14 hours for the ground-induced activity. One of the phenomena of nuclear weapons is the high flux of neutrons of all energies. The mumber of fast or high energy neutrons resulting from the detonation of larger weapons is sufficient to cause a large fission yield from uranium-238 when used as a tamper for the implosion process. In addition to the fission reaction, the capture process of neutrons of intermediate energies by the uranium-236 yields appreciable quantities of neptumium-239. This decays with a 2.3 day half-life to plutonium-239, emitting a hard beta and gamma. Thus, the quantity of plutonium-239 ultimately formed depends on the quantity of uranium-238 used in the tamper. Part of the radiation hazard found following the CASTLE series shots, particularly after Bravo, resulted from the large quantities of uranium-238 which were irradiated. The energy of the neptunium decay radiations is in the biologically hazardous range. The proportion of neptunium radiations emitted relative to the fission product emissions was sufficient to alter the gt decay rate appreciably during the period from about H+10 hours to D+10 days, go that data studied must consider this variation. but the quantity is still insufficient to be an independent hazard in a close-in fall-out field. B. Radioactive Particle Formation. When a nuclear weapon is detonated, all fission fragments, unfissioned active material, the bomb tamper, and the bomb casing are included under the name of bomb debris. The proximity of the fireball to the ground, the nature of the terrain, and the yield of the weapon used, largely determine the ultimate amount of soil which is mixed with the bomb debris to form radioactive fall-out particles. The final fate of bomb debris which is mixed with the dirt may be influenced to a considerable extent by the chemical composition of the fall-out vehicle. Detonations at a sufficient height so that the fireball does not reach the ground result in a distribution of atoms of bomb debris with a —_.