72 THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD would be approximately 70 percent at the midline and 35 percent at the exit. This particularly, since the curves for Co” gamma (1.3 Mev) and a cesium-137 source (0.7 Mev) agree within 3 percent at distances corresponding to the midline of the phantom [13]. Infinite _ Inedia build up factors corrected for the diserepancy noted between theoretical and measured curves for Co” gamma yielded midline and exit doses of 71 and 40 r, respectively. The build up facters for a water barrier [10], applied empirically, yielded corresponding per- centages of 68 and 40 percent. Use of o, only results in values of 70 and 50 percent. It is reasonable to assume, then, that the unilateral eurve for the fallout spectrum is approximately 70 percent at the midline, and 40 percent at the exit. Construction of a curve from this for the fallout field yields an expected depth dose pattern in the field that is essentially flat, with values of approximately 73 percent at the surfaces and 70 percent at the midline. A depth dose curve experimentally obtained in # fallout field is shown as curve a, Figure 4-C. Doses were measured with Sievert-type ionization chambers. The high surface doses include beta radiation measured by the thinwalled ionization chambers. The air dose was determined by covering the ionization chambers with sufficient copper (approximately 800 mg/ em?) to exclude beta radiation. As expected, the gammatissue dose throughout the phantom was essentially constant. The tissue gamma dose was approximately equal to the air dose, however, as opposed to the approximately 70 percent predicted from theory. The reason for this discrepancy probably lies in the manner in which the air dose was measured. The thickness of copper, equivalent to the wall thickness of some “gamma” monitoring instruments, undoubtedly excluded some gammaas well as beta radiation. Bomb, initial gamma radiation.—The curve to be expected with the immediate bomb gamma radiation was approximated in two ways. The linear absorption coefficient for bomb immediate gamma radiation observed at distance of biological interest: (quoted on page 97, ref. 14) can GEOMETRICAL, ENERGY FACTORS---EFFECT OF RADIATIONS ON MAN be converted to the mass absorption coefficient, by correcting for the small difference in electron density and for inverse square (no detectable fall off through the 26 cm phantom). Application of the absorption coefficient thus derived yields a decrease in tissue dose at the exit side to approximately 50 percent of the entrance tissue dose. A very similar result is obtained if the mass absorption coefficient for several Mev gammarays (about 0.03) is used with the appropriate build up factor. The factors for infinite media. apply closely here, since the large air mass constitules an adequate scatter medium. A measured depth-dose curve in phantom material exposed to the immediate gamma radiation from the bombis shown as curve c, Figure 3. The phantom employed was a cylinder measuring 25 cm. in diameter, and measurements were taken approximately 3 feet above the ground. The agreement with prediction is good. It is apparent that while the rate of fall off of dose in tissue is still appreciable in a thickness of tissue approximating man, the exit tissue dose of approximately 55 percent is well above the value of approximately 20 percent for cobalt-60 gammaradiation in the laboratory. It is pointed out that with both initial and fallout gamma ray exposures, the dose is essentially uniform as one goes from one end of the phantom to the other. This is in contrast to all of the laboratory geometries described, and is approached only with “4 Pi” exposure. Bomb, fast neutron irradiation.—Since fast neutrons are attenuated rapidly in traversing hydrogenous material, the considerations set would result in some departure from a monodirectional beam; however, it is probable that the beam would befar from isotropic, Thero- fore, the curves caleulated by Snyder [15] for a terrain configurations. 73 The depth dose pattern may thus be essentially unilateral rather than flat as observed in the semi-infinite plane. As will be seen, the biological offects are reduced plane monodirectional source would apply approximately. It is seen that the rate of fall off is quite rapid in hydrogenous material such as with unilateral exposure. It is highly probable that movement of the individual will result in a highly complex and unpredictable depth- energy of about 0.8 Mev, and the very large majority of neutrons below 3 Mev, the dose DISCUSSION water. For a fission spectrum with average dose pattern. could be expected to fall to the order of 10 to 15 percent of the surface dose at the midline, and considerably less than this at the exit surface. It is emphasized that this is only a rough approximation, and morerefined calculations or measured curves should be obtained. From X-ray data, however, it can be said that such shallow curves are relatively quite ineffective in producing acute illness or death in large animals (consider the very large monitored doses of beta rays required to produce acute effects). The relative biological effectiveness for fast neutrons, determined with essentially uniform tissue dose distribution in mice, appears to be of the order of 2 [16}, i. ¢., neutrons are twice as effective as X-rays for the same tissue dose in small animals in which essentially all tissues receive the same dose. Because of the shallow depth dose pattern in large animals, however, the neutrons maybe less effective for Comparison af depth-dose patterns.--In the preceding results, the marked difference in tissue dose, obtained with different exposure geometries for the same air dose as conventionally expressed, have been stressed. The large discrepancies possible must be kept in mind when only the air dose is quoted or is available. It is seen that no laboratory radia-~ tions as they have been employed quantitatively simulate the initiel or fallout gamma radiations from the atomic bomb. Perhaps morestriking than the differences, however, is the marked similarity of the depth-dose patterns for most of the exposure situations, and their essential identity if the artifact of expressing dose in terms of that received by the air rather than the tissues could be abandoned. The geome- tries fall into two basic categories—unilateral exposure, and a second to include all of the other types considered. With the exception of acute endpoints than penetrating X- or gamma radiation by a factor several times greater than the RBE determined in mice. It also becomes apparentthatit is not possible to add theeffects unilateral exposure, all those considered yield reasonably flat or uniform depth-dose patterns forth for gammia radiations apply to fast neu- gamma radiation in a one-to-one ratio. measured neutron depth dose curves for the the depth-dose problems are those of partial to the entrance air dose, for any exposure geometry, will vary considerably with beam energy, target-to-skin distance and animal sible to estimate how the curve might look. It can be assumed that the source spectrum for fallout material. Some degree of partial shielding probably will be commoninthefalloutfield. Shielding of a relatively small region of the body, particularly if bone marrow is contained in the shielded portion, will markedly reduce the effect of given radiation dose. “Hot spots’ probably will be common in a fallout field because of drifting, buildings and local trons from the atomic bomb as well. No field situation are available; however, it ia pos- relatively small weaponsis not unlike thefission spectrum measured in the laboratory. In traversing approximately 1,000 meters to air to arrive at distances of biological interest, it is doubtful that the spectrum would change appreciably. Elastic multiple scattering in air of the relatively nonpenetrating bomb neutrons and the very penetrating bomb immediate Body shielding, “local” geometry.—Allied to body shielding, and localized concentrations of {11. 17]. The relationship of the midline tissue dose thickness. The shape of the depth-dose curves (essentially fiat) for all geometries except unilateral exposure is remarkably insensitive to these factors for radiations and exposure conditions commonly used for large animals irradiation (200 to 2,000 KVP X-rays, cobalt60 gamma rays). As the beam energy becomes low (practically at about 100 KVP, 30 kev effective), or with animals of very large diam- eter (as with burros), the midline tissue dose