OFFICIAL USE ONLY there may be a significant difference in estimating dose rates a year or more after detonation and in estimating doses that might occur at these later periods. This is because (tame) "?*? 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. During the first two weeks after fallout there was no rainfall and the winds were light. storm occurred. About the end of the second week a tropical 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 a much greater reduction in contamination was produced by the first weathering events than for later ones. The theoretical curve of Graph One would flatten out with time due to the dominance of Cesium-137 with its 33 year half-life. The last survey of Rongelap Island in late July 1956 indicates a range of gamma dose rates at three feet above the ground of 0.2 - 0.5 milliroentgens per hour with an average of 0.4 mr/hr. The continued drop in actual dose rates versus theoretical might be explained on the basis of the effects of weathering. OFFICIAL USE ONLY