Which particle initiates the fission process when absorbed by a uranium atom?

• A. Neutron
• B. Electron
• C. Proton
• D. Photon

Answer: (A)

Neutrons are the particles that can most readily initiate fission in fissile nuclei like uranium-235. Because neutrons carry no electric charge, they aren’t repelled by the positively charged nucleus and can be captured by it when they collide. Once uranium-235 absorbs a neutron, the resulting uranium-236 is driven into an excited state and quickly splits into lighter fragments, releasing energy and additional neutrons. Those new neutrons can go on to cause more fission events, creating the chain reaction that powers reactors and weapons alike.

Other particles don’t fit this role as well. Charged particles such as protons or electrons face the Coulomb barrier from the nucleus, making it hard for them to be absorbed and to trigger fission. Photons would require very high energies to produce fission and do so far less efficiently in typical conditions. Hence, the neutron is the most effective and common initiator of the fission process.