CHAPTER 5 SECONDARY FALL-OUT For many days following a detonation, radioactive debris falls out over the surface of the earth. Previous tests have shown no reason for considering this secondary fall-out to be of military significance; a certain amount of documentation in this area is necessary, however, as a check and to provide information on upper air movements. DISTRIBUTION OF SECONDARY FALL-OUT From 2 to 8 days after the detonation, secondary fall-out arrived over an extensive area of F 5.1 the Pacific around Eniwetok Atoll. This fall-out was measured on Majuro, Kwajalein, Bikini, Kusaie, Eniwetok, and Ponape, as well as on task force ships and the free-floating stations. Johnston Island recorded no measurable fall-out, but Guam, which was not instrumented, is believed to have received a small amount. Figure 5.1 shows the concentration of particulate received at Bikini Atoll on 4 November 1952. This is typical of the particle density received at all the outer islands, 5.2 LEVELS OF RADIOACTIVITY Noneof the stations at which secondary fall-out was collected reported a gamma dose rate over 10 mr/hr. These dose rates were determined with gamma-survey instruments. In many The secondary fall-out over the area within 600 miles of Eniwetok Atoll lasted for severai days. Figure 5.2 shows the distribution of activity with time for the outer islands. There isa trend that indicates the easternmost islands experienced fall-out first, at M+2 day. The secondary fall-out drifted to the west and south, arriving at Ponape, the westernmost island, m M+5 day. 5.4 PARTICLE SIZE OF SECONDARY FALL-OUT The particle size of the secondary fall-out was investigated by observation of the radioactive particulate collected on several of the outer islands. In no case wasit larger than 25 . No determination was attempted on the distribution of particle size. 48 oy are PERIOD OF SECONDARY FALL-OUT wa een 5.3 WR are Emeneeet instances the level of radiation at the stations was so low that the instrumentsfailed to detect the presence of any fall-out. Detection of fall-out at these stations was made by thin-window counting with laboratory beta and gamma instruments.