8
The seventh Solvay Conference, held in Brussels in late October 1933, was George Gamow’s ticket of escape from a Soviet Union rapidly becoming inhospitable to theoretical physicists who persisted in modern views. The previous summer the tall, blond, powerfully built Odessan and his wife Rho, also a physicist, had tried to escape by paddling a faltboat—a collapsible rubber kayak—170 miles south from the Crimea to Turkey across the Black Sea without benefit of a weather report. They took a pocket compass, carefully hoarded hard-boiled eggs, cooking chocolate, two bottles of brandy and a bag of fresh strawberries, set out in the morning ostensibly on a recreational excursion and paddled hard all day and into the night. The only document they carried was Gamow’s Danish motorcycle-driver’s license, souvenir of the 1930 winter he spent in Copenhagen after working with Rutherford at the Cavendish. Gamow planned to show the Turks the document, announce himself in Danish to be a Dane, head for the nearest Danish consulate and put himself long-distance in Bohr’s capable hands. But the Black Sea is named for its storms. The wind thwarted the Gamows’ escape, drenching them in heavy seas, exhausting them through a long, cold night and finally blowing them back to shore.736
Back in Leningrad the following year Gamow received notice from his government that he was officially delegated to the Solvay Conference. “I could not believe my eyes,” he writes in his autobiography.737 It was an easy way out of the country—except that Rho had not been included. Gamow determined to acquire a second passport or defiantly stay home. Through the Bolshevik economist Nikolai Bukharin, whom he knew, he arranged an interview with Party Chairman Vyacheslav Molotov at the Kremlin. Molotov wondered that the theoretician could not live for two weeks without his wife. Gamow feigned camaraderie:
“You see,” I said, “to make my request persuasive I should tell you that my wife, being a physicist, acts as my scientific secretary, taking care of papers, notes, and so on. So I cannot attend a large congress like that without her help. But this is not true. The point is that she has never been abroad, and after Brussels I want to take her to Paris to see the Louvre, the Folies Bergère, and so forth, and to do some shopping.”738
That Molotov understood. “I don’t think this will be difficult to arrange,” he told Gamow.
When the time arrived to collect the passports Gamow found that Molotov had changed his mind, preferring not to set an awkward precedent. Gamow stubbornly refused to cooperate. The passport office called him three times to pick up his passport and three times he insisted he would wait until there were two. The fourth time “the voice on the telephone informed me that both passports were ready. And indeed they were!” (After the conference the young defectors sailed to America.739 Gamow taught at the University of Michigan’s summer school in pleasant Ann Arbor and from there moved to accept a professorship at George Washington University in Washington, D.C.)
The Solvay Conference, devoted for the first time to nuclear physics, drew men and women from the highest ranks of two generations: Marie Curie, Rutherford, Bohr, Lise Meitner among the older physicists; Heisenberg, Pauli, Enrico Fermi, Chadwick (eight men in all from Cambridge and no one from devastated Göttingen), Gamow, Irene and Frédéric JoliotCurie, Patrick Blackett, Rudolf Peierls among the younger. Ernest Lawrence, his cyclotron humming, was the token American that year.
They debated the structure of the proton. Other topics they discussed may have seemed more far-reaching at the time. None would prove to be. On August 2, 1932, working with a carefully prepared cloud chamber, an American experimentalist at Caltech named Carl Anderson had discovered a new particle in a shower of cosmic rays. The particle was an electron with a positive instead of a negative charge, a “positron,” the first indication that the universe consists not only of matter but of antimatter as well. (Its discovery earned Anderson the 1936 Nobel Prize.) Physicists everywhere immediately looked through their files of cloud-chamber photographs and identified positron tracks they had misidentified before (the Joliot-Curies, who had missed the neutron, saw that they had also missed the positron). The new particle raised the possibility that the positively charged proton might in fact be compound, might be not a unitary particle but a neutron in association with a positron. (It was not; there proved not to be room in the nucleus for electrons positive or negative.)
After they had identified the positrons they had missed before, the Joliot-Curies had started up their cloud chamber again and looked for the new particle in other experimental arrangements. They found that if they bombarded medium-weight elements with alpha particles from polonium, the targets ejected protons. Then they noticed that lighter elements, including in particular aluminum and boron, sometimes ejected a neutron and then a positron instead of a proton. That seemed evidence for a compound proton. They presented their evidence with enthusiasm as a report to the Solvay Conference.
Lise Meitner attacked the Joliot-Curies’ report. She had performed similar experiments at the KWI and she was highly respected for the cautious precision of her work. In her experiments, she emphasized, she had been “unable to uncover a singleneutron.”740Sentiment favored Meitner. “In the end, the great majority of the physicists present did not believe in the accuracy of our experiments,” Joliot says. “After the meeting we were feeling rather depressed.” Fortunately the theoreticians intervened. “But at that moment Professor Niels Bohr took us aside . . . and told us he thought our results were very important. A little later Pauli gave us similar encouragement.”741 The Joliot-Curies returned to Paris determined to settle the issue once and for all.
Husband and wife were then thirty-three and thirty-six years old, with a small daughter at home. They sailed and swam together in summer, skied together in winter, worked together efficiently in the laboratory in the Latin Quarter on the Rue Pierre Curie. Irene had succeeded her mother as director of the Radium Institute in 1932: the long-widowed pioneer was mortally ill with leukemia induced by too many years of exposure to radiation.
It seemed likely that the appearance of neutrons and positrons rather than protons might depend on the energy of the alpha particles attacking the target. The Joliot-Curies could test that possibility by moving their polonium source away from the target, slowing the alphas by forcing them to batter their way through longer ranges of air. Joliot went to work. Without question he was seeing neutrons. When he shifted the polonium away from the aluminum-foil target “the emission of neutrons [ceased] altogether when a minimum velocity [was] reached.” But something else happened then to surprise him.742 After neutron emission ceased, positron emission continued—not stopping abruptly but decreasing “only over a period of time, like the radiation . . . from a naturally radioactive element.” What was going on? Joliot had been observing the particles with a cloud chamber, catching their ionizing tracks in its supersaturated fog. Now he switched to a Geiger counter and called in Irene. As he explained to a colleague the next day: “I irradiate this target with alpha rays from my source; you can hear the Geiger counter crackling. I remove the source: the crackling ought to stop, but in fact it continues.”743 The strange activity declined to half its initial intensity in about three minutes. They would hardly yet have dared to think of that period as a half-life. It might merely mark the erratic performance of the Geiger counter.
A young German physicist who specialized in Geiger counters, Wolfgang Gentner, was working at the institute that year. Joliot asked him to check the lab instruments. The couple went off to a social evening they could find no excuse to avoid. “The following morning,” writes the colleague to whom Joliot spoke that day, “the Joliots found on their desk a little hand-written note from Gentner, telling them that the Geiger counters were in perfect working order.”744
They were nearly certain then that they had discovered how to make matter radioactive by artificial means.
They calculated the probable reaction. An aluminum nucleus of 13 protons and 14 neutrons, capturing an alpha particle of 2 protons and 2 neutrons and immediately re-emitting 1 neutron must be converting itself into an unstable isotope of phosphorus with 15 protons and 15 neutrons (13 + 2 protons = 15; 14 + 2 - 1 neutrons = 15). The phosphorus then probably decayed to silicon (14 protons, 16 neutrons). The 3-minute period was the half-life of that decay.
They could not chemically trace the infinitesimal accumulation of silicon. Joliot explained why in 1935, when he and his wife accepted the Nobel Prize in Chemistry for their discovery: “The yield of these transmutations is very small, and the weights of elements formed . . . are less than 10-15 [grams], representing at most a few million atoms”—too few to find by chemical reaction alone.745 But they could trace the radioactivity of the phosphorus with a Geiger counter. If it did indeed signal the artificial transmutation of some of the aluminum to phosphorus, they should be able to separate the two different elements chemically. The radioactivity would go with the new phosphorus and leave the untransmuted aluminum behind. But they needed a definitive separation that could be carried out within three minutes, before the faint induced radioactivity faded below their Geiger counter’s threshold.
The request perplexed a chemist in a nearby laboratory—“never having envisaged chemistry from that point of view,” says Joliot—but he contrived the necessary procedure.746 The Joliot-Curies irradiated a piece of aluminum foil, dropped it into a container of hydrochloric acid and covered the container. The acid dissolved the foil, producing, by reaction, gaseous hydrogen, which should carry the phosphorus with it out of solution. They drew off the gas into an inverted test tube. The dissolved aluminum fell silent then but the gas made the Geiger counter chatter: whatever was radioactive had been carried along. A different chemical test proved that the radioactive substance was phosphorus. Joliot bounded like a boy.
