CHAPTER 17 a= KL, EBe x We can define and r/hr 5 - 2 hr do « xamE, = : (do)y r-cm*/ (do); = £4 (do) (17-14) (17-15) where dy = a symbolic dose-rate measure of source strength. f4 = the fraction of 4, due to the source energy Ey: Goldetein and Wilkins © present a method of calculating the deep penetration of photons in infinite homogeneous media for point-isotropic or infinite uniform-plane mono-directional sources. This "Moments" method employs a different dose buildup factor for each energy and mediun. Because of their complexity, the calculations were performed on a computer and the results are presented in both tabular and graphical form in Ref. 52. Differential energy spectra for point-isotropie and plane monodirectional sources for various energies from 0.5 to 10 Mev and for pene- trations up to 20 mean free paths in several media, as well as buildup factors, are included. To determine the exposure-dose rate and the shielding effectiveness of a ship at e below-decks location when the ship is enveloped by a base surge, the unshielded dose rate due to a monoenergetic point source (Eq.17-13) mist be extended to represent the corresponding dose rate due to & volume source, and then mst be modified by a factor that accounte for the attenuation of the dose rate by the shielding afforded by the ship's structure. Finally, it mst be summed over all emitted energies. The theoretical method that has been developed at NRDL for this purpose is based on an idealized concept that considers the ex- posure point shielded by a slab from a semi-infinite volume of uniformlydistributed radioactive point sources. The basic slab geometry considered in the mathematical derivation is that of a circular truncated cone, and mumerical techniques are used to convert results for circular slabs of radius R to rectangular slabs that give the same dose-rate reduction. The conversion technique is explained in Ref. 53. The basic dose-rate equation for the monoenergetic point source can be extended to express the volume-source case; that is, to express the exposure-dose rate at a perpendicular distance h below a slab of finite thickness and infinite extent, while an infinite radioactive volume source above the slab is emitting n (photons/cm3-sec) quanta of energy 17-57 ne ee ee ee eern enee <a Urea ane re nee ae