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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 and modification of beams after production edit free neutron beams are obtained from neutron sources by neutron transport for access to intense neutron sources researchers must go to a specialized neutron facility that operates a research reactor or a spallation source the neutron s lack of total electric charge makes it difficult to steer or accelerate them charged particles can be accelerated decelerated or deflected by electric or magnetic fields these methods have little effect on neutrons but some effects may be attained by use of inhomogeneous magnetic fields because of the neutron s magnetic moment neutrons can be controlled by methods that include moderation reflection and velocity selection thermal neutrons can be polarized by transmission through magnetic materials in a method analogous to the faraday effect for photons cold neutrons of wavelengths of 6 7 angstroms can be produced in beams of a high degree of polarization by use of magnetic mirrors and magnetized interference filters 118 applications edit science with neutrons foundations neutron temperature flux radiation transport cross section absorption activation neutron scattering neutron diffraction small angle neutron scattering gisans reflectometry inelastic neutron scattering triple axis spectrometer time of flight spectrometer backscattering spectrometer spin echo spectrometer other applications neutron tomography activation analysis prompt gamma activation analysis fundamental research with neutrons ultracold neutrons interferometry fast neutron therapy neutron capture therapy infrastructure neutron sources research reactor spallation neutron moderator neutron optics reflector supermirror detection neutron facilities america hfir lansce nist cnr sns oceania opal asia j parc hanaro europe ber ii frm ii ill isis neutron and muon source jinr sinq rid historic ipns high flux beam reactor under construction ess v t e nuclear energy edit because of the strength of the nuclear force at short distances the nuclear energy binding nucleons is many orders of magnitude greater than the electromagnetic energy binding electrons in atoms 44 4 in nuclear fission the absorption of a neutron by some heavy nuclides such as uranium 235 can cause the nuclide to become unstable and break into lighter nuclides and additional neutrons 44 the positively charged light nuclides or fission fragments then repel releasing electromagnetic potential energy 119 if this reaction occurs within a mass of fissile material the additional neutrons cause additional fission events inducing a cascade known as a nuclear chain reaction 44 12 13 for a given mass of fissile material such nuclear reactions release energy that is approximately ten million times that from an equivalent mass of a conventional chemical explosive 44 13 120 ultimately the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible most of the energy released from fission is the kinetic energy of the fission fragments 119 44 12 the neutron plays an important role in many nuclear reactions for example neutron capture often results in neutron activation inducing radioactivity in particular knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons the fissioning of elements like uranium 235 and plutonium 239 is caused by their absorption of neutrons other uses edit cold thermal and hot neutron radiation is commonly employed in neutron scattering facilities for neutron diffraction small angle neutron scattering and neutron reflectometry slow neutron matter waves exhibit properties similar to geometrical and wave optics of light including reflection refraction diffraction and interference 121 neutrons are complementary to x rays in terms of atomic contrasts by different scattering cross sections sensitivity to magnetism energy range for inelastic neutron spectroscopy and deep penetration into matter the development of neutron lenses based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron gamma ray tomography 122 123 124 125 a major use of neutrons is to excite delayed and prompt gamma rays from elements in materials this forms the basis of neutron activation analysis naa and prompt gamma neutron activation analysis pgnaa naa is most often used to analyze small samples of materials in a nuclear reactor whilst pgnaa is most often used to analyze subterranean rocks around bore holes and industrial bulk materials on conveyor belts another use of neutron emitters is the detection of light nuclei in particular the hydrogen found in water molecules when a fast neutron collides with a light nucleus it loses a large fraction of its energy by measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei a neutron probe may determine the water content in soil medical therapies edit main articles fast neutron therapy and neutron capture therapy of cancer because neutron radiation is both penetrating and ionizing it can be exploited for medical treatments however neutron radiation can have the unfortunate side effect of leaving the affected area radioactive neutron tomography is therefore not a viable medical application fast neutron therapy uses high energy neutrons typically greater than 20 mev to treat cancer radiation therapy of cancers is based upon the biological response of cells to ionizing radiation if radiation is delivered in small sessions to damage cancerous areas normal tissue will have time to repair itself while tumor cells often cannot 126 neutron radiation can deliver energy to a cancerous region at a rate an order of magnitude larger than gamma radiation 127 beams of low energy neutrons are used in boron neutron capture therapy to treat cancer in boron neutron capture therapy the patient is given a drug that contains boron and that preferentially accumulates in the tumor to be targeted the tumor is then bombarded with very low energy neutrons although often higher than thermal energy which are captured by the boron 10 isotope in the boron which produces an excited state of boron 11 that then decays to produce lithium 7 and an alpha particle that have sufficient energy to kill the malignant cell but insufficient range to damage nearby cells for such a therapy to be applied to the treatment of cancer a neutron source having an intensity of the order of a billion 10 9 neutrons per second per cm 2 is preferred such fluxes require a research nuclear reactor health risks edit exposure to free neutrons can be hazardous since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms and can also cause reactions that give rise to other forms of radiation such as protons 44 the normal precautions of radiation protection apply av...
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