indicated that the greatest activity was in the core, so that the activity must have been picked up early in the cycle of hailstone formation. Another indication of the ability of rain to scavenge at great heights was a fall-out upon Albany, N.Y., from a portion of the cloud which ws stabilized at 40,000 feet and not at lower levels. Gummed peper collec- tors counted 1.6 x 10! disintegrations per minute at Albany following a@ severe thunderstorm, but the count was low at the six stations near Albany. While not a hazard to health, this disintegration level is very much higher than background. Although thunderstorms are the largest single source of rain in the United States east of the Rocky Mountains, their very limited size geographically and the infrequency of their formation suggest that they do not accowt for much of the total radioactivity that is returned to earth. F. Fractionation, The probability that fractionation exists in a detonation of a nuclear weapon has long been suspected by investigators, and each careful study of the radionuclides formed from detonation of test weapons helps to substantiate the hypothesis. There are several definitions of the term; however, for purposes of this report the one proposed by Stanford Research Institut 10 that "fractionation can be defined as any variation from the expected value in the relative fission product nuclide abundance” will be used. The degree of fractionation is a function of the particle size and chemical composition of the vehicle for fall-out, the environment of the particle after fall-out, and the physical and chemical properties of the radioactive elements formed in the fission process as well as those of daughter elements formed in the decay chain. The fission process yields constant relative amounts of radioactive isotopes, depending upon the fissionable materiel used. The half-lives of the 170 isotopes from 64 known decay chains contribute to the over-all fission 10/ Cadle, R.D., The Effects of Soil, Yield, and Scaled Depth on Contamination from Atomic Bombs, Stanford Research Institute, S.R.I. Project Cu-641, 29 June 1953. SECRET Restricted Data. 29