where I, is the radiation intensity at the point in question when the approaching radioactive cloud reaches this point. If the cloud continues past the point a distance y, the intensity becomes: J, (t) lip = Gu, OK = A and | Iupy — « = Ip + J, (t) A In [- Ei (- uy] + (+0) (1. ga Ia(t) = Pha . A ) MAY (1+K) where Iy is the radiation intensity at the center of an infinite hemisphere of radioactive cloud. To simulate actual conditions, however, clouds of finite thickness must be considered. The difference between values obtained for two infinite rectangular clouds at different distances from the detection point approximates the desired intensity for clouds of finite thickness. Base surge is thus approximated by a vertical wall of radioactive material infinite in length and height but of finite thickness. Values of K corresponding to energies of 1.0 and 1.25 Mev were selected, and the intensity as a function of distance to the leading edge was calculated for various thicknesses s and expressed as a fraction of Iy . These results are presented in Figures 1.8 and 1.9. By assuming a surface wind speed, relative intensities as a function of time may be obtained from these plots. However, most photographs of base surge from underwater detonations reveal that, although the vertical wall approximation may be reasonable for the upwind case, the surge front at downwind and crosswind positions usually approaches at an obtuse angle. According to Reference 46, this angle is approximately 120°, a value which is usually substantiated by photographic measurements. The general expression for a wall approaching at a 120° angle could not be integrated. However, approximate solutions for a number of thicknesses were obtained by geometric means fully described in Section A.3. The computed intensities relative to I, are presented in Figure 1.10 as a function of distance to the leading edge. Both the vertical and the 120° approach curves proved useful in the determination of base surge velocities and in the definition of time of arrival (Section 3.3.4). Analysis of the gamma dose rate histories at late times (5 minutes or greater) revealed peak activities that can best be explained by assuming the presence of radioactive water or foam in the vicinity of the coracles. The shape of these later peaks could not be reproduced by areas of upwelling, which were large in comparison to the mean free path of 1-Mev gammas, a configuration which has been calculated in Reference 47. Consequently, a special case of the model currently being investigated (Reference 48) was extended to dimensions that would approximate the passage of a relatively small patch of radioactive water or foam. The approximation used yielded the intensities due to passage of a thin disk of uniformly distributed activity beneath a point whose distance above the plane of the disk was equivalent to that of the GITR detector above the ocean surface. The computed intensities normalized to the intensity at the center of the circular radioactive area are plotted against the distance from this center for a number of radii (Figure 1.11). These curves were employed to determine whether the dose rate peaks observed could indeed have been caused by such bodies of radioactive water or foam. 1.3.3 Supplementary Measurements. The basic instrumentation of the project consisted therefore of GITR’s and 1C’s mounted in pairs on coracles arrayed about surface zero. This array was supplemented at specific locations by other instruments designed for more specialized measurements. Several underwater gamma detectors were used to detect activity due to upwelling contaminated water. Their locations were selected on the basis of the predicted movement of radioactive water (Section A.5). The data obtained was intended primarily for the correction of gross gamma records in cases where both the radiating cloud and heavily con- taminated water arrived simultaneously. However, these water corrections were never applied, since on both Wahoo and Umbrella the base surge rapidly outdistanced the contaminated water 39