Chapter 3
RESULTS and DISCUSSION
3.1
RESIDUAL-RADIATION MEASUREMENTS
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The data obtained from the residual-radiation stations are shown in Figures 3.1
through 8.18 in the form of log-log 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 represented 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 remainder of the group of Figures 3.1 through
3.18. On Figures 3.1 through J.18 the slopes are shown as dashed lines which were
drawn through the linear portion of the curves. In drawing these dashed lines, early
times were avoided when the concentration of gamma-ray sources was still building up
because of continuing depositionof failout material, and other data points were ignored
in cases where rain or wind hadredistributed the fullout material and caused perturbations in the decay curve.
Measured residual-gamma-radiation doses for each of the four shots are plotted cn
maps of Bikini Atoll in Figures 3.19 through 5.22. Free-field exposures shownon these
figures were extrapolated to infinite time using Equation 1.1, 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 rate, totai exposure, and decay expo-
nent.
The average decay exponent was found to be 1.1 for Shots Zuni and Tewa,
ineglecting the resu'ts from Station 221.04C,
which received two littleexposurefor uccurate evaluation).
In the many cases where
there was early rain leaching, the slope indicated by the data points taken after rain
had ceased was used to heip determine the best-fit straight line.
In these curves, the gamma-exposure 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 and 3.10 after about + 500
minutes. This effect was probably not due to instrumentation errors because these
curves represented the data from 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 whose half life was suchthat the decay was slower than the
combined fission fragment decay of t!-?| and the decay slope was dominated by this
isotope from about +500 minutes until the end of the record. However, the instrumen28