The Radioactive Cloud
In February 1954 preparations were made on Bikini Atoll for the
explosion of a hydrogen bomb. March 1 was the “ready” date. It did not
seem probable that the shot would actually be fired on that date because
the shot could be fired only under quite favorable wind conditions.
Large amounts of radioactivity, especially fission products, were
expected from the explosion. The shot could be fired only if no
inhabited places lay in the downwind direction.
Bikini is an oval-shaped coral reef, an atoll. It is one of several such
atolls belonging to the group called the Marshall Islands. If you look
at the map, you will see that west of Bikini at a distance of 200 miles
lies Eniwetok, on which our people were making preparations for further
tests.
To the east of Bikini, a hundred miles or so, is Rongelap Atoll. At that
time 64 people were living there. They lived primitively in palm houses
on the southern part of the atoll. The northern part was uninhabited.
On nearby Ailinginae Atoll 18 of the Marshallese islanders were on a
fishing expedition, while farther to the east on Rongerik 28 American
servicemen were stationed. The servicemen lived and worked in aluminum
huts. Their main job was to collect weather data.
[Illustration: Map of the Marshall Islands]
Much farther to the east, 300 miles from Bikini, is Utirik. One hundred
and fifty-seven Marshallese people lived on this atoll.
Early on the morning of March 1, a Japanese fishing boat lay somewhere
to the north of Rongelap. Her name was Fukuryu Maru, which means in
English the Fortunate Dragon. There were 23 men on board. Actually she
was in a patrolled zone but had not been sighted by the patrol aircraft.
Operations for the test were being directed from ships of Joint Task
Force 7. For several days prior to the morning of March 1, the
weathermen had been mapping the winds. A wind to the west would be bad
for Eniwetok. A wind to the east might hurt Rongelap and Rongerik. A
wind to the south could affect Kwajalein. The ideal direction would have
been due north, but this probably would not happen for months. On “shot”
morning the wind was blowing to the northeast. The meteorologists gave
their “O.K.” It was at dawn, the first of March, 1954.
The firing crew of nine people led by a man of considerable experience,
Jack Clark, were responsible for the final arrangements. They were in a
blockhouse on the south side of the atoll 20 miles from the bomb.
Others, more than 1000 people, watched from shipboard under the
direction of Al Graves, who was responsible for the technical phases of
the operation. The ships lay south and a little east of Bikini.
The firing mechanism was set into operation in the blockhouse. One after
another signals indicated that the various experiments and observations
were set to work. Finally a red light went off and a green light
appeared on the panel. This meant that the bomb had been detonated.
The men on shipboard watched the enormous fireball through darkened
glasses. The firing crew, sealed off in the blockhouse, saw nothing. A
couple of long seconds and Graves’ voice announced over their radio: “It
was a good shot.” A quick estimate indicated 15 megatons.
Some more slow seconds and the expected ground shock arrived. It was
like a big earthquake. A bad moment passed. The blockhouse rocked but
held.
Another minute or so and the air shock passed over. One could hear the
hinges groan—but this was no longer frightening.
Would the water wave pour over the blockhouse? Everything was
watertight. After fifteen minutes a porthole was opened—no water came
in. The men in the blockhouse emerged to look at the drifting atomic
cloud.
While they watched, Jack Clark’s radiation instrument began to show a
reading. The firing crew was called back into the blockhouse. There, in
the lowest corner shielded by a considerable amount of sand, they were
safe. Outside, the evaporated and condensing coral came down in pellets
carrying more and more radioactivity.
In the meantime there was fallout on the ships too. The wind had
definitely veered after shot time. Quickly the activity was washed down.
No one got a dangerous exposure. But it was wiser to sail away. A
message was sent to the blockhouse: “We will come back for you in the
evening.”
After a little more than an hour the activity around the blockhouse
started slowly to decrease. The firing crew waited patiently inside
without communication, without light for the rest of the day.
