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From the 1946 book, Dawn Over Zero.
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I HAD the privilege of spending the entire day on Sunday, July 15, 1945, and all those hours up to 5.30 of the morning of July 16, in the company of Professor Chadwick. I rode with him on the first leg of our journey from Los Alamos to Albuquerque, where we spent the day waiting for the rest of our caravan. I stood next to him when the great moment for the neutron arrived.
Never before in history had any man lived to see his own discovery materialize itself with such telling effect on the destiny of man, for the immediate present and all the generations to come. The infinitesimal neutron, to which the world paid little attention when its discovery was first announced, had cast its shadow over the entire earth and its inhabitants.
On that ride, as I sat there beside the neutron’s discoverer, this shadow sat between us. We conversed with it, largely through silences interrupted here and there by words.
He became more silent as the agonizing hours dragged on, and as the zero hour approached, I saw him standing there alone, looking off to the east, in the manner of a man awaiting a new apocalypse.
As the great moment came and the primitive pandemonium broke all around him, he was still standing there, rooted to the earth, looking (to use the phrase of Dr. Charles A. Thomas) “very inanimate” Then a human spirit, electrified by the great burst of neutrons, actuated a hand and made it descend with a resounding slap on Dr. Chadwick’s slight back. He grunted, leaped lightly into the air, and was still again.
A few miles to the east of us, in a lead-lined Sherman tank, sat Dr. Fermi, riding in the new no-man’s-land, scorched as no earth had ever been scorched before. He was measuring the radiations on the surface of this newly created jadelike earth, the ocean of invisible light that had just been liberated by the neutrons.
To Dr. Chadwick and Dr. Fermi, more than to any other men present that morning in the New Mexico desert, the great burst of light and the earth’s thunder represented a consummation, the greatest materialization ever granted to a mortal of what had started as a pure idea.
The light and the thunder were both the result of the neutron cracking open the atom of uranium, or the atoms of an element derived from uranium, and Dr. Fermi had been the first to shoot neutrons at uranium atoms, and the first to create new elements beyond uranium.
The story began at the University of Rome in 1934, when Dr. Fermi was thirty-three years old. He and his colleagues had been industriously bombarding most of the elements with neutrons as projectiles and observed many strange Alice-Through-the-Looking-Glass events.
As was expected, the neutron, because it has no electric charge, penetrated the barrier of the nucleus and frequently lodged there. Since the neutron is about equal in mass to the proton, the addition of one neutron meant an increase in the atomic weight of the element by one unit.
But the elements in which the neutrons lodged, or, as the physicists say, the nuclei that captured a neutron, became unbalanced because the extra neutron gave them more energy than they could hold. To regain their stability, the nuclei began throwing off that extra energy in the form of radiations.
Here came surprise number one. The radiations came out in the form of electrons, particles with a negative charge of electricity. But by that time it had been established that the nucleus consisted of only protons and neutrons. There are no electrons as such in the nucleus.
Where could these electrons have come from?
Then came surprise number two. On examining the elements bombarded with the neutrons it was found that many of them were no longer the same elements, as their nuclei contained an extra proton.
Where could this proton have come from?
There could be only one logical explanation of both mysteries. Both the electron and the proton came from the neutron.
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|Lead lined tank in which Enrico Fermi measured the radiations after the Trinity test blast.|
At least some neutrons, if not all, these experiments seemed to indicate, are in a state of electrical neutrality because they carry both a positive and a negative electrical charge, which balance each other.
Under certain conditions a neutron could be made to vibrate so violently that its electron was broken off and bounced out of the nucleus. This left the neutron positively charged; in other words, it became a proton.
If that is the case, Fermi reasoned, what would happen if a neutron were fired into the nucleus of a uranium atom? Uranium is the last and heaviest element made by nature. It has 92 protons and 146 neutrons in its nucleus, with a total atomic weight of 238. The shooting of an extra neutron into its nucleus would raise its atomic weight to 239.
But if one of its neutrons parted with its electron, it would become a proton, and the nucleus would have 93 instead of 92 protons. That would mean the creation of an entirely new element.
Scientists are supposedly individuals who leave their emotions behind when they enter their laboratories, but that day when Dr. Fermi and his associates, all young men, first fired neutrons at uranium was tense with expectations and high hopes. It was an adventure into the ultimate, a journey to a hitherto unknown continent of matter, bound to push back the frontiers of knowledge no matter what happened.
