SLIDE 18 Exposure rate measurements. Mean values for the external exposure rate were based on many measurements reported at the time of the evacuations. A Radsafe Officer plus a 3-man team took readings in the air, throughout the islands and especially in the village areas with Army, Navy catalogue AN/PDR-27E instruments. Readings tended to be the same with and without the beta shield (0C68). Readings were interpreted to be taken at waist level, about 1 meter (3 ft) from the surface (0C68). 10 -9 Held (He65) indicated C kg s~ a mean exposure rate at Rongelap Island of 2.9 x (40 mR h*) at 26 days post detonation. He reported a storm with heavy rain two weeks post detonation (He65). This was followed by a reduction in exposure rate greater than what would have been expected from decay of BRAVO fallout. Glasstone (G162) repored a 40% reduction in the exposure rate, attributed to weathering alone, during the first 25 days post BRAVO in certain areas of the Marshall Islands. We estimated the reduction in exposure rate due to weathering at Rongelap Island based on measurement taken 2.2 days post detonation by the USS PHILIP radsafe team. It was assumed to be a measurement on unweathered fallout and to decay with decay at a rate based on the nuclide mixture present at Rongelap 2.2 to 26 days post detonation. This decline was based on the gamma decay of 142 nuclides. For the estimate of decline, we accoupt¢d for 5h§ contribution to exposure rate from: 1} ,the tragsuranic nuclides U and Np, 2) the neutron induced nuclides S and ““Ca, 3) the day 26 fission products which had fractionated according to Japanese results (Ya56, Ts55) and 4) the fission The day 26 value of the exposure product and transuranic product precursors. rate extrapolated from the 2.2 day measurement made b per cent over that reported by Held. Thus, 3.4 x 10° qdsafe team was 18 37 (47 mR ho!) was the mean unweathered exposure rate estimated to have existed on day 26 had the rain storm not occurred. EXPOSURE RATE MEASUREMENTS Rh! Time Post Detonation 54 78 62 26 hours hours hours days Rongelap Island Mean Maximum Mean 1.5 _ _ 0.047 1.9 _ _ —_ _ 0.41 _— — 23 Sifo Island Utirik Island Maximum Mean Maximum =_ 0.48 — — ~_ _ 0.10 _ _0.13 _