The discovery might serve as an offering to Irène’s ailing mother, who had prepared the daughter and sponsored the son-in-law:
Marie Curie saw our research work and I will never forget the expression of intense joy which came over her when Irene and I showed her the first artificially radioactive element in a little glass tube. I can still see her taking in her fingers (which were already burnt with radium) this little tube containing the radioactive compound—as yet one in which the activity was very weak. To verify what we had told her she held it near a Geiger-Müller counter and she could hear the rate meter giving off a great many “clicks.” This was doubtless the last great satisfaction of her life.747
The Joliot-Curies reported their work—“one of the most important discoveries of the century,” Emilio Segrè says in his history of modern physics—in the Comptes Rendus on January 15, 1934, and in a letter to Nature dated four days later.748 “These experiments give the first chemical proof of artificial transmutation,” they concluded proudly.749 Rutherford wrote them within a fortnight: “I congratulate you both on a fine piece of work which I am sure will ultimately prove of much importance.” He had tried a number of such experiments himself, he said, “but without any success”—high praise from the master of experiment.750
They had demonstrated that it was possible not only to chip pieces off the nucleus, as Rutherford had done, but also to force it artificially to release some of its energy in radioactive decay. Joliot foresaw the potential consequences of that attack in his half of the joint Nobel Prize address. Given the progress of science, he said, “we are entitled to think that scientists, building up or shattering elements at will, will be able to bring about transmutations of an explosive type. . . . If such transmutations do succeed in spreading in matter, the enormous liberation of useful energy can be imagined.” But he saw the possibility of cataclysm “if the contagion spreads to all the elements of our planet”:751
Astronomers sometimes observe that a star of medium magnitude increases suddenly in size; a star invisible to the naked eye may become very brilliant and visible without any telescope—the appearance of a Nova. This sudden flaring up of the star is perhaps due to transmutations of an explosive character like those which our wandering imagination is perceiving now—a process that the investigators will no doubt attempt to realize while taking, we hope, the necessary precautions.
Leo Szilard received no invitation to the Solvay Conference. By October 1933 he had not accomplished any nuclear physics of note except within the well-equipped laboratory of his brain. In August he had written a friend that he was “spending much money at present for travelling about and earn of course nothing and cannot possibly go on with this for very long.”752 The idea of a nuclear chain reaction “became a sort of obsession” with him. When he heard of the Joliot-Curies’ discovery, in January, his obsession bloomed: “I suddenly saw that the tools were on hand to explore the possibility of such a chain reaction.”753
He moved to a less expensive hotel, the Strand Palace, near Trafalgar Square, and settled in to think. He had “a little money saved up” after all, “enough perhaps to live for a year in the style in which I was accustomed to live, and therefore I was in no particular hurry to look for a job”—the excitement of new ideas thus relieving his August urgency.754 The bath was down the hall. “I remember that I went into my bath . . . around nine o’clock in the morning. There is no place as good to think as a bathtub. I would just soak there and think, and around twelve o’clock the maid would knock and say, ‘Are you all right, sir?’ Then I usually got out and made a few notes, dictated a few memoranda.”755
One of the “memoranda” took the form of a patent application, filed March 12, 1934, relating to atomic energy.756, 757 It was the first of several, that year and the next, all finally merged into one complete specification, “Improvements in or Relating to the Transmutation of Chemical Elements.” (The same day Szilard applied for a patent, never issued, proposing the storage of books on microfilm.758) Szilard had already realized—in September, in the context of inducing a chain reaction—that neutrons would be more efficient than alpha particles at bombarding nuclei. He applied that insight now to propose an alternative method for creating artificial radioactivity:
In accordance with the present invention radio-active bodies are generated by bombarding suitable elements with neutrons. . . . Such uncharged nuclei penetrate even substances containing the heavier elements without ionization losses and cause the formation of radio-active substances.759
That was a first step. It was also a cheeky piece of bravado. Szilard had only theoretical grounds for believing that neutrons might induce radioactivity artificially. He had not done the necessary experiments. Only the Joliot-Curies had carried out such experiments so far, and they used alpha particles. Szilard was pursuing more than artificial radioactivity. He was pursuing chain reactions, power generation, atomic bombs. He had not yet found patentable form for these excursions. He wondered which element or elements might emit two or more neutrons for each neutron captured. He decided at some point, he said later, “that the reasonable thing to do would be to investigate systematically all the elements. There were ninety-two of them. But of course this is a rather boring task, so I thought that I would get some money, have some apparatus built, and then hire somebody who would just sit down and go through one element after the other.”760
The task would hardly be boring. The truth is that Szilard lacked the resources for such work—access to a laboratory, a dedicated crew, sufficient financial support. “None of the physicists had any enthusiasm for this idea of a chain reaction,” he would remember. Rutherford threw him out. Blackett told him, “Look, you will have no luck with such fantastic ideas in England.761 Yes, perhaps in Russia. If a Russian physicist went to the government and [said], ‘We must make a chain reaction,’ they would give him all the money and facilities which he would need. But you won’t get it in England.” Soaking in his bath against the London chill, Szilard turned back to mapping the future. The opportunity to explore the elements systematically for surprises by bombarding them with neutrons passed him by.
* * *
It fell instead to Enrico Fermi and his team of young colleagues in Rome. Fermi was prepared.762 He had all on hand that Szilard did not. He saw as soon as Szilard that the neutron would serve better than the alpha particle for nuclear bombardment. The point was not obvious. One used alphas to generate neutrons (as the Joliot-Curies had done along the way to chasing down their positrons). Since not all the alphas found targets, the neutral particles were correspondingly that much more scarce. As Otto Frisch would write: “I remember that my reaction and probably that of many others was that Fermi’s was really a silly experiment because neutrons were much fewer than alpha particles. What that simple argument overlooked of course was that they are very much more effective.”763
Fermi was prepared because he had been organizing his laboratory for a major expedition into nuclear physics for more than four years. If Italy had been one of the hot centers of physical research he might have been too preoccupied to plan ahead so carefully. But Italian physics was a ruin as sere as Pompeii when he came to it. He had no choice but to push aside the debris and start fresh.
Both Fermi’s biographers—his wife Laura and his protégé and fellow Nobel laureate Emilio Segrè—assign the beginning of his commitment to physics to the period of psychological trauma following the death of his older brother Giulio when Fermi was fourteen years old, in the winter of 1915.764 Only a year apart in age, the two boys had been inseparable; Giulio’s death during minor surgery for a throat abscess left Enrico suddenly bereft.
That same winter young Enrico browsed on market day among the stalls of Rome’s Campo dei Fiori, where a statue commemorates the philosopher Giordano Bruno, Copernicus’ defender, who was burned at the stake there in 1600 by the Inquisition. Fermi found two used volumes in Latin, Elementorum physicae mathematicae, the work of a Jesuit physicist, published in 1840. The desolate boy used his allowance to buy the physics textbooks and carried them home. They excited him enough that he read them straight through. When he was finished he told his older sister Maria he had not even noticed they were written in Latin. “Fermi must have studied the treatise very thoroughly,” Segrè would decide, looking through the old volumes many years later, “because it contains marginal notes, corrections of errors, and several scraps of paper with notes in Fermi’s handwriting.”765
From that point forward Fermi’s development as a physicist proceeded, with a single significant exception, rapidly and smoothly. A friend of his father, an engineer named Adolfo Amidei, guided his adolescent mathematical and physical studies, lending him texts in algebra, trigonometry, analytical geometry, calculus and theoretical mechanics between 1914 and 1917. When Enrico graduated from the liceo early, skipping his third year, Amidei asked him if he preferred mathematics or physics as a career and made a point of writing down, with emphasis, the young man’s exact reply: “I studied mathematics with passion because I considered it necessary for the study of physics, to which I want to dedicate myself exclusively. . . . I’ve read all the best-known books of physics.”766
Amidei then advised Fermi to enroll not at the University of Rome but at the University of Pisa, because he could compete in Pisa to be admitted as a fellow to an affiliated Scuola Normale Superiore of international reputation that would pay his room and board. Among other reasons for the advice, Amidei told Segrè, he wanted to remove Fermi from his family home, where “a very depressing atmosphere prevailed . . . after Giulio’s death.”767
When the Scuola Normale examiner saw Fermi’s competition essay on the assigned theme “Characteristics of sound” he was stunned. It set forth, reports Segrè, “the partial differential equation of a vibrating rod, which Fermi solved by Fourier analysis, finding the eigenvalues and the eigenfrequencies . . . which would have been creditable for a doctoral examination.”768 Calling in the seventeen-year-old liceo graduate, the examiner told him he was extraordinary and predicted he would become an important scientist. By 1920 Fermi could write a friend that he had reached the point of teaching his Pisa teachers: “In the physics department I am slowly becoming the most influential authority. In fact, one of these days I shall hold (in the presence of several magnates) a lecture on quantum theory, of which I’m always a great propagandist.”769 He worked out his first theory of permanent value to physics while he was still a student in Pisa, a predictive deduction in general relativity.
The exception to his rapid progress came in the winter of 1923, when Fermi won a postdoctoral fellowship to travel to Göttingen to study under Max Born. Wolfgang Pauli was there then, and Werner Heisenberg and the brilliant young theoretician Pascual Jordan, but somehow Fermi’s exceptional ability went unnoticed and he found himself ignored. Since he was, in Segrè’s phrase, “shy, proud, and accustomed to solitude,” he may have brought the ostracism on himself.770 Or the Germans may have been prejudiced against him by Italy’s poor reputation in physics. Or, more dynamically, Fermi’s visceral aversion to philosophy may have left him tongue-tied: he “could not penetrate Heisenberg’s early papers on quantum mechanics, not because of any mathematical difficulties, but because the physical concepts were alien to him and seemed somewhat nebulous” and he wrote papers in Göttingen “he could just as well have written in Rome.”771, 772 Segrè has concluded that “Fermi remembered Göttingen as a sort of failure. He was there for a few months. He sat aside at his table and did his work. He didn’t profit. They didn’t recognize him.”773 The following year Paul Ehrenfest sent along praise through the intermediary of a former student who looked up Fermi in Rome. A three-month fellowship then took the young Italian to Leiden for the traditional Ehrenfest tightening. After that Fermi could be sure of his worth.
