EFFECTS OF EXPOSURE GEOMETRY 363 factors, in accordance with the theories of Spencer and Fano (/3, 15), as extended and applied by others (74; also Loevinger et al. in reference 8). Bomb, fallout y-radiation. The depth-dose curve obtained for exposure in a y-ray field from fallout is shown as curve @ in Fig. 4C (the curve is truncated and does not indicate the skin dose). This curve was measuredin a fallout field (1), and the air dose is that measured by Sievert ionization chambers enclosed in sufficient copper to exclude @-radiation. The phantom actually used was 25 ¢m in diameter; the curve was flat throughout the phantom as measured with thin- walled Sievert chambers that measured y-radiation, as well as energetic 3-radiation if present. The flat central portion of the curve is, of course, due to y-radiation only. The relatively high doses at the edges (as high on the surface as fifty times the midline dose) resulted from addition of y-radiation and @- or very low energy y-radiation, not measured by the y-ray survey instruments used to determine air dose. It is apparent from the curve that the y-radiation dose throughout the phantomis essentially constant, except at the skin surface. It is possible to construet very approximately the depth-dose curve to be expected in the semi-infinite plane fallout situation, using a source spectrum for the fallout field (1), approximate build-up factors (2) calculated from the theories of Spencer and Fano (13, /6), and the geometrical considerations developed in the present paper. The resulting curve is essentially flat as in the experimental curve; however, the midline tissue dose with the ealeulated curve is approximately 75 % of the entrance air dose. The explanation for this difference between the calculated and observed curve is not apparent. It is pointed out that with both initial and fallout y-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 the laboratory geometries described and is approached only with dq exposure. DISCUSSION Comparison of depth-dose patterns. In the preceding results, the marked differences 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 oris available. It is seen that no laboratory radiations as they have been employed quantitatively simulate the initial or fallout y-radiations from the atomie bomb. Perhaps more striking 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 geometries fall into two basic categories—unilateral exposure, and a second to include all the other types considered. With the exception of unilateral S01 2492