section_path: Neutron temperature/Thermal neutrons
section_name: Thermal neutrons
paragraph_text: Thermal neutrons
  
  Thermal neutrons are free neutrons whose energies have a Maxwell–Boltzmann distribution with kT = 0.0253 eV (4.0 × 10 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 10–20) with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.
  
  In many substances, thermal neutron reactions show a much larger effective cross-section than reactions involving faster neutrons, and thermal neutrons can therefore be absorbed more readily (i.e., with higher probability) by any atomic nuclei that they collide with, creating a heavier – and often unstable – isotope of the chemical 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 fission energy neutrons directly.