What energy range defines Fast Neutrons?

What energy range defines Fast Neutrons?

• A. 10 keV - 20 MeV
• B. 0.01 eV
• C. 0.3 eV - 10 keV
• D. > 20 MeV

Answer: (A)

Fast neutrons are the high-energy portion of the neutron spectrum, far above the thermal energies neutrons have when they’re in a room-temperature environment. In reactor and fission physics, these fast neutrons come out with energies on the order of keV to tens of MeV, with most of the prompt fission neutrons carrying roughly a few MeV but extending up toward about 20 MeV. That makes the range from about 10 keV up to 20 MeV the spot where fast neutrons live: it clearly excludes the thermal region (those are at very small fractions of an eV) and also avoids the lower end epithermal range, while staying within the high-energy tail produced by fission events. Energies above 20 MeV are not typically considered “fast neutrons” in reactor practice because they are rare and come from other processes, so this upper limit helps keep the definition focused on the practical fast-neutron spectrum.

Fast neutrons are the high-energy portion of the neutron spectrum, far above the thermal energies neutrons have when they’re in a room-temperature environment. In reactor and fission physics, these fast neutrons come out with energies on the order of keV to tens of MeV, with most of the prompt fission neutrons carrying roughly a few MeV but extending up toward about 20 MeV. That makes the range from about 10 keV up to 20 MeV the spot where fast neutrons live: it clearly excludes the thermal region (those are at very small fractions of an eV) and also avoids the lower end epithermal range, while staying within the high-energy tail produced by fission events. Energies above 20 MeV are not typically considered “fast neutrons” in reactor practice because they are rare and come from other processes, so this upper limit helps keep the definition focused on the practical fast-neutron spectrum.