mic response, so that a wide range of radiation intensity could be recorded without a change of scales. The Top Hat aerial radiation detector was developed by HASL to overcome these problems. Units were installed in three AD-5N aircraft and field-tested at the Nevada Test Site (NTS) during Operation Teapot. The AD-5SN aircraft used during Operation Teapot were transferred to an aircraft carrier for surveys following the undersea test during Operation Wigwam (Reference 4). A wide rangeof radiation intensities were encountered in this operation. The first pass over surface zero was shortly after H + 11 minutes, and measurements were made which extrapolated to approximately 400 r/hr at the surface. At the other extreme, Surveys were made at D + 4 days to delineate the edges of the contaminated area, where the dose rates were approximately 0.1 mr/hr. The Top Hat system was modified for Operation Redwing, and additional units were constructed. No changes were made in the basic detecting elements; however, the hermetic sealing was improved in anticipation of the humidity at the Eniwetok Proving Ground. 1.3 THEORY The heat re ulting from an atomic explosion vaporizes the products of the explosion and the bomb casing. Soil and water in the vicinity of ground zero are also vaporized and picked up by the updraft produced by the rise of the ball of incandescent gases. On cooling, the material in the firevall condenses into particles that include the radioisotopes resulting from the fission process and fromneutron activation of inert materials. The energy released in the explosion will iafluence not only the quantity of particulate material but also its altitude distribution in the vicinity of ground zero. The portion of the yield related to the fission process is represented by the amount of radioactive contamination carried by the particles. Once the particles are formed, they fall and, influenced by the winds, will reach the surface displaced frcm ground zero. The radioactive fallout from megaton shots may contaminate thousands of square miles of surface. The shot conditions influence the form and quantity of the fallout. When shot is exploded on ‘aad, a large amount of soil is picked up and much of it is vaporized by the intense heat. This immaterial condenses in a wide range of particle sizes. Some of the radioactive products are con- ‘tensed around large particles that were picked up in the updraft but not vaporized. These larger particles fall rapidly and reach the surface relatively close to ground zero. When a shot takes place at the surface of deep water, vaporized water can carry some of the activity away from the site. The large particulate fallout encountered in the land shot will be missing, ard this will be reflected in the distribution of fallout on the surface. An air shot is one in which the fireball does not touch the surface, so that compared with surface shots relatively little foreign material is vaporized. Because there are no available particulates on which the fission products can condense, most of the active material remains in the upper atmosphere and little fallout is likely to be detected in the vicinity of the shot site. 1.3.1 Fallout Contamination of a Water Volume. Whenthe contamination falls into the sea, dispersion and dilution carry much of the material below the surface (Reference 3). The intervening water acts as a shield between the surface and much of the gammaactivity. Thus, the radiation dose rates measured above the surface are reduced many orders of magnitude; however, sensitive detectors can be used to delineate the area of contamination. Also, if samples are taken at various depths, the quantity of radioactivity present can be integrated to the maxtmum depth of mixing, and in this manner, it is possible to secure isodose distributions of the fallout as they would appear on an equivalent land surface. The location of detector and source volume on a coordinate system is shown in Figure 1.1. Because of the absorption of the gamma rays by the water, radiation detected above the surface comes from the top 10 to 20 cm of the sea. The following equation describes the variation of dose rate, I,, above such a contaminated volume (Appendix A, Equation A.1Q). 12