To sustain a fission reaction, a critical mass is required. This critical mass refers to the minimum amount of fissile material (such as uranium-235 or plutonium-239) needed to maintain a chain reaction. When the critical mass is achieved, the fission process can continue as each fission event produces neutrons that can trigger additional fission reactions, leading to a self-sustaining chain reaction.
In a nuclear reactor or a nuclear bomb, maintaining the critical mass is crucial for controlling the rate and intensity of the fission reactions. By adjusting the amount of fissile material present, scientists and engineers can regulate the energy output and prevent uncontrolled reactions.
Therefore, out of the options given, the most relevant requirement for sustaining a fission reaction is the critical mass. The other options such as gamma emission, leptons, and beta emission do not directly contribute to the sustainability of a fission reaction as the critical mass does.
