Although the magnitudes of the uncertainties in the mean unweathered activity per unit area were not well defined, we thought that the estimates for Rongelap Island had a standard deviation based partly on the original Bikini Ash activity measurements (e.g., 957~ S.D. = 220% (1s56)) and partly on the variation in exposure-rate measurements (e.g., radsafe team survey S.D. = t20% (0C68)). A measurement of activity per unit area based on a single soil sample would be highly uncertain, because of random fallout deposition and because of physical mechanisms which move deposited fallout. This variation was dampened considerably by our use of exposure-rate survey results to estimate mean surface activity rather than use of a few gross beta measurements on soil. Our estimate of the standard deviation for the few soil samples collected by the radsafe team was +140% of the mean value, based on the surface activity measuremente reported by O'Conner (0C68) for surface samples taken shortly after detonation from one island in the Northern Marshall Isiands. If soil sample results were used in the final estimate of dose, this large standard deviation would propagate through the calculations. Thus, we chose a method which offered greater certainty in the result. Our estimate of each nuclide's mean unweathered activity per unit area of Rongelap Island was extrapolated back to 0.5 day post-detonation. Results are listed in Table 13. The 0.5-day post-detonation time was chosen as the time at which the fallout at Rongelap Island had effectively ceased (Sh57). We used first-order linear kinetics for serially related nuclide species (Bal0) and decay schemes from the Table of the Isotopes (Le78) in order to calculate the 0.5-day activity from the day~-26 activity. iii. Areal Activity of Nuclides Without Descendants inBikini Ash. Many short-lived nuclides did not have daughter radionuclides present on day 26. We based the activity of these short-lived nuclides on the activity of a reference nuclide. Equation (3) was used by us to relate the unknown activity of the short-lived nuclide with no daughters present on day 26 to the known activity of a nuclide which had fractionated in the same fashion as the unknown. Thus, if mo isobar was present on day 26, an isotope or an isotope of an isobar of the unknown was chosen to represent the fractionation behavior and be the reference nuclide for the estimate of activity per unit area. The equation used to relate activity of a short-lived nuclide to a reference nuclide was As Brg An (3) Xp Bn where A * activity of nuclide A per unit area at time t post-detonation, B = activity of nuclide B per unit area at time t post-detonation, Ag Ay A, Bh = 7 = ™ decay constant of decay constant of number of A atoms number of B atoms nuclide A, nuclide B, per unit fission at time t, per unit fission at time tc. The quantity A, or B, was calculated using 1) first-order lin- ear kinetics equations, 2) fission yields for 14-MeV fission of 238y obtained from the evaluated nuclear data files of the National Nuclear Data Center - 32 -