The Danish Peace Academy
SCIENCE AND SOCIETY
John Avery
H.C. Ørsted Institute, University of Copenhagen
Chapter 15 NUCLEAR FISSION
Artificial transmutations
During the First World War, Rutherford’s young men had joined the army, and he had been forced to spend most of his own time working on submarine detection. In spite of this, he had found some spare time for his scientific passion - bombarding matter with alpha particles. Helped by his laboratory steward, Kay, Rutherford had studied the effects produced when alpha particles from a radium source struck various elements. In a letter to Niels Bohr, dated December 9, 1917, Rutherford wrote:
“I have got, I think, results that will ultimately have great importance. I wish that you were here to talk matters over with me. I am detecting and counting the lighter atoms set in motion by alpha particles, and the results, I think, throw a good deal of light on the character and distribution of forces near the nucleus... I am trying to break up the atom by this method. In one case, the results look promising, but a great deal of work will be required to make sure. Kay helps me, and is now an expert counter. Best wishes for a happy Christmas.” In July, 1919, Bohr was at last able to visit Manchester, and he heard the news directly from his old teacher: Rutherford had indeed produced artificial nuclear transmutations! In one of his experiments, an alpha-particle (i.e. a helium nucleus with nuclear charge 2) was absorbed by a nitrogen nucleus. Later, the compound nucleus threw out a proton with charge 1; and thus the bombarded nucleus gained one unit of charge. It moved up one place in the periodic table and became an isotope of oxygen.
Bohr later wrote: “I learned in detail about his great new discovery of controlled, or so-called artificial, nuclear transmutations, by which he gave birth to what he liked to call ‘modern Alchemy’, and which in the course of time, was to give rise to such tremendous consequences as regards man’s mastery of the forces of nature.”
Other scientists rushed to repeat and extend Rutherford’s experiments. Particle accelerators were built by E.O. Lawrence (1901-1958) in California, by J.H. van de Graff (1901-1967) at the Massachusetts Institute of Technology and by John Cockcroft (1897-1967), working with Rutherford at the Cavendish Laboratory. These accelerators could hurl protons at energies of a million electron-volts. Thus, protons became another type of projectile which could be used to produce nuclear transmutations.
Neutrons
During the 1920’s, nuclear transmutations could be achieved only with light elements. The charges on the nuclei of heavy elements were so large that, with the energies available, alpha particles and protons could not react with them. The positively charged projectiles were kept at a distance by the electrostatic repulsion of the heavy nuclei: They could not come close enough for the powerful but short-range nuclear attractive forces to become effective. However, in 1932, a new projectile was discovered - a projectile which was destined to unlock, with grave consequences, the colossal energies of the heavy nuclei. This new projectile was the neutron.
Rutherford and Bohr had for some time suspected that an electrically neutral particle with roughly the same mass as a proton might exist. The evidence for such a particle was as follows: Each isotope was characterized by a nuclear charge and by a nuclear weight. The nuclear charge was an integral multiple of the proton charge, while the nuclear weight was approximately an integral multiple of the proton weight. For example, the isotope carbon-12 had charge 6 and weight 12. This might be explained by supposing the carbon-12 nucleus to be composed of twelve protons and six electrons. However, there were theoretical objections to a model in which many electrons were concentrated within the tiny volume of a nucleus. Therefore, in 1920, Rutherford postulated the existence of neutrons - elementary particles with almost the same mass as protons, but no electrical charge. Then (for example) the carbon-12 nucleus could be thought of as being composed of six protons and six neutrons.
In 1930, the German physicist, Walter Bothe (1891-1957), discovered a strange, penetrating type of radiation coming from beryllium which had been bombarded with alpha particles. In 1931 and 1932, Bothe’s experiments were repeated in Paris by Ir`ene Joliot-Curie (1897- 1956) and her husband Fr´ed´eric (1900-1958). The Joliot-Curies noticed that the mysterious rays emanating from the bombarded beryllium could easily penetrate lead. They also noticed that when the rays hit a piece of paraffin, hydrogen nuclei were knocked out.
