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CUAPTER 7
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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
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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).
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