Finally the ships came back. At sundown a helicopter went out to the
island using the last of daylight and allowing as much time as possible
for the activity to decay. Clark and his friends rushed out of the
blockhouse wrapped in sheets to stop the beta rays and keep off the
radioactive dust. They moved as fast as possible to avoid unnecessary
exposure.
It was a hard experience but they got no more than two roentgens—no more
reason to worry than if they had had a medical X-ray. Toward the east,
however, some people were in real trouble.
Six or seven hours after the shot the American servicemen on Rongerik
noticed a mistlike fallout of highly radioactive dust. The wind had
veered enough to carry the atomic cloud over the occupied islands of
Ailinginae, Rongelap, and Rongerik. In the anxious hours which followed
no one could say how much damage had been done.
The Americans on Rongerik had had some education in the dangers of
radioactivity. They washed themselves, put on extra clothes, and
remained inside of the aluminum huts as much as possible. These actions
helped to protect them against beta ray burns on the skin. The
Marshallese on Rongelap and Ailinginae knew nothing of the danger and
took no precautions. Many of them suffered quite severe skin burns.
All of the exposed persons were evacuated to Kwajalein as soon as the
Task Force facilities would permit. But it was not until a week or so
after the explosion that arrangements could be made for men with
radiation measuring instruments to tour the atolls and determine what
the levels of exposure had been.
On the southern tip of Rongerik they measured the activity and
calculated that the American servicemen had received approximately 78
roentgens. This was good news because a dosage of 50 to 100 roentgens is
not lethal and only in rare cases leads to any sickness. In any event
full recovery could be expected within a few days.
As they prowled around Rongerik atoll, the measuring crew found places
where the radiation levels had been much higher. At the northern end a
person would have received more than 200 roentgens.
On Ailinginae the measured values were comparable to those on Rongerik.
The estimated dosage to the Ailinginae people was 69 roentgens.
On Rongelap the situation was much worse. Measurements in the southern
part of the atoll showed that the Rongelap people had gotten a dose of
about 175 roentgens. Such a dose would not be fatal, but at least some
of the people would probably be sick.
The crew then went on to explore the rest of the atoll. As they moved
north, the dose levels rose higher and higher. In the middle of the
atoll, only ten or fifteen miles from the inhabited part, a person would
have received 400 roentgens of radiation. At this level he would have a
fifty-fifty chance of surviving.
On the northern tip of the atoll, about thirty miles away, the dose
would have been over a thousand roentgens. Such a dose means certain
death in less than a month.
The following table contains a summary of what happened:
_Number _Time of _Time of _Dose
of fallout evacuation (roentgens)_
persons_ after after shot
shot (hours)_
(hours)_
Rongelap 64 4 to 6 51 175
Ailinginae 18 4 to 6 58 69
Rongerik 28 7 32 78
Utirik 157 22 65 14
Fortunate Dragon 23 4 200
On Kwajalein the Marshallese were cared for and underwent medical
observation. As soon as possible their skin and hair were scrubbed with
soap and water. The coconut oil in their hair made decontamination
difficult.
During all this time the presence of the Japanese fishing boat in the
area was not even suspected. Not until two weeks after the explosion,
when the little boat returned to Yaizu harbor, did the world find out.
By this time the 23 fishermen were pretty sick. We do not know precisely
what dose the fishermen received, but the best guess is about 200
roentgens. Unhappily, one of the fishermen died, presumably from
complications associated with the exposure to radiation.[10] The other
22, however, are in good health and back at work.
Our medical information on the Marshallese islanders is complete. After
staying three months on Kwajalein they were removed to Majuro atoll,
where homes were built for them and where they have been cared for and
under continuous surveillance since the incident. Frequent and thorough
medical examinations have been conducted, handicapped somewhat by the
problem of communicating through an interpreter.
