Thermal neutrons are needed to cause fission in U-235.

• A. Fast neutrons are needed to cause fission in U-235
• B. Thermal neutrons have no energy
• C. Thermal neutrons are needed to cause fission in U-235
• D. Neutrons cannot cause fission

Answer: (C)

Thermal neutrons cause fission of U-235 very efficiently because the fission cross-section for U-235 is highest at those low (thermal) energies. In a reactor, a moderator slows fast neutrons down to about 0.025 eV, which greatly increases the chance that a neutron will be captured by a U-235 nucleus and induce fission rather than just scattering or passing by. Thermal neutrons do have energy; they’re not zero energy, just very small. Fast neutrons can cause fission too, but with much lower probability for U-235, so they’re far less effective in sustaining fission chains. Neutrons can indeed cause fission, so saying they cannot is incorrect.