A thermal neutron is a free neutron with a kinetic energy level of ca. 0.025 eV (approx. 4.0e-21 J; 2.4 MJ/kg, hence a speed of 2.2 km/s). They are named 'thermal' as this level of kinetic energy is similar to the average kinetic energy of the molecules in a room-temperaturegas (see kinetic theory for energies and rms speeds of molecules). After a number of collisions with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.
Thermal neutrons have a much larger effective cross-section than fast neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier - and often unstable - isotope of the 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. Others, called fast breeder reactors, use fast neutrons directly.
Neutrons from fission have very high speeds and must be slowed greatly by water "moderation" to maintain the chain reaction.
Neutrons with energies less than one electron volt are commonly referred to as "thermal neutrons" since they have energies similar to what particles have as a result of ordinary room-temperature thermal energy.
A nuclear power reactor controls the fission chain reaction by moderating the neutrons and with the use of control rods which may be inserted in the reactor core to absorb neutrons and slow down the reaction.
Although the neutron has zero net charge, it may interact electromagnetically in two ways: first, the neutron has a magnetic moment of the same order as the proton; second, it is composed of electrically charged quarks.
Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules.
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