In the first twenty-four hours some of the victims complained of nausea,
fever, and stomach-ache. But these symptoms abated promptly in every
case without treatment. There was also some complaint of skin itching
and a burning sensation, but these symptoms also lasted only a couple of
days. Then followed a week or so of comfort and no complaint. After that
skin lesions and loss of hair began to occur.
Fifty to eighty per cent of the beta rays during the exposure period had
an average energy of 0.3 million electron-volts. Much of this energy was
stopped in the outer layer of skin, which is two thousandths of an inch
thick. The remainder of the beta rays had an average energy of 0.6
million electron-volts; these beta rays could easily penetrate into the
deeper layer of live skin. The most important fact, however, was that
clothing of any kind, even a thin cotton fabric, provided protection
against all the beta rays. Lesions developed only on the exposed parts
of the body and in a few other places such as the armpits and the
creases of the neck where material tends to accumulate. Bare feet were
especially bad. During the acute period some of the people walked on
their heels.
At the end of six months lost hair had grown out again unaltered in
texture and color, and the skin lesions had healed. Everyone appeared
healthy and normal with no apparent after effects.
There had been four pregnancies amongst Rongelap women at the time of
the exposure. One baby was born dead, but the other three were quite
normal. There was no evidence that the stillbirth had been due to
radiation effects. In fact the percentage of stillbirths amongst the
Rongelapese is normally high. Statistically, one in four is not an
unusual ratio.
Today, more than three years since the accident, all of the Marshallese
and American victims seem to be fully recovered. No malignancies or
leukemias have shown up, but these long-term effects are still being
carefully watched for by an AEC medical group.
All in all some serious but limited harm has been done. It was a close
shave. To see how close, one only needs to glance at the map below,
which shows the roentgen dosage for 48 hours of exposure. At the
southern tip of Rongelap, where the inhabitants lived, the dosage was
175 roentgens. But at the northern tip, less than thirty miles away, the
dosage was more than a thousand roentgens. If the wind had veered just a
little bit farther to the south, probably all of the people on
Ailinginae, Rongelap, and Rongerik would have been killed.
[Illustration: Dosage in First 48 Hours After Fallout Began]
This shot proved what had been argued for many years: that radioactivity
is not just an incidental part of an atomic explosion. The people on
Rongelap were far outside the area of danger from blast and thermal
effects. But they received a sizeable dose of radiation. In fact, a
person could have stood unprotected at a distance of thirty miles from
the explosion and been perfectly safe from the blast and thermal
radiation. But at that same distance in a downwind direction he would
have accumulated a lethal dose of radiation within a matter of minutes
after the fallout began.
Because of the radioactive fallout, the test sites must be located in
remote parts of the world. It would be desirable if sites could be found
which are so remote from populous areas that the tests could be
conducted without regard to the direction of the winds. Unfortunately
the bombs are too big and the planet is too small.
As a result the winds must be watched before every test; and the tests
must be delayed until the winds are favorable. What happened to the
Marshallese was an accident which might have been avoided if the winds
had been blowing more directly toward the north at shot time. Since this
accident the wind requirements for the tests have become far more
stringent, our knowledge of the danger has increased, and the rules of
safety have in all respects improved. Many large yield weapons have been
tested since March 1, 1954, but no other accidents have occurred. We can
be confident that accidents of this kind are now very improbable.
At the U. S. test site in Nevada there has been no instance of a major
fallout on a populated area. Probably the most worrisome situation which
has occurred there was in the spring of 1953 during the Upshot-Knothole
test series. After the ninth shot of the series the cloud drifted
eastward over St. George, Utah, a town of about 5000 people. Some
fallout occurred shortly before nine o’clock in the morning. About
nine-thirty AEC officials issued a warning advising the residents to
stay indoors. By noon the warning was withdrawn and people were allowed
to continue with their normal affairs. The incident left everyone a
little bit scared, but no one had received a radiation dose greater than
two or three roentgens.
