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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 within the neutron in one of the early successes of the standard model in 1964 mirza a b beg benjamin w lee and abraham pais calculated the ratio of proton to neutron magnetic moments to be 3 2 or a ratio of 1 5 which agrees with the experimental value to within 3 90 91 92 the measured value for this ratio is 1 459 898 05 34 8 the above treatment compares neutrons with protons allowing the complex behavior of quarks to be subtracted out between models and merely exploring what the effects would be of differing quark charges or quark type such calculations are enough to show that the interior of neutrons is very much like that of protons save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton the neutron magnetic moment can be roughly computed by assuming a simple nonrelativistic quantum mechanical wavefunction for baryons composed of three quarks a straightforward calculation gives fairly accurate estimates for the magnetic moments of neutrons protons and other baryons 89 for a neutron the result of this calculation is that the magnetic moment of the neutron is given by μ n 4 3 μ d 1 3 μ u where μ d and μ u are the magnetic moments for the down and up quarks respectively this result combines the intrinsic magnetic moments of the quarks with their orbital magnetic moments and assumes the three quarks are in a particular dominant quantum state baryon magnetic moment of quark model computed μ n displaystyle mu _ mathrm n observed μ n displaystyle mu _ mathrm n p 4 3 μ u 1 3 μ d 2 79 2 793 n 4 3 μ d 1 3 μ u 1 86 1 913 the results of this calculation are encouraging but the masses of the up or down quarks were assumed to be 1 3 the mass of a nucleon 89 the masses of the quarks are actually only about 1 that of a nucleon 93 the discrepancy stems from the complexity of the standard model for nucleons where most of their mass originates in the gluon fields virtual particles and their associated energy that are essential aspects of the strong force 93 94 furthermore the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment 95 but the nucleon magnetic moment has been successfully computed numerically from first principles including all of the effects mentioned and using more realistic values for the quark masses the calculation gave results that were in fair agreement with measurement but it required significant computing resources 96 97 electric charge edit the total electric charge of the neutron is 0 e this zero value has been tested experimentally and the present experimental limit for the charge of the neutron is 2 8 10 22 e 7 or 3 13 10 41 c this value is consistent with zero given the experimental uncertainties indicated in parentheses by comparison the charge of the proton is 1 e electric dipole moment edit main article neutron electric dipole moment the standard model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent electric dipole moment 98 but the predicted value is well below the current sensitivity of experiments from several unsolved puzzles in particle physics it is clear that the standard model is not the final and full description of all particles and their interactions new theories going beyond the standard model generally lead to much larger predictions for the electric dipole moment of the neutron currently there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment including cryogenic neutron edm experiment being set up at the institut laue langevin 99 n2edm experiment under construction at the ucn source at the paul scherrer institute 100 nedm experiment being envisaged at the spallation neutron source 101 102 nedm experiment being built at the institut laue langevin 103 antineutron edit main article antineutron the antineutron is the antiparticle of the neutron it was discovered by bruce cork in 1956 a year after the antiproton was discovered neutrons have baryon number equal to 1 while antineutrons have 1 while all measured particle interactions conserve baryon number matter dominates over antimatter in the cosmos suggesting that there must be some way to change the baryon number one proposed mechanism is neutron antineutron oscillations which might be detectable 104 105 the lower limit on the period of oscillations 0 86 10 8 s 90 cl was obtained using cold neutrons 106 ultracold neutrons may increase the sensitivity by 10 40 times depending on the model of neutron reflection from walls 107 detection edit main article neutron detection the common means of detecting a charged particle by looking for a track of ionization such as in a cloud chamber does not work for neutrons directly neutrons that elastically scatter off atoms can create an ionization track that is detectable but the experiments are not as simple to carry out other means for detecting neutrons consisting of allowing them to interact with atomic nuclei are more commonly used the commonly used methods to detect neutrons can therefore be categorized according to the nuclear processes relied upon mainly neutron capture or elastic scattering 108 neutron detection by neutron capture edit a common method for detecting neutrons involves converting the energy released from neutron capture reactions into electrical signals certain nuclides have a high neutron capture cross section which is the probability of absorbing a neutron upon neutron capture the compound nucleus emits more easily detectable radiation for example an alpha particle which is then detected the nuclides 3 he 6 li 10 b 233 u 235 u 237 np and 239 pu are useful for this purpose neutron detection by elastic scattering edit neutrons can elastically scatter off nuclei causing the struck nucleus to recoil kinematically a neutron can transfer more energy to a light nucleus such as hydrogen or helium than to a heavier nucleus detectors relying on elastic scattering are called fast neutron detectors