UNCLASSIFIED CHAPTER1 INTRODUCTION The gamma-radiation hazard associated with radioactive debris from nuclear explosions constitutes an important capability of atomic weapons. The degree to which this capability can be exploited depends upon the magnitudeof the militarily significant gamma-radiation fields produced and upon the ability to predict the location and extent of these fields. The phenomenon, commonly referred to as fall-out, varies with weapon yield and conditions of detonation. The present work proposes to extend the knowledge of such variations by investigating the fall-out material from Mike shot, Operation Ivy. The information derived will be useful for nw both offensive and defensive planning. 1.1 PREVIOUS FALL-OUT STUDIES Fall-out from surface and subsurface nuclear detonations has been documented at previous test programs. The phenomenon wasfirst observed after the detonation of the Alamogordo device in 1945.’ Since that time it has becomewell established that the gamma hazard resulting from fall-out must be seriously considered as a problem of military significance for all types of detonations except the air burst.* Fall-out was first fully documented at Operation Jangle, but limited data were obtained at Operations Crossroads and Greenhouse. we ee. 1.1.1 At Operation Greenhouse The fall-out study conducted at Operation Greenhouse revealed significant residual contamination from the Dog and Easy tower shots. This investigation was the first comprehensive study of fall-out forecasting.? These forecasting techniques, together with the work of J. O. Hirschfelder,® are the basis for the theories presented in the discussion of the fall-out at Operation Ivy. 1.1.2 At Operation Jangle we The surface shot at Operation Jangle more nearly represented a miniature Mike shot than any previous detonation. Fall-out studies were made at this operation, and complete data were obtained to a distance of several miles from ground zero.‘ The results were used in planning for Operation Ivy, and certain data to be found herein were extrapolated from information gained from the fall-out studies of Operation Jangle. *An air burst is defined for the purposes of this report as an explosion detonated at an elevation of such height that the resulting fireball at no time touches the surface of the earth. UNCLASSIFIED