The strange rays were, in fact, neutrons, as the Joliot-Curies would have realized immediately if they had been familiar with Rutherford’s prediction of the neutron’s existence. The Joliot-Curies might have made the correct identification of the rays given time; but Rutherford’s assistant, James Chadwick (1891-1974), was faster. On February 17, 1932, he published a paper in Nature reporting a series of experiments: Chadwick had studied not only the velocities of the hydrogen nuclei knocked out of paraffin by Bothe’s rays but also the velocities of nuclei knocked out of many other materials. In every case, he found that the velocities were consistent with the identification of the rays as neutrons. Chadwick completed his proof by showing that the rays moved with one-tenth the velocity of light, so that they had to be material particles rather than radiation; and he showed that the rays could not be deflected by a magnet. Therefore they carried no charge.
Fermi
Although Irène and Frédéric Joliot-Curie narrowly missed discovering the neutron, they soon made another discovery of major importance - artificial radioactivity. The Joliot-Curies had been bombarding an aluminum target with alpha-particles and studying the resulting radiation. One day in 1934, they noticed to their astonishment that the aluminum target continued to radiate even after they had stopped the alpha-particle bombardment. They discovered that some of the aluminum atoms in the target had been converted to a radioactive isotope of phosphorus!
In 1934, news of the startling discoveries of Bothe, Chadwick and the Joliot-Curies reached a brilliant young professor of theoretical physics in Rome. Although he was only 33 years old, Enrico Fermi (1901-1954) already had a worldwide reputation for his work in quantum theory. He also had attracted a school of extremely talented young students, the first physicists in Italy to enter the new fields of quantum mechanics and relativity: Persico, Amaldi, Rasetti, Segr`e, Pontecorvo, Majorana, Racah and Wick. It was a happy, informal group of young men.
Because of his reputation for scientific infallibility, Enrico Fermi was nicknamed “the Pope”, while Franco Rasetti was “the Cardinal” and Emilio Segr`e was “the Basilisk”. A medical colleague, Professor Trabacci, who generously supplied the group with equipment and chemicals, was known as “the Divine Providence”.
In 1934, Fermi was feeling somewhat discouraged with theoretical work, and in the mood to try something new. His paper on the theory of beta-decay (later regarded as one of his major achievements) had just been rejected by Nature. At that moment, he heard of Chadwick’s neutrons and the Joliot-Curie’s artificial radioactivity. Putting the two things together, Fermi decided to try to produce artificial radioactivity by bombarding elements with neutrons.
There were good theoretical reasons why Fermi’s plan should work, as well as practical reasons why it should fail. The argument in favor of neutrons was that they had no charge. Therefore they should be able to approach the nuclei of even heavy elements without being repelled by the electrostatic potential. The practical argument against neutrons was that it was difficult to produce them in worthwhile numbers. The yield of neutrons was only one for every hundred thousand alpha-particles.
Although he had no experience in working with radioactivity, Fermi managed to make his own Geiger counter. He also made a neutron source for himself by condensing radon gas (donated by “the Divine Providence”) into a small glass tube of powdered beryllium held at liquid air temperature.
Being a methodical person, Fermi began at the bottom of the periodic table and worked systematically upwards. The first eight elements which he bombarded with neutrons showed no artificial radioactivity, and Fermi almost became discouraged. Finally, he came to fluorine, and to his delight, he succeeded in making it strongly radioactive by neutron bombardment. He succeeded also with several other elements beyond fluorine; and realizing that the line of research was going to be very fruitful, he enlisted help from Segr`e, Amaldi, and the chemist, d’Agostino. Fermi also sent a cable to Franco Rasetti, who was on vacation in Marocco.
In order that the source should not disturb the measurements, the room where the elements were irradiated was far from the room where their radioactivity was measured - at the other end of a long corridor. The half-life of the induced radioactivity was very short in some elements, which meant that Fermi and Amaldi had to run full tilt with their samples, from one end of the hallway to the other.
One day a visitor arrived from Spain and asked to see “Sua Eccellenza Fermi”. (Fermi was a member of the Royal Academy of Italy, and therefore had the title “Excellency”, which much embarrassed him). “The Pope is upstairs”, said Segr`e, and then, realizing that the visitor did not know this nickname, he added: “I mean Fermi, of course.” The Spanish visitor arrived on the second floor of the institute just in time to see “Sua Eccellenza Fermi” dash wildly down the length of the corridor. After this fashion, Fermi and his group finally reached the top of the periodic table. They carefully purified uranium from its disintegration products and bombarded it with neutrons. A new radioactivity was induced, quite different from the ordinary activity of uranium. The question was: to what element or elements had the uranium been converted? With the help of the chemist, d’Agostino, they analysed the uranium target, and proved definitely that neutron bombardment had not converted uranium to any of the nearby heavy elements at the top of the periodic table. It seemed most likely that what they had produced by bombarding uranium was a new, unstable element, which had never before existed - element number 93! However, they lacked definite proof; and Fermi, always cautious, refused to jump to such a sensational conclusion.
