DNA 1240H-2 17.5 TRANSIT RADIATION 17.5.1. introduction Transit radiation has been defined (Section 17.3) as the gamma radiation* from airborne particles suspended in the cloud and base surge formed by water burets. Assessment of effects of such radiation is based on the dose or the time-integrated dose rate received at the exposure point. Thus, all available weapons-test dose and dose-rate date are of value in devising scaling techniques that would permit estimation either of dose or of dose-rate histories due to transit radiation at various ranges from surface zero for detonations of any yield. Transit-radiation data measured at weapons teats at unshielded (topside) shipboard locations are discussed in 17.52, and similar deta obtained at below-decks locations are diecussed in 17.53. In some cases, specific measurements of transit radiation were made; in other cases, where only one total-dose or dose-rate history was recorded, attempts were made to separate the transit from the deposit radiation. When the washdown system was in operation, deposit radiation was reduced; thus, the relative contrib- ution of transit radiation to the total exposure was greater on a washed ship than on an unprotected ship, although the absolute amount of transit radiation did not change. Weapons-test data available from the few water shots at which measurements have been made are insufficient to permit reliable ex- trapolations or scaling techniques. Therefore, attempte have been made to develop semi-theoretical models for predicting transit-radiation doses, employing available data to correct and verify the models. Two such models for predicting transit radiation at unshielded locations sbdoard ship are discussed in 17.5. 4. Transit dose rates and doses at interior locations in a ship will always be less than those recorded at the same time on the ship's weather deck, because of the attenuation afforded by the intervening structure. Such attenuation is generally expressed in terms of shielding factors, where the shielding factor for a given location is usually de- fined as the ratio of the dose rate at the given location to the dose rate at 3 ft above the weather deck. As noted in Ref. 41, the shielding factors depend not only on the arrangement and thickness of ship structure and materials, but also on the distribution of radioactive particles in space as well as on the radiation energy spectrum. The spectrum varies slightly with bomb type, but may vary considerably through fractionation of the different isotopes involved. It also varies with time after burst. A theoretical method for calculating ship-shielding factors and thus dose rates at interior shipboard locations is presented in 17.5.4. The effect *Beta radiation from transit sources contributes only a negligible amount to the total dose received at unshielded locations, and none at all at shielded locations. 17-34 eee ne en re a en a ee eee eee