CHAPTER 7 EXPOSURE GEOMETRY EFFECTS 7el DISCUSSICN In clinical and laboratery exposures, the radiation flux usually follows a narrow beam or at least a point-source "divergent" geometry, when an air-dose is used to specify the exposure conditions for a thick target, it is generally measured at the point subsequently oc~ cupied by the center of the proximal surface of the patient or experimental animal with respect to the source, For field exposures such as occurred on the islands, the radiation source is not a poir.t and the exposure geometry is "diffuse" rather than "diver;ent," when a cloud or a large planar area is the source, all surfaces of the irradiated individual are "proximal," in the sense that the air- dose measured anywhere in the space subsequently occupied by the individual is the same, Itis this airedcse which is measured ty a field instrument; it dees not bear the same relationship to the skin dose and depth dose as does the air-dose measured in a point source geometry. If a bilateral exposure is made in the laboratory, one-half the dose is usually given with one side of the individual facing the source and one-half with the other, This is a closer approach to the fiela geometry, But, if the air-dose has teen measured at the center of the proxinal surface as above, it is still not related to the depth dose in the same way as is the fielc air-dese, The doses received by the individuals on the islands were from both the cloud itself and the fallout deposited on the ground, It is believed likely, as discussed in Chapter 6, that the cloud dose was only a small part of the total dose and that the dose frcem the plane ground source contributed the major portion. This ccrresponds to the assumption of early maximum activity and short effective fallout time which was made in Chapter 6 for the maximum dose case, Alternatively, if a long fallout actually occurred, the source would have remained a cloud longer and the cloud volume, rather than the surface ¢cistribution, would have accounted for more of the total dose, In either case, it would appear that the midline dose, rather than the cose measured in air, would be the better common parameter in terms of which to pre- @ict biological effect, Since most existing data tacitly assumes narrow beam geometry, this distinction becomes important in relating field air-doses and their consequences to known clinical or experimen- tal results (Feferences 11, 12). O32 C18 Oo