prompt neutron
Technical term in nuclear physics: a neutron emitted immediately during fission, as opposed to delayed neutrons.
Prompt neutrons are emitted almost instantaneously during nuclear fission.
Delayed neutrons are emitted seconds to minutes after fission, while prompt neutrons appear immediately.
Compound of 即発 (immediate emission) and 中性子 (neutron), directly describing a neutron emitted promptly in fission.