Free neutron 1695130 222318722 2008-06-28T18:17:33Z GregorB 179697 Link fmt {{Expand-section|date=June 2008}} {{Elementbox |name=neutron |number=0 |symbol=n<sup>0</sup> |left=- |right=[[hydrogen]] |above=- |below=[[helium]] |series=noble gas |group=18 |period=0 |block=- |appearance=colourless |atomic mass=[1] |electron configuration=0s<sup>0</sup> |electrons per shell=0 |phase=gas |density gplstp= |melting point K= |melting point C= |melting point pressure= |boiling point K= |boiling point C= |boiling pressure= |triple point K= |triple point kPa= |critical point K= |critical point MPa= |heat fusion= |heat fusion pressure= |heat vaporization= |heat vaporization pressure= |heat capacity= |heat capacity pressure= |vapor pressure 1= |vapor pressure 10= |vapor pressure 100= |vapor pressure 1 k= |vapor pressure 10 k= |vapor pressure 100 k= |vapor pressure comment= |crystal structure=hexagonal close-packed<br />(pressumed) |oxidation states=0 |oxidation states comment= |electronegativity=0.00 |number of ionization energies=0 |atomic radius calculated= |covalent radius= |Van der Waal's radius= |magnetic ordering= |thermal conductivity= |speed of sound= |CAS number= |isotopes= {{Elementbox_isotopes_decay |mn=1 |sym=Nt |na=[[synthetic radioisotope|syn]] |hl=10.2&nbsp;[[minute|min]] |dm=[[beta emission|β<sup>−</sup>]] |de=0.782353 |pn=1 |ps=[[hydrogen|H]]}}}} A '''free neutron''' is a [[neutron]] that exists outside of an [[atomic nucleus]]. While neutrons can be stable when bound inside nuclei, free neutrons are unstable and [[beta decay|decay]] with a [[mean lifetime|lifetime]] of just under 15 minutes (885.7 ± 0.8 s).<ref>[http://pdg.lbl.gov/2006/tables/bxxx.pdf Particle Data Group's Review of Particle Physics 2006]</ref> Because the neutron consists of three [[quark]]s, the only possible decay mode without a change of [[baryon number]] requires the [[flavour changing processes|flavour changing]] of one of the quarks via the [[weak nuclear force]]. The neutron consists of two [[down quark]]s with charge -1/3 and one [[up quark]] with charge +2/3, and the decay of one of the down quarks into a lighter up quark can be achieved by the emission of a [[W boson]]. By this means the neutron decays into a [[proton]] (which contains one down and two up quarks), an [[electron]], and an [[electron antineutrino]] ([[antineutrino]]), with the proton and electron potentially forming a [[hydrogen]] atom: <center><math>\hbox{n}\to\hbox{p}+\hbox{e}^-+\overline{\nu}_{\mathrm{e}}.</math></center> Even though it is not a [[chemical element]], the free neutron is often included in tables of nuclides. It is then considered to have an [[atomic number]] of zero and a [[mass number]] of one. ==Production== Various [[nuclides]] become more stable by expelling neutrons as a [[decay mode]]; this is known as [[neutron emission]], and happens commonly during [[spontaneous fission]]. [[Cosmic ray|Cosmic radiation]] interacting the earth's atmosphere continuously generates neutrons that can be detected at the surface. [[nuclear reactor|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 [[fusion power|nuclear fusion reactors]] produce free neutrons as a waste product. However, it is these neutrons that possess most of the energy, and converting that energy to a useful form has proved a difficult engineering challenge to nuclear physicists. This also explains why this form of energy is likely to create around twice the amount of radioactive waste of a fission reactor, but with a short (50-100 years) decay period (as opposed to the 10,000 years for fission waste). [http://news.bbc.co.uk/1/hi/sci/tech/4627237.stm] [http://en.wikipedia.org/wiki/Nuclear_power#Solid_waste] ==Thermal neutron== A [[neutron temperature|'''thermal neutron''']] is a [[free neutron]] that is [[Maxwell–Boltzmann distribution|Boltzmann distributed]] with kT = 0.024 eV (4.0×10<sup>-21</sup> [[Joule|J]]) at room temperature. This gives characteristic (not average, or median) speed of 2.2 km/s. The name 'thermal' comes