We have been talking about the local fallout which occurs within a few
hundred miles of the test site. Not all the radioactivity which is made
in the explosion goes into this fallout. Some of it travels for really
long distances, not hundreds but actually thousands of miles from ground
zero. This part of the radioactivity is disseminated world-wide and
completely escapes the control of man. To be sure, by the time this
radioactivity is distributed over a large fraction of the earth’s
surface, the dosage levels of radiation are very tiny, less than a ten
thousandth of a roentgen for a megaton explosion. There is no danger
whatever that a person would die or even become mildly sick from this
amount of radiation. There is, however, the possibility of long-range
effects such as bone cancer, leukemia, and genetic mutation.
The world-wide danger is, of course, primarily due to the big bombs. The
little ones, such as are tested in Nevada, release about ten kilotons
(TNT equivalent) of fission energy. Some of the big ones in the Pacific
release a few megatons of fission energy. Since the amount of
radioactivity is proportional to the fission energy released, one big
bomb is equivalent to several hundred or possibly a thousand little
ones. Altogether in Nevada, to date, there have been only sixty or
seventy shots. It may be desirable to minimize the world-wide fallout
from the big shots in the Pacific. But for the little shots in Nevada,
it is probably more important to minimize the local fallout. How much
radioactivity goes into the local fallout, how much into the world-wide,
and how these relative amounts can be controlled, are the main topics
for the remainder of this chapter.
Not all the radioactivity which is made in the explosion contributes to
the fallout, either local or world-wide. Some of the radioactive fission
fragments (gamma emitters) have such short half-lives[11] that they
actually disintegrate before the bomb has disassembled. A great many
others disintegrate in the first few minutes while the atomic cloud is
rising. The energetic beta and gamma rays released in these early, rapid
disintegrations are stopped in short distances and merely add to the
havoc at the scene of the explosion.
[Illustration: _USAEC—Joint Office of Test Information_
1. A shallow underground explosion. The radioactivity and the ground
dirt are thoroughly mixed.]
[Illustration: _USAEC—Lookout Mountain Laboratory, USAF_
2. An atomic test tower—five hundred feet high.]
[Illustration: _USAEC_
3. A tower shot. Ground dirt rises along the stem, but very little
actually mixes with the fireball.]
[Illustration: _Elton P. Lord—USAEC_
4. An air shot—3,500 feet above ground. No dirt.]
[Illustration: 5. Leg bone of a three-month-old rabbit killed ten
minutes after injection of Sr⁸⁹. The darkened areas show where the
strontium has been deposited. Sr⁹⁰ and normal Sr⁸⁸ would be
deposited in the same places. It is an important fact that the
deposition is fairly uniform in the calcified portions of the bone.
_From a chapter by Vaughan, Tutt, and Kidman in the book_ Biological
Hazards of Atomic Energy, _edited by Haddow, published by Oxford
University Press, 1952_]
[Illustration: 6. Leg bone of a woman who died of radium poisoning.
The bright regions show where the radium has been deposited. Hot
spots are clearly visible.
_From an article, “The Late Effects of Internally Deposited
Radioactive Materials in Man,” by Aub et al., in_ Medicine—_a
professional journal, Vol. 31, No. 3, September, 1952_]
[Illustration: _USAEC—Knolls Atomic Power Laboratory_
7. Capsules of cobalt⁶⁰, shielded in a water tank. One hundred and
thirty million dollars’ worth of radium, twice the world’s present
supply, would be needed to equal the rays from this powerful gamma
source.]
[Illustration: _USAEC_
8. Cobalt irradiation.]
[Illustration: 1. The metallic element cobalt is machined into
wafers slightly larger than a dime.]
[Illustration: 2. The wafers are placed edge to edge in aluminum
containers, then inserted into an atomic furnace, or reactor.]
[Illustration: 3. Under bombardment of neutrons, the nuclei of the
cobalt atoms become excited and emit radiation, or rays.]
[Illustration: 4. After “cooking” in the reactor a certain time, the
cobalt is removed and placed in shielded containers for shipment.]