What they were actually doing, without knowing it, was making miniature atomic bombs. For that is what the atomic bomb is—a species of uranium, or uranium derivative, violently exploded by an avalanche of neutrons.
They were also building a model atomic power plant, for the explosion of that species of uranium, whose existence was not known at the time though it was present in the samples of uranium they were using, is accompanied by the liberation of enormous amounts of atomic energy.
Without knowing it, they were even laying the foundation for controlling the atomic power plant of the future, for by that time Dr. Fermi had developed the method of taming the neutron by slowing it down, a method essential for producing atomic power under control.
But none of them even remotely suspected any of this at the time, and, as was learned later, it was well that nature kept her secret a little while longer.
Like Columbus these young pioneers discovered a great deal more than they had set out to find, and like him they were unaware of the vastness and riches of the new continent they had opened up. But what they did find, and other things they believed at the time they had found, created one of the greatest scientific sensations of the day. It made the name of Enrico Fermi internationally known and won for him the Nobel prize for physics in 1938.
The results they observed far exceeded even their most optimistic expectations. The neutrons fired at element 92 seemed to produce not only element 93 but, strange to behold, also element 94. Though this was somewhat more than they had bargained for, it fitted well into the scheme of things.
But then something seemed to go berserk, as though old Mother Nature had gone on a mad spree. For closer examination revealed that the mating of neutrons with uranium had given birth to at least a score of “illegitimate” substances that came popping out like genii from a magic bottle. It was as though an elemental bull had gone wild in the cosmic china shop.
For a time it was believed that three of these “illegitimates” might be elements 95, 96, and 97, but it soon became evident that none of them could be fitted properly into any spot in the restricted spaces of the periodic table in the vicinity of uranium, where they properly belonged if they were really elements beyond uranium. And, according to the best knowledge available at the time, they could not be anything else.
Actually, as was learned five years later, they were the fragments of uranium atoms that had been split by the neutrons, forming radioactive varieties of “legitimate” elements much lighter than uranium, and occupying spots 36 to 56 spaces farther back than uranium in the periodic table. But that was too revolutionary a concept even to think about in the antediluvian period of the atomic age, A.D. 1934.
The trans-uranium elements born through the fertilization of uranium with neutrons became the most tantalizing mystery among atom-smashers the world over. For five years all the major nuclear physics laboratories in many lands worked on its solution. And the mystery seemed to grow more profound as more facts came to the surface.
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|Enrico Fermi (right) with Rome University students, 1930’s.|
The stage for the final solution of the mystery was not set until the autumn of 1938, at about the time of the Pact of Munich. The scene was the Kaiser Wilhelm Institute in Berlin. The dramatis personae: Otto Hahn, Fritz Strassmann, and Lise Meitner.
By that time method had been worked out for the chemical analysis of the minute bits of matter produced by modern alchemy in the course of transmuting one atomic nucleus into another or in the creation of artificially radioactive elements, Drs. Hahn, Strassmann, and Meitner began by repeating the original Fermi experiment, bombarding uranium with neutrons. With the new technique for analyzing the products of such bombardment they proceeded on the tedious, and until then fruitless, task of identifying the mysterious substance for which no proper home could be found in the infinite cosmos.
At last there was a glimmer of hope, a ray of recognition. One of the illegitimate offspring of the uranium-neutron union gave off gamma rays very similar to those of radium, though much more powerful.
One way in which this could happen would be if the neutron caused the bombarded uranium atom to emit two alpha particles, or a total of four protons and four neutrons.
This would mean an unheard-of phenomenon—that the original uranium nucleus had lost four of its 92 protons and four of its 146 neutrons, leaving a nucleus of 88 protons and 142 neutrons, or four more neutrons than the normal radium nucleus. These four extra neutrons would make the nucleus much more unstable and would account for its radioactivity being greater than that of normal radium.
Now, to separate radium from other elements the normal procedure is through the addition of barium, element No. 56.
The barium combines chemically with the radium in the mixture of other substances and carries it down with it in the form of a precipitate. The radium-barium precipitate is then separated from the solution, and the barium is then in turn separated from the radium by a specific chemical process.
The Hahn-Strassmann-Mejtner team therefore proceeded in the usual way to isolate the substance they believed to be a new form of radium by adding barium to the mixture of strange uranium progeny. As they had expected, the barium combined with the new radium-like substance and precipitated it out of the solution containing the other substances.
