Neutron temperature

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Science with neutrons
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Neutron scattering
Other applications
Infrastructure
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The neutron detection temperature, also called the neutron energy, indicates a

de Broglie relation. The long wavelength of slow neutrons allows for the large cross section.[1]

Neutron energy distribution ranges

Neutron energy range names[2][3]
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.[4]

The following is a detailed classification:

Thermal

A thermal neutron is a free neutron with a kinetic energy of about 0.025

eV (about 4.0×10−21 J or 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), the mode of the Maxwell–Boltzmann distribution for this temperature, Epeak = k
T.

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

Thermal neutrons have a different and sometimes much larger effective

unstable isotope of the chemical element as a result. This event is called neutron activation
.

Epithermal]

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

Cadmium]

  • Neutrons which are strongly absorbed by cadmium
  • 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 properties[5][6] e.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.[7]

Resonance]

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

Intermediate]

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

Fast

A fast neutron is a free neutron with a kinetic energy level close to 1 
TJ/kg), hence a speed of 14,000 km/s or higher. They are named fast neutrons
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:

  • center of momentum frame of the disintegration, and the mode of the energy is only 0.75 MeV, meaning that fewer than half of fission neutrons qualify as "fast" even by the 1 MeV criterion.[8]
  • californium-252
    .
  • actinides
    .
  • Neutron emission occurs in situations in which a nucleus contains enough excess neutrons that the separation energy of 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 temperature neutron moderator is used for this process. In reactors, heavy water, light water, or graphite are typically used to moderate neutrons.

See caption for explanation. Lighter noble gases (helium and neon depicted) have a much higher probability density peak at low speeds than heavier noble gases, but have a probability density of 0 at most higher speeds. Heavier noble gases (argon and xenon depicted) have lower probability density peaks, but have non-zero densities over much larger ranges of speeds.
A chart displaying the speed probability density functions of the speeds of a few noble gases at 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 for neutrons upon moderation.

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

Most

cross sections than light water.[9]

An increase in fuel temperature also raises uranium-238's thermal neutron absorption by Doppler broadening, providing negative feedback to 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 negative void 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 the

thorium cycle
, which has a good fission/capture ratio at all neutron energies.

fast breeder reactor
can 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 the

Chernobyl accident due to low prices in the uranium market, although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.[when?
]

See also

References

  1. ^ de Broglie, Louis. "On the Theory of Quanta" (PDF). aflb.ensmp.fr. Retrieved 2 February 2019.
  2. Bibcode:2007ipep.book.....C
    .
  3. ^ "Neutron Energy". www.nuclear-power.net. Retrieved 27 January 2019.
  4. ^ H. Tomita, C. Shoda, J. Kawarabayashi, T. Matsumoto, J. Hori, S. Uno, M. Shoji, T. Uchida, N. Fukumotoa and T. Iguchia, Development of epithermal neutron camera based on resonance-energy-filtered imaging with GEM, 2012, quote: "Epithermal neutrons have energies between 1 eV and 10 keV and smaller nuclear cross sections than thermal neutrons."
  5. S2CID 243745548
    , retrieved 2022-11-11
  6. S2CID 229156429
    .
  7. S2CID 249056691
    .
  8. ISBN 978-0-486-48238-5
    (pbk.) p. 259.
  9. ^ Some Physics of Uranium. Accessed March 7, 2009

External links

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