The chain reaction, however, was still far from a reality. Niels Bohr and John Wheeler proved that a neutron could not cause fission in U²³⁸ unless its energy were greater than about one million electron-volts. When the neutrons are first made in the fission process, many of them do have energies greater than one million electron-volts. But before they can cause a fission, they usually make a few nonfission collisions with uranium nuclei, giving part of their energy to the nuclei and escaping with the remainder. The nuclei are then left with too little energy to undergo fission and the neutrons with too little energy to cause fissions in their next encounters. Thus too few neutrons reproduce themselves and no chain is possible.
Bohr and Wheeler suggested, however, that the rare isotope of uranium, U²³⁵, can undergo fission when any neutron, even a slow neutron, hits it. Thus a chain reaction is possible in U²³⁵. This was confirmed experimentally shortly afterwards by John Dunning and Alfred Drier and their co-workers at Columbia University.
Why the isotopes 235 and 238 behave so differently, is not difficult to understand. The 235 is more explosive and more prone to undergo fission than 238 because it is smaller and therefore its protons repel each other more strongly. More important still, when a neutron is captured by 235, it acquires a greater kinetic energy by virtue of the short-range nuclear attraction than a neutron acquires when it is captured by 238. This happens for the simple reason that nuclei tend to be more stable when they have an even number of neutrons (or protons) than when they have an odd number. U²³⁵, having an odd number of neutrons, is more eager to receive an additional neutron than 238, which already has an even number of neutrons. Consequently, the capture of a slow neutron by 235 almost always eventuates in the fission process; while in 238, the excess energy, introduced by theneutron, is merely ejected from the nucleus in the form of a gamma ray, and U²³⁸ becomes U²³⁹.
A chain reaction is possible in U²³⁵ , but it is necessary to separate this rare isotope from the abundant U²³⁸. The separation process is anything but simple since isotopes of the same element are chemically indistinguishable. Even the weight difference in this case, is little more than one per cent. Bohr rejected the idea of a large-scale separation with the remark: “You would have to turn the whole country into a factory.” Of course it is now a matter of history that the job was actually done under the Manhattan project during World War II. During the war Bohr (alias Nicholas Baker) again visited the United States and was shown the separation plants. He said: “You see I was right. Youdidturn the country into a factory.”
Natural uranium contains U²³⁵ in the ratio of 1 part to 139 of U²³⁸. It was hoped at first that this concentration would be sufficient to make a chain reaction, and that the expensive enrichment processes could be avoided. This seemed possible because at energies of a fraction of an electron-volt the neutrons are much more easily caught by U²³⁵ than by U²³⁸, which compensates for the low concentration. Actually neutrons are slowed down until their energy is as low as the energy of all other particles participating in the general agitation caused by the temperature. This energy is low enough for the purpose.
However, the neutrons are made in the fission process with an energy of about a million electron-volts. Before they slow down sufficiently, they must pass through a stage in which their energy is about 7 electron-volts. In the neighborhood of this energy, it happens that the U²³⁸ has an extremely high probability for capturing a neutron and changing into U²³⁹. Near some other energies, similar though smaller absorption hurdles must be passed. Therefore natural uranium by itselfcannot be used to make a chain reaction. In 1940, Fermi and Szilard, working now in the United States, found a way around this difficulty.
Their trick was to mix the natural uranium with a material whose nuclei are so lightweight that they suffer a big recoil when struck by a neutron and thus absorb a large fraction of the neutron energy. The neutron is thusmoderateddown to a low energy, rapidly and in big energy jumps, so that either it does not spend much time at the unfavorable energies where it can be caught by U²³⁸ or else it misses these energies altogether. By imbedding the uranium in lumps in the moderating material instead of making a homogeneous mixture of the two, the absorption can be circumvented even better.
For the purpose of making acontrolledchain reaction, one may use the method of enrichment, or the method of moderation, or both. But to produce aviolentchain reaction, an atomic bomb, only the enrichment method will work. The reason is that all the energy of the bomb must be generated in a time that is as short as the time it takes the bomb to fly apart, which is a fraction of a microsecond. If natural uranium were used, the reaction would be slow and sluggish and would be extinguished before a substantial fraction of the nuclei could have reacted.
