72
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
would be approximately 70 percent at the
midline and 35 percent at the exit. This particularly, since the curves for Co” gamma (1.3
Mev) and a cesium-137 source (0.7 Mev) agree
within 3 percent at distances corresponding to
the midline of the phantom [13].
Infinite
_ Inedia build up factors corrected for the diserepancy noted between theoretical and measured curves for Co” gamma yielded midline
and exit doses of 71 and 40 r, respectively.
The build up facters for a water barrier [10],
applied empirically, yielded corresponding per-
centages of 68 and 40 percent. Use of o, only
results in values of 70 and 50 percent. It is
reasonable to assume, then, that the unilateral
eurve for the fallout spectrum is approximately
70 percent at the midline, and 40 percent at
the exit. Construction of a curve from this
for the fallout field yields an expected depth
dose pattern in the field that is essentially flat,
with values of approximately 73 percent at
the surfaces and 70 percent at the midline.
A depth dose curve experimentally obtained
in # fallout field is shown as curve a, Figure
4-C. Doses were measured with Sievert-type
ionization chambers. The high surface doses
include beta radiation measured by the thinwalled ionization chambers. The air dose was
determined by covering the ionization chambers
with sufficient copper (approximately 800 mg/
em?) to exclude beta radiation. As expected,
the gammatissue dose throughout the phantom
was essentially constant. The tissue gamma
dose was approximately equal to the air dose,
however, as opposed to the approximately 70
percent predicted from theory. The reason for
this discrepancy probably lies in the manner in
which the air dose was measured. The thickness of copper, equivalent to the wall thickness
of some “gamma” monitoring instruments, undoubtedly excluded some gammaas well as beta
radiation.
Bomb, initial gamma radiation.—The curve
to be expected with the immediate bomb gamma
radiation was approximated in two ways.
The
linear absorption coefficient for bomb immediate
gamma radiation observed at distance of biological interest: (quoted on page 97, ref. 14) can
GEOMETRICAL, ENERGY FACTORS---EFFECT OF RADIATIONS ON MAN
be converted to the mass absorption coefficient,
by correcting for the small difference in electron
density and for inverse square (no detectable
fall off through the 26 cm phantom). Application of the absorption coefficient thus derived
yields a decrease in tissue dose at the exit side
to approximately 50 percent of the entrance
tissue dose.
A very similar result is obtained
if the mass absorption coefficient for several
Mev gammarays (about 0.03) is used with the
appropriate build up factor. The factors for
infinite media. apply closely here, since the large
air mass constitules an adequate scatter
medium.
A measured depth-dose curve in phantom
material exposed to the immediate gamma radiation from the bombis shown as curve c, Figure
3. The phantom employed was a cylinder
measuring 25 cm. in diameter, and measurements were taken approximately 3 feet above
the ground. The agreement with prediction is
good. It is apparent that while the rate of fall
off of dose in tissue is still appreciable in a
thickness of tissue approximating man, the exit
tissue dose of approximately 55 percent is well
above the value of approximately 20 percent
for cobalt-60 gammaradiation in the laboratory.
It is pointed out that with both initial and
fallout gamma ray exposures, the dose is essentially uniform as one goes from one end of the
phantom to the other. This is in contrast to
all of the laboratory geometries described, and
is approached only with “4 Pi” exposure.
Bomb, fast neutron irradiation.—Since fast
neutrons are attenuated rapidly in traversing
hydrogenous material, the considerations set
would result in some departure from a monodirectional beam; however, it is probable that
the beam would befar from isotropic,
Thero-
fore, the curves caleulated by Snyder [15] for a
terrain configurations.
73
The depth dose pattern
may thus be essentially unilateral rather than
flat as observed in the semi-infinite plane. As
will be seen, the biological offects are reduced
plane monodirectional source would apply approximately. It is seen that the rate of fall off
is quite rapid in hydrogenous material such as
with unilateral exposure. It is highly probable
that movement of the individual will result in
a highly complex and unpredictable depth-
energy of about 0.8 Mev, and the very large
majority of neutrons below 3 Mev, the dose
DISCUSSION
water.
For a fission spectrum with average
dose pattern.
could be expected to fall to the order of 10 to
15 percent of the surface dose at the midline,
and considerably less than this at the exit surface. It is emphasized that this is only a rough
approximation, and morerefined calculations or
measured curves should be obtained.
From X-ray data, however, it can be said that
such shallow curves are relatively quite ineffective in producing acute illness or death in large
animals (consider the very large monitored
doses of beta rays required to produce acute
effects). The relative biological effectiveness
for fast neutrons, determined with essentially
uniform tissue dose distribution in mice, appears to be of the order of 2 [16}, i. ¢., neutrons
are twice as effective as X-rays for the same
tissue dose in small animals in which essentially
all tissues receive the same dose. Because of
the shallow depth dose pattern in large animals,
however, the neutrons maybe less effective for
Comparison af depth-dose patterns.--In the
preceding results, the marked difference in
tissue dose, obtained with different exposure
geometries for the same air dose as conventionally expressed, have been stressed. The
large discrepancies possible must be kept in
mind when only the air dose is quoted or is
available. It is seen that no laboratory radia-~
tions as they have been employed quantitatively
simulate the initiel or fallout gamma radiations
from the atomic bomb. Perhaps morestriking
than the differences, however, is the marked
similarity of the depth-dose patterns for most
of the exposure situations, and their essential
identity if the artifact of expressing dose in
terms of that received by the air rather than
the tissues could be abandoned. The geome-
tries fall into two basic categories—unilateral
exposure, and a second to include all of the
other types considered. With the exception of
acute endpoints than penetrating X- or gamma
radiation by a factor several times greater than
the RBE determined in mice. It also becomes
apparentthatit is not possible to add theeffects
unilateral exposure, all those considered yield
reasonably flat or uniform depth-dose patterns
forth for gammia radiations apply to fast neu-
gamma radiation in a one-to-one ratio.
measured neutron depth dose curves for the
the depth-dose problems are those of partial
to the entrance air dose, for any exposure
geometry, will vary considerably with beam
energy, target-to-skin distance and animal
sible to estimate how the curve might look. It
can be assumed that the source spectrum for
fallout material. Some degree of partial shielding probably will be commoninthefalloutfield.
Shielding of a relatively small region of the
body, particularly if bone marrow is contained
in the shielded portion, will markedly reduce
the effect of
given radiation dose. “Hot
spots’ probably will be common in a fallout
field because of drifting, buildings and local
trons from the atomic bomb as well.
No
field situation are available; however, it ia pos-
relatively small weaponsis not unlike thefission
spectrum measured in the laboratory. In traversing approximately 1,000 meters to air to
arrive at distances of biological interest, it is
doubtful that the spectrum would change appreciably. Elastic multiple scattering in air
of the relatively nonpenetrating bomb neutrons
and the very penetrating bomb immediate
Body shielding, “local” geometry.—Allied to
body shielding, and localized concentrations of
{11. 17].
The relationship of the midline tissue dose
thickness.
The shape of the depth-dose curves
(essentially fiat)
for all geometries except
unilateral exposure is remarkably insensitive
to these factors for radiations and exposure
conditions commonly used for large animals
irradiation (200 to 2,000 KVP X-rays, cobalt60 gamma rays).
As the beam energy becomes
low (practically at about 100 KVP, 30 kev
effective), or with animals of very large diam-
eter (as with burros), the midline tissue dose