rate method for analyzing fall-out patterns from surface burst 15 MI yield weapons. The method should be modified to permit scaling to other yields and programmed for an electronic computer to make it use- ful for military analysis. This is the method the Chief of Staff, U.S. Air Force recommended be considered by the AFSWP in the preparation of this study, in a memorandum for the Joint Chiefs of Staff dated 22 June 1955.25 The Air Research end Development Command method and the Tech- nical Operations Inc. models are also in this post-shot analysis category, and appear to give somewhat less accurate results than the other methods for an equivalent expenditure of computational effort. I. World-wide Distribution. Close-in fall-out, as previously defined, is thatfall-out_ within an isodose line which markstheboundary of the area within which thedoseaccumusated.to infinitetime4sroentgens or more for exposed ‘This area is bounded for a considerable distance by gradually decreasing amounts resulting from fall-out from the primary cloud. The cloud gradually spreads over a very large area and diffuses to such an extent that it is no longer a single entity but may be a series of ribbons several miles wide and hundreds of miles long. These cloud sections continue to diffuse and follow the wind patterns to such an extent that measurable distributions may be found continent- wide for smaller shots and world-wide for large yield detonations. After several months the distribution for all intents and purposes can be considered uniformly spread over the upper atmosphere, but not necessarily uniform to the top of the stratosphere. Thus, world-wide fall- out decreases in deposition rate after reaching a post-detonation peak, but probably does not reach zero as long as radioactive material is present in the atmosphere at any altitude. The external level of radiation from world-wide fall-out is not e biological hazard. However, the beta particles from internally deposi- ted radioactive material are of considerable biological importance. 16 J.c.S. 1716/18 55