64 THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD case suggest that, the method is valid for consideration of fallout’ gamma radiations also. 2. Application of transport theory to the initial garmma radiations shows that the majority of the air dose delivered at distances of a thousand-or-so yards and further is deposited by very energetic photons, ranging up to the 10.8 Mev gamma rays emitted by the nitrogen capture component of the bomb gammasource. a. It farther appears that for these composite energetic radiations the air acts more as a filter than as a scattering medium, so that the initial bomb gammaspectra “harden” with increasing distance. b. In the case of very large yield detonations blast wave radiation enhancement factors may vitiate the theoretical predictions and produce larger total doses with softer energy spectra. ce. Nonetheless, the exceedingly hard spectra present in most cases of initial bomb gamma radiation from which biological radiation damage criteria have been derived must be taken into account before applying these criteria directly to fallout or other situations. 3. Calculation of fallout gamma spectra has been less extensive. Generally fallout dose spectra must be far less energetic than are initial gamma spectra. a. Theoretical calculations of beth dose and 4a-spectrumfrom fallout, based on either measured or predicted gamma source data as a function of time, and of weapon and of environmental parameters should prove feasible but apparently have not been attempted. GEOMETRICAL AND ENERGY FACTORS INFLUENCING THE EFFECT OF PENETRATING RADIATIONS ON MAN‘ By V. P. Bonp REFERENCES 1, Spencer, L. V. and Fano, V.: “Penetration and Diffusion of X-rays; Calculations of Spatial Distributions by Polynomial Expansion”; Journal of Research of the National Bureau of Standards, 46; 446, 1951. Also, Phys. Rev. 81, 464, 1951. 2. Garns, L. D., Jr., and Ersennauzr, C.: “Spectral Brookhaven National Laboratory INTRODUCTION In considering the degree ofeffect to be ex- Distribution of Gamma Rays in Air’; AFSWP pected in man exposed to penetrating radiations from the atomic bomb,it is necessary to examine 3. Marasax, R. E.: “Theory of the Slowing Down of Neutrons by Elastic Collision with Atomic Nuclei”; Revs. Modern Phys. 19, 185, 1947. possible exposure situations and the energy or 4. Morerr, J.: These factors are known from laboratory ex- Report No. 502A, 1954. “Fission Product Decay Gamma Energy Spectram”; APEX-134, 1953. General Electric Report 5, Kinspy, et. al: “Gamma Rays Produced by Slow Neutron Capture in Beryllium Carbon and Nitrogen”; Canad. J. Phys. 29, 1, 1951. 8. Hirscuretper, J. 0., et al., editors: “The Effects of Atomie Weapons”; The Combat Forces Preas, Washington, D. C., 1950. 7. Ovcgrerson, A. and Warrsn,S.: ‘Medical Effects of the Atomic Bomb in Japan”; McGraw-Hill, New York, 1056. 8. Cronuits, E. P.; Bono, V. P.; and Dunnam, C. L., editors: “Some Effects of Ionizing Radiation on Human Beings”; TID 5358, 1956. the extent to which the geometry of the various spectrum of the beam may influence theresult. perience to be of considerable importance, and must be taken into account when efforts are made to compare quantitatively the results under different conditions of exposure. In this paper, the patterns of dose deposition through a man-sized phantom to be expected theoretically are developed for a variety of exposure conditions, and these are compared with the experimentally determined depth dose patterns. The degree to which biological effect is influenced by the various patterns of dose deposition are then considered. It is shown that such considerations can result in a difference of a factor of 5 or more in the degree of effect to be expected under various conditions of exposure, for the same monitored air dose. The laboratory situation will be considered first for two reasons. The simpler situations in the laboratory allow a basis for developing the situations to be expected under the more complex field conditions. In addition, the hazard to man in the field of necessity must be evaluated in terms of laboratory experience with large animals and man, In general, laboratory biological data are far more reliable than those obtained under trying field conditions. ' Research supported by U. 8. Atomic Energy Commission In thefield situation, the immediate and fallout gamma radiation from the atomic bomb will be dealt with mainly. Fast neutrons will be considered briefly. Someof the present material is presented in more detail elsewhere [1]. A rather obvious fact must be introduced initially. Monitoring instruments measure the free-in-air dose. However, there is no real interest in the dose received by the ambient air—the degree of biological effect is determined by the radiation dose received by the tissue. It is this dose, and its distribution in the body that governs the degree of biological response. This basic fact has, of course, been long recognized by radiologists, and the recommendations for many years in the reports of National and International Committees on Radiation Units in that dose be reported in terms of tissue dose ? rather than the free-in-air dose (2, 3]. Thus some of what I say has long been known by radiologists; however, much of it has not been brought to the attention of radiobiologists and others concerned with hazard evaluation in man. , The use of tissue dose hes gone far in resolving apparent quantitative differences in biological response in radiology, and in radiobiology concerned with small animals. Both, in general, are concerned with radiation effects in a relatively small, circumscribed volume of * See refs. 2,4,and5. Tissue dose refers only to tho ionization measured by the detector embedded In the material being irradiated and usually does not indicate aceurately the absorbed dose, i. ¢., the energy per unit mass deposited in the irradiated material, here tissue or unit density material Over much of the range of radiation energies usually of interest in large animal work, from 260 KVP to 1.6 Mev or higher, the tissue dose will be equal to the absorbed dose in soft tissue, expressed as. rads (100 ergs/gram), to within 10 percent or better, Much larger ¢tiserepancies ocour in bone. 65