recoiling nuclei can ionize and excite further atoms through collisions charge and or scintillation light produced in this way can be collected to produce a detected signal a major challenge in fast neutron detection is discerning such signals from erroneous signals produced by gamma radiation in the same detector methods such as pulse shape discrimination can be used in distinguishing neutron signals from gamma ray signals although certain inorganic scintillator based detectors have been developed 109 110 to selectively detect neutrons in mixed radiation fields inherently without any additional techniques fast neutron detectors have the advantage of not requiring a moderator and are therefore capable of measuring the neutron s energy time of arrival and in certain cases direction of incidence sources and production edit main articles neutron source neutron generator and research reactor free neutrons are unstable although they have the longest half life of any unstable subatomic particle by several orders of magnitude their half life is still only about 10 minutes so they can be obtained only from sources that produce them continuously natural neutron background a small natural background flux of free neutrons exists everywhere on earth 111 in the atmosphere and deep into the ocean the neutron background is caused by muons produced by cosmic ray interaction with the atmosphere these high energy muons are capable of penetration to considerable depths in water and soil there in striking atomic nuclei among other reactions they induce spallation reactions in which a neutron is liberated from the nucleus within the earth s crust a second source is neutrons produced primarily by spontaneous fission of uranium and thorium present in crustal minerals the neutron background is not strong enough to be a biological hazard but it is of importance to very high resolution particle detectors that are looking for very rare events such as hypothesized interactions that might be caused by particles of dark matter 111 recent research has shown that even thunderstorms can produce neutrons with energies of up to several tens of mev 112 recent research has shown that the fluence of these neutrons lies between 10 9 and 10 13 per ms and per m 2 depending on the detection altitude the energy of most of these neutrons even with initial energies of 20 mev decreases down to the kev range within 1 ms 113 even stronger neutron background radiation is produced at the surface of mars where the atmosphere is thick enough to generate neutrons from cosmic ray muon production and neutron spallation but not thick enough to provide significant protection from the neutrons produced these neutrons not only produce a martian surface neutron radiation hazard from direct downward going neutron radiation but may also produce a significant hazard from reflection of neutrons from the martian surface which will produce reflected neutron radiation penetrating upward into a martian craft or habitat from the floor 114 sources of neutrons for research these include certain types of radioactive decay spontaneous fission and neutron emission and from certain nuclear reactions convenient nuclear reactions include tabletop reactions such as natural alpha and gamma bombardment of certain nuclides often beryllium or deuterium and induced nuclear fission such as occurs in nuclear reactors in addition high energy nuclear reactions such as occur in cosmic radiation showers or accelerator collisions also produce neutrons from disintegration of target nuclei small tabletop particle accelerators optimized to produce free neutrons in this way are called neutron generators in practice the most commonly used small laboratory sources of neutrons use radioactive decay to power neutron production one noted neutron producing radioisotope californium 252 decays half life 2 65 years by spontaneous fission 3 of the time with production of 3 7 neutrons per fission and is used alone as a neutron source from this process nuclear reaction sources that involve two materials powered by radioisotopes use an alpha decay source plus a beryllium target or else a source of high energy gamma radiation from a source that undergoes beta decay followed by gamma decay which produces photoneutrons on interaction of the high energy gamma ray with ordinary stable beryllium or else with the deuterium in heavy water a popular source of the latter type is radioactive antimony 124 plus beryllium a system with a half life of 60 9 days which can be constructed from natural antimony which is 42 8 stable antimony 123 by activating it with neutrons in a nuclear reactor then transported to where the neutron source is needed 115 institut laue langevin ill in grenoble france a major neutron research facility nuclear fission reactors naturally produce free neutrons their role is to sustain the energy producing chain reaction the intense neutron radiation can also be used to produce various radioisotopes through the process of neutron activation which is a type of neutron capture experimental nuclear fusion reactors produce free neutrons as a waste product but it is these neutrons that possess most of the energy and converting that energy to a useful form has proved a difficult engineering challenge fusion reactors that generate neutrons are likely to create radioactive waste but the waste is composed of neutron activated lighter isotopes which have relatively short 50 100 years decay periods as compared to typical half lives of 10 000 years 116 for fission waste which is long due primarily to the long half life of alpha emitting transuranic actinides 117 some nuclear fusion fission hybrids are proposed to make use of those neutrons to either maintain a subcritical reactor or to aid in nuclear transmutation of harmful long lived nuclear waste to shorter lived or stable nuclides neutron beams 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