He was always averse to philosophical physics; a rigorous simplicity, an insistence on concreteness, became the hallmark of his style. Segrè thought him inclined “toward concrete questions verifiable by direct experiment.”774 Wigner noticed that Fermi “disliked complicated theories and avoided them as much as possible.”775 Bethe remarked Fermi’s “enlightening simplicity.”776 Less generously, the sharp-tongued Pauli called him a “quantum engineer”; Victor Weisskopf, though an admirer, saw some truth in Pauli’s canard, a difference in style from more philosophical originals like Bohr.777 “Not a philosopher,” Robert Oppenheimer once sketched him.778 “Passion for clarity. He was simply unable to let things be foggy. Since they always are, this kept him pretty active.” An American physicist who worked with the middle-aged Fermi thought him “cold and clear. . . . Maybe a little ruthless in the way he would go directly to the facts in deciding any question, tending to disdain or ignore the vague laws of human nature.”779
Fermi’s passion for clarity was also a passion to quantify. He seems to have attempted to quantify everything within reach, as if he was only comfortable when phenomena and relationships could be classified or numbered. “Fermi’s thumb was his always ready yardstick,” Laura Fermi writes. “By placing it near his left eye and closing his right, he would measure the distance of a range of mountains, the height of a tree, even the speed at which a bird was flying.”780 His love of classification “was inborn,” Laura Fermi concludes, “and I have heard him ‘arrange people’ according to their height, looks, wealth, or even sex appeal.”781
Fermi was born in Rome on September 29, 1901, into a family that had successfully made the transition during the nineteenth century from peasant agriculture in the Po Valley to career civil service with the Italian national railroad. His father was a capo divisione in the railroad’s administration, a civil rank that corresponded to the military rank of brigadier general. In accord with a common Italian practice of the day, the infant Enrico was sent to live in the country with a wet nurse. So was his brother Giulio, but because Enrico’s health was delicate he did not return to his mother and father until he was two and a half years old. Confronted then with a roomful of strangers purporting to be his family, and “perhaps,” writes Laura Fermi, “missing the rough effusiveness of his nurse,” he began to cry:782
His mother talked to him in a firm voice and asked him to stop at once; in this home naughty boys were not tolerated. Immediately the child complied, dried his tears, and fussed no longer. Then, as in later childhood, he assumed the attitude that there is no point in fighting authority. If they wanted him to behave that way, all right, he would; it was easier to go along with them than against.
In 1926, when he was twenty-five years old, Fermi was chosen under the Italian system of concorsos, national competitions, to become professor of theoretical physics at the University of Rome. An influential patron had seen to the creation of the new post, a Sicilian named Orso Mario Corbino, a short, dark, volatile man, forty-six when Fermi sought him out in 1921, the director of the university physics institute, an exceptional physicist and a Senator of the Kingdom. Since the old guard of Italian physicists resentedFermi’s rapid promotion, he especially welcomed the protection of Corbino’s patronage. Corbino found support for his efforts to improve Italian physics from the Fascist government of the bulletheaded former journalist Benito Mussolini, although the senator was not himself a party member.
In the later 1920s Corbino and his young professor agreed that the time was ripe for the small group they were assembling in Rome to colonize new territory on the frontier of physics.783 They chose as their territory the atomic nucleus, then finding description in quantum mechanics but not yet experimentally disassembled. Fermi’s tall, erudite Pisa classmate Franco Rasetti signed on as Corbino’s first assistant early in 1927. Rasetti and Fermi together recruited Segrè, who had been studying engineering, by taking him along to the Como conference and explaining the achievements of the assembled luminaries to him—by then, Segrè saw, Pauli and Heisenberg recognized Fermi’s talents and included him among their friends. The son of the prosperous owner of a paper mill, Segrè contributed elegance to the group as well as brains.
Corbino added Edoardo Amaldi, the son of a mathematics professor at the University of Padua, by frankly raiding the engineering school. The group quickly nicknamed Fermi “the Pope” for his quantum infallibility; Corbino, like Rutherford at the Cavendish, called them all his “boys.” Rasetti departed to Caltech, Segrè to Amsterdam, for seasoning. Fermi sent them out again in the early 1930s, after the decision to go into nuclear physics: Segrè to work with Otto Stern in Hamburg, Amaldi to Leipzig to the laboratory of the physical chemist Peter Debye, Rasetti to Lise Meitner at the KWI. By 1933, with a departmental budget above $2,000 a year, ten times the budget of most Italian physics departments, with a well-made cloud chamber and a nearby radium source and KWI training in the vagaries of Geiger counters, the group was ready to begin.
In the meantime, two months after the Solvay Conference, Fermi completed the major theoretical work of his life, a fundamental paper on beta decay. Beta decay, the creation and expulsion by the nucleus of highenergy electrons in the course of radioactive change, had needed a detailed, quantitative theory, and Fermi supplied it entire. He introduced a new type of force, the “weak interaction,” completing the four basic forces known in nature: gravity and electromagnetism, which operate at long range, and the strong force and Fermi’s weak force, which operate within nuclear dimensions. He introduced a new fundamental constant, now called the Fermi constant, determining it from existing experimental data. “A fantastic paper,” Victor Weisskopf later praised it, “ . . . a monument to Fermi’s intuition.”784 In London the editor of Nature rejected it on the grounds that it was too remote from physical reality, which Fermi found irritating but amusing; he published it instead in the little-known weekly journal of the Italian Research Council, Ricerca Scientifica, where Amaldi’s wife Ginestra worked, and later in the Zeitschrift fúr Physik.785, 786 With only minor adjustments Fermi’s theory of beta decay continues to be definitive.
The Comptes Rendus reporting the Joliot-Curies’ discovery of artificial radioactivity reached Rome shortly after Fermi returned from skiing in the Alps, in January 1934.787 “We had not yet found any [nuclear physics] problems to work on,” Amaldi reminisces. “ . . . Then came out the paper of Joliot, and Fermi immediately started to look for the radioactivity.”788 Like Szilard, Fermi saw the advantages of using neutrons. I. I. Rabi catalogues those advantages in a lecture:
Since the neutron carries no charge, there is no strong electrical repulsion to prevent its entry into nuclei. In fact, the forces of attraction which hold nuclei together may pull the neutron into the nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth. The nucleus is violently shaken up by the blow, especially if the collision results in the capture of the neutron. A large increase in energy occurs and must be dissipated, and this may happen in a variety of ways, all of them interesting.789
When Fermi began his neutron-bombardment experiments he was thirty-three years old, short, muscular, dark, with thick black hair, a narrow nose and surprising gray-blue eyes. His voice was deep and he grinned easily. Marriage to the petitely beautiful Laura Capon, the daughter of a Jewish officer in the Italian Navy, had encouraged him in methodical habits: he worked for several hours privately at home, arrived at the physics institute at nine, worked until twelve-thirty, lunched at home, returned to the institute at four and continued work until eight in the evening, returning home then for dinner. With marriage he had also gained weight.
He and his team of young colleagues occupied the south section of the second floor of the institute, sharing the space with Corbino and with the chief physicist of Rome’s Sanitá Pubblica—its health department—a generous soul named G. C. Trabacchi who lent Corbino’s boys some of the instruments and supplies they needed for their experiments (in return they cherished him, nicknaming him “Divine Providence”). Antonino Lo Sordo, a frustrated old-guard physicist, fended off the encroaching horde from an office at the north end of the floor. Corbino and his family lived above, the residence overlooking a private garden in back with a goldfish pond at its focus. The first floor served students; the basement held electrical generators and a lead-lined safe for the Sanitá’s gram of radium, worth 670,000 lire—about $34,000—in that year of its most historic use. Glass pipes passed through a wall of the special safe to carry radon, formed in the decay of radium, to a compact extraction plant, a modest refinery of glasspipe towers that purified and dried the radioactive gas. The residential upper story of the institute, contracted above the longer lower floors to make room at one end for the dome of a small rotunda, was roofed with tile. “The location of the building in a small park on a hill near the central part of Rome was convenient and beautiful at the same time,” Segrè recalls. “The garden, landscaped with palm trees and bamboo thickets, with its prevailing silence (except at dusk, when gatherings of sparrows populated the greenery), made the institute a most peaceful and attractive center of study.”790 A gravel path that shone white in the golden Roman sun led down to the Via Panisperna.
As usual, Fermi hewed the neutron experiments by hand. In February and early March he personally assembled crude Geiger counters from aluminum cylinders acquired by cutting the bottoms off tubes of medicinal tablets.791 Wired, filled with gas, their ends sealed and leads attached, the counters were slightly smaller than rolls of breath mints and a hundred times less efficient than modern commercial units, but with Fermi to operate them they served. While he built Geiger counters he asked Rasetti to prepare a neutron source in the form of polonium evaporated onto beryllium. Since polonium emits relatively low-energy alpha particles, the resulting source emitted relatively few neutrons per second, and Fermi and Rasetti irradiated several samples without success.
At that point Rasetti, showing a surprising lack of eagerness for historic experiment, went off to Morocco for Easter vacation. Fermi cast about for some way to acquire a stronger neutron source. The rationale for using polonium in the first place, in Paris and Cambridge and Berlin as well as in Rome, had been that a stronger alpha emitter like radon also emitted strong beta and gamma radiation, which disturbed the instruments and interfered with measurements. Fermi realized suddenly that since he was trying to observe a delayed effect, he was measuring only after he removed the neutron source in any case—and therefore any beta and gamma radiation wouldn’t matter and he could use radon. Trabacchi had the radon to spare and willingly dispensed it; with a half-life of only 3.82 days it was perishable in any case and his glowing gram of radium continually exhaled a fresh draft.
