Chapter 3 | RESULTS aad DISCUSSION 3.1 RESIDUAL-RADIATION MEASUREMENTS The data obtained from the residual-radiation stations ure shown in Figures 3.1 through 3.18 in the form of log-iog plots for convenience of presentation and for ease of determination of the decay exponent. The decay exponent was equal to the slope of a straight line drawn through the data points that were considered to be related to each other only by radioactive decay. All residuai data was analyzed in detail for this report. The instruments for those stations vepre:ented by Figures 3.3, 3.11, and 3.12 were operating at levels below their high-resolution region and did not yield the essentially continuous curves shown in the remaindcrof the group of Figures 3.1 through 3.18. On Figures 3.1 through 5.18 the slopes are shown us dashed lines which were drawn through the linear portion of the curves. In drawing these dashed lines, early times were avoide:| when the concentration of gamma-ray sources was still building up because of continuing deposition of fallout material, and other data points were ignored in cases where rain or wind had redistributed the fullout n:aterial and caused pertur- bations in the decay curve. Measured residual--gzummua-radiation doses for each of the four shots are plotted on maps of Bikini Atoll in Figures 3.19 through 3.22. -Free-field exposures shown on these figures were extrapolated to infinite time using Equation 1.], Section 1.3.2, of this report, Tables 3.1 through 3.4 summarize the data on residual-stution locations, time of arrival of fallout, maximum-observed-exposure vate, total exposure, and decay expo- nent. The average decay cxponent was found to be 1.1 for Shots Zuni and Tewa, neglecting the resu'ts from Station 221.04C, which received tuo littleexposurelor accurate evaluation). In the many cases where there was early rain leaching, the slope indicated by the data paints taken after rain had ceased was used to help determine the best-fit straight line. In these curves, the gamma-expusure rate after rainfall was approximatelyhalf of that expected if the normal radioactive decay were the only cause of change of exposure rate. In Figures 3.3 and 3.18, the buildup of the exposure rate was apparently more complex than the monotonic buildup presented by most of the other figures. It appears that fallout ceased to arrive for a short period at 60 minutes (in Figure 3.18) and then began to arrive again. Slope changes are evident in the curves in Figures 3.9 :ind 3.10 after about +500 minutes. This effect was probably not due to instrumentation errors because these curves represented the data frorn two independent instruments located at the same station. A possible explanation of these slope changes was the presence of one (or more than one) radioactive isotope whuse half life was suchthat the decay was slower than the combined fission fraginent decay of t~!-?, und the decay slope was dominated by this isotope from about +500 minutes until the end of the record. 28 However, the instrumen-