Chopter | INTRODUCTION 1.1 OBJECTIVE The objective of Project 2.1 was to determine gamma exposures versus distance from the point of detonation of various high-yield devices. A secondary objective was to determine the gamma exposures received in several discrete time intervals between time of arrival of the thermal pulse and 1 minute after time of detonation. 1.2 BACKGROUND Initial-gamma radiation may be considered as that emitted during the first 30 seconds after detonation. The initial-gamma radiation output for nuclear devices with yields up to 250 kt has been well documented in previous test operations rs Gamma-radiation measurements from high-yield nuclear devices during Operation Ivy showed that the initialgamma radiation did not follow the same scaling laws that had been established for smaller de- vices} ~~} This was attributed in part to the hydrodynamic effect, which results in an enhancementofthe garumaradiation. This effect is caused by the passage of the shock front through the detector station, resulting in a reduced air density between detector and radiating source. Section 1.3.4 gives a simplified treatment of the hydrodynamic effect. Measurements were made during Operation Castle by the U.S. Army Signal Engineering Laboratories to determine the empirical relation between yield and hydrodynamic enhancement : Some high-yield Operation Castle devices provided data points; however, it was thatadditional data were needed at a number of suitably spaced points for various yields and types of nuclear devices to determine more valid scaling laws. The present scaling laws for initial-gamma radiation from high-yield thermonuclear devices were based on data from relatively low-yield fission devices (1 to 500 kt), a few data points from Operation Ivy, and the sparse data from Operation Castle. Initial-gamma radiation appearedto be of little significance compared to damage caused by blast and thermal effects. Residual-gammaradiation is here defined as that which reaches the detector 30 seconds or more after time of detonation. Residual-gamma exposure measurements have been made by various organizations at previous test operations (References 2, 3, 5, and 6). During Operation Buster-Jangle, the Signal Corps, in conjunction with the National Bureau of Standards (NBS), made residual-gamma exposure measurementsof a 1-kt surface blast and a 1-kt device detonated at a depth of 17 feet (Reference 7). During Operation Teapot the U.S. Army Signal Engineering Laboratories made measurements of residual-gamma exposure resulting from an underground blast of a low-yield device (Reference 3). The advent of high-yield thermonuclear devices has resulted in a manifold increase in the radiological hazard, and gammaradiation from fallout has become of greater military significance. Operation Castle demonstrated that large quantities of radioactive material could be deposited by high-yield devices over areas of several thousand square miles, This led toa military requirement for fallout data for devices of various types and yields. Project 2.1 was charged with documenting the residual-gamma radiation exposures from the fallout at land stations at Bikini Atoll during Operation Redwing.