But when they came to separate the “radium” from the barium by the usual standard methods, they had the surprise of their scientific lives—in fact, one of the greatest surprises in the entire history of science. For the barium refused to be separated from the so-called “radium” by any method known to chemistry. The two clung together in an indissoluble union.
There could be only one inescapable conclusion. The fact that the barium could not be separated could mean only that the mysterious substance that had been believed to be a form of radium could not be anything but barium, a radioactive barium that had been present before they added the normal barium to serve as the carrier.
They had believed they were about to solve a mystery and found themselves confronted with an even greater mystery. Where could this super-radioactive barium have come from? It was like observing a chicken hatch from a duck’s egg.
Before the mystery could be solved, Lise Meitner found herself forced to leave Germany as a “non-Aryan.” She went first, via Holland, to Copenhagen, a broken woman, her most important lifework interrupted at its climactic stage.
She could not get the mystery of the barium out of her mind. And the more she thought about it, the more convinced she became that this submicroscopic bit of barium held the key to one of the greatest discoveries of all time. On reaching Copenhagen she communicated at once with another fellow exile, Dr. O. R. Frisch, who had found a haven in the physics laboratory of Niels Bohr, pioneer explorer of the atom’s structure.
With Dr. Frisch she discussed her daring hypothesis as to the origin of the barium, too revolutionary to be accepted by her conservative German colleagues. Meantime, on January 6, 1939, Drs. Hahn and Strassmann reported in a German scientific publication the strange phenomenon they had observed, stating that while they could not doubt the presence of the radioactive barium, they could not at the time offer any explanation of its origin.
To Dr. Meitner there could be only one possible explanation, and Dr. Frisch was quick to realize the enormous implications of that hypothesis. As so often happens, the explanation was very simple. Barium has an atomic weight nearly half that of uranium. Ergo, the barium must have resulted from the neutron’s splitting the uranium atom into two nearly equal parts.
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|Lise Meitner and Otto Hahn engaged in research. Lise was forced to leave Germany in 1938 in the middle their super-radioactive barium research.|
It seemed impossible, inconceivable, contrary to all accepted concepts of the nature of the forces that held the atom together. These forces were known to be so vast that no power at the disposal of man was deemed great enough to overcome them, any more than one could conceive of a power to hold back the tides or to move the earth itself.
Could it be that the uranium atom was an exception, that it, the last of the elements, stood on the edge of the cosmos, as it were, so that, like a massive rock precariously balanced on the rim of a precipice, it could be pushed off with very little force, splitting in twain as it struck bottom?
The presence of the barium seemed to indicate that this was so. And if it was, they knew, the splitting should be accompanied by the release of tremendous amounts of energy.
They also knew that this energy should impart itself, in the form of kinetic energy (energy of motion), to the uranium fragments, so that they would come flying out with tremendous speeds.
This suggested other experiments that would provide positive proof, or disproof, of their hypothesis. Through these experiments the exact amounts of the energy liberated could be measured, and these amounts, if high enough, would tell for certain whether the uranium atom had been split.
When Dr. Meitner arrived in Copenhagen, Dr. Bohr, who four years later was to help in the production of the atomic bomb, was preparing to leave for the United States, intending to spend several months at the Institute for Advanced Study, in Princeton, New Jersey. He was particularly anxious to discuss some abstract problems with Einstein, who, since his exile from Nazi Germany, had been a member of the faculty of the institute.
But the news brought by Dr. Meitner, and the conferences during which she and Dr. Frisch had outlined to him their guess about the splitting of the uranium atom by a neutron, a process they had christened uranium “fission,” had provided Dr. Bohr with problems much more concrete to discuss with the man who had been first to fathom the depths of the ocean of energy within the atom.
When he arrived in the United States on January 16, 1939, he found a cable from Drs. Meitner and Frisch waiting for him. They had performed the experiments and had obtained conclusive proof of the correctness of their guess. The uranium atom had been split.
The two fragments flew apart with the unheard-of energies of 100 million volts each, liberating a total of 200 million volts. In simple terms this means that a source of energy had been tapped three million times greater than that liberated in the burning of coal and twenty million times more powerful as an explosive than TNT.
