Neutron temperature

Theneutron detection temperature,also called theneutron energy,indicates afree neutron'skinetic energy,usually given inelectron volts.The termtemperatureis used, since hot, thermal and cold neutrons aremoderatedin a medium with a certain temperature. The neutron energy distribution is then adapted to theMaxwell distributionknown for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. Themomentumandwavelengthof the neutron are related through thede Broglie relation.The long wavelength of slow neutrons allows for the large cross section.

Neutron energy distribution ranges

Neutron energy range names

Neutron energy	Energy range
0.0 – 0.025 eV	Cold (slow) neutrons
0.025 eV	Thermal neutrons (at 20°C)
0.025–0.4 eV	Epithermal neutrons
0.4–0.5 eV	Cadmium neutrons
0.5–10 eV	Epicadmium neutrons
10–300 eV	Resonance neutrons
300 eV–1 MeV	Intermediate neutrons
1–20 MeV	Fast neutrons
> 20 MeV	Ultrafast neutrons

But different ranges with different names are observed in other sources.

The following is a detailed classification:

Thermal

Athermal neutronis a free neutron with a kinetic energy of about 0.025eV(about 4.0×10⁻²¹Jor 2.4 MJ/kg, hence a speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F), themodeof theMaxwell–Boltzmann distributionfor this temperature, E_peak=kT.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, thoseneutronswhich are not absorbed reach about this energy level.

Thermal neutrons have a different and sometimes much larger effectiveneutron absorptioncross-sectionfor a givennuclidethan fast neutrons, and can therefore often be absorbed more easily by anatomic nucleus,creating a heavier, oftenunstableisotopeof thechemical elementas a result. This event is calledneutron activation.

Epithermal

• Neutrons of energy greater than thermal
• Greater than 0.025 eV

Cadmium

• Neutrons which are strongly absorbed bycadmium
• Less than 0.5 eV.

Epicadmium

• Neutrons which are not strongly absorbed by cadmium
• Greater than 0.5 eV.

Cold (slow) neutrons

• Neutrons of lower (much lower) energy than thermal neutrons.
• Less than 5 meV.

Cold (slow) neutrons are subclassified into cold (CN), very cold (VCN), and ultra-cold (UCN) neutrons, each having particular characteristics in terms of their optical interactions with matter. As the wavelength is made (chosen to be) longer, lower values of the momentum exchange become accessible. Therefore, it is possible to study larger scales and slower dynamics. Gravity also plays a very significant role in the case of UCN. Nevertheless, UCN reflect at all angles of incidence. This is because their momentum is comparable to the optical potential of materials. This effect is used to store them in bottles and study their fundamental propertiese.g. lifetime, neutron electrical-dipole moment etc... The main limitations of the use of slow neutrons is the low flux and the lack of efficient optical devices (in the case of CN and VCN). Efficient neutron optical components are being developed and optimized to remedy this lack.

Resonance

• Refers to neutrons which are strongly susceptible to non-fission capture byU-238.
• 1 eV to 300 eV

Intermediate

• Neutrons that are between slow and fast
• Few hundred eV to 0.5 MeV.

Fast

Afast neutronis a free neutron with a kinetic energy level close to 1MeV(100TJ/kg), hence a speed of 14,000 km/sor higher. They are namedfastneutronsto distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators.

Fast neutrons are produced by nuclear processes:

• Nuclear fissionproduces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as "fast". However the range of neutrons from fission follows aMaxwell–Boltzmann distributionfrom 0 to about 14 MeV in thecenter of momentum frameof the disintegration, and themodeof the energy is only 0.75 MeV, meaning that fewer than half of fission neutrons qualify as "fast" even by the 1 MeV criterion.
• Spontaneous fissionis a mode of radioactive decay for some heavy nuclides. Examples includeplutonium-240andcalifornium-252.
• Nuclear fusion:deuterium–tritiumfusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% of thespeed of light) that can easily fissionuranium-238and other non-fissileactinides.
• Neutron emissionoccurs in situations in which a nucleus contains enough excess neutrons that theseparation energyof one or more neutrons becomes negative (i.e. excess neutrons "drip"out of the nucleus). Unstable nuclei of this sort will often decay in less than one second.

Fast neutrons are usually undesirable in a steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it. Ideally, a room temperatureneutron moderatoris used for this process. In reactors,heavy water,light water,orgraphiteare typically used to moderate neutrons.

A chart displaying the speed probability density functions of the speeds of a fewnoble gasesat a temperature of 298.15 K (25 C). An explanation of the vertical axis label appears on the image page (click to see). Similar speed distributions are obtained forneutronsuponmoderation.

Ultrafast

• Relativistic
• Greater than 20 MeV

Other classifications

Pile

• Neutrons of all energies present in nuclear reactors
• 0.001 eV to 15 MeV.

Ultracold

• Neutrons with sufficiently low energy to be reflected and trapped
• Upper bound of 335 neV

Fast-neutron reactor and thermal-neutron reactor compared

Mostfission reactorsarethermal-neutron reactorsthat use aneutron moderatorto slow down ( "thermalize") the neutrons produced bynuclear fission.Moderation substantially increases the fissioncross sectionforfissilenuclei such asuranium-235orplutonium-239.In addition,uranium-238has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by²³⁸U. The combination of these effects allowslight water reactorsto uselow-enriched uranium.Heavy water reactorsandgraphite-moderated reactorscan even usenatural uraniumas these moderators have much lowerneutron capturecross sectionsthan light water.

An increase in fuel temperature also raises uranium-238's thermal neutron absorption byDoppler broadening,providingnegative feedbackto help control the reactor. When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negativevoid coefficient), depending on whether the reactor is under- or over-moderated.

Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception is theuranium-233of thethorium cycle,which has a good fission/capture ratio at all neutron energies.

Fast-neutron reactorsuse unmoderatedfast neutronsto sustain the reaction, and require the fuel to contain a higher concentration offissile materialrelative tofertile material(uranium-238). However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so afast breeder reactorcan potentially "breed" more fissile fuel than it consumes.

Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only a handful of reactors built in the decades since theChernobyl accidentdue to low prices in theuranium market,although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.

See also

• Absorption hardening
• List of particles
• Neutron detection
• Neutron source
• Nuclear reaction
• Scintillator

External links

• Language of the Nucleus