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n the nuclei of atoms atoms of a chemical element that differ only in neutron number are called isotopes free neutrons are produced copiously in nuclear fission and fusion they are a primary contributor to the nucleosynthesis of chemical elements within stars through fission fusion and neutron capture processes neutron stars formed from massive collapsing stars consist of neutrons at the density of atomic nuclei but a total mass more than the sun neutron properties and interactions are described by nuclear physics neutrons are not elementary particles each is composed of three quarks a free neutron spontaneously decays to a proton an electron and an antineutrino with a mean lifetime of about 15 minutes the neutron is essential to the production of nuclear power dedicated neutron sources like neutron generators research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments free neutrons do not directly ionize atoms but they do indirectly cause ionizing radiation so they can be a biological hazard depending on dose a small natural neutron background flux of free neutrons exists on earth caused by cosmic rays and by the natural radioactivity of spontaneously fissionable elements in the earth s crust discovery edit main article discovery of the neutron the story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century leading ultimately to the atomic bomb in 1945 the name derives from the latin root for neutralis neuter and the greek suffix on a suffix used in the names of subatomic particles e g electron and proton 9 10 and references to the word neutron can be found in the literature as early as 1899 in connection with discussion on the nature of the atom 11 in the 1911 rutherford model the atom consisted of a small positively charged massive nucleus surrounded by a much larger cloud of negatively charged electrons in 1920 ernest rutherford suggested that the nucleus consisted of positive protons and neutrally charged particles suggested to be a proton and an electron bound in some way 12 electrons were assumed to reside within the nucleus because it was known that beta radiation consisted of electrons emitted from the nucleus 12 13 about the time rutherford suggested the neutral proton electron composite several other publications appeared making similar suggestions and in 1921 the american chemist w d harkins first named the hypothetical particle a neutron 14 11 throughout the 1920s physicists assumed that the atomic nucleus was composed of protons and nuclear electrons 15 16 beginning in 1928 it became clear that this model was inconsistent with the then new quantum theory confined to a volume the size of a nucleus an electron consistent with the heisenberg uncertainty relation of quantum mechanics would have an energy exceeding the binding energy of the nucleus 17 18 the energy was so large that according to the klein paradox 19 discovered by oskar klein in 1928 an electron would escape the confinement of a nucleus 17 furthermore the observed properties of atoms and molecules were inconsistent with the nuclear spin expected from the proton electron hypothesis protons and electrons both carry an intrinsic spin of 1 2 ħ and the isotopes of the same species were found to have either integer or fractional spin by the hypothesis isotopes would be composed of the same number of protons but differing numbers of neutral bound proton electron particles this physical picture was a contradiction since there is no way to arrange the spins of an electron and a proton in a bound state to get a fractional spin 17 in 1931 walther bothe and herbert becker found that if alpha particle radiation from polonium fell on beryllium boron or lithium an unusually penetrating radiation was produced the radiation was not influenced by an electric field so bothe and becker assumed it was gamma radiation 20 21 the following year irène joliot curie and frédéric joliot curie in paris showed that if this gamma radiation fell on paraffin or any other hydrogen containing compound it ejected protons of very high energy 22 neither rutherford nor james chadwick at the cavendish laboratory in cambridge were convinced by the gamma ray interpretation 23 chadwick quickly performed a series of experiments that showed that the new radiation consisted of uncharged particles with about the same mass as the proton 24 25 26 these properties matched rutherford s hypothesized neutron chadwick won the 1935 nobel prize in physics for this discovery 2 models depicting the nucleus and electron energy levels in hydrogen helium lithium and neon atoms in reality the diameter of the nucleus is about 100 000 times smaller than the diameter of the atom models for an atomic nucleus consisting of protons and neutrons were quickly developed by werner heisenberg 27 28 29 and others 30 31 the proton neutron model explained the puzzle of nuclear spins the origins of beta radiation were explained by enrico fermi in 1934 by the process of beta decay in which the neutron decays to a proton by creating an electron and a then undiscovered neutrino 32 in 1935 chadwick and his doctoral student maurice goldhaber reported the first accurate measurement of the mass of the neutron 33 34 by 1934 fermi had bombarded heavier elements with neutrons to induce radioactivity in elements of high atomic number in 1938 fermi received the nobel prize in physics for his demonstrations of the existence of new radioactive elements produced by neutron irradiation and for his related discovery of nuclear reactions brought about by slow neutrons 35 in december 1938 otto hahn lise meitner and fritz strassmann discovered nuclear fission or the fractionation of uranium nuclei into lighter elements induced by neutron bombardment 36 37 38 39 in 1945 hahn received the 1944 nobel prize in chemistry for his discovery of the fission of heavy atomic nuclei 40 41 42 the discovery of nuclear fission would lead to the development of nuclear power and the atomic bomb by the end of world war ii it was quickly realized that if a fission event produced neutrons each of these neutrons might cause further fission events in a cascade known as a nuclear chain reaction 43 460 461 44 these events and findings led fermi to construct the chicago pile 1 at the university of chicago in 1942 the first self sustaining nuclear reactor 45 just three years later the manhattan project was able to test the first atomic bomb the trinity nuclear test in july 1945 45 occurrence edit atomic nucleus edit main articles atomic nucleus and nuclear physics see also valley of stability beta decay stable isobars and neutron emission nuclear physics nucleus nucleons p n nuclear matter nuclear force nuclear structure nuclear reaction models of the nucleus liquid drop nuclear shell model interacting boson model ab initio nuclides classification isotopes equal z isobars equal a isotones equal n isodiaphers equal n z isomers equal all the above mirror nuclei z n stable magic 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 framew...
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