68

THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD

DEPTH IN (cm)
fo
'
20
T
T
T

5

25
T

GEOMETRICAL, ENERGY FACTORS-~EFFECT OF RADIATIONS ON MAN

up factor varies markedly with energy and
depth, of penetration. Build up is rapid over
the first mean free path, which results in a low
energy beam appearing to be more penetrating
thanitis over the first few em. of unit density
materia], X-ray beams, with their broad and
continuous spectra, cannot be handled in this
fashion. The considerations developed above
areof particular importancelater in considering

30
T

bombradiatious.

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oO

od

&
a

&

&
5
a

Qo
x

fi

4
(0) 2 =0.064
(b) 1/d?
(¢) MEASURED CURVE
TSD toocm
J

L

I

(caleulated curve higher than measured curve

by factor of 1.3 at the midline and 1.6 at the

The build up factors for a water barrier

describe the measured curve within 10 percent,
however this appears to be fortuitous andlittle
theoratical justification exists for applying
barrier build up factor to the present geometri-

The true absorption coefficient

(o,, approximately 0.03) predicts the midline

dose within 5 percent but overestimates the

exit dose by a factor of 1.8.

Thus the depth-

dose curves to be expected with gamma rays

cannot be predicted precisely from presently

available theoretical data; however a basis
does exist for approximating the curve to be

expected from a monochromatic beam (a beam
composed of several monoenergic components
can be handled by treating each component

separately and adding the results).

The build

very small pereentage of the “dose” that the
phantomor animal, by convention, is said to
have received. The marked fall off in dose
results frem both absorption in the phantom
and from the inverse square effect.
Bilateral exposure—In an effort to overcome

the marked lack of uniformity of depth dose

obtained with unilateral exposure, a numberof
investigators have employed the “bilateral exposure’ technique (sea the excellent work of
Tullis, ref. 11). This procedure is identical to
the unilateral exposure, except that one-half

of the “totel dose” is administered from one
side.

Thus, if a total of “300 r” is to be given,

150 r as measured free in air at the proximal

exposure to cobalt-60 gamma rays and the
total obtained by combining the values obtained with each separate exposure are shown
in Figure 4~A.

Tt can be seen from the curve that the tissue

dose throughout the phantom is remarkably

uniform when contrasted with that obtained
with unilateral exposure, and that a maximum
variation of only 10 percent is obtained in tra-

[id}----- Co® GAMMA RADIATION
1

s

1

10

1

1

15
20
DEPTH (cm)

4

25

4
i

30

Figure 3.-~-Unilateral-exposure depth-dose curves in a
Masonite phantom for different energy radiations;
depth-dose expressed as percent of entrance air dose.
3 The term “unilateral”is applied for convenience to the exposureto the
initial gamma radiation from the atomic bomb, even though an appreciable component of the total dose undoubtedly ts recelved from the lateral
anddistal aspects of the phantom,

limiting situation involves rotating the source
about the phantom at TSD of 100 cm., or
equivalent, rotating the phantom placed 100
em. in front of the stationary sources. It is
easily shown [1] that these procedures do not

differ materially in effect from bilateral exposure, and the depth dose patterns obtained
(curve d, fig. 4—A) superimpose essentially on

the bilateral curve.

Crossfire technique-—With the crossfire tech-

nique, only a single exposure using two opposing
“point” sources energized simultaneously is
used, as opposed to the bilateral technique in
which two exposures,first one side and then the
other, are made with a single source.

The

resulting dose pattern is shown as curve a,
Figure 4-B. It is apparent that the shape of
the curve is negligibly different from that ob-

tained with bilateral, multilateral or rotational

however, is the fact that the tissue at no point
in the phantom exceeds 62 percent of the entrance air dose, the dose that the phantom, by
for this discrepancy lies mainly in the fact that

considers that as soon as the animal or phantom

Of equal importance,

convention, is said ta have received.

4

69

to equalize the tissue dose and the totalair dose,
but does not accomplish this fully.
Multilateral and Rotational exposure.—In
these techniques, instead of giving one-half
the dose from each of 2 sides, the dose is administered one-fourth from each of 4 “sides,”
one-eighth from each of 8 “sides,” etc. The

techniques, and that the tissue dose is still
considerably below the air exposure dose that
the phantom is said to have received.
The reason for the low tissue dose relative to
air dose may not be immediately apparent, since
with crossfire technique the air exposure dose
throughout the exposure volumeis essentially

versing the phantom.

«
|(a) -—— 250 KVP X-RAY
NOTA.
(b)—--— 2000 KVP X-RAY
a
20/-(c)—~-— BOMB, INITIAL GAMMA
RADIATION
°

posure, the distal surface may receive only a

skin surface is given from side A (fig. 1). The
remaining 150 r is then administered from side
B. The depth-dose pattern for each separate

T

ie
3
L

tion, unilateral exposure, TSD of 100 om.

cal situation.

T

140

025 05 075 10 125 15) L7S
DEPTH IN MEAN FREE PATH LENGTHS
Fravre 2,—Depth-dose curve for cobalt-60 gamma radia-

exit).

T

g

|

T

g

1

T

5

|

T

PER CENT ENTRANCE AIR DOSE
ey

10

In Figure 3, the measured curves for Co®
gamma and other radiations are shown for
comparison,
In all cases the total dose is
delivered in a single exposure from one side of
the phantom? It is apparent from the figure
that marked nonuniformity of dose deposition
results even with highly energetic radiations,

and that with this type of “total body” cx-

The reason

during each half-exposure, the distal side of the
phantom is receiving only a very small per-

centage of the dose received by the proximal
side, and on adding the half-exposures, the
total falls far short of the dose said to have
been given (see under “crossfire” exposure below for additional reasons).

If the midlineair dose, instead of the entrance

air dose, is taken as the total exposure, the

resulting curve retains the shape noted above,
but becomes 70 percent (instead of 55 percent}
at the midline. Thusit is seen that use of the
midline rather than the entrance air dose tends

constant.

It is easily seen, however, if one

is introduced, the entrance tissue dose at either

side (and throughout the phantom) immediately
drops considerably because of absorption in the
tissue or phantom. Thus, the entire curve is
well below the entrance air dose.
Thecrossfire curveis higher than the bilateral
curve because of what might be regarded as an
artifact of dosimetry resulting from the manner
in which atr dose is measured with the two
techniques. This can be seen as follows: with
the bilateral technique, the total air “dose”
given is the sum of two entrance air doses from
the two half-exposures. With the crossfire

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