Chapter | INTRODUCTION 1.1 OBJECTIVES The specific objectives of Project 2.61 were to: (1) proof test a system using rocket-borne detection units with telemetering transmitters to explore the spatial distribution of radioactivity in the stem and cloud resulting from a nuclear detonation; (2) measure gammaintensities along several continuous known trajectories passing through tre stem and cloud at 7 and 15 minutes after detonation; and (3) estimate the extent to which the rocket became contaminated as it passed through the stem or cloud. 1.2 BACKGROUND AND THEORY Although various mathematical models for the fallout process have been presented (Reference 1), gross differences exist among the assumptions as to spatial distribution of radioactive emitters in the cloud and stem. Determination of the distribution which actually exists is essential to the development of a correct model and the eventual realistic predictions of fallout patterns. Without such knowledge there wouid be continuing uncertainties as to the spatial positions of active particles prior to fall, resulting in unreliable predictions cf the spread and extent of activity. Besides being essential to the development of fallout theory, a knowledge of the distribution of radioactivity in the cloud and stem at early times may be important for interception, countermeasures, and long-range-detection studies. Construction of an effective fallout model requires knowledge of the size, activity, and spatial distribution of radioactive particles in the stem and cloud. Of these parameters, particle size distribution and related activity were determined from particles collected as fallout at the surface of the earth by Projects 2.63 and 2.65. If, in addition, measurements of gammaintensities in the stem and cloud are made, gross distribution of active particles in the stem and cloud may be inferred. Restrictions due to time and equipment available before the operation precluded measurement by this project of any parameter except gammaactivity as a function of time and position. Measurements of radiation fields existing in clouds resulting from detonations of devices in the kiloton range have been made previously. The first measurement of cloud-radiation fields was made during Operation Greenhouse by the use of drone aircraft. These measurements were made in the stems of clouds resulting from explosions whose yields ranged from ~ Fields of avout 10‘ r/hr were observed at 3 to 5 minutes after detonation and of about 350 r/hr at 30 minutes after detonation (Reference 2). During Operation Upshot-Knothole, cannisters and drone aircraft operated in the mushroom tups resulting from 11 tc 26 kt explosions. Fields of about 10‘ c//hr existed at 2 to 6 minutes after detonation (Reference 3). During Operation Redwing, aircraft were flown through the stem and lower portion of six clouds resulting from detonations in the megaton range. Reference 4 gives as the average dose rates encountered when correctcd to 100 percent-fission yield: D = 1.0 x 108 yo? (1d)