CHAPTER 17 other materials in which radioactivity has been induced by neutron capture. In the literature, early radiation has been called "initial," and has been rather arbitrarily defined as all radiation emitted within the first mimite. Such a definition may be true for water-surface bursts, but cannot hold for underwater bursts and conform with the above definition of residual radiation, since the base surge may be clearly distinguishable and the fission products in the surge may be emitting radiations by 30 sec after burst. Therefore, this report defines "fireball-plume-cloud radiation” as above, with no fixed time limit. For brevity, the initials, F.P.C. radiation, will be used in following discussions. F.P.C. radiations of significance to the total nuclear-radiation exposure dose for surface or very shallow underwater bursts include (1) prompt gamma rays and prompt neutrons emitted at the time of fission or fusion; (2) gamma rays resulting from inelastic scattering of neutrons; (3) nitrogen-capture gamma rays; (4) early time fissionproduct gamma rays. The prompt ganmmas and neutrons are liberated in the process of fission or fusion in a time of less than a microsecond, and are thus emitted at ea time when the bomb is still almost completely compacted. Most of the prompt gamma rays are absorbed by the bomb fy materials and casing and thus do not contribute significantly to the total F.P.C. radiation. Although many of the prompt neutrons are slowed down and captured by the bomb residues, a significant number of neutrons escape to the atmosphere. As these neutrons traverse the atmosphere, they may undergo either capture or scatter reactions with atomic nuclei along their paths. If neutron capture occurs, the energy of the captured neutron raises that of the capturing nucleus, and the excess energy of the nucleus may be emitted as ganma radiation. In the two types of scattering collisions, the incident neutron loses part of its energy to the struck nucleus, and a neutron degraded in energy results from the reaction. Inelastic scattering occurs when part of the kinetic energy of the incident neutron is converted into excitation energy of the struck nucleus. This energy is then emitted as gamma radiation. Elastic scattering occurs when a portion of the neutron kinetic energy is transferred to the struck nucleus. In this case the total kinetic energy of both particles after collision is the same as before, al- though the energy distribution may be different. The gamna rays resulting from inelastic scattering of those neutrons that escape to the atmosphere can contribute significantly to F.P.C. radiation, particularly for bursts of fusion weapons, where large numbers of high-energy neutrons are emitted. The high-energy nitrogen-capture gammas result from the nuclear capture reactions between atmospheric nitrogen and prompt neutrons at or near thermal energies. The early-time fission-product gammas are emitted by 7-15 ewe wren ee ee ee ee ee eee mem ee cae ee