To the basement of the physics institute on the Via Panisperna, in his gray lab coat, in mid-March, Fermi thus carried a snippet of glass tubing no larger than the first joint of his little finger. It was flame-sealed at one end and partly filled with powdered beryllium. He set the sealed end of this capsule into a container of liquid air. The radon, directed from the outlet of the extraction plant into the capsule, condensed on its walls in the—200 °C cold. Fermi then had to attempt quickly to heat and draw closed the other end of the capsule, without cracking the glass, before the radon evaporated and escaped. When he succeeded, he finished preparing the neutron source by dropping it into a two-foot length of glass tubing of larger diameter and sealing it into the far end so that it could be handled at a distance safe from dangerous exposure to its gamma rays. For all the tedious preparation its useful life was brief.
In the beginning Fermi worked alone. He intended eventually to irradiate most of the elements in the periodic table and he started methodically with the lightest. His source, he calculated, supplied him with more than 100,000 neutrons per second.792 “Small cylindrical containers filled with the substances tested,” he would explain in his first report, “were subjected to the action of the radiation from this source during intervals of time varying from several minutes to several hours.”793 Fermi first irradiated water—testing hydrogen and oxygen at the same time—then lithium, beryllium, boron and carbon without inducing them to radioactivity. Laura Fermi says he wavered then, discouraged by the lack of results, but Fermi seldom talked shop at home and doubt seems unlikely: he knew from the Joliot-Curie work that aluminum, a little farther along, reacted with alphas, and neutrons should prove even more effective.
In any case he succeeded on his next attempt, with fluorine: “Calcium fluoride, irradiated for a few minutes and rapidly brought into the vicinity of the counter, causes in the first few moments an increase of pulses; the effect decreases rapidly, reaching the half-value in about 10 seconds.”
Soon he found a radioactivity in aluminum with a half-life of twelve minutes, different from the Joliot-Curies’ discovery. Putting aluminum first to link his work with theirs, he reported his findings in a letter to the Ricerca Scientifica on March 25, 1934.
A Roman numeral I distinguishes that first report on “Radioactivity induced by neutron bombardment.” The search was on. To move it along Fermi recruited Amaldi and Segrè and cabled Rasetti in Morocco to rush home. Segrè writes:
We organized our activities this way: Fermi would do a good part of the experiments and calculations. Amaldi would take care of what we would now call the electronics, and I would secure the substances to be irradiated, the sources, etc. Now, of course, this division of labor was by no means rigid, and we all participated in all phases of the work, but we had a certain division of responsibility along these lines, and we proceeded at great speed. We needed all the help we could get, and we even enlisted the help of a younger brother of one of the students (probably 12 years old), persuading him that it was most interesting and important that he should prepare some neat paper cylinders in which we could irradiate our stuff.794
The next letter that went to the Ricerca Scientifica (and in summary form to Nature) reported artificially induced radioactivity in iron, silicon, phosphorus, chlorine, vanadium, copper, arsenic, silver, tellurium, iodine, chromium, barium, sodium, magnesium, titanium, zinc, selenium, antimony, bromine and lanthanum.795 By then they had established a routine: they irradiated substances at one end of the second floor and tested them under the Geiger counters at the other end, down a long hall. That shielded the counters from stray radiation from the neutron source. But it also meant, whenever the half-life of an induced radioactivity was short, that someone had to run down the hall. “Amaldi and Fermi prided themselves on being the fastest runners,” Laura Fermi notes, “and theirs was the task of speeding short-lived substances from one end of the corridor to the other. They always raced, and Enrico claims that he could run faster than Edoardo. But he is not a good loser.”796 A dignified Spaniard showed up one day to confer with “His Excellency Fermi.” Rome’s young professor of theoretical physics, a dirty lab coat flying out behind him, nearly knocked the visitor down.
They came, finally, to uranium. They had roughly classified the effects they were seeing. Light elements generally transmuted to lighter elements by ejecting either a proton or an alpha particle. But the electrical barrier around the nucleus works against exits as well as entrances, and that barrier increases in strength with increasing atomic number. So heavy elements got heavier, not lighter: they captured the bombarding neutron, threw off its binding energy by emitting gamma radiation, and thus, with the addition of the neutron’s mass, but with no added or subtracted charge, became a heavier isotope of themselves. Which then decayed by the delayed emission of a negative beta ray to an element with one more unit of atomic number. Uranium did the same; after a delay it emitted a beta electron. That should mean, Fermi realized, that bombarding uranium with neutrons was producing first a heavier isotope, uranium 239, and then a new, man-made transuranic element, atomic number 93, something never seen on earth before.
It was necessary to purify their uranium sample (uranium nitrate in solution, a light yellow liquid) of the obscuring beta activity its natural decay products gave off. (Uranium decays naturally through a series of fourteen complex steps down the periodic table to thorium, protactinium, radium, radon, polonium and bismuth to lead.) Trabacchi in his generosity had by then even lent the group a young chemist, Oscar D’Agostino, fresh from training in radiochemistry on the Rue Pierre Curie; D’Agostino accomplished the laborious purification in early May. They were using stronger sources now, up to 800 millicuries of radon, about a million neutrons per second.797 Irradiating the uranium nitrate gave “a very intense effect with several periods [of half-lives]: one period of about 1 minute, another of 13 minutes besides longer periods not yet exactly determined”—thus their May 10 report.798
These several induced radioactivities were all beta emitters. They made whatever atom was emitting them heavier by one atomic number. It seemed to follow, then, that they were transmutations up the periodic table into the uncharted new region of man-made elements. To confirm that stunning possibility Fermi needed to demonstrate with chemical separations that the neutron bombardment was not unaccountably creating elements lighter than uranium. The one-minute half-life was too short to work with, so he concentrated on the thirteen-minute substance. D’Agostino diluted the irradiated uranium nitrate with 50 percent nitric acid, dissolved into the acid a small amount of manganese salt and set the solution to boil. By adding sodium chlorate to the boiling solution he precipitated crystals of manganese dioxide. When he filtered the crystals from the solution the radioactivity went with the manganese, much as the radioactivity the Joliot-Curies had induced in aluminum went off with the hydrogen gas. If the radioactivity could be precipitated out of the uranium solution along with a manganese carrier, then it must not be uranium anymore.
By adding other carriers and precipitating other compounds D’Agostino proved that the thirteen-minute substance was neither protactinium (91), thorium (90), actinium (89), radium (88), bismuth (83) nor lead (82). Its behavior excluded elements 87 (then known as ekacesium), and 86 (radon). Element 85 was unknown. Perhaps because the half-lives were different, Fermi made no attempt to check polonium (84). But he felt he had been sufficiently thorough. “This negative evidence about the identity of the 13 min-activity from a large number of heavy elements,” he reported cautiously in Nature in June, “suggests the possibility that the atomic number of the element may be greater than 92.”799
Corbino injudiciously announced “a new element” at the annual convocation, the King of Italy in attendance, that closed the academic year, which set the press baying and gave Fermi a few sleepless nights.800 Having so splendidly accomplished Szilard’s “rather boring task,” the weary physicist was happy to depart after that with his wife and their small daughter Nella for a summer lecture tour sponsored by the Italian government through Argentina, Uruguay and Brazil.
* * *
Leo Szilard had emerged from his bath that spring of 1934 to pursue his favorite causes, not yet joined, of releasing the energy of the nucleus and of saving the world. In a late-April memorandum condemning the recent Japanese occupation of Manchuria he seemed to look ahead to a far future: “The discoveries of scientists,” he wrote, “have given weapons to mankind which may destroy our present civilization if we do not succeed in avoiding further wars.”801 He probably meant military aircraft; the horrors of strategic bombing and even its potential for deterrence through a balance of terror were much bruited at mid-decade. But almost certainly he was also thinking of atomic bombs.
Several weeks earlier, looking for a patron, he had sent Sir Hugo Hirst, the founder of the British General Electric Company, a copy of the first chapter of The World Set Free. “Of course,” he wrote Sir Hugo with a touch of bitterness, still brooding on Rutherford’s prediction, “all this is moonshine, but I have reason to believe that in so far as the industrial applications of the present discoveries in physics are concerned, the forecast of the writers may prove to be more accurate than the forecast of the scientists. The physicists have conclusive arguments as to why we cannot create at present new sources of energy for industrial purposes; I am not so sure whether they do not miss the point.”802
That Szilard saw beyond “energy for industrial purposes” to the possibility of weapons of war is evident in his next patent amendments, dated June 28 and July 4, 1934. Previously he had described “the transmutation of chemical elements”; now he added “the liberation of nuclear energy for power production and other purposes through nuclear transmutation.” He proposed for the first time “a chain reaction in which particles which carry no positive charge and the mass of which is approximately equal to the proton mass or a multiple thereof [i.e., neutrons] form the links of the chain.”803 He described the essential features of what came to be known as a “critical mass”—the volume of a chain-reacting substance necessary to make the chain reaction self-sustaining.804 He saw that the critical mass could be reduced by surrounding a sphere of chain-reacting substance with “some cheap heavy material, for instance lead,” that would reflect neutrons back into the sphere, the basic concept for what came to be known (by analogy with the mud tamped into drill holes to confine conventional explosives) as “tamper.” And he understood what would happen if he assembled a critical mass, spelling out the results simply on the fourth page of his application:805
If the thickness is larger than the critical value . . . I can produce an explosion.
