assumedto be so in the calculation. The total dose rate d ata height 3 feet above a uniformly contaminated infinite plane is given by: ; i=m d= YD nidi i=l Where: dj = dose rate in Mev min~!cm™at height, photonsof initial energy, (4-1) x above an infinite plane emitting Ej isotropically at the rate of 1 photon min™'cm~?, hy = number of photons min~!cm~?of initial energy, Ei: The dose rate, dj is defined by: oo _ Ej h(Ei) aE e~§ ds Bi (ti) ti (4.2) 5 Where: E; = initial photon energy. h(E;) = “true” linear absorption coefficient for air or fractional energy loss per unit path length. ty = Mix x = 3 feet Hy = total linear absorption coefficient for photons of energy Ej. Bi(ty) = i. buildup factor or ratio of dose from a]l photons to that from un1 - yj scattered photons. yi = fraction of dose from source energy E;, delivered by scattered photons; y; is obtained from Curve A, Figure 20, Reference 1. The value of the exponential integral may be found in prepared mathematical tables (let s =tj). Values of yy and h(E;) are compiled in Reference 1. E; was taken as the mean energy of the ith finite energy interval in the experimentally determined spectrum (Reference 2). The actual calculations were carried out as described below. Let R gamma energy emission rate per unit area of the plane source in units of Mev min~'!cm~?. A = gammaactivity per unit area of the plane source in units of counts min7‘!cm~?. I = gammaactivity per unit area of the plane source measured in a gamma ionization detector whose response at various energies is known in arbitrary units of mv em~?. d, = dose rate at 3 feet from a reference source for which R = 1 Mev min™'em=*. 54