Chapter 1 .- ’ INTRODUCTION 1.1 OBJECTIVES The general objective was to estimate, from analytical data on cloud samples, the relative distribution of certain radionuclides between the local and worldwide fallout formed by megatonrange detonations on land and water surfaces, with particular emphasis on the distribution of Sr® and Cs!” between local and worldwide fallout. Specific objectives were to: (1) obtain airborne particle and gas samples by rocket and aircraft sampling techniques, (2) determine the distribution of radionuclides between two groups of particles that differed from one another in their falling rates in air and that could be consid-~ ered representative of local and worldwide fallout, (3) attempt to determine an early time distribution of radionuclides and particles between the upper and lower halves of the cloud and radially outward from the cloud axis, and (4) estimate the extent of separation of fallout from gaseous fission products by fission determinations on gas and particle samples collected coincidentally near the top of the cloud at various times following the shota. 1.2 BACKGROUND AND THEORY Data on the geographical distribution of fallout is particularly needed to assess the global hazards associated with the testing of auclear devices, but the information is also important for an appraisal of the effects of nuclear weapons used in warfare. It has been recognized Since the earliest weapon tests that a substantial portion cf the radio- nuclides formed in a nuclear detonation are deposited throughout the worid, thereby becoming available for general biological assimilation. The total fallout is usually considered as being divided into two classes, designated as local and worldwide fallout. In a general way, local fallout is thought of as consisting of relatively large particles, which reach the earth’s surface in a few hours, whereas worldwide fallout is composed of finely divided material, which may remain suspended in the atmosphere for months or years and be deposited at long distances from the source. A more precise differentiation is needed for specific gituations—one of the most important considerations being the location of the detonation site in relation to world cen- ters of population. For explosions at the Eniwetok Proving Ground (EPG), the boundary between the two classes has been chosen at a particle falling velocity of 3 inches per second; material settling out more slowly than this is likely to be transported beyond the ocean areas and deposited in inhabited regions, if it attains an altitude of 100,000 feet. The ratio of local to worldwide fallout is also governed by the height attained by the nuclear cloud and the size distribution of the particles in the nuclear cloud, which act as collectors for the radioactive fission-product atoms. If many large particles with fast failing rates are present, as is the case for underground or surface shots where the fireball contacts the ground, the local fallout will be large. Local fallout can be expected to decrease as the detonation height increases and to become a negligible quantity for an airburst high above the ground. 11