_,
)3
different elements.
All of these
are radioactive, decaying by the
emission of electrons, accompanied
by one or more gamma rays.
It has been calculated that at one
minute after the detonation of a
“ 4
a
of radium. This is an energy corresponding to about 67 billionths
of a kilowatt-hour.
The total energy of the electrons
(beta
particles)
produced
in
the
fission process is similar in magni-
20-kiloton TNT equivalent atomic
bomb, when the residual nuclear
radiation begins, the fission products wil! be emitting gamma radiation at the enormous rate of 934.5
kilowatt-hours of energy per second. Even after an hour, the rate
of emission of gamma radiation
tude to that of the gamma radiation. However, because the electrons do not penetrate to such
great distances as do the gamma
rays, the energy of the former
would, aS a general rule, be of
per second, so that, although the
has remained in a region in which
the dosage rate after one hour
will be nearly 7.12 kilowatt-hours
gamima activity has decreased by
a factor of about 130, it is still
extremely large.
A
widely
used
method
of
ex-
pressing rates at which radioactive
atoms decay is in terms of a unit
called the curie, named after the
discoverers of radium. A curie is
defined as a quantity of radioactive
material undergoing 37 billion disintegrations per second, which is
equal to the rate of disintegration
of one gram (1/28th ounce} of pure
radium. A megacurie is a million
curies, corresponding to disintegrations at the rate of 37 quadrillion
atoms per second, namely, that of
1,000 kiiograms (2,200 pounds) of
radium.
The gamma activity of the fission products one minute after the
explosion of the nominal 20-kiloton
TNT equivalent atomic bomb, is
equal to the gamma radiation
emitted by 1,804,000,000 pounds
(902,000 tons) of radium. This, as
we have seen, is at rate of 934.5
kilowatt-hours of energy per sec-
ond. After one hour the gamma
radiation from the fission products
significance
cases.
Data
in
borderline
show that a person
who
from the explosion is one r per
hour, will have received a total of
23.8 r of residual radiation within
two hours from the explosion, and
24.9 r after three hours from the
detonation.
Neutrons from an atomic explo-
sion which reach the earth's surface may interact with elements
there and make them radicactive.
Radioactivity induced by the neutrons may persist for some time,
contributing fo the residual radia-
curies,
is
products will still equal the gamma
day. An activity of one megacurie
per square mile would be attained
if at the end of one day these prod-
will be the equal of 600 milligrams
burst only a portion of the fission
products would have descended by
is equivalent to about 100 r per
month, and less than four ounces
after a year. Even after ten years
the gamma activity of the fission
ucts were spread uniformly over
133 square miles. In a normal air
energy given off by eight grams
of radium, While after the lapse of
106 years the gamma activity
32
the
to each 200 square miles of the
earth’s surface.
Estimates on Plutonium
An estimate of the possibility of
Absorption of Average Human
world-wide contamination by plu-
It has been estimated that at
tonium is more difficult, because
of the uncertainty concerning the
sea level a human being absorbs,
from all the aforementioned background sources, something like
0.003 r of radiation per week
proportion which escapes fission.
In order to take the extreme case
it is supposed that the whole of
the plutonium originally present
in the bomb is uniformly distribu-
This is about
one-hundredth part of the accepted
tolerance
dose
believed
to
be
ted in the top centimeter of soil.
This plutonium may then be pre-
sumed to be absorbed
Tays is increased three-fold at
15,000 feet, the total background
radiation is appreciably higher.
absorbed as 4 result of X-ray or
similar treatment. The same state
On
something like a million atomic
bombs, of the nominal size, would
have to be detonated, roughly one
and in the soil
throughout his life.
out.
basis of these postulates, it has
been calculated that in order to
constitute a world-wide hazard
These radioactive
Third Source of Radiation
in
tially to the faJl-out, sufficient
time must be allowed for all the
particles to settle
Species are also present in plants
radiation over the whole body, in
measured
sion products would be due essen-
body contains not insignificant
amounts of radioisotopes of carbon
and potassium,
Fur-
ther, since contamination from fis-
This
plosions present special problems.
A third possible source of residual nuclear radiation is the uranium 235 or plutonium which may
have escaped fission. Their radio-
very short period of time.
“background
radiation’
is
due
partly to the high energy particles,
known as cosmic rays, originating
in outer space, and partly to radium and its disintegration products
which are present in the earth and
in the air.
In addition, it is not
generally realized that the human
It appears that during the average lifetime every individual receives from 10 to 15 r or more of
pounds of radium; 28,600 pounds
after a week, 5,060 pounds after a
After one day it equals 292,600
being exposed to radiations.
air burst, and then at distances
not too great from ground zero.
Underwater and underground ex-
duced activity would probably be
significant only fer relatively low
the fission products.
For a contamination of one
megacurie Per square mile due to
fission products, the dosage rate
at about three feet above the
ground, caiculations show, is approximately 4 r per hour, which
Active After Ten Years
It is of interest to note that
even under norma] circumstances
long before X-rays or
atomic
bombs were even dreamed of, all
living organisms were continually
harmiess.
At
high
altitudes,
where the intensity of the cosmic
very small compared with that of
off by 13,200,000 pounds (6,600
tons) of radium, an energy equal
of 7.12 kilowatt-hours per second.
than 133 square miles.
tion activity. As the neutron’s intensity at the earth’s surface decreases rapidly with increasing
distance from the bomb, the in-
activity,
equals those that would be given
only
hazard due to plutonium which has
escaped fission.
If the whole surface of the earth
is to be contaminated, with a minimum number of bombs, they
would have to be exploded within a
the end of one day, and the area
covered would probably be greater
by
plants
and thus find its way into the human bedy in the form of food. Inhalation of dust represents another
possibility. It appears from the
calculations that for plutonium to
constitute a world-wide hazard
millions of atomic bombs would
have to be exploded.
addition to amounts that may be
World-wide radioactive contamination would thus appear to be extremely unlikely, but local contam-
of affairs has undoubtedly persisted during the whole period of
ination due to a relatively small
man’s existence on earth, although
number of bombs might be a seri-
the total radiation absorbed in a
lifetime has increased as the average lifespan has lengthened.
Fears have been expressed in
some quarters concerning the dan-
ous problem over a large area. The
fact that the fall-out may he so
widely dispersed means that radioactive particles will descend hundreds and even thousands of miles
ger of world-wide contamination
by radioactivity resulting from
atomic explosions. That such fears
from the point of detonation, Although they may not necessarily
estimating the number of bombs
. which would have to be detonated
lustration is the case of radioactive dust from the test explosion
do
are groundless can be shown by
any
physiological
harm,
the
particles may cause trouble. An il-
to produce enough activity to cover
the earth. Such calculations may
be made for externa] gamma radiation from the fission products, on
the one hand, and for the internal
at Alamogordo appearing in strawboard manufactured over a thouSand miles away and spoiling sensitive photographic film wrapped
in this material.
33