By this time, the summer of 1934 had begun. The university year ended, as was traditional, with a meeting of the Accademia dei Lincei, attended by the King of Italy. In 1934, the speaker at this meeting was Senator Corbino, who had been a talented physicist before he became a politician. Corbino had been responsible for raising money to support Fermi’s group of young physicists; and he was justly proud of what they had achieved. In his 1934 speech before the king, Senator Corbino glowingly described their production of neutron-induced radioactivity; and he ended the speech with the words:
“The case of uranium, atomic number 92, is particularly interesting. It seems that, having absorbed the neutron, it converts rapidly by emission of an electron, into the element one place higher in the periodic system, that is, into a new element having atomic number 93...
However, the investigation is so delicate that it justifies Fermi’s prudent reserve and a continuation of the experiments before an announcement of the discovery. For what my own opinion on this matter is worth, and I have followed the investigations daily, I believe that production of this new element is certain.”
Corbino had not cleared this announcement with Fermi. It was immediately picked up by both the Italian and international press and given great publicity. A new element had been made by man! The official newspapers of fascist Italy, in particular, made much of this “great discovery” which, they claimed, showed that Italy was regaining the glorious position which it had held in the days of the Roman Empire.
Fermi was thrown into a mood of deep despair by this premature publicity. He could not sleep, and woke his wife in the middle of the night to tell her that his reputation as a scientist was in jeopardy. Next morning, Fermi and Corbino prepared a statement attempting to halt the publicity: “The public is giving an incorrect interpretation to Senator Corbino’s speech... Numerous and delicate tests must still be performed before the production of element 93 is actually proved.” Before the question of element 93 could be cleared up, the attention of Fermi’s group was distracted by an accidental discovery of extreme importance. They had been obtaining inconsistent and inexplicable results.
The radioactivity induced in a sample depended in what seemed to be a completely illogical way on the conditions under which the experiment was performed. For example, if the target was bombarded with neutrons while standing on a wooden table, the induced activity was much stronger than when the target was on a marble table. Fermi suspected that these strange results were due to scattering of neutrons by surrounding objects. He prepared a lead wedge to insert between neutron source and the counter to measure the scattering. However, he did not use the lead wedge which he had so carefully prepared.
“I was clearly dissatisfied with something”, Fermi remembered later, “I tried every excuse to postpone putting the piece of lead in its place. I said to myself, ‘No, I do not want this piece of lead here; what I want is a piece of paraffin.’ It was just like that, with no advance warning, no prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.”
The effect of the paraffin was amazing. The radioactivity increased a hundredfold! Puzzled, the group adjourned for lunch and siesta. When they reassembled a few hours later, Fermi had developed a theory to explain what was happening: The neutrons had almost the same mass as the hydrogen atoms in the paraffin. When they collided with the hydrogen atoms, the neutrons lost almost all their energy of motion, just as a billiard ball loses almost all its speed when it collides with another ball of equal mass. What Fermi and the others had discovered by accident was that slow neutrons are much more effective than fast ones in producing nuclear reactions.
“What we need”, said Fermi, “is a large amount of water.” The group excitedly took the neutron source and targets to Senator Corbino’s nearby garden, where there was a goldfish pond. The hydrogencontaining water of the pond produced the same result: It slowed the neutrons, and greatly enhanced their effect.
That evening, at Edouardo Amaldi’s house, they prepared a paper reporting their discovery. Fermi dictated, while Segr`e wrote. Meanwhile, Rasetti, Amaldi and Pontecorvo walked up and down, all offering suggestions simultaneously. They made so much noise that when they left, the maid asked Mrs. Amaldi whether her guests had been drunk. The happy and carefree days of the little group of physicists in Rome were coming to an end. They had thought that they could isolate themselves from politics; but in 1935, it became clear that this was impossible.
One day, in 1935, Segrè said to Fermi: “You are the Pope, and full of wisdom. Can you tell me why we are now accomplishing less than a year ago?”
