The initial and rapid measurements taken by the Manhattan Engineer District served the critically important purpose of allowing the American occupation of Hiroshima and Nagasaki to proceed as scheduled. The more extensive surveys by the Manhattan Project Atomic Bomb Investigating Group and the Naval Mission to Japan resulted in reports since regarded as basic source documents and included in the references appended to the end of this chapter. 6.2 RESIDUAL RADIATION IN HIROSHIMA AND NAGASAKI. After the bombings, one area of low-level residual radioactivity remained around ground zero in each city and in areas downwind of each city. The former was induced radioactivity, and the latter was caused by fallout. 6.2.1 Induced Radioactivity at the Hypocenters. Roughly circular patterns of residual radiation were created at the times of detonation, when the high-intensity burst of neutrons from the bomb encountered elements in the soil and building materials, such as concrete, # metal, and tile, in the area beneath the detonation and caused them to become radioactive. (Examples of elements in which radioactivity can be induced are aluminum, sodium, manganese, cobalt, and cesium.) The induced radioactivity was of relatively low intensity because the detonation heights minimized the number of neutrons reaching the ground and because many of the induced activity radionuclides had short half-lives (the time required for the radiation intensity to be reduced from any given value to one-half that value). For example, aluminum-28 has a half-life of about 2.3 minutes, and manganese-56 has a half-life of about 2.6 hours. When the first occupation troops entered Hiroshima 60 days after the detonation, the intensity of induced radioactivity around the hypocenter was 0.03 milliroentgen per hour,* as shown in figure 18. within this area was about 0.1 milliroentgen per hour. The highest intensity About the same levels of induced radioactivity. remained in Nagasaki when the main body of occupation troops arrived 45 days after the bombing. Figure 19 shows an isointensity *A milliroentgen equals one-thousandth of a roentgen. 155