It is interesting to consider that chain-reacting substances could have been obtained easily six billion years ago, before the U²³⁸ had time to decay and become a rare isotope. (The U²³⁵ was then about as abundant as U²³⁸.) A chemical separation would still have been necessary and so we do not need to imagine that chain-reacting mixtures accumulated spontaneously on the young earth.
On the other hand, six billion years from now U²³⁵ will have become so rare that it will be impossible to get a reactor going by moderation. At the same time the isotope separation will have become most expensive since the isotope to beseparated will be present in an abundance of less than 100 parts in a million. For those who like to worry about the distant future we should hasten to add that other methods of obtaining atomic energy will remain possible. And in any case there is good reason to believe that some stellar explosions produce fresh supplies of U²³⁵ which space merchants could undoubtedly make available.
As to our present terrestrial supplies: uranium, like other heavy elements, is quite rare. But the earth is divided into layers of which the topmost 10 miles, forming something of a slag or scum, contain quite a few rare compounds. In particular almost all of the uranium in our planet is conveniently collected right under our feet, for us to use as we see fit.
When an energetic particle moves through matter (living or nonliving), what happens is a question of chemistry. Chemistry is the subject that deals with the arrangement and rearrangement of electrons in atoms and molecules. A chemical rearrangement generally requires an energy in the neighborhood of a few electron-volts. (As we have seen, an electron-volt is the energy released when an electron moves through a potential of one volt, i.e., a little less than one per cent of the driving force in a standard electric outlet.) An energetic particle, such as might be emitted in a radioactive decay, typically has an energy of a few million electron-volts. Thus a single such particle has the potentiality of about a million chemical rearrangements.
Energetic particles may be charged or neutral, light or heavy, or electromagnetic in nature. Because of this diversity one might think there would be no common grounds for comparing the action on matter of different particles. Each particle might conceivably make its own inimitable variety of chemical rearrangements. Actually this is not the case.
Unlike some chemical poisons, which seek out specific molecules in our body, the energetic particles strike at whatever atoms or molecules happen to get in their way. Theyact, in this sense, like a sledge hammer. Their effects can be measured directly from the strength (or energy) of the blow. Which particle delivers the blow is of little consequence provided the same amount of energy is delivered and provided the same tissues are affected (in the case of living matter). After the blow, however, some specific chemical effects may occur. When water or some other molecule in the body is broken up by radiation, the fragments produced may themselves be chemical poisons and attack the biologically important large molecules in a secondary way. In fact, it seems probable that a considerable part of the radiation damage caused in living systems, both healthwise and genetically, occurs in this manner.
Although the energetic particles are all similar in their ultimate action on matter, namely in producing wholesale destruction of atoms and molecules, they differ somewhat in the way in which they bring about this destruction. Charged particles act in one way, gamma rays in another, and neutrons in still another. It is simplest to begin our discussion with the charged particles.
The most important charged particles are those connected with the natural background of radioactivity and cosmic rays, and the fission process. These include alpha rays, beta rays, mesons, and fission fragments. For review, a table of the weights and charges of these particles, as well as a few others, is shown. As usual, we have used the weight and charge of the proton as units.
If the fission fragments were completely stripped of their orbital electrons, they would have charges even greater than the values indicated in the table. The reader will recall that the average charge of the nucleus of the light fission fragment is 38, and of the heavy, 54. But such highly positively charged particles exert an enormous attraction on electrons. Some of these remain attached even during the fission process itself. As the fission products lose their speed during passage through matter, they pick up more electrons and gradually lose their charge.
When any of these energetic charged particles moves through matter, it interacts with electrons in the atoms. As a result of this interaction, the electrons may be dislodged from their usual states of motion. If the interaction is gentle—either because the charged particle passes the atom at a considerable distance or else because the particle is moving so rapidly that the interaction lasts for only a short time—the electron may be left undisturbed. If the interaction is more violent, however, the electron may be excited to a more energetic state of motion while still remaining in the same atom or molecule; or it may actually be ejected, ending up at some other atomic site. In this latter event the original atom is left with a residual positive charge and is said to beionized. At the same time the displaced electron is apt to unite with whatever atom or molecule happens to be nearby, creating in this way a negative ion. The whole process may be described as forming an ion pair. In the wake of the charged particle one finds, therefore, ionized and excited atoms and molecules. A rearrangement of atoms will now ensue which leads to new chemical compounds. The important thing for us is, however, that these chemical changes do not depend very much on the type of particle which produced the ionization; the proportion between ionization, excitation, and eventual chemical reaction remains more or less the same. Roughly speaking, the more ion pairs that areformed in living cells, the greater is the extent of biological damage.