[Illustration: 5. The now radioactive cobalt goes from the Savannah
River Plant to Oak Ridge for re-shipment to medical centers all over
the country.]
[Illustration: 6. At medical centers, it is placed in tele-therapy
machines. Its powerful rays aid medical specialists in the fight
against cancer.]
[Illustration: _NTO—Lookout Mountain Laboratory Photo_
9. The smoke-ring cloud from the air-defense atomic weapon.]
[Illustration: _Wide World Photo_
10.]
[Illustration: _University of California Radiation Laboratory_
11. The streaks are condensation trails produced by charged
particles in a Wilson Cloud Chamber. They appear bright because the
chamber is illuminated and the condensation trails reflect light
just as an ordinary cloud does.]
[Illustration: _University of California Radiation Laboratory_
12. Another picture in the Wilson Cloud Chamber. A large number of
closely-spaced tracks form a cloud. (The tracks are curved because
of the presence of a magnetic field.)]
[Illustration: _USAEC—Argonne National Laboratory_
13. Cutaway section of a nuclear reactor. The heart of the reactor
is a small region at the center where the fission energy is
generated. Most of the weight and volume are needed for cooling
apparatus and shielding material to keep in nuclear radiation.]
For the radioactivity to affect areas at a large distance from the point
of the explosion, considerable time must elapse while the atomic cloud
rises and drifts in the horizontal winds. During this time more
disintegrations occur, due mainly to the short-lived nuclei. The rate at
which they occur keeps diminishing as the short-lived nuclei disappear.
Roughly speaking, the rate diminishes simply in proportion to the time.
More precisely, the rate drops somewhat faster, decreasing by a factor
of ten when the time increases by a factor of seven. A minute after the
explosion the activity is less than one per cent of what it is at a
second. After an hour it is less than one per cent of its value at a
minute. This law for the decrease in activity of fission products is, of
course, quite different from the simple law of radioactive decay. The
latter law applies to a single radioactive species. The fission products
consist at any instant of many different radioactive species. Each one
obeys the simple law of radioactive decay, but the totality follows a
different law.
It should be kept in mind that the product nucleus of a radioactive
disintegration may itself be radioactive with a different half-life. For
example, there is strontium⁹⁰. Only a small amount of this isotope is
made directly in the fission process. The fission process yields large
quantities of krypton⁹⁰, which decays with a half-life of one-half
minute into rubidium⁹⁰. The latter has a half-life of three minutes and
decays into strontium⁹⁰. This is how practically all of the strontium⁹⁰
is made in the explosion. Thus both the intensity and the nature of the
radioactivity keep changing with time.
These facts are important because they determine the magnitude and the
character of the danger when the radioactivity finally falls out of the
cloud and is deposited on the surface of the earth. Those radioactive
particles which disintegrate while still in the cloud need not worry us
since this radiation can have no effect on living organisms that may be
underneath. Provided that the cloud is more than a few hundred feet
above the ground, the beta and gamma rays released in these
disintegrations merely dissipate their energy in ionizing the air.
The time which the radioactive debris spends in the cloud depends most
critically on one factor: the proximity of the explosion to the ground
surface. The nature of the surface, whether it is soil or water, also
plays a role. If the explosion has taken place right on the ground, on a
soil surface, a lot of big, heavy dirt particles become incorporated
into the fireball and begin to fall under the action of gravity even
before the cloud stops rising. This fallout continues for a period of
several hours to perhaps a half day. At the same time some of the
radioactive fission products which have adhered to these dirt particles
also fall out. This is the origin of the so-called close-in or local
fallout, which extends for a distance downwind of the explosion of a few
miles to a few hundred miles, according to the energy of the bomb and
the strength of the winds. Approximately eighty per cent or so of all
the fission products are accounted for by this close-in fallout in the
case of a surface explosion. The shot on March 1, 1954 was of this
variety.
