Thermal neutron
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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-temperature gas (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.