r
§
EFFECTS
OF IONIZING
RADIATION
dose rate had fallen to less than 40 percent of
its jnitial value. Thus the dose rate also dif-
the dose at the center of the body is approximately 50 percent higher than would result
from a given air dose with narrow beam geom-
laboratory.
curve from an experimental situation using
90 percent of the dose had been received, the
fered
from
the
usual
constant
rate
in
etry.
the
Figure 1.4 illustrates the depth dose
=
EVACUATION
r
169
aT 5) HR
~ 100
=
<
z
Llu
1
—
nw
o
Qa
Le
ai
<
L
oO
r
bw
sor
—
3
12 HR FALL-OUT
4-|I6
HR
—
i
.
ot
Oo
er
Poo
10
20
30
TIME
AFTER H-HOUR
40
59
(HR!
Ficurke 1.3—The accuntalation of gamma dose os a function of time after
commencement Gf fallout on Rongclap atoll.
1.23. Geometry of the Exposure
In addition to the dose rate and energy dif-
ferences the geometry of the exposure to fallout radiation is significantly different from: the
usual laboratory sources.
Since fallout radia-
tion is delivered from a planar source 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 oa bilateral exposure to an
N-ray beam becuuse falloff from inverse square
is in effect neutrulized. For the same energy,
5002 1bb
spherically oriented Co® sources with a phantom placed at their center, compared with 4
conventional bilateral depth dose curve ob-
tained with a single source (4). 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.
For the held cuse,
atl 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 measured
by a field instrument; it does not bear the same