Fermi answered without hesitation: “Go to the physics library. Pull out the big atlas that is there. Open it. You shall find your explanation.” When Segrè did this, the atlas opened automatically to a muchthumbed map of Ethiopia.
In 1935, Mussolini’s government had attacked Ethiopia, and Italy had been condemned by the League of Nations. For thinking Italians, this shock revealed the true nature of Mussolini’s government. They could no longer ignore politics. Within a few years, Enrico Fermi and most of his group had decided that they could no longer live under the fascist government of Italy. By 1939, most of them were refugees in the United States.
Hahn, Meitner and Frisch
Without knowing it, Enrico Fermi and his group had split the uranium atom; but four years were to pass before this became apparent. All the experts agreed that Fermi’s group had undoubtedly produced transuranic elements. There was only one dissenting voice - that of the German chemist, Ida Noddack, who was an expert in the chemistry of rare elements. Knowing no nuclear physics, but a great deal of chemistry, Ida Noddack saw the problem from a different angle; and in 1934 she wrote:
“It would be possible to assume that when a nucleus is demolished in this novel way by neutrons, nuclear reactions occur which may differ considerably from those hitherto observed in the effects produced on atomic nuclei by protons and alpha rays. It would be conceivable that when heavy nuclei are bombarded with neutrons, the nuclei in question might break into a number of larger pieces, which would, no doubt, be isotopes of known elements, but not neighbors of the elements subjected to radiation.”
No one took Ida Noddack’s suggestion seriously. The energy required to smash a heavy nucleus into fragments was believed to be so enormous that it seemed ridiculous to suggest that this could be accomplished by a slow neutron.
Many other laboratories began to bombard uranium and thorium with slow neutrons to produce “transuranic elements”. In Paris, Ir`ene Joliot-Curie and Paul Savitch worked on this problem, while at the Kaiser Wilhelm Institute in Berlin, Otto Hahn (1879-1968), Lise Meitner (1878-1968) and Fritz Strassmann (1902-) did the same.
Meanwhile, night was falling on Europe. In 1929, an economic depression, caused in part by the shocks of the First World War, began in the United States; and it soon spread to Europe. Without the influx of American capital, the postwar reconstruction of the German economy collapsed. The German middle class, which had been dealt a severe blow by the great inflation of 1923, now received a second heavy blow. The desperation produced by economic chaos drove the German voters into the hands of political extremists.
On January 30, 1933, Adolf Hitler was appointed Chancellor and leader of a coalition cabinet by President Hindenberg. Although Hitler was appointed legally to this post, he quickly consolidated his power by unconstitutional means: On May 2, Hitler’s police seized the headquarters of all trade unions, and arrested labor leaders. The Communist and Socialist parties were also banned, their assets seized and their leaders arrested. Other political parties were also smashed. Acts were passed eliminating Jews from public service; and innocent Jewish citizens were boycotted, beaten and arrested.
On March 11, 1938, Nazi troops entered Austria. Lise Meitner, who was working with Otto Hahn in Berlin, was a Jew, but until Hitler’s invasion of Austria, she had been protected by her Austrian citizenship. Now, she was forced to escape from Germany. Saying goodbye only to Otto Hahn and to a few other close friends, she went to Holland for a vacation, from which she did not plan to return. From there, she went to Stockholm, where she had been offered a post by the Nobel Institute.
Meanwhile, Hahn and Strassmann continued to work on what they believed to be production of transuranic elements. They had been getting results which differed from those of the Paris group, but they believed that Irène Joliot-Curie must be mistaken. When Strassmann tried to show Hahn one of the new papers from Paris, he continued to puff calmly on his cigar and replied: “I am not interested in our ladyfriend’s latest writings”. However, Strassmann would not be deterred, and he quickly summarized the most recent result from Paris. “It struck Hahn like a thunderbolt”, Strassmann said later, “He never finished that cigar. He laid it down, still glowing, on his desk, and ran downstairs with me to the laboratory.”
Hahn and Strassmann quickly repeated the experiments which Irène Joliot-Curie had reported. They now suspected that one of the products which she had produced was actually an isotope of radium. Since radium has almost the same chemical properties as barium, they tried percipitating it together with a barium carrier. This procedure worked: The new substance came down with the barium.
