. from the fallout probably settled to the bottom of the lagoon within weeks after the nuclear explosion, it certainly would have settled to the bottom by now. During the past 15 years, much of the colloid-soluable fallout material could be expected to have passed out to the open ocean. Early Time Radioactivity Measurements The early time radiations at Eniwetok Atoll during the tests are now only a matter of historical interest because their effects have long since passed in importance; however, a short review is included } ! here for completeness. _ The prompt neutrons and gamma rays are emitted well within a second following the explosion. radiation. This is sometimes called the initial As noted apove, the induced aciivitics produced by these prompt neutrons are now only a minor consideration in the radiological condition of the Atoll. Following the initial radiation is the fallout radiation. However, some time is involved before this local fallout reaches its maximum, a maximum resulting from radioactivity decreasing with time as seen in Figure 1 (labeled as Fig 9.16b) as a result of nuclear decay (Glasstone, 1964, p. 420) being offset initially by the delay until fallout reaches the ground (Glasstone, 1964, p. 454). The mean arrival time of early fallout at Eniwetok Atoll in the test series has been observed to be roughly 30 minutes for explosions in the megaton range. This time-applies for upwind fallout and results from 14 '