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

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

Answer: (B)

Neutron absorption drives fission in fissile nuclei like uranium-235. When a slow (thermal) neutron is captured, the nucleus becomes an excited uranium-236 system that splits into two smaller nuclei and releases energy plus several more neutrons. The neutron is the best initiator because it has no electric charge, so it can approach and be captured by the nucleus without needing to overcome a Coulomb barrier that charged particles must face. This makes neutron-induced fission the dominant and most practical way to start the process. Other particles—electrons, photons, or protons—can, in principle, cause fission only under special high-energy conditions or unlikely interactions, but they are not the typical initiators in standard fission reactions.