OFFICIAL USE ONLY I. EXTERNAL GAMMA RADIATION Gamma dose rates were taken periodically on several islands in the Pacific over a time ranging from about two days to over two years. The attached map is an estimate of the gamma dose rates at three feet above the ground at D+ 1 (one day after the detonation). A very rough approximation of the degree of contamination may be made by dividing these readings by four to arrive at units of gamma megacuries per square mile. (The beta to gamma ratio varies with time but at one day may be near unity, so these values may also be thought of beta activities.) However, the gamma dose rates do indicate the relative degrees of contamination on the islands and therefore are useful in this respect when evaluating the data in subsequent sections of this report. Graph One shows the decay with time of gamma dose rates on the Island of Rongelap. Similar decay curves were found on other islands in the Atoll and in nearby Atolls (Ailinginae and Rongerik). The decay of activity of mixed fission products is assumed to follow (tame) 7°" principle. atoms. This is intended to apply to disintegrations of However, in estimating the reduction of gamma dose rates above a plane with time there must be considered the changing numbers and energy spectra of gamma photons released per disintegration, and the effects of weathering. When computing the infinity radiation doses from fallout that occurs within afew hours after detonation, integration of the (time )71** curve gives a fair approximation since most of this total dose is accumulated during the early periods when this curve lies near the theoretical gamma decay curve. However, in extrapolating by (time)7?? OFFICIAL USE ONLY