All four types are present at the time of the detonation, but the gamma and neutron are by far the most important clinically. All but the neutron radiation are present in fallout and, in this instance, the gamma is the most important. Ionizing radiation is emitted both at the time of the nuclear detonation and for a considerable time afterward. The significant part of prompt radiation consists of a mixture of gamma and neutron radiation, most of which is emitted within a few seconds of the onset of the detonation. However, the duration of significant emission may be longer, particularly with larger weapons.
Residual radiation is that which persists beyond the first minute after detonation.
Its source is the variable amount of residual radioactive material produced by a nuclear detonation. In addition, small amounts of unfissioned bomb material, and material in which neutron radiation has induced radioactivity, are present. All of these residually radioactive materials will be found in fallout. Fission products are the major radiation hazard in fallout, since a large number of them emit penetrating gamma radiation and, as a result, can be hazardous even at great distances. They have half lives varying from fractions of seconds to several years, but most have half lives in the range of days to weeks.