Question: what is the difference between fast and thermal neutrons?
Answer: A free thermal neutron has energy in the order of 0.025 eV (minor deviation possible). Fast neutron has significantly higher energy, in a range of 1 – 20 MeV. Velocity of thermal neutron is close to 2.2 km/s. Fast neutron has very high velocity, typically in the order of 2×104 km/s.