To make an ion pair requires the expenditure of a certain amount of energy. It might seem as though this amount should depend crucially on the weight, charge, and energy of the particle, and also on the medium through which the particle is moving. This is not so. There is some dependence, of course, but only slight. Any charged particle, irrespective of its energy, moving in any medium—air, water, soil, or living tissue—creates ion pairs at the rate of about one per 32 electron-volts. A one-million-electron-volt particle produces about 30,000 ion pairs before losing all of its energy. (When it does lose its energy, if it is a positively charged particle, it will pick up enough electrons to become neutral. An alpha particle, for example, will become an ordinary helium atom; a proton will become an atom of hydrogen.)
We have said that two charged particles having the same energy, produce the same total number of ionizations. There is an important respect, however, in which charged particles of the same energy may differ. That is, in the density of ionization along their paths. In particular, the more slowly the particle is moving and the greater its charge, the more ionization and damage it will produce in a given distance. At the same time it will lose energy at a greater rate. If we compare two charged particles of the same energy plowing into matter, the one which leaves the deeper furrow will be stopped more quickly.
For a greater charge it is easy to understand that the electrical interaction is increased and hence each atomic electron is more strongly disturbed. If, on the other hand, the particle moves more slowly (which is usually the case if it is heavy) it spends a longer time in the neighborhood of the atomic electrons. The electrical interaction thus has a longer duration and is more effective in ejecting an electron. For this same reason the density of ionization along the path ofa particular charged particle should tend to become greater and greater as the particle slows down. Actually this tendency is opposed in the case of a fission fragment by the increased likelihood of the particle’s picking up electrons and reducing its charge. As a result, the ionization density for these fragments is rather uniform. If a heavily charged, slow particle moves through matter it leaves so many disturbed and disrupted molecules behind that now these molecules may react with each other. Therefore heavy ionization may lead to peculiar effects. Nevertheless all ionizing particles give rise to roughly similar chemical change and destruction.
Except for the beta rays, all the charged particles are very heavy compared to the electron. Consequently, as they move through matter and interact with the atomic electrons, their paths are not perceptibly deflected from the original direction. The beta rays, on the other hand, having the same weight as the atomic electrons, are appreciably affected by their encounters and are frequently forced to change direction. Their paths are thus winding and random.
Because the beta ray does not travel in a straight line, its ability to penetrate matter must not be measured by its total path length. As a rule of thumb, therangeof a beta particle, being the distance it travels along the line of its original direction, is about one half of its total path length. For heavier charged particles, however, no distinction need be made between range and actual distance traveled.
The most important fact about the ranges of charged particles is that they are small. An alpha particle, for instance, with a typical radioactive energy of a few million electron-volts, has a range in water (or living tissue) of a few thousandths of an inch. Such a particle could not penetrate a sheet of paper. A fission fragment, despite its great energy, is even less penetrating than the alpha particle. The proton has a somewhat greater range than the alpha particle. But the beta ray, because of its low weight, has by far the greatestrange of any of the charged particles. Even it, however, goes only a fraction of an inch in solid or liquid materials.
The following table shows the ranges (in inches) of some of the charged particles in air and water as a function of energy (in millions of electron-volts):
The table shows that charged particles travel only short distances in matter. For this reason these particles are not a serious external radiation hazard. The protons and the alpha rays are usually stopped by less than a foot of air. Ordinary clothing or even the outer layer of our skin (which is composed of nonliving cells) will stop them completely.
Beta rays are stopped by less than seventy feet of air or an inch or less of solid material. (Actually most of the beta rays produced in the fission process have energies less than a million electron-volts or so, and hence their ranges are even smaller.) Radioactive contamination of beta emitters directly on one’s clothes or body could cause trouble; but a good scrubbing soon after exposure will eliminate this problem. The interior of a house or building should be quite safe from any outside source of charged particles emitted by radioactive substances except possibly the most energetic beta rays. Only if the source of charged particles is inside the body so that in spite of their limited ranges the particles can find their way to sensitive tissues, is there any danger. In this case, as we shall see in a later chapter, the danger may be considerable.
