Editing Neutron (section)

== Neutron temperature ==
{{Main|Neutron temperature}}
The energy of free neutrons are characterized by their temperature as given by their [[Maxwell–Boltzmann distribution]]. For example, [[Neutron temperature|thermal neutrons]] have with kT&nbsp;=&nbsp;{{val|0.0253|ul=eV}} ({{val|4.0|e=-21|ul=J}}) corresponding to room temperature, giving them a characteristic (not average, or median) speed of 2.2&nbsp;km/s. 
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 nucleus|atomic nuclei]] that they collide with, creating a heavier – and often [[unstable isotope|unstable]] – [[isotope]] of the [[chemical element]] as a result.
Most [[nuclear reactor|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. 

Cold and even ultra-cold neutrons can be created by thermalizing with cryogenic materials.
Higher temperature neutrons arise from nuclear fission and nuclear fusion. The highest energies arise from cosmic ray collisions.