As if to mark in some distant inhuman ledger the end of one age and the beginning of another, Marie Sklodowska Curie, born in Warsaw, Poland, on November 7, 1867, died that day of Szilard’s filing, July 4, 1934, in Savoy. Einstein’s was the best eulogy: “Marie Curie is,” he said, “of all celebrated beings, the only one whom fame has not corrupted.”806
There is nothing in the documentary record to indicate that Szilard was yet thinking of uranium. His June amendment describes a possible chain reaction using light, silvery beryllium, element number 4 on the periodic table.
To study that metal Szilard needed access to a laboratory and a source of radiation. The beryllium nucleus was so lightly bound he suspected he could knock neutrons out of it not only with alpha particles or neutrons but even with gamma rays or high-energy X rays. Radium emitted gamma rays and radium was available conveniently at the nearest large hospital. So Szilard, an unusually practical visionary, dropped in to see the director of the physics department at the medical college of St. Bartholomew’s Hospital. Couldn’t he use St. Bart’s radium, “which was not much in use in summertime,” for experiments? Something of value to medicine might emerge.807 The director thought he could if he teamed up with someone on the staff. “There was a very nice young Englishman, Mr. [T. A.] Chalmers, who was game, and so we teamed up and for the next two months we did experiments.”
Their first experiment demonstrated a brilliantly simple method for separating isotopes of iodine by bombarding an iodine compound with neutrons. They then used this Szilard-Chalmers effect (as it came to be called), which was extremely sensitive, as a tool for measuring the production of neutrons in their second experiment: knocking neutrons out of beryllium using the gamma radiation from radium. “These experiments,” Szilard reminisces wryly, “established me as a nuclear physicist, not in the eyes of Cambridge, but in the eyes of Oxford. [Szilard had in fact applied to Rutherford that spring to work at the Cavendish and Rutherford had turned him down.] I had never done work in nuclear physics before, but Oxford considered me an expert. . . . Cambridge . . . would never had made that mistake. For them I was just an upstart who might make all sorts of observations, but these observations could not be regarded as discoveries until they had been repeated at Cambridge and confirmed.”808, 809
If Szilard’s summer work helped establish his Oxford reputation, it was also a personal disappointment: beryllium proved an unsatisfactory candidate for chain reaction. The problem, not settled until 1935, lay with the established mass of helium.810 The one stable isotope of beryllium consists of two helium nuclei lightly bound by a neutron. Its apparently high mass, which was calculated from Francis Aston’s measurements of the mass of helium, seemed to indicate that it should be unstable. But the mass spectrograph was a skittish instrument even in the hands of its inventor, and as Bethe, Rutherford and others were about to demonstrate, Aston’s measurements were inaccurate: he had set the mass of helium too high. One casualty of that error was beryllium’s candidacy for chain reaction, for nuclear power and atomic bombs.
* * *
Emilio Segrè and Edoardo Amaldi pilgrimaged to Cambridge early in July, short on English but carrying with them a comprehensive report on the Rome neutron-bombardment investigations.811 They met Chadwick, Kapitza and the other regulars at the Cavendish; observed the retired J. J. Thomson making his rounds; noted Aston, says Amaldi innocently, “going on improving the accuracy of his measurements of atomic masses”; and had a memorable meeting with Rutherford, whose “strong personality dominated the whole laboratory.”
The two young physicists had come to compare experiments with two of Rutherford’s boys. An unanswered question hung over the neutron work, a question that called existing nuclear theory into doubt.812 The Nature paper they brought with them discussed the difficulty frankly. It concerned what is called “radiative capture,” the typical reaction of the heavy elements to neutron bombardment: a nucleus captures a neutron, emits a photon of gamma radiation to stabilize itself energetically and thus becomes an isotope one mass unit heavier.
Theory at the time treated the nucleus as if it was one large particle. As such, it had a definite diameter, which was modest enough that a speeding neutron could go in one side and exit out the other in about 10-21 seconds, a billion times less than a trillionth of a second. Any capture process would have to work within that brief interval of time. Otherwise the neutron would be gone. Capturing a neutron means stopping it within a nucleus. To do that the nucleus has to absorb the neutron’s energy of motion. The nucleus in turn has to get rid of the excess energy. Which it does: by emitting a gamma photon.
But the gamma-emission times Fermi’s group had measured were different from what theory said they ought to be. The nuclei the Rome group had studied took at least 10-16 seconds to get around to gamma emission—one hundred thousand times too long. And that was unaccountable.
Definite proof of radiative capture would sharpen the challenge to theory. That required proving beyond doubt, by experiment, that a heavier isotope really forms when a heavy nucleus captures a neutron. The Cavendish team Segrè and Amaldi came to visit in the summer of 1934 accomplished the first part of the proof, using sodium, while the Italians were on hand. They then returned to Rome and enlisted D’Agostino’s help to perform the confirming chemistry. In the heat of Roman August they looked for additional clear-cut examples and won a double prize: “We also found a second case of ‘proven’ radiative capture,” Amaldi writes, “which was based on the discovery of a new radioisotope of [aluminum] with a lifetime of almost 3 minutes.”813
Fermi planned to stop off in London for an international physics conference on his way home from South America. His young colleagues sent him word of their aluminum discovery. He reported to the conference on the neutron work. (Szilard also attended, happy to hear praise for his summer experiments and well launched toward a paying fellowship at Oxford.) Fermi said his group had studied sixty elements so far and had induced radioactivity in forty of them. Discussing the radiative-capture problem he cited the Cavendish results “and those of Amaldi and Segrè on aluminium,” which were both, he said, “to be considered particularly important.”814, 815 Segrè describes the tempestuous aftermath:
Shortly afterwards I caught a cold and could not go to the laboratory for several days. Amaldi tried to repeat our experiments and found a different [half-life] for irradiated aluminum which showed that our so-called (n,γ) reaction [i.e., neutron in, gamma photon out] did not occur. This was hurriedly relayed to Fermi who resented having communicated a result which now looked to be in error. He strongly criticized us and did not conceal his displeasure. The whole business was becoming very troublesome because we could not find any fault with the various experiments which gave inconsistent results.816
The chastened junior members had their work cut out for them. A new recruit joined them, a tall, broad-shouldered, handsome tennis champion from Pisa named Bruno Pontecorvo, as they set about polishing their first rough work. Neutron bombardment activated some elements more intensely than others. They had previously categorized that activation only generally as strong, medium or weak. Now they proposed to establish a quantitative scale of activibility. They needed some standard intensity against which to measure the intensity of other activations. They chose the convenient 2.3-minute half-life period that neutron bombardment induced in silver.
Amaldi and Pontecorvo got the assignment. They immediately found, to their surprise, that their silver cylinders activated differently in different parts of the laboratory. “In particular,” writes Amaldi, “there were certain wooden tables near a spectroscope in a dark room which had miraculous properties, since silver irradiated on those tables gained much more activity than when it was irradiated on a marble table in the same room.”817
That was a mystery worth exploring. On October 18 they started a systematic investigation, a series of measurements made inside and outside a lead housing. By October 22 they were prepared to measure what might happen when only a lead wedge separated the neutron source from its target. But the experimenters had to give student examinations that morning and Fermi decided to go ahead on his own. He described the historic moment late in life to a colleague curious about the process of discovery in physics:
I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came into the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something: I tried every excuse to postpone putting the piece of lead in its place. When finally, with some reluctance, I was going to put it 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 conscious prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.818
The extraordinary result of substituting paraffin wax for a heavy element like lead was a dramatic increase in the intensity of the activation. “About noon,” Segrè remembers, “everybody was summoned to watch the miraculous effects of the filtration by paraffin. At first I thought a counter had gone wrong, because such strong activities had not appeared before, but it was immediately demonstrated that the strong activation resulted from the filtering by the paraffin of the radiation that produced the radioactivity.”819Laura Fermi says “the halls of the physics building resounded with loud exclamations: ‘Fantastic! Incredible! Black magic!’ ”820
Not even his most important discovery kept Fermi from going home for lunch. He was alone; his wife and daughter would not return from a visit to the country until the following morning. He pondered in solitude and may have considered the difference between wood and marble tables as well as between paraffin and lead. When he returned in midafternoon he proposed an answer: the neutrons were colliding with the hydrogen nuclei in the paraffin and the wood. That slowed them down. Everyone had assumed that faster neutrons were better for nuclear bombardment because faster protons and alpha particles always had been better. But the analogy ignored the neutron’s distinctive neutrality. A charged particle needed energy to push through the nucleus’ electrical barrier. A neutron did not. Slowing down a neutron gave it more time in the vicinity of the nucleus, and that gave it more time to be captured.
The simple way to test Fermi’s theory was to try some other material besides paraffin that contained hydrogen (other light nuclei would also work to slow neutrons down, but hydrogen would work best: its nuclei are protons, about the same size and mass as neutrons, and they therefore bounce hardest and soak up the most energy per collision). Down to the first floor and out the back door they marched with their silver cylinder and their neutron source extended in its long glass tube, to the pond in Corbino’s garden where Rasetti had experimented with raising salamanders, where they had all caught the fad one summer of sailing candle-powered toy boats, where the dark, curving leaves and leathery gray drupes of an almond tree shaded the lively goldfish.
The hydrogen in water (and in goldfish) worked as well as paraffin.821 Back in the lab they quickly tested whatever they could lay hands on to irradiate: silicon, zinc, phosphorus, which did not seem to be affected by the slow neutrons; copper, iodine, aluminum, which did. They tried radon without beryllium to make sure the paraffin was affecting neutrons and not gamma rays. They replaced the paraffin with an oxygen compound and found much less increase in induced radioactivity.