There are several possibilities for influencing the amount of close-in
fallout. One is to explode the bomb over deep water. In this case the
close-in fallout amounts to between thirty and fifty per cent. This is
because many of the water drops to which radioactive particles have
adhered evaporate before they hit the ground. Over shallow water,
however, if the fireball actually touches the bottom, the close-in
fallout resembles the case of a land explosion and is again about eighty
per cent or so. The close-in fallout for underground or underwater
explosions will be even higher than for the surface explosions. In fact
a really deep underground or underwater explosion would be completely
contained and no activity would be spread around.
Another possibility for reducing the close-in fallout is to detonate the
bomb on a tower so tall that the fireball cannot touch the surface. In
this case the amount of close-in fallout is reduced from eighty per cent
to approximately five per cent. Of course, it is not feasible to build
towers for really big bombs whose fireballs may be a mile or so in
diameter. In this case the bomb might be dropped from an airplane to
produce the same effect. The Hiroshima explosion was an example of an
air burst of a small bomb. The close-in fallout in that case was very
small. Such radiation sickness as occurred there was due to the direct
gamma rays and neutrons released in the explosion itself.
In the case of a near-surface explosion, where the fireball almost
touches the ground, the close-in fallout is also only about five per
cent. This is a somewhat surprising fact since in this case photographs
show large quantities of surface material being sucked up into the
cloud, just as they are in a true surface explosion.
This material certainly consists of large, heavy dirt particles which
subsequently fall out of the cloud. Yet most of them somehow fail to
come in contact with the radioactive fission products.
This peculiar phenomenon can be understood by looking at the details of
how the fireball rises. At first the central part of the fireball is
much hotter than the outer part and thus rises more rapidly. As it
rises, however, it cools and falls back around the outer part, creating
in this way a doughnut-shaped structure. The whole process is analogous
to the formation of an ordinary smoke ring. In most of the photographs
one sees, the doughnut is obscured by the cloud of water that forms, but
sometimes when the weather is particularly dry, it becomes perfectly
visible. During the rather orderly circulation of air through the hole,
the bomb debris and the dirt that has been sucked up remain separated.
(See pictures 1-4.)
The close-in fallout accounts for only a portion of the radioactivity,
ranging from less than a per cent for a high altitude shot to almost
complete deposition for some ground shots. For the world-wide fallout we
are interested in what happens to the remainder. This depends on how the
atomic cloud is carried by the upper winds for long distances. In this
connection it is important to distinguish between a big bomb and a
little bomb. It is also important to distinguish between the lower and
higher portions of the atmosphere called, respectively, the troposphere
and the stratosphere.
The atmosphere is heated by the sun in an indirect way. The sun’s rays
pass through air without warming it. They heat up instead the bottom of
the atmosphere, that is, the solid ground. The atmosphere is heated in
the same manner in which a boiling pot is heated on the kitchen range.
The heat is delivered from below and is carried in rising currents to
the top.
Only in the case of the atmosphere there is no sharp upper limit. The
currents rise to an altitude of thirty to fifty thousand feet, then turn
and descend. This boiling part of the atmosphere is called the
troposphere or region of heat. Above it there is less vertical motion.
The upper region is called the stratosphere or stratified region.
For a little bomb the atomic cloud stops rising before it reaches the
stratosphere. For a big bomb, above about a megaton of energy (a million
tons of TNT equivalent), the cloud pokes right into the stratosphere and
keeps going to a height of a hundred thousand feet or so.
The most important fact about the stratosphere is this: It has very
little weather. Most of the weather phenomena such as clouds, rain,
snow, fog, mist, etc., are confined to the lower portion of the
atmosphere, the troposphere. The stratosphere, however, contains
practically no water.
Now suppose a little bomb whose cloud will remain in the troposphere has
been exploded at one of the United States test sites. The Nevada test
site is at a latitude of 37°N and the Pacific test site at 12°N. In
these middle latitudes, in the troposphere, the winds blow mainly from
west to east with an average speed of approximately 20 miles an hour.