Immediately on his arrival at Princeton Dr. Bohr communicated the news to his former student Dr. John A. Wheeler and to other physicists at the university. From them the news spread quietly by word of mouth to neighboring laboratories.
Without anyone realizing it at the time, the work on the atomic bomb got under way.
By a historic coincidence, Dr. Fermi, fleeing Fascist Italy, arrived in the United States almost simultaneously with Dr. Bohr and had accepted a position in the physics laboratories of Columbia University. Since it was he who had initiated the chain of events that led eventually to the fission of uranium, it was natural that he should be among the first to be informed about it.
Dr. Fermi called a conference of the Columbia atom-smashers, headed by Dr. John R. Dunning, and informed them of the big news. Plans were laid for a series of experiments to obtain an independent check on the Meitner-Frisch results. A tentative program for further exploration was outlined.
The Columbia experiments were designed to reveal the heavy electrical (ionization) pulses that would be from the flying fragments of the split uranium atoms.
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|Cyclotron used by the Columbia University "atom-smashers," was operational in 1939.|
These pulses register the energy of the flying particles on an instrument known as the oscilloscope, which may be described as a species of atomic thermometer.
The energy registers itself through lines in the oscilloscopes, in a manner similar to the rise of the mercury column in the thermometer with the rise of temperature.
Late on Wednesday night, January 25, the experiments at Columbia were concluded after a hectic twenty-four hours in the vaultlike laboratory resembling a medieval alchemist’s chamber.
A tired group of scientists stood anxiously in front of the oscilloscope. At a signal someone pushed a button. Up jumped the line in the atomic thermometer to the staggering height registering 200 million volts.
Before the experiments were concluded Dr. Fermi had left New York to attend a conference on theoretical physics, which opened in Washington, D. C., January 26, under the auspices of George Washington University and the Carnegie Institution of Washington.
Dr. Fermi sat next to Dr. Bohr when the latter rose to tell the assembly the news of the smashing of the uranium atom. And it did not take long for the significance of what Dr. Bohr was reporting in low, measured tones to be realized by the group of young atom-smashers at the conference.
One by one they were seen leaving the lecture hall. The long-distance lines to research laboratories in various parts of the country became exceedingly busy. It was almost as though the young nuclear physicists, most of whom later played important roles on the Atomic Bomb Project, had a premonition of the need for haste. There was no time to lose.
Without their being aware of it, the race was on.
In the famous Radiation Laboratory of the University of California, birthplace of the atom-smashing cyclotron, young Dr. Philip H. Abelson had also been working on the mystery of the strange brood of elements given birth by uranium bombarded with neutrons. He had devised an ingenious X-ray “microscope” with which he was hoping to identify these elements by telltale lines specific for each element.
Like the Hahn-Strassmann-Meitner team in Germany, Abelson was amazed to find X-ray spectrum lines corresponding to elements with only about half the atomic weight of uranium. Bewildered, he confided his difficulties to a colleague, young Dr. Luiz W. Alvarez, who was later to become, along with Dr. Abelson, one of the leading contributors to the Atomic Bomb Project.
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|Cyclotron completed in 1939 at the Radiation Laboratory of the University of California. Dr. Luiz Alvarez (among many other renown West Coast scientists) is at the top.|
On the morning after the conference at Washington, Dr. Alvarez was in the midst of a haircut at the Stevens Union on the University of California campus when his eye fell on a newspaper account of the discovery of uranium fission. Out of the barber’s chair dashed Professor Alvarez clutching the newspaper in his hand.
His blond hair only half cut, he rushed up the hill to the Radiation Laboratory. The much startled Dr. Abelson quickly forgot the strange appearance of his colleague, when he heard what he had to tell.
The discovery that uranium could be split and made to yield energies millions of times greater than those released by ordinary chemical processes, great and revolutionary as that was, would nevertheless not have been of any practical importance were it not for another, equally vital fundamental discovery that came almost simultaneously with the discovery of fission. It made it possible to utilize the vast energy released, both as a source of tremendous power and as the most devastating explosive the world had ever known.
This was the discovery that when the uranium atom splits, a few of the neutrons in its nucleus are set free. Since uranium could be split only with neutrons, this opened the staggering possibility that once the first uranium atom was split, it would liberate other neutrons that would split other uranium atoms, which in turn would liberate more neutrons.
Thus what is known as a "chain reaction" would be set off similar to the chemical chain reaction occurring in lighting an ordinary fire with a match.
