II. PHYSICAL ASPECTS OF THE FALL-OUT PROBLEM A. Radioactive Materials Formed In A Nuclear Detonation. The quantity of radioactive material created as fissicn products in the detcnation of a fission weapon is known to depend upon the total energy release of the weapon. Some methods of calculation of total fission yields utilize the ntmber of fissions per kiloton, which gives approximately 1.38 x 10°) fissions occurring per KT of fission yiela/, resulting in 2.76 x 10°? radioactive fission fragments per KT. This value includes all of the immediate fission products, some of which decay with very short half-lives, while others my give rise to decay chains consisting of several isotopes, some of which may have longer half-lives, resulting in their decay over a period of months or years. All emit one or more beta particles and most emit one or more gamma rays. There sare approximately 170 isotopes ultimately formed from a total of 35 elements known to result from the fission process. Some of these isotopes are formed directly from fission; others form in part from direct fission and partly from decay chains. If the same isotope is formed by the two processes, the total isotope concentration is the sum of the two. Two exemples of decay chains which form the same elements but different isotopes as indicated by their different halflives are: Germeniun-77 pont Arsenic-77 win Selenium-77 (Stable) Germaniun-78 ="———» Arsenic-78 Gs Selenium-78 (Stable) It is convenient to discuss the quantity of radioactive materials formed in a nuclear detonation in terms of the value at one hour after detonation, expressed in curies per KT. At this H+l hour reference time, there are approximately 3.0x 108 curies of gamma-active fission 1/ Spence, R. W., Bowman, M.C., Radiochemical Efficiency Results of SANDSTONE Tests. Scientific Director's Report of Atomic Weapons Test Annex 1. SECRET Restricted Data. 3