3036 Decay Exnonent Variations The variation in decay exponent from sample to sample results from a real or apparent variation in the zero time activity of various nuclides, This may result from changes in fission yield because of different fission processes, from differential deposition of various nuclides (fractionation), geographic limitations of the station layout , and limitations of the collecting instruments themselves. However, no ons of these factors has been determined to be the primary cause of these decay variations. 304 BETA ACTIVITY 3401 Interval and Cumulative Activities from Intermittent Fallout Collector Samples The activities of the IFC samples were corrected to the mdpoint of each sampling time by the methods described in Section 3.3 and calculated in terms of activity in disintezrations per min per 0.6 in.2,. The averaged agtivty valuss are based upon samples having a total area of 264 ine2. Figures 3.5 through 3.17 present these data. It is to be noted that "in many of these graphs the early intervals of the leand-5=—min interval collectors show higher initial fallout activities than the first intervals of the 30min interval collectors on the same island. The correction for decay is reflected in these results. Obviously, the midpoint of the sampling intervals for the first few l<and=5~min intervals is much closer to the actual time of detonation than the midpoint of the first 30-min interval. However, it is believed that the method used is a reasonable method of showing the relative activity at about the actual time of sampling. Activity results from Shots 1 and 3 were more complete than from the other shots. Data from selected intervals from these two shots can be expressed in approximate units of disintegrations per minute per square foot, using the relations 2 d d activity aT = 1.67 Seu x 144 in. ES (3-19) These results (in Tables 3.5 ind 3.6) indicate the concentration of beta activity which could be expected over land areas, assuming that the material falling into the collector trays fell uniformly over the land mss being considered.* The results indicate that when significant fallout occurred at an island on the shot atoll after any of these shots, it apparently began to arrive there within six minutes after the detonation. The maximum activity per sampling time interval resulting from Shot 1 and % This assumption has not been investigated extensively. Several groups of two IFC's ten feet apart and with identical timing intervals were set up at IVY. 3/ There was a variation in the re- sults of the two instruments; it was much less pronounced where the station was subject to heavy fallout than where fallout was sparsee At CASTLE, no instruments were available to check this assumption.e 49