r § EFFECTS OF IONIZING RADIATION dose rate had fallen to less than 40 percent of its jnitial value. Thus the dose rate also dif- the dose at the center of the body is approximately 50 percent higher than would result from a given air dose with narrow beam geom- laboratory. curve from an experimental situation using 90 percent of the dose had been received, the fered from the usual constant rate in etry. the Figure 1.4 illustrates the depth dose = EVACUATION r 169 aT 5) HR ~ 100 = < z Llu 1 — nw o Qa Le ai < L oO r bw sor — 3 12 HR FALL-OUT 4-|I6 HR — i . ot Oo er Poo 10 20 30 TIME AFTER H-HOUR 40 59 (HR! Ficurke 1.3—The accuntalation of gamma dose os a function of time after commencement Gf fallout on Rongclap atoll. 1.23. Geometry of the Exposure In addition to the dose rate and energy dif- ferences the geometry of the exposure to fallout radiation is significantly different from: the usual laboratory sources. Since fallout radia- tion is delivered from a planar source the usual narrow beam geometry is not applicable. In such a diffuse 360° field, the decrease of dose, with depth in tissue is less pronounced than that resulting from oa bilateral exposure to an N-ray beam becuuse falloff from inverse square is in effect neutrulized. For the same energy, 5002 1bb spherically oriented Co® sources with a phantom placed at their center, compared with 4 conventional bilateral depth dose curve ob- tained 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 held cuse, atl 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