bis . CHAPTER 4 PRIMARY FALL-OUT 38 zero, This fall-out has considerable military significance, The areas of primary fall-out, particularly from superweapons,are quite extensive, and many hours can elapse before the fall-out gammafield is completéty 4.1 defined. GAMMA FIELD 4 otf ar Vey poy Pay 4 taleci ie = Primaryfall-out following anuclear.detonation may be defined as the particulate which arrives at relatively early timesand forms a well-delineated pattern downwind from ground The gammafield following Mike shot was well documented within the lagoon. An analysis of the wind profile at shot time indicated that the downwindfall-out lay over the open sea ina swath west-northwest to north of the island where the shot occurred. The data collected at Eniwetok on the Atoll islands and within the lagoon represent primarily the cross-wind pattern and a portion of the upwind region. Observed Gamma Field, Comprehensive data on the gammafield were obtained within the bounds of Eniwetok Atoll and represent the cross-wind and upwind field. Figure 4.1, showing the gammafield, was compiled from island gamma-survey measurements and lagoon-station gamma-} ackground readings corrected to values représentative of the field that would be experienceu on an extensive land mass. The a values indicated for the lagoon stations are the observed readings multiplied by 7. Thi ltiplying factor results from the relation obtained at Operation Jangle between field gamma readings and gamma measurementsof the fall-out from this field as read in a region having a gamma-free background. *! Cessation of cross-wind fall-out was at approximately M+2 hr. The field reaches its maximum intensity? at about this time. Figure 4.1 represents the field at M+2 hr. Extrapolation of gammaintensity to M+2 hr was based on the (t7!-?) decay law. No activity was detected from gamma-survey measurements taken over the open water in the lagoon, An examination, primarily of the density of fall-out particulate, indicates that the particulate fell rapidly into the lagoon, where it settled on the bottom and left a zero field at the surface, There was some evidence that the lagoon currents carried a small amountof activity southward from the crater; this was measured by actual water sampling, but the activity had such low intensity that it did not generate a gammafield at the surface. *It is to be understood that the extension of Jangle relations to the soil and water condi- tions existing at Eniwetok is open to question. The data presented for the lagoon stations tn Fig. 4.1 are simply the best approximations. As indicated on the Project 5.3 fall-out gamma time-intensity records. 33