Which statement best describes delayed neutrons?

Which statement best describes delayed neutrons?

• A. They appear sometime after fission.
• B. They are emitted during fission.
• C. They are emitted before fission occurs.
• D. They are never emitted.

Answer: (A)

Delayed neutrons come from the fission fragments themselves, not from the fission event exactly as it happens. After a nucleus splits, some of the resulting fragments are radioactive and decay by beta emission, and in the process they release neutrons. These neutrons appear after some delay—ranging from a fraction of a millisecond up to minutes—so they are not part of the immediate fission flash. This delayed component is small compared to the prompt neutrons, yet it is crucial for reactor control because the reactor’s neutron population responds on those longer time scales, allowing control systems to adjust reactivity in a manageable way. In short, delayed neutrons are emitted sometime after fission as the fission fragments decay, not during or before fission.

Delayed neutrons come from the fission fragments themselves, not from the fission event exactly as it happens. After a nucleus splits, some of the resulting fragments are radioactive and decay by beta emission, and in the process they release neutrons. These neutrons appear after some delay—ranging from a fraction of a millisecond up to minutes—so they are not part of the immediate fission flash. This delayed component is small compared to the prompt neutrons, yet it is crucial for reactor control because the reactor’s neutron population responds on those longer time scales, allowing control systems to adjust reactivity in a manageable way. In short, delayed neutrons are emitted sometime after fission as the fission fragments decay, not during or before fission.