particle the possibility that the neutron was a spin 3 2 particle lingered the interactions of the neutron s magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron 82 in 1949 hughes and burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin 1 2 83 in 1954 sherwood stephenson and bernstein employed neutrons in a stern gerlach experiment that used a magnetic field to separate the neutron spin states they recorded two such spin states consistent with a spin 1 2 particle 82 84 as a fermion the neutron is subject to the pauli exclusion principle two neutrons cannot have the same quantum numbers this is the source of the degeneracy pressure which counteracts gravity in neutron stars and prevents them from forming black holes 85 see also delta baryon magnetic moment edit main article nucleon magnetic moment even though the neutron is a neutral particle the magnetic moment of a neutron is not zero the neutron is not affected by electric fields but it is affected by magnetic fields the value for the neutron s magnetic moment was first directly measured by luis alvarez and felix bloch at berkeley california in 1940 86 alvarez and bloch determined the magnetic moment of the neutron to be μ n 1 93 2 μ n where μ n is the nuclear magneton the neutron s magnetic moment has a negative value because its orientation is opposite to the neutron s spin 87 the magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution 88 in the quark model for hadrons the neutron is composed of one up quark charge 2 3 e and two down quarks charge 1 3 e 88 the magnetic moment of the neutron can be modeled as a sum of the magnetic moments of the constituent quarks 89 the calculation assumes that the quarks behave like point like dirac particles each having their own magnetic moment simplistically the magnetic moment of the neutron can be viewed as resulting from the vector sum of the three quark magnetic moments plus the orbital magnetic moments caused by the movement of the three charged quarks wit...
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