Charged particles of one type stand pretty much by themselves. These are the mesons found in cosmic rays. These particles move as fast as energetic beta rays and, like the beta rays, carry unit charge. Their biological effects are thereforethe same as the biological effects of beta radiation, with one important difference. The cosmic ray mesons carry much more energy and therefore have a much greater range. Whereas the beta rays are stopped in the skin, the mesons can cause damage throughout the entire body. The mesons produce the same effects as a substance which emits beta radiation uniformly in the whole body. This fact is important. It puts us in the position to compare effects of man-made radioactivity with effects of the cosmic rays to which we are constantly exposed.
Not all the energy in cosmic rays is carried by mesons. We also find showers of electrons. These are almost the same as beta rays except that they have more energy and arrive frequently in fairly sizeable numbers traveling along nearly parallel tracks. Their effects, however, are the same as the effects of the mesons.
We have been talking now about the interactions between charged particles and the atomic electrons. No mention has been made of interactions between the charged particles and nuclei. Nuclear interactions do occur sometimes, but by and large they have only a negligible influence in slowing down the charged particle. They do affect, however, beta rays.
When a beta ray collides with a highly charged nucleus, the beta particle is violently deflected. The violence of this process is due to the heavy charge of the nucleus and the small mass of the beta particle. In the sudden change of velocity which occurs, part of the electric force field which surrounds the electron breaks loose; the result is high-frequency radiation called X-rays. The importance of such electromagnetic radiation is that it can penetrate more deeply into matter. In our bodies, for typical beta-ray energies, only a small part of the beta-ray energy is converted into X-rays. But in many radioactive processes gamma rays (which are physically the same as X-rays) are produced quite abundantly.These rays may carry as much or more energy than the beta rays.
Unlike charged particles, which constantly interact as they move through matter, gamma rays can go for long distances without having a single encounter. The actual distance depends on the energy of the gamma ray, the medium in which it moves, and pure chance. On the average, a one-million-volt gamma ray goes about six inches in water before anything at all happens to it. A four-million-volt gamma ray goes about a foot. In living matter the distances are approximately the same. Thus gamma rays from an external source can find their way deep inside the body.
Of course living matter is not injured by the mere presence of a gamma ray. There is a small probability that the gamma ray could go right through the body without a single encounter. If so, there would be no biological effect. An effect is produced only when the gamma ray interacts with the matter. There are three most important ways in which such an interaction may occur.
One way is simpleabsorptionof the gamma ray by one of the atomic electrons. The gamma ray disappears in this process, and the electron acquires all of its energy. A tiny bit of this energy is used for the electron to break its bond with the atom. The remainder goes into kinetic motion of the electron. The electron is now on the loose and can cause biological damage by exciting and ionizing other atomic electrons. In fact it is now the same thing which we used to call a beta ray.
A second way in which the gamma ray may interact with matter is byscattering. In this case the gamma ray does not disappear but merely loses a part of its energy to the atomic electron. Again the electron is free to cause biological damage, while the gamma ray goes on to its next encounter.
The third way requires that the gamma ray be near anucleus and have an energy greater than a million electron-volts. (Ordinary X-rays such as are used in medical practice are not energetic enough for this process to occur.) Under these conditions the gamma ray may disappear, with the simultaneous appearance of an electron and a positron. This is an example of the creation of matter out of pure energy. In accordance with the formula E = mc², a part of the gamma-ray energy is consumed in producing particles with definite masses. This amounts to about one million electron-volts. The remainder of the gamma-ray energy goes into kinetic motion of the two particles. Again biological damage results from the subsequent ionization due to the charged particles. After the positron has expended its kinetic energy in the ionization process, it will join with an electron in a disappearing act. The energy reappears in the form of two or three gamma rays (each having less energy than the original gamma ray).
In no case is the gamma ray directly responsible for any biological damage. The damage is always made by electrons (or positrons) to which the gamma ray has transferred some or all of its energy. But this only makes gamma rays the more dangerous. They can first penetrate to the sensitive tissues of the body, and then cause ionization.
