CHAPTER| Ly a. External Gamma Radiation x & “& 2» ~ = te * Aon # eyuetes LE GAMMA DOSE RATES WEREtaken periodically on several islands in the Pacific over a time ranging from about two days to more than 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 on March 1, 1954). 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 gammaratio varies with time but at one day may be near unity, so these values may also be thought of as beta activities.) However, the gamma dose rates do indicate the relative degrees of contamination on the islands, therefore are useful in this respect when evaluating the data in subsequentsections of this report. Graph 1 shows the decay with time of gamma dose rates on the Island of Rongelap. Similar decay curves were found on otherislands in the Atoll and in nearby Atolls (Ailinginae and Rongerik). The decay of activity of mixed fission products is assumed to follow (time)—)? principle. This is intended to apply to disintegrations of atoms. 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 computingthe infinity radiation doses from fallout that occurs within a few hours after detonation, integration of the (time)-)? curve gives a fair approxima- tion 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)~'? there may be @ significant difference in estimating dose rates a year or more after detonation and in estimating doses that might occur at these later periods. ; During thefirst two weeks after fallout there was no rainfall and the winds were light. About the end of the second week a tropical storm occurred. For these reasons, a straight line was drawn for the first two weeks followed by a break in the curve. The readings are not to be considered precise, due to the nature of such measurements, but the curves suggest that whatever was the reduction of gamma dose rates by weathering, it occurred principally with the first heavy rainfalls. Except for the last data point on Graph 1 for the Island of Rongelap, which may be somewhat high, the actual and theoretical decay curves correspond fairly well. The theoretical curve of Graph 1 would fiatten out with time due to the dominance of Cesium137 with its 27 year half-life. The last survey of Rongelap Island in late July 1956 indicates a range of gammadose rates at three feet above the ground of 0.2—0.5 milliroentgens per hour with an everage of 0.4 mrfhr. These values are higher than suggested by Graph 1 and are due to the small additional fallout resulting from Operation Redwing (Spring and Summer 1956). 1