In nuclear fission, neutrons are emitted in two distinct groups based on when they are released:
Prompt Neutrons:
- Emitted almost instantly (within about 10⁻¹⁴ seconds) as the nucleus splits.
- These neutrons are directly produced during the fission event itself.
- They play a critical role in sustaining a chain reaction by quickly causing further fissions.
Delayed Neutrons:
- Emitted seconds to minutes later from the beta decay of certain fission fragments (called precursor nuclei).
- Although only a small fraction of the total neutrons, delayed neutrons are vital for reactor control.
- Their delayed emission provides a time buffer, allowing operators and control systems to regulate the fission reaction safely.
Importance:
- The presence of delayed neutrons is crucial for the stable and controlled operation of nuclear reactors.
- Without delayed neutrons, the reaction would proceed too rapidly for safe control.
In summary, prompt neutrons drive the immediate chain reaction, while delayed neutrons provide the essential time delay that allows humans to control and harness nuclear fission safely.