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ere 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 avoid exposure stay as far from the source as possible and keep exposure time to a minimum but particular thought must be given to how to protect from neutron exposure for other types of radiation e g alpha particles beta particles or gamma rays material of a high atomic number and with high density makes for good shielding frequently lead is used however this approach will not work with neutrons since the absorption of neutrons does not increase straightforwardly with atomic number as it does with alpha beta and gamma radiation instead one needs to look at the particular interactions neutrons have with matter see the section on detection above for example hydrogen rich materials are often used to shield against neutrons since ordinary hydrogen both scatters and slows neutrons this often means that simple concrete blocks or even paraffin loaded plastic blocks afford better protection from neutrons than do far more dense materials after slowing neutrons may then be absorbed with an isotope that has high affinity for slow neutrons without causing secondary capture radiation such as lithium 6 hydrogen rich ordinary water effects neutron absorption in nuclear fission reactors usually neutrons are so strongly absorbed by normal water that fuel enrichment with a fissionable isotope is required the number of neutrons produced per fission depends primarily on the fission products the average is roughly 2 5 to 3 0 and at least one on average must evade capture in order to sustain the nuclear chain reaction the deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium normal light hydrogen deuterium is therefore used in candu type reactors in order to slow moderate neutron velocity to increase the probability of nuclear fission compared to neutron capture neutron temperature edit main article neutron temperature the energy of free neutrons are characterized by their temperature as given by their maxwell boltzmann distribution for example thermal neutrons have with kt 0 0253 ev 4 0 10 21 j corresponding to room temperature giving them a characteristic not average or median speed of 2 2 km s in many substances thermal neutron reactions show a much larger effective cross section than reactions involving faster neutrons and thermal neutrons can therefore be absorbed more readily i e with higher probability by any atomic nuclei that they collide with creating a heavier and often unstable isotope of the chemical element as a result most fission reactors use a neutron moderator to slow down or thermalize the neutrons that are emitted by nuclear fission so that they are more easily captured causing further fission cold and even ultra cold neutrons can be created by thermalizing with cryogenic materials higher temperature neutrons arise from nuclear fission and nuclear fusion the highest energies arise from cosmic ray collisions see also edit ionizing radiation isotope list of particles neutron radiation and the sievert radiation scale neutronium nuclear reaction nucleosynthesis neutron capture nucleosynthesis r process s process thermal neutron reactor neutron sources edit neutron generator neutron source processes involving neutrons edit neutron bomb neutron diffraction neutron flux neutron imaging neutron transport cosmogenic radionuclide dating references edit ernest rutherford archived 2011 08 03 at the wayback machine chemed chem purdue edu retrieved on 2012 08 16 1 2 1935 nobel prize in physics archived 2017 10 03 at the wayback machine nobelprize org retrieved on 2012 08 16 2022 codata value neutron mass the nist reference on constants units and uncertainty nist may 2024 retrieved 2024 05 18 1 2 2022 codata value neutron mass energy equivalent in mev the nist reference on constants units and uncertainty nist may 2024 retrieved 2024 05 18 1 2 2022 codata value neutron mass in u the nist reference on constants units and uncertainty nist may 2024 retrieved 2024 05 18 zyla p a 2020 n mean life pdg live 2020 review of particle physics particle data group archived from the original on 17 january 2021 retrieved 25 february 2021 1 2 olive k a particle data group et al 2014 review of particle physics pdf chinese physics c 38 9 1 708 arxiv 1412 1408 bibcode 2014chphc 38i0001o doi 10 1088 1674 1137 38 9 090001 hdl 10481 34376 pmid 10020536 s2cid 118395784 archived pdf from the original on 2020 06 01 retrieved 2017 10 26 1 2 3 4 mohr p j taylor b n and newell d b 2014 the 2014 codata recommended values of the fundamental physical constants archived 2013 10 09 at the way...
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