8 EFFECTS OF IONIZING RADIATION 90 percent of the dose had been received, the dose rate had fallen to less than 40 percent of its initial value. Thus the dose rate also differed from the usual constant rate in the laboratory. the dose at the center of the body is approximately 50 percent higher than would result from a given air dose with narrow beam geometry. Figure 1.4 illustrates the depth dose curve from an experimental situation using EVACUATION AT 51 HR y 150 Lf a 100 &a = zSU w o bea Q 2 << e Oo - 50 12 7 ol oO HR FALL-OUT 4-I6 HR IO Lt yy 20 py ft pe 30 tt py 40 tt 50 TIME AFTER H-HOUR (HR) Figure 1.3—The accumulation of gamma dose as a function of time after commencement of faliout on Rongelap atoll. 1.23 Geometry of the Exposure CF In addition to the dose rate and energy differences the geometry of the exposureto fallout radiationis significantly different from the usual laboratory sources. Since fallout radiation is delivered from a planar source the usual narrow beam geometry 18 not applicable. In such a diffuse 360° field, the decrease of dose with depth in tissue is less pronounced than that resulting from a bilateral exposure to an X-ray beam because falloff from inverse square is In effect neutralized. For the same energy, spherically oriented Co® sources with a phantom placed at their center, compared with a conventional bilateral depth dose curve obtained with a single source (4). In the latter case, the air dose is usually measured at the point subsequently occupied by the center of the proximal surface of the patient or animal with respect to the source. For the field case, all surfaces are “proximal,” in the sense that the air dose measured anywhere in the space subsequently occupied by the individual is the same. It is this air dose which is measured by a field instrument; it does not bear the same