They went home to dinner but met afterward at Amaldi’s, whose wife had a typewriter, to prepare a first report. “Fermi dictated while I wrote,” Segrè remembers. “He stood by me; Rasetti, Amaldi, and Pontecorvo paced the room excitedly, all making comments at the same time.”822 Laura Fermi recreates the scene: “They shouted their suggestions so loudly, they argued so heatedly about what to say and how to say it, they paced the floor in such audible agitation, they left the Amaldis’ house in such a state, that the Amaldis’ maid timidly inquired whether the guests had all been drunk.”823
Ginestra Amaldi delivered the typed paper, “Influence of hydrogenous substances on the radioactivity produced by neutrons—I,” to the director of the Ricerca Scientifica the next morning.824 Tucked away in its historic paragraphs was a quiet justification for the confusion over aluminum: “The case of aluminum is noteworthy. In water it acquires an activity showing a period slightly shorter than 3 minutes. . . . This activity under normal conditions is so weak that it almost disappears compared to other activities generated in the same element.”825
Amaldi and Segrè had not been wrong about aluminum. They had simply irradiated different samples of the element on different tables. The hydrogen in the wooden table had slowed down some of the neutrons and enhanced the almost-three-minute activity. As Hans Bethe once noted wittily, the efficiency of slow neutrons “might never have been discovered if Italy were not rich in marble. . . . A marble table gave different results from a wooden table.826 If it had been done [in America], it all would have been done on a wooden table and people would never have found out.”
The discovery of slow-neutron radioactivity meant that Fermi’s group had to work its way through the elements again looking for different and enhanced half-lives—which is to say, different isotopes and decay products.
While that work proceeded a paper appeared in the Physical Review criticizing the group’s earlier study of uranium.827 The paper’s primary author was Aristide von Grosse, who had been one of Otto Hahn’s assistants at the KWI and who had purified the first substantial sample of protactinium, the element Hahn and Meitner had discovered in 1917. Von Grosse argued that when Fermi irradiated uranium he had created protactinium, atomic number 91, not a new transuranic element. The Rome group took the paper as a challenge to further experiment. At the same time Hahn and Meitner decided proprietarily to repeat Fermi’s previous uranium work. “It was a logical decision,” Hahn explains in his scientific autobiography; “having been the discoverers of protactinium, we knew its chemical characteristics.”828 The increasing number of different half-lives that investigators in Berlin and Paris found when they irradiated uranium were puzzling; Hahn correctly felt that he was better qualified than anyone else in the world to accomplish the subtle radiochemistry necessary to sort everything out.
In January and February 1935, in the midst of other projects, Amaldi set to work looking for alpha-emitting reactions in uranium in addition to the beta reactions the group had originally found. If uranium emitted alpha particles when it captured neutrons it would be transmuting down the periodic table rather than up, which might indeed produce protactinium along the way. Amaldi chose to use an ionization chamber connected to a linear amplifier to capture and measure the radiation. “I began to irradiate some foil[s] of uranium,” he writes, “ . . . and put them immediately after irradiation in front of the thin-window ionization chamber.”829 Nothing happened. Conceivably the half-lives were too brief for the run down the hall from the irradiation area to the ionization chamber. Amaldi decided to try irradiating his samples directly in front of the chamber. That required screening out unwanted radiation. The gamma rays from his neutron source, which would have disturbed the ionization chamber, he blocked by setting a piece of lead between the source and the chamber: the desirable neutrons would find the lead no obstacle.
He also wanted to filter out uranium’s natural alpha background. To do that he took advantage of the basic law of radioactivity that shorter half-lives mean more energetic radiation. The half-life of natural uranium is about 4.5 billion years; its alphas are proportionately mild, mild enough to be blocked by a layer of aluminum foil. On the other hand, if there really were half-lives in his experiment so short that he had to irradiate directly in front of the ionization chamber to catch them, their alphas should be energetic enough to breeze easily through the aluminum and the chamber window and enter the chamber for counting. So Amaldi wrapped his uranium samples with aluminum foil. It did not occur to him that his shielding might also screen out other reaction products. In 1935, alpha, beta and gamma radiation were the only reaction products anyone knew. “The experiments,” Amaldi concludes, “gave negative results.”830 He found no artificially induced alphas from uranium.
The Italians thought it even more probable then that by irradiating uranium they were creating new, man-made elements. Hahn and Meitner reported they thought so too. Fermi’s group rounded up its work in the Proceedings of the Royal Society in a paper Rutherford approvingly passed along to that journal on February 15:
Through these experiments our hypothesis that the 13-minute and 100-minute induced activities of uranium are due to transuranic elements seems to receive further support. The simplest interpretation consistent with the known facts is to assume that the 15-second, 13-minute and 100-minute activities are chain products [i.e., one decays into the next], probably with atomic number 92, 93 and 94 respectively and atomic weight 239.831
But the truth was, uranium was a confusion, and no one yet knew.
What else besides beryllium? Leo Szilard asked himself in London. Beryllium looked suspicious. What other elements might chain-react? He answered with an amended patent specification on April 9, 1935: “Other examples for elements from which neutrons can liberate multiple neutrons are uranium and bromine.”832 He was guessing, and without research funds he saw no way to experiment. The physicists he talked to remained profoundly skeptical of his ideas. “So I thought, there is after all something called ‘chain reaction’ in chemistry. It doesn’t resemble a nuclear chain reaction, but still it’s a chain reaction. So I thought I would talk to a chemist.”833 The chemist he thought he would talk to was someone even more skillful than Leo Szilard at raising funds: Chaim Weizmann, who now lived and worked in London. Weizmann received Szilard and “understood what I told him.”834 He asked Szilard how much money he needed. Szilard said £2,000—about $10,000. Though he was certainly hard-pressed for funding himself, Weizmann said he would see what he could do. Szilard recalls:
I didn’t hear from him for several weeks, but then I ran into Michael Polanyi, who by that time had arrived in Manchester and was head of the chemistry department there. Polanyi told me that Weizmann had come to talk to him about my ideas for the possibility of a chain reaction, and he wanted Polanyi’s advice on whether he should get me this money. Polanyi thought that this experiment should be done.
A decade passed before Szilard and Weizmann met again, a gulf of history. Weizmann had not neglected Szilard’s request, he explained then in apology in late 1945; he had only not succeeded in raising the funds.
Since the beginning of his rescue work in England Szilard had been in occasional contact with the physicist Frederick Alexander Lindemann, who was professor of experimental philosophy at Oxford and director of the Clarendon Laboratory there.835 It was Lindemann, wealthy and well-connected, who was arranging a fellowship for Szilard, part of his continuing campaign to arm the decrepit Oxford science laboratory against its splendid Cambridge rival. Lindemann had made effective use in that campaign of the Nazi expulsion of the Jewish academics but had given as good as he got: immediately upon hearing of the civil service law he had gone to Imperial Chemical Industries and convinced its directors to establish a grant program, arguing that such an investment would be not charity but money well spent. ICI had already begun paying out its first grant on May 1, 1933, while Beveridge and Szilard were still laying plans. It was an ICI grant that Szilard missed winning the following August, perhaps because he had not yet accomplished his summer of impressive experiment at St. Bart’s, but Lindemann was paying attention now.
The tall, handsome Englishman, forty-nine years old in 1935, had been born in Germany, at Baden-Baden, because his mother chose not to allow advanced pregnancy to interfere with a visit to that fashionable spa. To provide their son with an outstanding education his English parents had sent him to the Gymnasium in Darmstadt. As a student before the Great War at the Darmstadt Technische Hochschule, where he was a protégé of the physical chemist Walther Nernst (the 1920 chemistry Nobelist), he had enjoyed such exceptional family connections that he found himself at times playing tennis with the Kaiser or the Czar. Inevitably the war made suspect such golden afternoons. Lindemann was chagrined and angered in 1915 to find that the British Army, noting his German birth certificate and German-sounding name, was unwilling to extend him a commission.
The Army’s decision injured him deeply and may have changed his life. He had served as a co-secretary to the 1911 Solvay Conference, standing up proudly with Nernst, Rutherford, Planck, Einstein, Mme. Curie, but even before that youthful apotheosis Nernst had predicted difficulty: “If your father were not such a rich man,” the blunt German had said, “you would become a great physicist.”836 When the Army questioned Lindemann’s patriotism, writes a refugee colleague, “he became withdrawn to avoid exposing himself to slights and insults. Secretiveness about his personal life developed into a mania and he discouraged personal approaches by a stand-offishness which was easily mistaken for arrogance.”837 Lindemann retreated from original work and became a talented administrator, “the Prof,” an “unbending Victorian gentleman,” always impeccable in bowler hat, summer gray suit, winter dark suit, rolled-up umbrella and long, dark coat.838 If he could not win a uniform he would adopt one of his own.
He worked for his country during the war at the Royal Aircraft Factory at Farnborough, designing what are now called avionics and doing aeronautical research. Tailspins were recognized maneuvers in air fighting by 1916, a good way to shake off an attacker. Lindemann was the first to study them scientifically. To do so he took flying lessons—only changing from civilian clothes to flying clothes on the runway beside the plane—then coolly flew spin after spin, memorizing his instrument readings as he plummeted and writing them down after he had recovered level flight.
