i clit Bn BiBhnzamee alte CUAPTER 7 atone chapadbea nb td EXPOSURE GEOMETRY EFFECTS in clinical and lahoraury ex_csures, the radiation flux usually w follows a marrow beam or ot least a point-source "divergent" peonetry. vhen an airedesée is used to specify the eyposure conditions for a thick tar,et, it is generally measured at the ;oint subsequently oc~ 4 . cupied by the center of the proaimal surface of the patient ui exporiw: - mental anical with respect to the source. eccurred cn the islands, For flela expesures such as the raciation seurce is not a point and the exposure ~waretry is "diffuse" rather than "“diverjpent." 7 when a cloud or a large planar crea is the scurce, all surfeces of the irreciated individual are "proximal," in the sense that the aircose measumec anywhere in the space subsequently occupied by the indi- vicial is the same, : It is this airedcese which is measured by a field instrurent; 1 it does net bear the same relaticnship to the skin dose as and depth cose as does the atir-cose measured in a puint source geoma) etry. if a tilateral exposure is made in the lateratcry, one-half the ccse is usually given with ome side of the individual facing the source ané one-half with the other. this is a closer appreach to the fiela ‘geometry, but, if the air-dose has teen measurec at the center of the Froximal surface as above, it is stil] not related to the depth dose in the same way as is the fiele air-dese, The ecses received ty the indivicusls on the islands were from teth the cloud itself and the fallout dej;osited on the pround, it is Lelieved likely, as discussed in Chapter 6, that the cloud dose was only a s.ail ;art of the total dose and that the dose frem the plane amine mee men BAM eetbas te A et Te ewe a thn what en ine eeee we ned Sam pyre vavite Mf sete ae ee eee See be yp hee OB ws hl ware assumption cf early maximum activity and shurt effective fallout time which was rade in Chapter 6 tor the maximum dose case, Alternatively, if a long fallout actually cccurred, the source wouid have remained a clcud longer and the cloud voluxe, rather than the curface distribeSdon, woule have accounted for more of the totai dose, In either cuse, Lt would arjear that the -idline cuse, rather than the cose measured in air, woud te the Letter conmon parameter in terms of which to pre- Gict biclogical efrect. row leam peanetry, Since =cst existing data tacitly assures nar~ tunis distinction Lecanes impertant in relating field air-coses and their counsepences to known clinical or exp-erimen= tal results (Feferences 11, 12). at