UMBRELLA EVENT OPERATION HARDTACK 1. The UMBRELLA device was detonated approximately three miles east of Keith Island, Eniwetok Atoll, 150 feet below the surface, at 1115M, 9 June 1958. The water cloud rose to 2,000 feet. As it settled, a base surge cloud of several thousand yards in diameter formed. Within a few minutes, all visible moisture settled to the surface. 2. A Pav afroraft (Wildroot 7) reported over Glenn at H+20 minutes. It was vectored on radials of 360 degrees, 340 degrees, 300 degrees, and 280 degrses for four miles, at 1,500 feet. The highest reading was made over ) ground zero at 1137M: 350 mr/hr. All other readings were insignificant, add R-Hour wes declared at 1145M, with the exception of the area west of the lagoon. A final radial from Glemn of 270 degrees for ten miles was flown, and 0.9 mr/hr was the maximm reading at 1150M, 3. Two RadSafe boats entered the target crea from the direction of Glenn at 1200M. High readings of extremely short duration were encountered as the RadSafe boats surveyed the six major vessels in the target array. The highest recorded readings were made off the stern of the EC-2 at 1225M: 1.5 r/hr, This operation was controlled from the USS Monticello, and by 1315M, the area was declared sufficiently clear to allow the project recovary operations to commence. 4. wench It is estimated that all fallout fell within the first hour on a Bearing of 260 degrees. This was well within the forecast fallout plot. ~ ” TAB A BEST cary AV AILARIER (7/