After the war Lindemann accepted appointment to an Oxford still donnishly disdainful of science. He escaped from that further condescension, says his colleague, into “gracious living,” enjoying weekends with the nobility that were seldom vouchsafed to less well-born Oxford dons. By then a Rolls-Royce was part of his regalia. In June 1921, on a weekend at the country estate of the Duke and Duchess of Westminster, Lindemann met Winston Churchill, twelve years his senior. “The two men, so different in background and character, took to each other immediately and their acquaintance soon turned into a close friendship.”839 Churchill recalled that he “saw a great deal of Frederick Lindemann” during the 1930s. “Lindemann was already an old friend of mine. . . . We came much closer from 1932 onwards, and he frequently motored over from Oxford to stay with me at Chartwell. Here we had many talks into the small hours of the morning about the dangers which seemed to be gathering upon us. Lindemann . . . became my chief adviser on the scientific aspects of modern war.”840
To this illustrious personage, a vegetarian who daily consumed copious quantities of olive oil and Port Salut, Szilard turned in the early summer of 1935 to discuss “the question whether or not the liberation of nuclear energy . . . can be achieved in the immediate future.” If “double neutrons” could be produced, Szilard wrote Lindemann on June 3, “then it is certainly less bold to expect this achievement in the immediate future than to believe the opposite.” That meant trouble, Szilard thought, if Germany achieved a chain reaction first, and he argued for “an attempt, whatever small chance of success it may have . . . to control this development as long as possible.”841 Secrecy was the way to achieve such control: first, by winning agreement from the scientists involved to restrict publication, and second, by taking out patents.
Michael Polanyi had cautioned Szilard late in 1934 that “there is an opposition to you on account of taking patents.”842 The British scientific tradition that opposed patents assumed that those who filed them did so for mercenary purposes; Szilard explained his patents to Lindemann to clear his name:
Early in March last year it seemed advisable to envisage the possibility that . . . the release of large amounts of energy . . . might be imminent. Realising to what extent this hinges on the “double neutron,” I have applied for a patent along these lines. . . . Obviously it would be misplaced to consider patents in this field private property and pursue them with a view to commercial exploitation for private purposes. When the time is ripe some suitable body will have to be created to ensure their proper use.843
For the time being, Szilard proposed to work at Oxford on finding his “double neutrons,” possibly raising £1,000 on the side “from private persons” so that he could hire a helper or two. To bait Lindemann’s Clarendon ambitions, he argued in conclusion that “this type of work could greatly accelerate the building up of nuclear physics at Oxford.”844 As indeed, had it gone forward, it might have done.
When he learned, possibly from Lindemann, that he could keep his patents secret only by assigning them to some appropriate agency of the British government, Szilard offered them first to the War Office. Director of Artillery J. Coombes turned them down on October 8, noting that “there appears to be no reason to keep the specification secret so far as the War Department is concerned.”845 If Lindemann heard of the rejection he must have remembered his own rejection by the Army in 1915. The following February 1936, he intervened on Szilard’s behalf with the Admiralty, Churchill’s old bailiwick, writing the head of the Department of Scientific Research and Development cannily:
I daresay you remember my ringing you up about a man working here who had a patent which he thought ought to be kept secret. I enclose a letter from him on the subject as you suggested. I am naturally somewhat less optimistic about the prospects than the inventor, but he is a very good physicist and even if the chances were a hundred to one against it seems to me it might be worth keeping the thing secret as it is not going to cost the Government anything.846
The patent, Szilard explained in the letter Lindemann enclosed, “contains information which could be used in the construction of explosive bodies . . . very many thousand times more powerful than ordinary bombs.”847 He was concerned about “the disasters which could be caused by their use on the part of certain Powers which might attack this country.” Wisely and withal inexpensively the Admiralty accepted the patent into its safekeeping.
* * *
Eight months in Copenhagen had suited Edward Teller. He met George Gamow on the Odessan’s last visit there, after the Solvay Conference of the previous autumn; the two of them roared across Denmark and back during Easter vacation on Gamow’s motorcycle, working over a problem in quantum mechanics. The Rockefeller Foundation did not approve of marriage during a fellowship period, but James Franck had interceded on his behalf and Teller had married his childhood sweetheart, Mici Harkanyi, in Budapest on February 26. He had also written an important paper. He returned to London with Mici in the summer of 1934 with his reputation enhanced and again took up his lectureship at University College. Assuming they would settle in England, the Tellers signed a nine-year lease just before Christmas on a pleasant three-room flat.
Two offers arrived in January, one of which changed Teller’s mind. The first was from Princeton: a lectureship. The second was from Gamow: a full professorship at George Washington University. GWU wanted to strengthen its physics department; Gamow wanted company and liked Teller’s verve.
Teller was twenty-six years old and a newlywed. He was less than sure about living in the United States, but a full professorship was not something he could sensibly refuse. His wife found someone to sublet the flat. The U.S. State Department refused nonquota immigration visas because Teller had only taught for one year—the Copenhagen time counted merely as a fellowship—and was required to have taught for two. He had not, however, tried for visas on the Hungarian immigration quota because he assumed the quota was full. In fact there was room. The Tellers followed the Gamows across the Atlantic in August 1935.
* * *
Niels Bohr celebrated his fiftieth birthday on October 7. “Bohr in those days seemed at the height of his powers, bodily and mentally,” Otto Frisch observes. “When he thundered up the steep staircase [of the institute], two steps at a time, there were few of us younger ones that could keep pace with him. The peace of the library was often broken by a brisk game of pingpong, and I don’t remember ever beating Bohr at that game.”848 To honor Denmark’s leading physicist, George de Hevesy organized a fund-raising campaign; the Danish people contributed 100,000 kroner to buy Bohr 0.6 gram of radium for his birthday. De Hevesy divided the radium, in liquid solution, into six equal parts, mixed each with beryllium powder and allowed them to dry, making six potent neutron sources. He had them mounted on the ends of long rods and stored them in a dry well in the basement of the institute that had been dug originally to supply vibration-free housing for a spectrograph.
The institute’s annual Christmas party continued to be held in the well room, Stefan Rozental recalls: “The lid of the well served as a table, a Christmas tree stood in the middle, and all the personnel were gathered, from the chief down to the youngest apprentice in the workshop, and served a modest meal of sausages and beer. During the party Niels Bohr used to make a speech in which he gave a sort of survey of the past year.”849 Safely below the sausages, stuck in a gallon flask of carbon disulphide, the neutron sources silently transmuted sulfur to radioactive phosphorus for de Hevesy’s biological radioisotope studies.
Bohr had won national distinction for his work and the enduring gratitude of refugees for his aid; he had also faced personal pain. In 1932 the Danish Academy offered him lifetime free occupancy of the Danish House of Honor, a palatial estate in Pompeiian style built originally for the founder of Carlsberg Breweries and subsequently reserved for Denmark’s most distinguished citizen (Knud Rasmussen, the polar explorer, was its previous occupant). By then the institute buildings included a modest director’s house, but the Bohrs shared it with five handsome sons. They moved to the mansion beside the brewery, the best address in Denmark after the King’s.
Two years later an accident took the Bohrs’ eldest son, Christian, nineteen years old. Father, son and two friends were sailing on the Öresund, the sea passage between Denmark and Sweden, when a squall blew up. Christian “was drowned by falling over[board] in a very rough sea from a sloop,” Robert Oppenheimer reports, “and Bohr circled as long as there was light, looking for him.”850 But the Öresund is cold. For a time Bohr retreated into grief. Exhausting as it was, the refugee turmoil helped him.
Everyone at the institute followed Fermi’s neutron work with fascination. Frisch, the only physicist on hand who knew Italian, was drafted to translate the successive papers aloud as soon as each issue of the Ricerca Scientifica arrived. The Copenhagen group was puzzled that slow neutrons affected some elements more intensely than others; on the one-particle model of the nucleus even a slow neutron should almost always shoot completely through a nucleus without capture.
From Cornell Hans Bethe published a paper calculating the slim odds of neutron capture. They conflicted squarely with observation. Frisch remembers the colloquium in Copenhagen in 1935 when someone reported on Bethe’s paper:
On that occasion Bohr kept interrupting, and I was beginning to wonder, with some irritation, why he didn’t let the speaker finish. Then, in the middle of a sentence, Bohr suddenly stopped and sat down, his face completely dead. We looked at him for several seconds, getting anxious. Had he been taken unwell? But then he suddenly got up and said with an apologetic smile, “Now I understand it.”851
What Bohr understood about the nucleus he embodied in a landmark lecture to the Danish Academy on January 27, 1936, subsequently published in Nature. “Neutron capture and nuclear constitution” exploited the phenomenon of neutron capture to propose a new model of the nucleus; once again, as he had with Rutherford’s planetary model of the atom, Bohr stood on the solid ground of experiment to argue for radical theoretical change.852
He visualized a nucleus made up of neutrons and protons closely packed together—a model now familiar—rather than a single particle. (Nuclear particles collectively are known as nucleons.) A neutron entering such a crowded nucleus would not pass through; it would collide with the nearest nucleons, surrender its kinetic energy (as a cue ball does at break in billiards) and be captured by the strong force that holds the nucleus together. The energy added by the neutron would agitate the nearby nucleons; they would collide in turn with other nucleons beyond; the net effect would be a more generally agitated, “hotter” nucleus but one where no single component could quickly acquire enough energy to push through the electrical barrier and escape. If the nucleus then radiated its excess energy by ejecting a gamma photon, “cooling off,” none of its nucleons could accrue enough energy to escape. The result, already confirmed by Fermi’s experiments, would be the creation of a heavier isotope of the original element being bombarded.
