68 THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD DEPTH IN (cm) fo ' 20 T T T 5 25 T GEOMETRICAL, ENERGY FACTORS-~EFFECT OF RADIATIONS ON MAN up factor varies markedly with energy and depth, of penetration. Build up is rapid over the first mean free path, which results in a low energy beam appearing to be more penetrating thanitis over the first few em. of unit density materia], X-ray beams, with their broad and continuous spectra, cannot be handled in this fashion. The considerations developed above areof particular importancelater in considering 30 T bombradiatious. wl oO od & a & & 5 a Qo x fi 4 (0) 2 =0.064 (b) 1/d? (¢) MEASURED CURVE TSD toocm J L I (caleulated curve higher than measured curve by factor of 1.3 at the midline and 1.6 at the The build up factors for a water barrier describe the measured curve within 10 percent, however this appears to be fortuitous andlittle theoratical justification exists for applying barrier build up factor to the present geometri- The true absorption coefficient (o,, approximately 0.03) predicts the midline dose within 5 percent but overestimates the exit dose by a factor of 1.8. Thus the depth- dose curves to be expected with gamma rays cannot be predicted precisely from presently available theoretical data; however a basis does exist for approximating the curve to be expected from a monochromatic beam (a beam composed of several monoenergic components can be handled by treating each component separately and adding the results). The build very small pereentage of the “dose” that the phantomor animal, by convention, is said to have received. The marked fall off in dose results frem both absorption in the phantom and from the inverse square effect. Bilateral exposure—In an effort to overcome the marked lack of uniformity of depth dose obtained with unilateral exposure, a numberof investigators have employed the “bilateral exposure’ technique (sea the excellent work of Tullis, ref. 11). This procedure is identical to the unilateral exposure, except that one-half of the “totel dose” is administered from one side. Thus, if a total of “300 r” is to be given, 150 r as measured free in air at the proximal exposure to cobalt-60 gamma rays and the total obtained by combining the values obtained with each separate exposure are shown in Figure 4~A. Tt can be seen from the curve that the tissue dose throughout the phantom is remarkably uniform when contrasted with that obtained with unilateral exposure, and that a maximum variation of only 10 percent is obtained in tra- [id}----- Co® GAMMA RADIATION 1 s 1 10 1 1 15 20 DEPTH (cm) 4 25 4 i 30 Figure 3.-~-Unilateral-exposure depth-dose curves in a Masonite phantom for different energy radiations; depth-dose expressed as percent of entrance air dose. 3 The term “unilateral”is applied for convenience to the exposureto the initial gamma radiation from the atomic bomb, even though an appreciable component of the total dose undoubtedly ts recelved from the lateral anddistal aspects of the phantom, limiting situation involves rotating the source about the phantom at TSD of 100 cm., or equivalent, rotating the phantom placed 100 em. in front of the stationary sources. It is easily shown [1] that these procedures do not differ materially in effect from bilateral exposure, and the depth dose patterns obtained (curve d, fig. 4—A) superimpose essentially on the bilateral curve. Crossfire technique-—With the crossfire tech- nique, only a single exposure using two opposing “point” sources energized simultaneously is used, as opposed to the bilateral technique in which two exposures,first one side and then the other, are made with a single source. The resulting dose pattern is shown as curve a, Figure 4-B. It is apparent that the shape of the curve is negligibly different from that ob- tained with bilateral, multilateral or rotational however, is the fact that the tissue at no point in the phantom exceeds 62 percent of the entrance air dose, the dose that the phantom, by for this discrepancy lies mainly in the fact that considers that as soon as the animal or phantom Of equal importance, convention, is said ta have received. 4 69 to equalize the tissue dose and the totalair dose, but does not accomplish this fully. Multilateral and Rotational exposure.—In these techniques, instead of giving one-half the dose from each of 2 sides, the dose is administered one-fourth from each of 4 “sides,” one-eighth from each of 8 “sides,” etc. The techniques, and that the tissue dose is still considerably below the air exposure dose that the phantom is said to have received. The reason for the low tissue dose relative to air dose may not be immediately apparent, since with crossfire technique the air exposure dose throughout the exposure volumeis essentially versing the phantom. « |(a) -—— 250 KVP X-RAY NOTA. (b)—--— 2000 KVP X-RAY a 20/-(c)—~-— BOMB, INITIAL GAMMA RADIATION ° posure, the distal surface may receive only a skin surface is given from side A (fig. 1). The remaining 150 r is then administered from side B. The depth-dose pattern for each separate T ie 3 L tion, unilateral exposure, TSD of 100 om. cal situation. T 140 025 05 075 10 125 15) L7S DEPTH IN MEAN FREE PATH LENGTHS Fravre 2,—Depth-dose curve for cobalt-60 gamma radia- exit). T g | T g 1 T 5 | T PER CENT ENTRANCE AIR DOSE ey 10 In Figure 3, the measured curves for Co® gamma and other radiations are shown for comparison, In all cases the total dose is delivered in a single exposure from one side of the phantom? It is apparent from the figure that marked nonuniformity of dose deposition results even with highly energetic radiations, and that with this type of “total body” cx- The reason during each half-exposure, the distal side of the phantom is receiving only a very small per- centage of the dose received by the proximal side, and on adding the half-exposures, the total falls far short of the dose said to have been given (see under “crossfire” exposure below for additional reasons). If the midlineair dose, instead of the entrance air dose, is taken as the total exposure, the resulting curve retains the shape noted above, but becomes 70 percent (instead of 55 percent} at the midline. Thusit is seen that use of the midline rather than the entrance air dose tends constant. It is easily seen, however, if one is introduced, the entrance tissue dose at either side (and throughout the phantom) immediately drops considerably because of absorption in the tissue or phantom. Thus, the entire curve is well below the entrance air dose. Thecrossfire curveis higher than the bilateral curve because of what might be regarded as an artifact of dosimetry resulting from the manner in which atr dose is measured with the two techniques. This can be seen as follows: with the bilateral technique, the total air “dose” given is the sum of two entrance air doses from the two half-exposures. With the crossfire