Definition - What does Thermal Neutron mean?
A Thermal Neutron is a neutron which is in thermal equilibrium with the ambient medium. These kinds of neutrons have a neutron speed of 2200 m/s at ambient temperature conditions of 293.6 Kelvin corresponding to energy of 0.0253 eV. The fast thermal neutrons which are generated during nuclear fission are slowed down by collisions with the atoms of moderator material to thermal energy which are further 'thermalized'. The moderator material can be water, heavy water or graphite.
Petropedia explains Thermal Neutron
Generally, neutrons are elementary particles that have no charge and have a mass somewhat greater than that of protons. They are the integral part of the nuclei of all atoms except hydrogen nuclei. There are free Neutrons present outside the nucleus which are unstable and have a life of around 14 minutes and 42 seconds. Neutrons have a total electric charge of 0 which has been experimentally tested. The mass of Neutrons cannot be determined directly by mass spectrometry because of the lack of electronic charge.
They play a crucial role in several nuclear reactions. For instance, captured Neutrons result in the Neutron activation that induces radioactivity. One of the great uses of Neutrons is to excite prompt and delayed gamma rays from the elements in materials. They can also be used in the detection of light nuclei, for example, hydrogen found in the water molecules.