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 cloudse The muclear 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 sanples ig determined by analyzing the gas for 2.8~hour Kr88 and the solid for a representative mclide such as Mo??, . . 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 sampled portion of the cloud at the time of sampling. At a very early time, if no separation of gas and particles occurs, this ratio ‘should be one. Later it would be expected to decrease as the falling particles remove the bound fission products. Hence, if the early ratio is one, the frac- tion of the material in world-wide fallout may be determined if the time is known at which particles having a falling velocity of 3 inches per second leave [4 the sampling or if the ratio approaches a ccnstant with time. ae) region, o2¢5 Prior Estimates of Local Fallout. Determinations of local fallcut have been made at virtually ell the muclear weapons tests conducted by the United States. Estimates of the fraction of the radioactivity deposi- ted locally have been made for Operations JANGIE (References 16, 23, 25, 26