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Froora 4.—Depth-close curves for cobalt-60 gamma radiation in Masonite phantom material for several exposure
geometries, depth-dose expressed as percent of entrance air dose.
of the entrance air dose from one machine and
the exit air dose from the opposite machine
(less by inverse square). Thus the air “dose”
with crossfire is less with bilateral and the tissue
dose, in terms of percent of air “dose,” is
correspondingly greater.
It should be noted
that exposure with crossfire for one half the
total time for both half-exposures with bilateral
(two tubes on simultaneously with crossfire)
yields a tissue dose curve that superimposes on
the bilateral curve. However, since as noted,
trum of course varies with time and place;
however, that given by Sondhaus [12] can he
present purposes.
It is seen to consist of a
as might result if inverse square were negligible.
group of monoenergic sources, that can be
considered to be composed of energies grouped
at approximately 100 to 200 kev (11 percent),
0.75 kev (67 percent) and 1.5 Mev (22 percent).
Seatter of radiation from partially-buried
exit, doses is used as the ‘air dose,” instead of
scatter from the ground will be neglected since
considering only the undegraded beam will
any sense eliminated the effect.
It has aver-
aged the entrance and exit exposure doses, and
thus has raised the depth-dose curve, somewhat
Anidentical superimposed curve is obtained if,
with bilateral, the average of the entrance and
the entrance air dose with each half-exposure.
If the midline air dogo is used with bilateral
tially a spherical configuration [1]. The depthdose pattern for both exposures is shown as
4
5 10 5 20 26
DEPTH (cm)
technique, the total air “dose” given is the sum
technique has taken into account to a degree
the inverse square effect, it has not, of course, in
approximately the level of the crossfire curve,
is considerably flatter than the crossfire curve
(70.5 percent at the edges, 69.0 percent at the
midline).
Ring and “4 Pi” exposures.—-With ring geometry, the phantom is at the centerof a concentric
ring of fixed sources [1]. With “4 Pi” geometry, the phantom is placed in the geometric
center of a group of sources arranged in essen-
40 ID) RING,OR 4 7 EXPOSURE]
f
taken as sufficiently representative for the
Thus, the difference noted is seon to result
are corrected for inverse squarefall off before
addition, the resulting curve, while placed at
6015) CROSSFIRE EXPOSURE|
\
from the inverse square effect. However, it is
important to note that while the crossfire
shape to the crossfire curve, but is placed a
short distance above it. Of importance later
in considering the curve for fallout radiation,
if the half-exposure curves for bilateral radialion
(b)
20+
the total time for bilateral, and the depth dose
curve is thus above that for bilateral.
are of importance in considering the curve to
be expected with fallout gamma radiation,
The fallout field in the simplest case can be
considered as a semi-infinite plane uniformly
contaminated with gamma emitters. The spec-
crossfire, (he exposure time with crossfire for
the same total air “close” is longer than one-half
exposure, the curve is essentially identical in
“|
80
the air dose for (he same total time is Jess with
71
curve b, Figure 4-B. They are essentially
identical and are negligibly different from those
obtained with the crossfire technique.
These
types of exposure can be considered to bear a
similar relationship to crossfire exposure, as does
multilateral or rotational exposure to the bilateral technique. Inverse square is taken into
account to a degrea, but is not corrected or
eliminated.
Bomb, fallout gamma radiation. —The geometrical and other considerations noted above
isotopes in the overlying ground, and secondary
result in the largest possible dose to the phan-
tom. The radiation at any given point in air
above the plane will of course be coming from
all directions; however the primary source
can be considered as an infinite number of
concentric ring sources and ean be treated as
such. As noted above, the crossfire or ring
depth-dose curve’ can be constructed from
the unilateral curve, adding together two
half-exposures from each side. No corrections
for inverse square should be made in the
unilateral curve since, as shown above, the re-
sulting paltern on adding the half-curves is
thus placed in correct relation to the air dose.
Also, two separate calculations by Drs. Robertson and Brennan have indicated that the
bulk of the radiation comes from several
meters or more which tends to flatten the curve
but not alter its relation to the air dose. The
unilateral curves for the components of the
fallout gamma spectrum were approximated in
several ways as follows: since the bulk of
the fallout radiation is approximately 0.75
kev (67 percent) and 1.5 Mev (22 percent),
a curve closely approximating the unilateral
eurve for Co® gamma would be expected.
Uncorrected for inverse square, the curve
+ The phantom inthe fallautfleld is ubave the plano of the ring souross,
44 opposed to in the same plane In the laboratory situation. [¢ ean be
easily shown, however, that thts dees not appreciably affect the mean
path length of radiation in the phantom in reaching a given polnt, and.
thus absorption in the phantom is not significantly altered.