We have already mentioned that X-rays are the same as gamma rays. The latter are produced by an excited nucleus, the former in the collision of an electron (or a beta ray) with a nucleus. The man-made X-rays are obtained by first accelerating a stream of electrons and then letting them impinge on a target containing highly charged nuclei.
The usefulness of X-rays is, of course, due to their power of penetration; that is the same property which renders X-rays dangerous. One can use X-rays to find out what happens to be inside the human body. But this cannot be done without producing some disruption and rearrangement in the tissues which lie in the path of the X-rays. The damage isof the same kind as that caused by radioactivity or cosmic rays.
The effects of neutrons on matter are rather similar to the effects of gamma rays. Like gamma rays, neutrons can travel long distances in matter without interacting. On the average, a million-volt neutron goes a few inches in water before having a collision of any kind. Also like the gamma rays, the neutrons are not themselves directly responsible for any biological damage. Being neutral, they interact only with the atomic nuclei to which they are strongly attracted. By far the most important of these interactions is with the nuclei of hydrogen. There are a great number of these in living tissue in the form of protein and water molecules.
The collisions with hydrogen nuclei (i.e., protons) are important because a large fraction of the neutron energy is transferred in the process. This happens because the neutron and the proton have very nearly the same weight. If the neutron hits a heavy nucleus, it loses only a small fraction of its energy in the impact.[9]After colliding with hydrogen or a heavier nucleus, the neutron continues on to other such collisions. The nucleus, however, being charged and energetic, now causes excitation and ionization of atomic electrons. Thus, like gamma rays, energetic neutrons are exceedingly dangerous, because they can first penetrate and then cause ionization.
Neutrons are dangerous even when they are not energetic. A nonenergetic neutron may react with nuclei of living matter in a number of ways of which two are particularly probable. Either the neutron may be captured by a proton to form a deuteron, in which case the excess energy will be emitted in the form of a two-million-volt gamma ray that will cause further damage. Or the neutron may react with a nucleus of nitrogen¹⁴ (abundantly present in living matter)to produce a nucleus of carbon¹⁴ and an energetic proton. Thus a nonenergetic neutron will have a biological effect equivalent to an energetic gamma ray, or to an energetic proton plus an energetic carbon¹⁴ ion.
In summary, all particles, charged or not, have a similar action on matter. Directly or indirectly, they produce excited atoms, molecules, and ion pairs. These processes always occur in practically the same proportions, and therefore the number of ion pairs formed can be used as a measure of the radiation effects. The more ion pairs produced in living matter, the greater the extent of biological damage. For this reason it is customary to describe radiation effects in terms of the number of ion pairs created per gram of living tissue in various parts of the body. Since each ion pair corresponds to an energy transfer of about 32 electron-volts, an alternative description may be given in terms of the amount of energy deposited. The unit in common usage for this purpose is theroentgen, which means specifically an energy equivalent to lifting the body (in which the radiation is deposited) by one twenty-fifth of an inch. This is equivalent to about 60 million million ion pairs in each ounce. It is less exact but more significant to say that one roentgen deposits in a cell of our body a few thousand ion pairs.
Of course the amount of ionization within individual cells is not a quantity that is easily measured. What one usually knows instead, is the roentgen dosage to a piece of tissue, which consists of many cells. If the charged particles inducing the ionization are electrons (as they are when the primary radiation is a beta ray or a gamma ray), the ionization will be distributed more or less uniformly among the cells in the affected neighborhood. If the charged particle is heavy—a proton or an alpha ray—the density of ionization which it produces is much greater, so that some cells receive a good many more ion pairs, while others nearby may receive none. For this reason it is sometimes important to specify not justhow many roentgens the tissue has been exposed to, but also which kind of radiation has been responsible.
In a later chapter we shall discuss the biological effects of various amounts of radiation. We may mention here, however, that 1000 roentgens of X-rays or gamma rays delivered more or less uniformly over the whole body of a human being in a time less than a few hours or so, will lead to almost certain death. And it is a remarkable fact that nature has not provided us with a warning. Radiation does not hurt. The greater is the need that we understand this process which affects our well-being but not our senses.