The discovery of fission may be compared to the discovery by ancient man of how to produce a spark. The second discovery, that in the process of uranium fission a number of extra neutrons are emitted, may be compared to the prehistoric discovery that a spark can be used for the purpose of starting a fire that will keep going without the need of any further sparks.
It was this discovery, not that of fission itself, that made the atomic bomb possible and opened the way to the utilization of atomic energy on a practical scale, for it gave us for the first time a practically inexhaustible source of neutrons at the expenditure of hardly any energy at all.
Just as lighting a match starts the chemical chain reaction of the wood or coal fire, in which the energy in the outer electronic shell of the atom is liberated, so the neutron serves as a cosmic match, as it were, for starting the chain reaction that produces a cosmic fire in which the energy locked up in the inner core of the atom is set free.
Prior to this discovery of the fountain of neutrons gushing from split uranium atoms, neutrons could be obtained only on a very small scale at the expenditure of relatively enormous amounts of energy. This was done by allowing either the alpha particles naturally emitted from radium at high speeds, or a stream of charged atomic particles accelerated by a giant cyclotron, to strike a beryllium target.
By these methods only a few out of the billions of bullets fired would hit a bull’s-eye in the beryllium atom and thus knock a few neutrons out of its nucleus. It was like firing a million bullets to bring down one duck.
This was why before the discovery of fission, with its liberation of a self-multiplying stream of neutrons, no physicist believed that atomic energy could ever be utilized on a practical scale.
In fact, only a short time before the discovery of fission became known, Einstein assured me that it could never be done. “We are,” he told me, “poor marksmen shooting at birds in the dark in a country where there are very few birds.”
A few months later Einstein was urging President Roosevelt to give serious consideration to the possibilities of uranium as the most destructive weapon ever conceived by man. We were no longer “poor marksmen.”
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|Einstein and Dr. Leo Szilard compose a letter to President Roosevelt delivered in October 1939.|
In fact, we couldn’t miss. For it was realized at once that to start the chain reaction it would not even be necessary to produce the initial neutron to serve as the cosmic match. There are always a few stray neutrons present all around as the result of violent collisions between atoms in the air and highly penetrating cosmic-ray particles that constantly enter the atmosphere from outer space, or particles emitted from radioactive substances floating about everywhere in minute amounts.
Neutrons thus liberated by such collisions immediately enter the nucleus of the nearest atom. Such a free neutron would serve as the match for automatically starting the cosmic fire.
But, one may ask, if that is the case, why does not this phenomenon occur spontaneously in nature? In fact, why didn’t the uranium in the earth’s crust blow up long ago, even before man appeared on the earth?
The answer is that the chain reaction can take place only under special conditions that do not exist in nature. One of these is that the uranium must be of an extremely high degree of purity, such as had never before been attained with any metal.
The possibility that a geyser of neutrons would come gushing out on the splitting of the uranium atom suggested itself at once to Dr. Leo Szilard, Dr. Fermi, and others as soon as the fission phenomenon was discovered.
The nuclei of the elements in the middle of the periodic table, resulting from the splitting of the uranium atom of atomic weight 238 (92 protons and 146 neutrons), can hold only a certain number of neutrons in excess of their protons.
Any extra neutrons beyond that maximum would cause the nucleus to become unstable, and in such cases nature begins taking steps to regain her balance. This she may do in two ways:
• Some of the extra neutrons may give up their electrons, thus increasing the number of protons to the point where the proper ratio between protons and neutrons in the particular nucleus is restored.
• Or she may take more drastic steps by evicting some of the intruding neutrons bodily from her nucleus. She may also restore her balance by a combination of these two methods.
In the case of such an unprecedented cosmic catastrophe as the splitting of the uranium atom, mere theoretical considerations suggested that some neutrons would be set free in the act, either at the very instant of fission or soon thereafter. Since the first fragment resulting from fission to be identified was barium, of atomic number 56, the other half would obviously be krypton, of atomic number 36.
Now, the heaviest isotope of barium found in nature has an atomic mass of 138, while the heaviest natural krypton isotope has a mass of 86, making a total of 224, or a mass fourteen neutrons less than the total in the U.238 nucleus, and eleven less than in U.235.
The difference could be accounted for in two ways:
• either the uranium fragments consisted of new, highly unstable isotopes of barium and krypton, of atomic masses ranging up to 143 and 146 for the barium and 92 for the krypton,
• or they consisted of new isotopes of atomic masses totaling less than 238 or 235.
