netsininininisinininteinennye 0018933 ing aloft for long periods. In the application of this method, coincident samples of gas and particles are taken by an isokinetic collector during the first few hours of existence of the clouds, The nuclear aerosol is sucked through a filter to remove the suspended material and the particle-free gas is then pumped into a storage bottle. The number of fissions in the two samples is deter- mined by analyzing the gas for 2, 8-hour Kr°8 and the solid for a representative nuclide such as Mo?9, The ratio of sample fissions calculated from a bound nuclide to those from an unattached rare gas nuclide will give the fraction of the. . reference substance which is in the cloud at the time of sampling. Ata very early time, before any appreciable fallout has taken place, this ratio should be one, but later it would be expected to decrease as the falling particles remove the bound fission products. Hence the fraction of the material in world-wide fallout may be determined if the time is knownat which particles having a falling velocity of 3 inches per second leave the sampling region. 1.2.5 Prior Estimates of Local Fallout. Determinations of local fallout have been made at virtually all the nuclear weapons tests conducted by the United States. Estimates of the fraction of the radioactivity deposi- ted locally have been made for Operations JANGLE (References 16, 23, 25, SAN BRUNO FRE 26 Ale