Testing of atomic explosives is usually carried out in beautiful surroundings. There is a good reason for this: the radioactive fallout.
Because of the fallout, the test site must be isolated. The presence of human population does not improve nature (with exceptions which are quite rare and the more notable). Also, to keep the site clean, tests must be carried out in the absence of rain. Therefore, at the site one usually finds sunshine and solitude.
For the participants the beauty of nature forms the back-drop to preparations of experiments which are difficult and exciting to everyone involved. At the end, the atomic explosion is always dwarfed by its setting. But the work that culminates in the detonation is rewarded by something quite different from a flash and a bang.
The really important results of a test consist in marks on photographic plates. Most of the apparatus that produced the plates has been destroyed in the explosion. But enough is saved so that one can conclude what has happened in the short fractions of a second that pass between the pressing ofthe button and the knowledge in the observer: this was it. In those fractions of a second another stone was added to the structure which we may call astrophysical engineering. What happens and what is observed in nuclear explosions are closely related to the behavior of matter in the interiors of the stars.
The details of the nuclear explosion cannot be described here for three reasons. First, the details are secret. Second, the size of this book and the forbearance of the reader set limitations. And third, we understand only a small part of the process. Within these limitations, this is what happens:
The actual nuclear reaction takes only a fraction of a microsecond (one microsecond = one millionth of a second). All the energy of the bomb is released in this short period. At the end of this period, the main body of the nuclear material is moving apart at a rapid rate and by this motion further nuclear reactions are stopped. In addition to the more or less orderly outward motion, considerable portions of the energy are found in the disorderly temperature motion, which has stripped most of the electrons off the nuclei and has transformed the atoms into a freely and chaotically moving assembly of charged particles. By this time many of the original nuclei have been transformed into nuclei of radioactive species, partly by the fission process and partly by the capture of neutrons in all sorts of atoms which had been originally present in the bomb materials.
Still another portion of the energy is present as electromagnetic radiation. This radiation closely resembles light except that it is of shorter wave length and is therefore not actually visible; but it can be absorbed and re-emitted by all sorts of materials, and is in a violent exchange of energy with the exploded bomb fragments.
All this perturbation spreads outward from the region where the nuclear reaction has taken place into the surrounding components of the bomb. During the outward spread,more atoms and more space get engulfed. The agitation and the radiation become somewhat less hot.
This hot region tends to be limited by a sharply defined boundary which is called a shock front and which is moving outward at a speed of several hundred miles per second. This front finally reaches the limits of the more or less dense material in which the whole bomb structure was originally encased. It then breaks through into the surrounding air. The air heats up in the immediate vicinity, and this is the beginning of the fireball.
From this point on, the energy spreads due to the push of the high-temperature air. A sharp shock front forms and keeps moving outward at a speed greatly surpassing ordinary sound speed. The radioactive material is contained within this hot and expanding sphere.
As the fireball expands and the temperature falls, more and more visible radiation is emitted. Actually, the surface is growing less brilliant as the structure expands and cools, but its greater size and the longer time that is available for the emission of radiation overcome this disadvantage. Finally, at a radius of perhaps a few hundred feet for a small bomb and a mile for a big one, the fireball expansion halts. This happens because the shock front is no longer strong enough to make the air luminous. The luminosity not only stops advancing but is actually partly dimmed by absorbing substances formed by the badly mistreated air molecules.
The time which has elapsed to reach this stage of the explosion depends on the bomb energy. If two explosions are compared, and the bigger one has a thousand times the explosive power of the smaller one, then the time needed to reach the extreme expansion of the fireball will be approximately ten times greater for the more violent event. In any case, a reasonably close observer has to use strongly absorbing glasses during this time if he is not to be blinded. For small bombs, the expansion of the fireball is too short to register.For the really big ones, you can see the expansion developing and you wonder when it will stop. To the unprotected eye the small bombs are almost as dangerous as the big ones, because there is not enough time to blink.
In the meantime, the shock wave, now separated from the fireball, travels through the air and carries with it a considerable fraction of the original explosive power. An important part of the damage which a bomb can cause is due to this invisible pressure wave which spreads with a speed close to that of sound, over a distance of miles, before it settles down into harmless rumbling.