There will be a slight southerly or northerly motion on top of this. But
by and large the radioactive cloud will stay in a pretty narrow band
around the latitude at which the explosion took place.
After the first few hours, when the close-in fallout has dwindled, the
radioactive particles remaining in the cloud are too light and too fine
to fall any more under the action of gravity. At this point the weather
becomes important. Rain, fog, or mist captures the radioactive
particles, and returns them to the ground in the rainfall. This results
in the so-called tropospheric fallout.[12] The average time for this
fallout to occur is approximately two weeks to a month. During this
time, while staying more or less in the latitude of the explosion, the
radioactive particles may actually have encircled the earth.
The clouds of the big bombs rise high into the stratosphere. The winds
in the stratosphere do not blow so predominantly in a latitudinal
direction. What is more important, they stay in the stratosphere for
years, in which time the radioactivity is distributed to all areas of
the globe. The fallout from the big bombs is thus really world-wide.
The tropospheric fallout takes about a month. The stratospheric fallout
takes 5 to 10 years. The reason for this difference is the weather, or
rather the lack of it. In the stratosphere there is no rain or fog to
catch the radioactive particles and hence no effective mechanism for
producing the fallout. In fact, since the radioactive particles are too
fine to fall by gravity, they must simply wait until some turbulent
motions impel them downward back into the troposphere. This process
requires a long time.
That rainfall is the most important mechanism for producing the
world-wide fallout has been shown by examining the fallout in certain
dry regions of southern California and South America. In every case the
fallout was found to be considerably sub-normal. In one place in Chile,
where there is never any rain, the fallout was found to be only one per
cent of what might be expected on the basis of the average fallout at
the same latitude.
In regions having at least a few inches of rain per year, the fallout
tends to be proportional to the rainfall on the average. However, the
proportionality to rainfall depends on the nature of the weather so
that, say, twenty inches of rain in one part of the world may not give
as much fallout as the same amount of rain in other weather zones. We
are rapidly learning about this.
Having said what the age is of the various kinds of fallout, we are in a
position to say which radioactive species are still present when the
radioactivity is deposited on the ground. The close-in fallout, being
only a few hours old, still includes many short-lived isotopes, which
disintegrate before there is a possibility of ingestion or inhalation
into the body. Consequently the danger from the close-in fallout results
from external exposure, mainly to gamma radiation on the whole body, and
to a lesser extent to energetic beta rays on the skin. Clothes and
ordinary housing provide relatively little shielding against gamma rays.
Special protective shelters are needed. During a war if the enemy were
to bomb our cities with super-megaton weapons surface-burst, the
close-in fallout would be a far greater agent of destruction against an
unsheltered populace than either blast or thermal radiation.
In the stratospheric world-wide fallout, however, all of the short-lived
radioactivity has disappeared, since a period of many years has elapsed
since the explosion. After a year or so the only gamma emitter which is
left in appreciable quantity is cesium¹³⁷, with a half-life of 30 years.
Its gamma ray, however, is not very penetrating. In spite of this fact
cesium¹³⁷ is considered to be the second most important hazard for the
long term fallout. The first is strontium⁹⁰, which is a beta emitter
with a half-life of 28 years. This is long enough so that most of these
nuclei will still be present even after spending a long time in the
stratosphere. Since strontium is chemically similar to calcium, it
contaminates our foodstuffs and is easily incorporated into our bodies.
Once inside it stays for long periods of time, deposited in our bones.
We shall see in a later chapter how serious this danger may be.
The tropospheric fallout, and to a lesser extent, the stratospheric,
includes some other radioactive species besides cesium¹³⁷ and
strontium⁹⁰, and we shall discuss these in the next chapter. But by and
large they are of little consequence (with the possible exception of
iodine¹³¹) either because they are not easily absorbed in the body or
else because their radiation is not very energetic. The world-wide
hazard is thus narrowed down to just two isotopes, an internal beta
emitter and a weak gamma emitter.