Otto Hahn was the most experienced radiochemist in the world, and many years previously he had developed a method for separating radium from barium. He and Strassmann now tried to apply this method. It did not work. No matter how they tried, they could not separate the active substance from barium.
Could it be that an isotope of barium had been produced by bombarding uranium with neutrons? Impossible! It would mean that the uranium nucleus had split roughly in half, against all the wellestablished rules of nuclear physics. It could not happen - and yet their chemical tests told them again and again that the product really was barium. Finally, they sat down and wrote a paper:
“We come to this conclusion”, Hahn and Strassmann wrote, “Our ‘radium’ isotopes have the properties of barium. As chemists, we are in fact bound to affirm that the new bodies are not radium but barium; for there is no question of elements other than radium and barium being present... As nuclear chemists, we cannot decide to take this step, in contradiction to all previous experience in nuclear physics.” On December 22, 1938, Otto Hahn mailed the this paper to the journal, Naturwissenschaften. “After the manuscript was mailed”, he said later, “the whole thing seemed so improbable to me that I wished I could get the document back out of the mail box.”
After making this strange discovery, Otto Hahn’s first act had been to write to Lise Meitner, who had worked by his side for so many years. She received his letter just as she was starting for her Christmas vacation, which was to be spent at the small Swedish town of Kungälv, near Göteborg.
It was even more clear to Lise Meitner than it had been to Hahn that something of tremendous importance had unexpectedly come to light. As it happened, Lise Meitner’s nephew, O.R. Frisch, had come to Kungälv to spend Christmas with his aunt, hoping to keep her from being lonely during her first Christmas as a refugee. Frisch was a physicist, working at Niels Bohr’s institute in Copenhagen. He was one of the many scientists whom Bohr saved from the terror and persecution of Hitler’s Germany by offering them refuge in Copenhagen.
When Frisch arrived, Lise Meitner immediately showed him Otto Hahn’s letter. “I wanted to discuss with her a new experiment I was planning”, Frisch said later, “but she wouldn’t listen. I had to read the letter. Its content was indeed so startling that I was at first inclined to be sceptical.”
Frisch put on his skis, and went out to get some air; but his aunt followed him over the snow, insisting that he think about the problem of uranium and barium. Lise Meitner knew the precision and thoroughness of Otto Hahn’s methods so well that she could not imagine him making a mistake of that kind. If Hahn said that bombarding uranium with neutrons produced barium, then it did produce barium. She insisted that her nephew should try to explain this impossible result, rather than shrugging it off as an error.
Finally, aunt and nephew sat down on a log in the middle of the snow-filled Swedish forest and tried to make some calculations on the back of an envelope. They continued their calculations back at their hotel, consulting some tables of isotopic masses which Frisch had brought with him. Gradually, they formed a picture of what had happened:
The uranium nucleus was like a liquid drop. Although the powerfully attractive short-range nuclear forces produced a surface tension which tended to keep the drop together, there were also powerful electrostatic repulsive forces which tended to make it divide. Under certain conditions, the nucleus could become non-spherical in shape, with a narrow waist. If this happened, the electrostatic repulsion would split the nucleus into two fragments, and would drive the fragments apart with tremendous energy of motion.
Frisch and Meitner calculated that for a single uranium nucleus, the energy of motion would be roughly two hundred million electron volts. What was the source of this gigantic energy? By consulting tables of isotopic masses, the two scientists were able to show that in the splitting of uranium, a large amount of the mass is converted to energy. If one of the fragments was an isotope of barium, the other had to be an isotope of krypton. Using Einstein’s formula relating energy to mass, they found that the lost mass was exactly equivalent to two hundred million electron volts. Everything checked. This had to be the explanation.
Meitner and Frisch were struck by the colossal size of the energy released in the fission of uranium. Ordinary combustion releases one or two electron volts per atom. They realized with awe that in the fission of uranium, a hundred million times as much energy is released! When O.R. Frisch returned to Copenhagen, Niels Bohr was preparing to leave for a lecture tour in America. Frisch had only a few minutes to tell him what had happened, but Bohr was quick to understand. “I had hardly begun to tell him”, Frisch said later, “when he struck his forehead and exclaimed, ‘Oh what idiots we all have been! But this is wonderful! This is just as it must be!’”