The rest of the energy is still sitting in the fireball near the point where the explosion occurred and the hot air now commences to ascend, breaking up into a turbulent mushroom as it goes. The hot interior portions get occasionally exposed and the object gives the appearance of an enormous flaming mass, at least when seen in a motion picture which slows down the action and reduces the size. The radiant tongues are too big and too fast for any ordinary flames.
During this stage the display gradually pales sufficiently so that it can be viewed with the naked eye. The originally hot masses have now emitted enough energy in the form of light and mixed with a sufficiently great mass of cool air that they no longer glow violently. This mass of central and rising gas contains practically all the radioactivity, not only that originally formed in the explosion but also some produced by neutrons which leaked out of the bomb and got captured by a variety of nuclei in the air, water, or ground within the neighborhood.
And now the aftermath of the explosion is turning into a display growing rapidly and yet in a measured manner so that not only the eye of the observer but his mind and his feelings can follow the events. The mushroom which has been formed by the first updraft develops into a column with more and more agitated boiling masses added on the top andwith slanting skirts of a snowy appearance descending toward the sides. What is this white mass that looks just like a cloud of peculiar shape and that has grown up to the high heavens (or as the meteorologists call it: the stratosphere) in a few minutes before our eyes?
It is actually a cloud: a collection of droplets of water too small to turn into rain but big enough to reflect the white light of the sun. And it is formed in a similar way to the cumulus clouds of a thunderstorm. Indeed it is a beautiful example of a many-storied castle of cumulus upon cumulus. But strangely enough what makes this cloud is not the heat of the bomb. It is the cooling of the air masses that have been sucked in as the remnants of the fireball rush upward like a giant balloon. Under this balloon air is drawn upward. As this air rises, it cools and water vapor contained in it condenses into droplets: precisely the same mechanism which gives rise to thunderheads on a hot summer day.
The white skirts (which are not always present) do not consist of any material that is falling out of the cloud. On the contrary, a moist layer of air is sucked up into the cloud from the side and the droplets which form in this layer give rise to a cloud-sheet with the appearance of a skirt.
In big bombs near the top a particularly smooth and white cap is seen. This is again condensation, not into droplets but into fine crystals of ice. In some explosions more than one of these caps are present.
Finally the cloud has gained its full height. Depending on the size of the bomb it may have grown to 20,000 feet, to 100,000 feet or more. Then the wind blowing at various levels in various directions tears the structure apart sweeping some of it to the east, some to the west. The radioactive debris in the cloud has started on its travel.
What this radioactivity will do, how it can affect living beings, how dangerous it actually is, we shall discuss in succeedingchapters. But one thing is clear and remains present in the minds of all participants in an atomic test: The danger of the test is nothing compared to the catastrophe that may occur if great numbers of these weapons should be used in an unrestricted nuclear war.
It has been frequently asserted that our present atomic explosives can wipe out the cities and industries of the greatest countries. Why continue with further development and testing?
The answer is simple: The main purpose of a war is not to destroy the enemy’s civilian centers but rather to defeat his armed forces, and for this purpose we need flexible refined weapons of all kinds and sizes. We also need weapons with which to defend our own cities. We need weapons with which to defend our allies and in particular we need weapons which will do their job against an aggressor and will do the least possible damage to the innocent bystander.
In this last respect, in particular, notable progress has been made. We are developing clean weapons which are effective by their blast and their heat, but which produce little radioactivity. Of course, blast and heat will do damage only near the point of detonation. Radioactivity may be carried by the winds and escape the control of man to a considerable extent.
It is clear that war is and always has been terrible. We refuse to believe that wars will always be with us but we cannot disregard the danger of war as long as the world is half free and half slave.
An atomic war, limited or even unlimited, need not be connected with more suffering than past wars. However, such a war would probably be more violent and it would be shorter.
The story is told that a war which turned out to be perhaps the most dreadful in the history of mankind was started with this message: “Thou hast chosen war. That will happenwhich will happen and what is to be we know not. God alone knows.” Perhaps the only possible path for a free people is to be well prepared for war but never to choose war as long as the choice is free. But what will happen God alone knows.
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 servicemenlived and worked in aluminum huts. Their main job was to collect weather data.
Map of the Marshall Islands
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, theevaporated 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.