from their energy being that of the room temperature gas or material they are permeating. (see ''[[kinetic theory]]'' for energies and speeds of molecules). After a number of collisions (often in the range of 10&ndash;20) with nuclei, [[neutron]]s arrive at this energy level, provided that they are not absorbed. In many substances, thermal neutrons have a much larger effective cross-section than faster neutrons, and can therefore be absorbed more easily by any [[atomic nucleus|atomic nuclei]] that they collide with, creating a heavier &mdash; and often [[unstable isotope|unstable]] &mdash; [[isotope]] of the [[chemical element]] as a result. Most [[nuclear reactor|fission reactor]]s 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. Others, called [[fast breeder]] reactors, use fission energy neutrons directly. ==Cold neutrons == These neutrons are thermal neutrons that have been equilibrated in a very cold substances such as liquid [[deuterium]]. These are produced in [[neutron scattering]] research facilities. ==Ultracold neutrons == [[Ultracold neutrons]] are produced by equilibration in substances with a temperature of a few kelvins, such as solid [[deuterium]] or superfluid [[helium]]. An alternative production method is the mechanical deceleration of cold neutrons. ==Fission energy neutron== A [[neutron temperature|'''fast neutron''']] is a free neutron with a kinetic energy level close to 2 [[Mega|M]][[electronvolt|eV]] (20 [[Tera|T]][[Joule|J]]/[[Kilogram|kg]]), hence a speed of 28,000 [[Kilometre|km]]/[[second|s]]. They are named ''fission energy'' or ''fast'' [[neutron]]s to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as [[nuclear fission]]. Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a [[neutron moderator]]. In reactors, typically [[heavy water]], [[light water]], or [[graphite]] are used to moderate neutrons. Fusion neutrons can have higher energies such as 14.1 MeV for D-T fusion, or 2.45 MeV for D-D fusion to <sup>3</sup>He. See [[Nuclear fusion#Criteria and candidates for terrestrial reactions]] for a list. ==Intermediate neutrons== A fission energy neutron that is slowing down is often said to have intermediate energy. There are not many non-elastic reactions in this energy region, so most of what happens is just slowing to thermal speeds before eventual capture. Intermediate energy neutrons are a hazard in reactors owing to the existence of a [[resonance]] region in the [[fission cross section]] of [[fissile]] elements. Within this region there exist many local minima and local maxima of probability of causing fission; this means that a reactor operating with a significant population of intermediate neutrons in contact with fuel nuclei could exhibit dangerous [[transient]] response. In such reactors, other mechanisms of inherent stability must be provided, such as large [[hydrogen]] populations to provide [[Doppler broadening]]. ==High-energy neutrons== These neutrons have more energy than fission energy neutrons and are generated in accelerators or in the atmosphere from cosmic particles. They can have energies as high as tens of joules per neutron. ==See also== * [[Neutron radiation]] * [[Neutron scattering]] * [[Neutron temperature]] * [[List of particles]] * [[Nuclear reaction]] * [[Thermal reactor]] * [[Dineutron]] * [[Fast neutron]] * [[Ionizing radiation]] * [[Isotope]] * [[Neutron flux]] * [[Neutron star]] * [[Neutronium]] * [[Tetraneutron]] * [[Neutron generator]] ==References== <references /> *Krane, K. S. (1998) ''Introductory Nuclear Physics''<br /> *G. L. Squires (1997) ''Introduction to the Theory of Thermal Neutron Scattering''<br> *M. S. Dewey, D. M. Gilliam, J. S. Nico, M. S. Snow and F. E. Wietfeldt '' NIST Neutron Lifetime Experiment''<br /> {{Isotope|element=neutron |lighter=Nothing |heavier=[[Dineutron]] |before=Many [[nuclear reactions]] |after=[[Hydrogen-1]] }} [[Category:Neutron]] [[es:Neutrón libre]] [[ko:자유 중성자]] [[pt:Nêutron livre]] [[ru:Свободный нейтрон]] [[pl:Neutron termiczny]]