only 6.4 percent and 11 percent of the total exposures were delivered before shock arrival at these two stations.
3.2
NEUTRON RADIATION
The basic neutron-flux measurements were made with activation detectors whose
indicated effective threshold energies were:
Detectcr:
Au, Au-Cd
Threshold: <1 ev
Ta, Ta-Cd
5
«iL ev
>3 Mev
Additional measurements were made with fission detectors and germanium crystals,
primarily to test their usefulness. The fission detectors were used in two ways: count-~
ing fission fragments in a photographic emulsion and counting gammaactivity fromfis~
sion products after recovery of the samples. The fission detectors used and their
effective t!:reshold energics were:
-Detector:
u*?
Threshold: 1.5 Mev
Np™?
0.64 Mev
Th
1.5 Mev
Pu?
200-1,000 ev!
The Shot 1 data from the activation and fission detectors are summarized in Figure
3.3; the fission detector data from Shot 2 are illustrated in Figure 3.4. The germanium
crystal (Ge) dose data agree in order of magnitude with the threshold detector data.
There was a large scatter in the Ge data, indicating that the detectors were not reliable
in the form used.
3.3
FALLOUT DISTRIBUTION
3.3.1 Instrumentation.
The following procedures were used to furnish information
on the distribution of fallout activity after each of the Castle shots (some of the colleciors
also provided samples for chemical, physical, and radiochemical studies of the fallout
material):
1. Survey readings were taken by project personnel and the Rad-Safe organization
at island stations at various times after the shots.
2. Readings of total residual-gamma exposure at island stations were taken from
film badge and chemical dosimeters.
3. The activity of samples from total fallout collectors was related to the infinitefield exposure rate by normalization at island stations. Total collectors of the funneland-bottle or gummed-paper type were placed at island stations, on rafts anchored in
the lagoons, and on free-floating buoys placed north of Bikini Atoll during the last few
days before shot time.
4. Gamma-exposure-rate recorders were placed at some island stations to provide
data on the time of arrival, rate of arrival, peak activity, and decay of fallout.
5. Incremental fallout collectors were used to collect samples during 5- to 30-minute
intervals and to provide data on time and rate of arrival of fallout.
6.
After Shots 5 and 6, surface and aerial surveys of the ocean fallout area were per-
formed to measure the activity in the surface layer of the ocean and its depth of penetration.
The existence of a mixed layer in the ocean down to the thermocline, with little
mixing below, enabled these measurements to be related to the total activity deposited.
1 Depending on amount of B” shielding around sample.
44