2 availablé. The low power lens system, 125 x magnification, was used. The estimation of the size was made by comparison with a red blood cell which is known to be 7 to 8 microns in size. Thus compared, the particles separated appeared to be 50 to 150 microns average diameter. Some of the very largest particles found were from the island of Rigili. One of these meas- ured between 1 and 2 mm. It was crushed be- tween two microscope slides and divided into roughly three portions. Each portion carried part of the activity which indicated more or less thorough mixing when the particle was made. The appearance of the average particle was that of a mixed amorphous substance. On the outer layer of most particles, there appeared to be black specks estimated at 5 microns or less. Some of the larger particles appeared to have a small tip on one edge with the tip containing the black material. Attempts were made to identify one or more parts of the amorphous system. Dilute hydrochloric acid was dropped onto the slide and carefully added to the crystal. Bubbles could be seen through the microscope, and after the evolution of all the gas there remained quartzlike threads and black particles, neither of which could be visually characterized. The bubbles were assumedto be carbon dioxide from the reaction of the acid on coral sand, Upon one occasion, to assist in identification, a Kleenex was used to remove the excess acid, and a drop of water was added to washoff the slide. No activity was carried on the Kleenex. Careful division of the remaining residue was made, and activity was carried with each division. It was thought that the black part could be iron, but it did not appear to dissolve, even in 6N hydrochloric acid after several minutes. Ammonium thiocyanate solution was added, and no identification of iron could be made. No further chemistry was attempted. When a micrometer scale becameavailabie, photomicrographs were made of about fifteen particles. The slides were then preserved with a piece of scotch tape. Samples that were collected and examined included those from islands of the Atoll from Parry around to Rigili. Microscopic studies of mechanically separated particles, which included samplesfromben eet seven southern isiands ci the Atoll, indicated that the fall-out of the first 6 hr after Dog shot was 20 to 250 microns. No particles smaller than that indicated were found, but a few larger ones were separated. 2.6.6 Intensity of Fall-out on Atoll, Easy Shot The systematic periodic intensity surveys of the islands following Easy shot show a number of characteristic features that can be compared with fall-out predictions made from wind soundings. Of the string of islands extending generally westward from Engebi, the heaviest fall-out (30 to 40 r/hr) occurred on the extreme western two, Bogallua and Bogombogo. These islands lie somewhatto the south of west, in the direction of the winds below 10,000 ft. Of the intermediate islands, those large enough to catch much fall-out lie somewhat to the north of west. These showed muchless intensity (around 1 r/hr). Wind soundings showed that fall-out in this sector would be scattered on account of very variable winds in the range 20,000 to 25,000 ft. Of the islands extending generally to the southeast of Engebi, the highest intensities (around 1 r/hr) were found on the closest, Muzin, Kirinian, and Bokon. Progressively lower intensities were observed on farther islands, with significant values (40 mr/hr) as far as Runit, at about 10 miles. According to the predictions, this fall-out should have come from the 40,000-ft region and should not have been completed at the time the first surveys were made. On Biijiri it was apparent that most of the fall-out occurred after about 3 hr. For a long time the high activity on Muzin, Kirinian, and Bokon was thought to have gotten there within 1 hr of shot time, whenthefirst survey was made. This early time coupled with the lower activity (400 mr/hr) at the southeast corner of Engebi, which is much closer to ground zero, was difficult to understand. An attempt was madeto account for it on the basis of blast wind phenomena. Subsequently, an analysis of the decay of island activities by Maj P. S. Harris, Assistant Radiological Safety Operations Officer (see Figs. 2.3 and 2.4) indicated that the fall-out on Muzin, Kirinian, and Bokon was not complete at the time supposed. It appears |that therelatively low intensity at the a eee . ee een an mm serene waa wna roo anes - om Bene wae a te OS en ve ns “( R GeE e T HORM R AD ON aan yy we ee