CACTUS EVENT OPERETION HARDTACK 1. CACTUS was detonated at O615M, 6 May 1958, at the surface on the north end of Yvonne Island, Eniwetok Atoll. The cloud rose to 19,000 feet in the first ten minutes, then stabilized at 15,000 feet by H+20 minutes. Radar tracking was employed while the cloud was in the lagoon and indicated a general drift to the west-southwest. The main body split into two parts at H+23 minutes and continued movement towards the west—southvest et 18 knots. By H+l hour the cloud stretched completely across the lagoon from Leroy Igiand (at 5,000 feet) to ground zero (at 10,000 feet). At this time a very thin dust cloud was noticed northeast of Yvonne at a high altitude but radar or aircraft reports were not obtainable. The cloud was well . scattered by H+2 hours and almost completely dispersed by H+3 hours. 2. The initial helicopter survey est +3 hours resulted in zero readings for ail of the main atoll islands except Yvonne, Leroy, and Meck. A reading of 440 r was obtained over debris in the water immediately west of ground sero. Other readings at Yyonne wore 1700 m at mid-island and 5 mr at the southern tip. leroy and Mack, which were directly in the cloud'’s path, indicated 850 mr ani 100 mr, respectively. 3. Ons P2V (Wildroot 16) started a survey over Yvonne at H+3 hours. An island survey and vectored mission were fiom over Eniyetok and Ujelang with the highest reading of 27 mr over Yvonne at 1,000 feet. iS