70 THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD 120 O&b)I/2 OF TOTAL DOSE RECEIVED C) ) rot!TOTAL D Cc ROOOSE d) MULTILATERAL OR ROTATIONAL EXPOSURE| 60} J (c) 7OF ] | cop) uJ 50 4+ (b) 8 40 L > 30; (a) aq 20; x 8 10 z tlm Ledeen 5 10 5 20 25 A =z Wl20 pT 6 1 - & zoL> e) Co®?, BILATERAL, Ne 4 el 4$ 60h OO © 50h z 4 - 4 | iW | é 40+ ~~ . ~ s He, 2 30| 2 20 '0 4 Lot 4 | ‘ Ll 5 10 (5 20 26 c | DEPTH (cm) aw {00r ts* a)FALLOUT GAMMA b) IMMEDIATE BOMB] | ©) Con CANIMA NILA d) Co®° CROSSFIRE 4 10 g1008 a eo & 2 gol DEPTH (cm) & uw GHOMETRICAL, ENERGY FACTORS--~EFFECT OF RADIATIONS ON MAN (a)Z qa 4 B \ \ 4 Froora 4.—Depth-close curves for cobalt-60 gamma radiation in Masonite phantom material for several exposure geometries, depth-dose expressed as percent of entrance air dose. of the entrance air dose from one machine and the exit air dose from the opposite machine (less by inverse square). Thus the air “dose” with crossfire is less with bilateral and the tissue dose, in terms of percent of air “dose,” is correspondingly greater. It should be noted that exposure with crossfire for one half the total time for both half-exposures with bilateral (two tubes on simultaneously with crossfire) yields a tissue dose curve that superimposes on the bilateral curve. However, since as noted, trum of course varies with time and place; however, that given by Sondhaus [12] can he present purposes. It is seen to consist of a as might result if inverse square were negligible. group of monoenergic sources, that can be considered to be composed of energies grouped at approximately 100 to 200 kev (11 percent), 0.75 kev (67 percent) and 1.5 Mev (22 percent). Seatter of radiation from partially-buried exit, doses is used as the ‘air dose,” instead of scatter from the ground will be neglected since considering only the undegraded beam will any sense eliminated the effect. It has aver- aged the entrance and exit exposure doses, and thus has raised the depth-dose curve, somewhat Anidentical superimposed curve is obtained if, with bilateral, the average of the entrance and the entrance air dose with each half-exposure. If the midline air dogo is used with bilateral tially a spherical configuration [1]. The depthdose pattern for both exposures is shown as 4 5 10 5 20 26 DEPTH (cm) technique, the total air “dose” given is the sum technique has taken into account to a degree the inverse square effect, it has not, of course, in approximately the level of the crossfire curve, is considerably flatter than the crossfire curve (70.5 percent at the edges, 69.0 percent at the midline). Ring and “4 Pi” exposures.—-With ring geometry, the phantom is at the centerof a concentric ring of fixed sources [1]. With “4 Pi” geometry, the phantom is placed in the geometric center of a group of sources arranged in essen- 40 ID) RING,OR 4 7 EXPOSURE] f taken as sufficiently representative for the Thus, the difference noted is seon to result are corrected for inverse squarefall off before addition, the resulting curve, while placed at 6015) CROSSFIRE EXPOSURE| \ from the inverse square effect. However, it is important to note that while the crossfire shape to the crossfire curve, but is placed a short distance above it. Of importance later in considering the curve for fallout radiation, if the half-exposure curves for bilateral radialion (b) 20+ the total time for bilateral, and the depth dose curve is thus above that for bilateral. are of importance in considering the curve to be expected with fallout gamma radiation, The fallout field in the simplest case can be considered as a semi-infinite plane uniformly contaminated with gamma emitters. The spec- crossfire, (he exposure time with crossfire for the same total air “close” is longer than one-half exposure, the curve is essentially identical in “| 80 the air dose for (he same total time is Jess with 71 curve b, Figure 4-B. They are essentially identical and are negligibly different from those obtained with the crossfire technique. These types of exposure can be considered to bear a similar relationship to crossfire exposure, as does multilateral or rotational exposure to the bilateral technique. Inverse square is taken into account to a degrea, but is not corrected or eliminated. Bomb, fallout gamma radiation. —The geometrical and other considerations noted above isotopes in the overlying ground, and secondary result in the largest possible dose to the phan- tom. The radiation at any given point in air above the plane will of course be coming from all directions; however the primary source can be considered as an infinite number of concentric ring sources and ean be treated as such. As noted above, the crossfire or ring depth-dose curve’ can be constructed from the unilateral curve, adding together two half-exposures from each side. No corrections for inverse square should be made in the unilateral curve since, as shown above, the re- sulting paltern on adding the half-curves is thus placed in correct relation to the air dose. Also, two separate calculations by Drs. Robertson and Brennan have indicated that the bulk of the radiation comes from several meters or more which tends to flatten the curve but not alter its relation to the air dose. The unilateral curves for the components of the fallout gamma spectrum were approximated in several ways as follows: since the bulk of the fallout radiation is approximately 0.75 kev (67 percent) and 1.5 Mev (22 percent), a curve closely approximating the unilateral eurve for Co® gamma would be expected. Uncorrected for inverse square, the curve + The phantom inthe fallautfleld is ubave the plano of the ring souross, 44 opposed to in the same plane In the laboratory situation. [¢ ean be easily shown, however, that thts dees not appreciably affect the mean path length of radiation in the phantom in reaching a given polnt, and. thus absorption in the phantom is not significantly altered.