C CHAPTER 17 A brief summary of the theory follows: When the point-source case (£q.17-13) 18 extended to express the ex- posure dose rate at a distance h below a slab of infinite extent, with radioactive monoenergetic sources distributed uniformly over the top surface, the dose rate dy)... due to the plane source emitting n (photons/ -sec) quanta of energy E, (Mev/photon) can be expressed as the integral: Be? (ux) ' Gext 4Neo = ku,nky aa (r/hr) (17-31) ° The source strength per unit area, may be expressed by nE, Mev/cm®-sec and x = distance (cm) from the exposure point to an incremental element of area, dA (ux)' = 41x) + pox, where x, 1s the path length in air and x2 is the peth length in the slab, and each p, is the total linear absorption coefficient for the corresponding medium. a The symbolic dose-rate measure of source strength, 4, may be expressed: dy = Kua, n Ey However, (r/hr) (17-32) since a ship is not infinite in extent, it is necessary to determine the effectiveness of a finite slab in shielding the exposure point from the radioactive material. Furthermore, for the idealized concept of the problem, it is assumed that the shielding layers (corresponding to the plating of the ship's decks) are con- tiguous. It was found more feasible to calculate the shielding provided by the rectangular slabs of ship structure by considering the ehielding provided by circular slabs that give the same dose-rate reductions. Graphs that equate circular shields to rectangular shields in terms of radius R and semi-length and semi-width a and b are given in Reference 71. Then, the dose rate at an exposure point shielded by a finite elab of radius R from the plane distributed source may be expressed by: «Re dnp nR « do Be” (Hx) ' Be. Iexxe aa (r/nr) (17-33) B5EST AYAILADLE COPY ©: 17-83 ee egye erm cue aoe me ene ne mr mmm