In the first instance nature would be likely to restore her balance by bodily ejecting some of the excess neutrons soon after the split occurred. In the second instance it would mean that some neutrons were liberated at the time of fission. In either case fission would be accompanied by the liberatin of neutrons.
At the Washington meeting in January 1939 Dr. Fermi discussed with Dr. Bohr this possibility of the liberation of neutrons in sufficient numbers to start a chain reaction. The first experimental proof that this was indeed the case came very shortly afterward.
By that a time laboratories all over the world were working on uranium fission. Among the first to announce the proof were Dr. Frédéric Joliot, son-in-law of Marie Curie, and his associates Drs. H. H. Halban and L. Kowarski, of Paris.
Their experiments revealed that as many as three free neutrons are produced by each uranium atom split. Experiments providing similar results were carried out simultaneously in the United States by Drs. Fermi, Szilard, H. L. Anderson, W. H. Zinn, and Henry B. Hanstein, who obtained independent confirmation that more than one free neutron is produced by each uranium nucleus undergoing fission.
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|Otto Hahn and Lise Meitner, discoverers of nuclear fission. A year later "laboratories all over the world were working on uranium fission."|
It was still not certain, however, that these free neutrons would necessarily act as the triggers to start a chain reaction.
By that time many of the characteristics of the behavior of neutrons had been learned.
It was known, for example, that they first emerge with speeds of thousands of miles per second. A large number of these highly penetrating neutrons would therefore escape into the open air without splitting any atoms.
Furthermore, certain elements present in unrefined uranium were known to devour neutrons. Such elements would therefore consume the free neutrons necessary to keep the chain reaction going.
Yet these two drawbacks, it was realized, could be remedied by taking certain measures. Since the distance that even the fastest, and hence most energetic, neutrons can travel has a definite limit, all one would have to do to prevent the free neutrons from escaping would be to make a uranium block of a size greater than the distance the neutron can travel.
To put it another way, since the production of neutrons increases with the volume of the uranium block, whereas the escape is but a surface effect, a size can be attained at which more neutrons will be born inside the block than will escape through the surface.
Such a size, the dimensions of which were not known until later, is referred to as the “critical size,” or “crit” for short.
To prevent the neutrons from being devoured by elements mixed with the uranium—that is, impurities—it would, of course, be necessary to use uranium of a very high degree of purity. It was realized from the beginning that this would be a most formidable task, since only a few parts of impurities per million would be enough to spoil a chain reaction.
Purity of such a high order had not been attained with any metal up to that time.
Even if these obstacles were to be completely overcome, however, it was soon realized that a third, even more formidable obstacle might completely frustrate the realization of the dream of utilizing atomic energy on a grand scale. This came with the discovery, regarded at first as a supreme tragedy and later as a blessing in disguise, that uranium itself eats up neutrons of certain energy levels without being split.
Would the cannibalistic uranium eat up enough of its offspring neutrons to prevent a self-perpetuating, self-sustaining cosmic fire? No one could tell in those early days of 1939, so fateful to the history of the world. It looked to some of the scientists as though nature had brought man to the very gates of the promised land of atomic energy only to slam them shut in his face just as he was about to enter.
To others, particularly to the eminent exiled scientists in our midst, who were already anxiously watching the shadow of the Nazi war machine above the horizon, the cannibalism of uranium held out the hope of at least a temporary reprieve for mankind.
For from the very beginning it became obvious to Dr. Fermi and other fellow exiles that if a self-perpetuating, self-sustaining chain reaction was possible, uranium could be used for producing the most devastating explosion ever attained on earth.
With the Nazis feverishly preparing for war while the democracies were still living under the delusion that Munich had brought “peace for our time,” it appeared to them almost inevitable that the Nazis would be the first to harness atomic energy to their military machine, since that would make certain the realization of their goal of world domination.
If uranium ate its young in large enough numbers, the joke, for the time being at least, would be on the Nazis.
But no one could tell for certain. There was still the possibility that there were certain limits to the uranium appetite and that the neutrons multiplied at a rate high enough to satisfy this appetite while still keeping the cosmic fire burning.
And so, eight months before the shooting war started, began the battle of the laboratories.
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|The French Cyclotron drops into Nazi hands.gif|