There was no time to talk, but as Niels Bohr entered the taxi which would take him to the liner, Drottningholm, he asked Frisch whether he had written a paper. Frisch handed some rough notes to Bohr, and said that he would write a paper immediately. Bohr promised that he would not talk about the new discovery until the paper was ready. Bohr’s assistant, Rosenfeld, had accompanied him on the trip, and the long sea voyage to New York gave the two physicists a good opportunity to think about the revolutionary new discovery of nuclear fission.
A blackboard was installed in Bohr’s stateroom on the Drottningholm. Bohr and Rosenfeld covered this blackboard with calculations, and by the end of the voyage, they were convinced that Otto Frisch and Lise Meitner had correctly analysed the problem of nuclear fission.
At the harbor in New York, they were met by Professor John Wheeler of Princeton, together with Enrico Fermi and his wife, Laura, who had become refugees in America. Laura Fermi remembered later the tense and worried expression with which Bohr described the rapidly deteriorating political situation in Europe. With her imperfect knowledge of English and the noise of the pier, she could only make out a few of the words - “Europe - war - Hitler - danger”.
Rosenfeld accompanied Wheeler to Princeton, while Bohr and his 19 year old son, Erik, remained a few days in New York. At Princeton, Rosenfeld was invited to address the “Journal Club”, a small, informal group of physicists. Bohr had neglected to tell Rosenfeld that he had promised not to talk about nuclear fission until the Hahn-Strassmann and Meitner-Frisch papers were out; and Rosenfeld spoke about the revolutionary new discovery to the physicists at Princeton.
The news spread with explosive speed. Telephone calls and letters went out to other parts of America. The physicist, I.I. Rabi, who happened to be at Princeton, returned to Colombia University, where Fermi was working, and told him the news. Fermi acted with characteristic speed and decisiveness. He devised an experiment to detect the high-energy fragments produced by uranium fission; and he suggested to his co-worker, Dunning, that the experiment should be performed as fast as possible. Fermi himself had to leave for a theoretical physics meeting in Washington, where Bohr would be present.
When Bohr heard that Rosenfeld had talked about fission, he was very upset, because he had promised Frisch to remain silent until the papers were out. He sent a telegram to Copenhagen urging Frisch to hurry with his manuscript, and urging him to perform an experiment to detect the fission fragments.
In fact, Otto Frisch had already performed this experiment, using a radium-lined ionization chamber containing a radium-beryllium neutron source. An amplifier connected with the chamber had shown giant bursts of ionization, which could only be due to the immensely energetic fission fragments.
On January 16, 1939, the same day that Rosenfeld had revealed the news about fission to the physicists at Princeton, Otto Frisch had mailed two papers to Nature. The first of these papers presented the theory of nuclear fission which he and Lise Meitner had developed, while the second described his experimental detection of the high-energy fragments.
On January 26, Bohr and Fermi arrived at the American capital to attend the Fifth Washington Conference on Theoretical Physics. The same day, Erik Bohr received a letter from his brother, Hans. The letter contained the news that Frisch had completed his experiment and had sent the paper to London. Simultaneously, Bohr learned from a reporter who was covering the conference that the Hahn-Strassmann paper had just been published in Naturwissenschaften. At last, Bohr felt free to speak. He asked the chairman whether he might make an announcement of the utmost importance; and he told the astonished physicists the whole story.
While Bohr was speaking, Dr. Tuve of the Carnegie Institution whispered to his colleague, Halfstead, that he should quickly put a new filament in the Carnegie accelerator. Several physicists rushed for the door to make long-distance telephone calls. Fermi decided to leave the conference immediately, and to return to New York. On the way out, Fermi met Robert B. Potter, a reporter from Science Service, who asked: “What does it all mean?” Fermi explained as well as he could, and Potter wrote the following story, which was released to newspapers and magazines:
“New hope for releasing the enormous energy within the atom has arisen from German experiments that are now creating a sensation among eminent physicists gathered here for the Conference on Theoretical Physics. It is calculated that only five million electron volts of energy can release two hundred million electron volts of energy, forty times the amount shot into it by a neutron (neutral atomic particle). World famous Niels Bohr of Copenhagen and Enrico Fermi of Rome, both Nobel Prize winners, are among those who acclaim this experiment as one of the most important in recent years. American scientists join them in this acclaim.”
Chapter 16: HIROSHIMA AND NAGASAKI.
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