~ ome eee ee or distance from surface zero. The regularity of the curve shapes is definite evidence of superstructure effect. It was found that "the total solid angle of unshielded base surge subtended by an absorbing volume bears a direct relationship to the total dose received." The average of film-pack doses for the platform film packs, and even for completely unshielded positions on the superstructure decks is high, because of the large solid angle subtended at the films due to their elevated positions. Where even a relatively thin section of the superstructure subtended more than 10% of the total solid angle (at the film), an epproximate shielding factor was estimated, using the ship's plans and a gamma energy of 1 Mev. The calculation of shipboard dosea from free field isodose contours requires the use of "conversion factors" that compensate for superstructure shielding. Such factors were calculated from film pack and GITR data for exposures aboard the target ships at Shots Wahoo and Umbrella, and are given in Table 3.33 of Ref. 32. The individual factors vary from a low of about 0.15 (for an exposure dose at frame 100 along the centerline of the super- structure deck of the DD474 for Shot Wahoo) to a high of 1 for an exposure dose between frames 120 and 130 on the superstructure deck of the DD592 at Shot Umbrella. The average variation of the factors (on the same ships) from the mean for both shots lies between 4% and 144%. It is suggested in Ref. 32 that use of the conversion factors may be extended to inner compartments, but that it is impossible to estimate the true accuracy of the procedure; therefore, the conversion factore should be used with caution, particularly in the case of moving ships. Conclusions state that a reduction equal to a factor of 2 or greater in weatherdeck dose, due to superstructure shielding, was observed at certain locations. The different geometries of the base-surge radiation fields for the two shots were responsible for the differences in gamma dose-rate records. Interpretation of the photographic data‘? indicates that at Shot Wahoo, there were probably both a primary and a secondary base surge. The passage of the two surges caused numerous significant peaks in the downwind dose-rate histories.°* The Shot Umbrella base surge appears to have formed a single ring relatively clear of airborne radiation material at its center, 4 and in most cases the Shot Umbrella records contain a single high peak in dose rate followed at a jater time by a prolonged and relatively low increase in dose rate.°° The differences between the Wahoo and Umbrella records indicate that depth of burst has a pronounced influence on the radiation fields produced, but it is impossible at this time to extrapolate from these two documented cases to predictions of effects of bursts at other depths, particularly since more pronounced differences probably occur as the depth of burst approaches zero. However, this effect has been taken into account in an approximate way in the base~surge model discussed in Section 17.5.4, The Model of Reference 48. 17-67 NO rr nee see woes - me et ee ee ee me em re me mete ee