The prompt neutron lifetime, l , is the average time between the emission of neutrons and either their absorption in the system or their escape from the system. The neutrons that occur directly from fission are called " prompt neutrons ", and the ones that are a result of radioactive decay of fission fragments are called " delayed neutrons ". The term lifetime is used because the emission of a neutron is often considered its "birth", and the subsequent absorption is considered its "death".