8
EFFECTS OF IONIZING RADIATION
the dose at the center of the body is approximately 50 percent higher than would result
from a given air dose with narrow beam geometry. Figure 1.4 illustrates the depth dose
curve from an experimental situation using
90 percent of the dose had been received. the
dose rate had fallen to less than 40 percent of
its initial value. Thus the dose rate also differed from the usual constant rate in the
laboratory.
EVACUATION
AT 5S! HR
¢
|
ua
°
TOTAL OOSE IN AIR
8
(¢)
150
-
°
(2 AR FALL-OUT
4-'6 HR
tt
10
20
|
i
{|
30
Lt
a
Lt
50
TIME AFTER H-HOUR (HR)
Fieurg 1.3—The accumulation of gamma dose as a function of time after
commencement of fallout on Rongelap atoll.
1.23
Geometry of the Exposure
Inaddition to the dose rate and energy differences the geometry of the exposure to fallout radiation is significantly different fromthe
usual laboratory sources. Since fallout radiation is delivered from a planarsource the usual
narrow beam geometry is not applicable. In
such a diffuse 360° field, the decrease of dose
with depth in tissue is less pronounced than
that resulting from a bilateral exposure to an
X-ray beam because falloff from inverse square
is in effect neutralized. For the same energy,
spherically oriented Co® sources with a phantom placed at their center, compared with a
conventional bilateral depth dose curve obtained with a single source (+). In the latter
case, the air dose is usually measured at the
point subsequently occupied by the center of
the proximal surface of the patient or animal
with respect to the source. Forthefield case,
all surfaces are “proximal,” in the sense that
the air dose measured anywhere in the space
subsequently occupied by the individual is the
same. It is this air dose which is meusured
by a field instrument; it does not bear the same