on the only two target ships that were instrumented (the instrument on DD 593 had been cannibalized at the last minute to replace a burned out solenoid on one of the closer ships). Because
thé dropping mechanism for GITR Station 15 proved unreliable, the underwater radiation de-
tectors were submerged in the water prior to Shot Umbrella in the hope that some data would
be obtained; however, the instruments on DD 474 and DD 592 were damaged by shock. Conse-
quently, the only data obtained was from DD 593 after Shot Umbrella.
The tabulated radiation data obtained from the underwater GITR on DD 593 for Shot Umbrella
is presented in Appendix D. During the period when the ship was enveloped by the base surge,
the peak dose rates
. are attributed to contaminants depositing in the water and possibly to contaminants washed off the ship.
lowuntil
Following this period, the underwater dose rates were very
6.4 hours after shot,
This late resurgence of underwater radiation is attributed to
a patch of contaminated water (detonation debris originally upwelling at surface zero) drifting
down upon DD 593.
Figure 3.41 presents ratios of dose rate in the water to average dose rate on the washed
weather decks of DD 593 after Shot Umbrella. Three curves were constructed because of a
possible uncertainty of 30 seconds in the timing. The results for al) three possibilities show
that the underwater dose rates were less than 0.2 percent of the washed-weather-deck dose
rates during the periods when the ship was enveloped by the base surge and were no more than
20 percent of the washed-weather-deck dose rates during the later periods when the deck dose
rates were very low. Therefore, although the contaminated water did not contribute significantly to the gamma doSe observed on DD 593 after Shot Umbrella, the radiation from the water
mayhave influenced the dose-rate ratios to a significant degree at later times.
3.4
GAMMA-IONIZATION DECAY
No data on gamma-ionization decay was obtained for Shot Wahoo, because the starting signal
was not received.
The gamma dose-rate data from the decay unit (GITR Station 22) aboard DD
592 after Shot Umbrella is presented in Appendix D.
Logarithms of the relative gamma dose rates are plotted as a function of logarithms of the
time-~after-shot in Figure 3.42. The decay curve was also separated into segments fitted to an
equation of the form
Dose rate = constant x (time)
n
The exponents n were evaluated for various time intervals and are represented by the slopes
of the log-log curve shown in the figure. Standard regression techniques were applied to the
logarithmic variables to obtain the slopes and their 95-percent confidence limits.
The background of external radiation affecting the dose rates inside the 6-inch-thick lead
cave was estimated to be negligible for the time periods under consideration.
The estimate was
based upon use of: (1) gamma energy variations listed in Reference 10; (2) gamma-radiation absorption coefficients and buildup factors from Reference 11; and (3) monodirectional attenuation
equations applied to the average deck-dose rates.
30