exact median size being a function of depth. However, due to the lower concentrations of vaporized material, the final particle size in either case was expected to be significantly smaller
than those typical of the underground case. The gamma field resulting from the dispersion of
such particles by meteorological processes was expected to differ significantly {n the area of
interest, because the mass subsidence of large amounts of water was expected to confine the
total event to a much smaller area than that normally expected from an underground burst.
The initial dose for an underwater shot could also be significantly altered both by shielding
effects of the water itself and by the absence of nitrogen, thus preventing the high-energy gam-
ma emission (average 6.5 Mev, Reference 20) due to the (n,y) reaction on N“*. Gamma fields
associated with the radiating cloud could be further altered by differences in gamma spectrum
due to the presence or absence of Specific induced radionuclides.
The similarity between underground and underwater bursts, therefore, appeared tenuous at
best; and while it was recognized that HE models might effectively simulate the dynamics of
clouds resulting from underwater nuclear detonations, insufficient data then existed for the
calculation of the associated gamma fields. Although the peak dose rates during transit for
Crossroads and Wigwam compare favorably (see Appendix E), the authors concluded that precise documentation of the total gamma environment resulting from the underwater detonation
of a nuclear device was definitely required.
1.3.
THEORY
The project proposed measurement of the gamma fields at 21 locations, selected after consideration of the best available information (References 21 through 25), to obtain data from
three substantially different areas of the total event, viz, base surge without fallout, combination of base surge and fallout, and fallout only. For convenience, these locations were given
nominal position designators stated in terms of the probable wind direction (References 26 and
27) as indicated in Table 1.1 and Figure 1.1. These nominal positions, which indicate theoriginally intended location of a station, are used throughout this chapter. The original nominal
positions are changed at the beginning of Chapter 2 (Table 2.1) to reflect changes in intended
position necessitated by operational conditions in the field. This second set of nominal position
designators is used throughout the remainder of the report.
At each location, a number of detecting and collecting instruments were placed on specially
designed floating platforms, termed “coracles” to distinguish them from skiffs previously used
as deep-anchored stations (Figures 1.2 and 1.3). The coracles were circular, to facilitate interpretation of instrument responses, and were held to the smallest practical diameter to minimize corrections to the free-field dose rate due to deposited activity (actual dimensions are
given in Figure A.1}. They were also designed (1) to minimize wash over the deck, (2) to with-
stand overpressures of 2,000 psi, and (3) to reduce a shock of 200 g delivered to the coracle
hull to 5 g delivered to instruments mounted in an internal instrument well. A fully instrumented
coracle weighed approximately 1,700 pounds and drew 14 inches of water.
The use of shielded detectors to eliminate contributions from deposited activity was considered as a means of obtaining free-fleld measurements, but the interpretation of the record
from single shielded detectors appeared difficult. Previous measurements of the directional
characteristics of nonhomogeneous gammafields (References 28 and 29) had indicated that the
greatest directional contribution could be expected in that direction which transected the greatest thickness of the radioactive cloud. The principal component of the complex gammafield at
most times and at most stations was therefore expected to be nearly horizontal. Although directional shielding has been attempted (References 30 and 31), the interpretation of records
from shielded detectors mounted on the rolling platform afforded by a coracle would involve
considerably more instrumentation than that allowed by funds then available to the project.
Therefore, unshielded gamma detectors were employed.
When using unshielded detectors (Figure 1.4), the project had to consider the possibility that
deposits of radioactive material on the coracle decks and the detector casing itself might be
25