More violent assaults on the nucleus, Bohr thought, would still disperse their energies throughout the compound nucleus created by their capture. Subsequent reconcentration of the energy might allow the nucleus to eject several charged or uncharged particles. Bohr did not think his compound model of the nucleus boded well for harnessing nuclear energy:
For still more violent impacts, with particles of energies of about a thousand million volts, we must even be prepared for the collision to lead to an explosion of the whole nucleus. Not only are such energies, of course, at present far beyond the reach of experiments, but it does not need to be stressed that such effects would scarcely bring us any nearer to the solution of the much discussed problem of releasing the nuclear energy for practical purposes. Indeed, the more our knowledge of nuclear reactions advances the remoter this goal seems to become.853
Thus by the mid-1930s the three most original living physicists had each spoken to the question of harnessing nuclear energy. Rutherford had dismissed it as moonshine; Einstein had compared it to shooting in the dark at scarce birds; Bohr thought it remote in direct proportion to understanding. If they seem less perceptive in their skepticism than Szilard, they also had a better grasp of the odds. The essential future is always unforeseen. They were experienced enough not to long for it.
In his lecture Bohr preferred to state only general principles, but to trace “the consequences of the general argument here developed” he had a specific mathematical model in mind.854 He published a discussion of that model the following year, in 1937. It reached all the way back to his doctoral dissertation on the surface tension of fluids to demonstrate the usefulness of treating the atomic nucleus as if it were a liquid drop.1
The tendency of molecules to stick together gives liquids a “skin” of surface tension. A falling raindrop thus rounds itself into a small perfect sphere. But any force acting on a liquid drop deforms it (think of the wobbles of a water-filled balloon thrown into the air and caught). Surface tension and deforming forces work against each other in complex ways; the molecules of the liquid bump and collide; the drop wobbles and distorts. Eventually the added energy dissipates as heat, and the drop steadies again.
The nucleus, Bohr proposed, was similar. The force that stuck the nucleons together was the nuclear strong force. Counteracting that strong force was the common electrical repulsion of the positively charged nuclear protons. The delicate balance between the two fundamental forces made the nucleus liquidlike. Energy added from the outside by particle bombardment deformed it; it wobbled like a liquid drop, oscillating complexly just as the braided streams of water Bohr had studied for his dissertation had oscillated. Which meant he could use Rayleigh’s classical formulae for the surface tension of liquids to understand the complex nuclear energy levels and exchanges that Fermi’s work had revealed. “This 1937 paper had to close with many issues not cleared up,” writes the American theoretical physicist John Archibald Wheeler, who helped Bohr clear up more of them later.855, 856 The liquid-drop model proved useful, however, and Frisch in Copenhagen and Meitner in Berlin, among others, took it to heart.
* * *
One fine October Thursday in 1937 Ernest Rutherford, a vigorous sixty-six, went out into the garden of his house on the green Cambridge Backs to trim a tree. He took a bad fall. He was “seedy” later in the day, Mary Rutherford said—nausea and indigestion—and she arranged for a masseur.857 Rutherford vomited that night. In the morning he called his family doctor. He suffered from a slight umbilical hernia, which he confined with a truss; his doctor found a possible strangulation, consulted with a specialist and directed the Rutherfords to the Evelyn Nursing Home for emergency surgery. Rutherford told his wife along the way that his business and financial affairs were all in order. She said his illness wasn’t serious and asked him not to worry.
Surgery that evening confirmed a partial strangulation, released the imprisoned portion of the small intestine and restored its circulation. Saturday Rutherford seemed to be recovering but he began vomiting again on Sunday and there were signs of infection, deadly in those days before antibiotics. Monday he was worse; his doctors consulted the surgeon, a Melbourne man, who advised against a second operation given the patient’s age and symptoms. Rutherford was made comfortable with intravenous saline, six pints by Tuesday, and a stomach tube. Tuesday morning, October 19, he was slightly improved, but though his wife judged him “a wonderful patient [who] bears his discomforts splendidly” and believed she discovered “just a thread of hope,” he began that afternoon to weaken.858 A bequest he decided late in the day suggests he found gratitude in those last hours reviewing his life. “I want to leave a hundred pounds to Nelson College,” he told Mary Rutherford.859 “You can see to it.” And again loudly a little later: “Remember, a hundred to Nelson College.” He died that evening. “Heart and circulation failed” because of massive infection, his doctor wrote, “and the end came peacefully.”
An international gathering of physicists in Bologna that week celebrated the 200th anniversary of the birth of Luigi Galvani; Cambridge cabled the news of Rutherford’s death on the morning of October 20. Bohr was on hand and accepted the grim duty of announcement. “When the meeting scheduled for that morning assembled,” writes Mark Oliphant, “Bohr went to the front, and with faltering voice and tears in his eyes informed the gathering of what had happened.”860 They were shocked at the abruptness of the loss. Bohr had visited Rutherford at Cambridge a few weeks earlier; the Cavendish men had seen their leader in fine fettle only days ago.
Bohr “spoke from the heart,” says Oliphant, recalling “the debt which science owed so great a man whom he was privileged to call both his master and his friend.” For Oliphant it was “one of the most moving experiences of my life.” Remembering Rutherford in a letter to Oppenheimer on December 20 Bohr balanced loss with hope, complementarily: “Life is poorer without him; but still every thought about him will be a lasting encouragement.”861 And in 1958, in a memorial lecture, Bohr said simply that “to me he had almost been as a second father.”862
The sub-dean of Westminster immediately approved interment of Rutherford’s ashes in the nave of Westminster Abbey, just west of Newton’s tomb and in line with Kelvin’s. Eulogizing Rutherford at a conference in Calcutta the following January, James Jeans identified his place in the history of science:
Voltaire said once that Newton was more fortunate than any other scientist could ever be, since it could fall to only one man to discover the laws which governed the universe. Had he lived in a later age, he might have said something similar of Rutherford and the realm of the infinitely small; for Rutherford was the Newton of atomic physics.863
Ernest Rutherford unknowingly wrote his own more characteristic epitaph in a letter to A. S. Eve from his country cottage on the first day of that last October. He reported of his garden what he had also done for physics, vigorous and generous work: “I have made a still further clearance of the blackberry patch and the view is now quite attractive.”864
* * *
In September 1934, in the wake of Fermi’s June Nature article “Possible production of elements of atomic number higher than 92,” a curious paper appeared in a publication seldom read by physicists, the Zeitschrift für Angewandte Chemie—the Journal of Applied Chemistry. Its author was a respected German chemist, Ida Noddack, co-discoverer with her husband (in 1925) of the hard, platinum-white metallic element rhenium, atomic number 75. The paper was titled simply “On element 93” and it severely criticized Fermi’s work.865 His “method of proof” was “not valid,” Noddack wrote bluntly. He had demonstrated that “his new beta emitter” was not protactinium and then distinguished it from several other elements descending down the periodic table to lead, but it was “not clear why he chose to stop at lead.” The old view that the radioactive elements form a continuous series beginning at uranium and ending at lead, wrote Noddack, was exactly what the Joliot-Curies’ discovery of artificial radioactivity had disproved. “Fermi therefore ought to have compared his new radioelement with all known elements.”
The fact was, Noddack went on, any number of elements could be precipitated out of uranium nitrate with manganese. Instead of assuming the production of a new transuranic element, “one could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which have not been observed previously.” In the past, elements have transmuted only into their near neighbors. But “when heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors.” They would be, rather, much lighter elements farther down the periodic table than lead.
Segrè remembers reading the Noddack paper.866 He knows, because he asked them, that Hahn in Berlin and Joliot in Paris read it. It made very little sense to anyone. “I think whatever chemists read it,” Frisch reminisces, “probably thought that this was quite pointless, carping criticism, and the physicists possibly even more so if they read it, because they would say, ‘What’s the use of criticizing unless you give some reason why that criticism would be valid?’ Nobody had ever found a nuclear disintegration creating far-removed elements.”867 Which was a point Noddack had carefully addressed, but was clearly one reason for the paper’s neglect. The summary report for Nature on artificial radioactivity that Amaldi and Segrè had delivered to Rutherford in midsummer 1934 makes the assumption explicit: “It is reasonable to assume that the atomic number of the active element should be close to the atomic number . . . of the bombarded element.”868
But Fermi seldom left anything to assumption, however reasonable. He would certainly not have left to assumption this issue, about which he was already acutely sensitive because of Corbino’s ill-timed speech (Noddack rubbed salt into that wound by referring to “the reports found in the newspapers”). He sat down and performed the necessary calculations. He later told at least Teller, Segrè and his American protégé Leona Woods that he had done so.869 Teller is quite sure he knows what those calculations were:
Fermi refused to believe [Noddack]. . . .870 He knew how to calculate whether or not uranium could break in two. . . . He performed the calculation Mrs. Noddack suggested, and found that the probability was extraordinarily low. He concluded that Mrs. Noddack’s suggestion could not possibly be correct. So he forgot about it. His theory was right . . . but . . . it was based on the . . . wrong experimental information.
Here Teller indicts Aston’s measurement of the mass of helium (the same that had misled Szilard to beryllium), which “introduced a systematic error into calculating the mass and energy of nuclei.”
Segrè finds Teller’s version of the story possible but not persuasive. The helium mass number problem would not necessarily have ruled out breaking up the uranium nucleus. “You know, occasionally Fermi would tell you things, then you asked him, ‘But really, how? Show me.’ And then he would say, ‘Oh, well, I know this on c.i.f’ He spoke Italian.871 ‘C.i.f.’ meant ‘con intuito formidable,’ ‘with formidable intuition.’ So how he did it, I don’t know. On the other hand, Fermi made a lot of calculations which he kept to himself.”
Leona Woods’ version sheds light on Teller’s:
Why was Dr. Noddack’s suggestion ignored? The reason is that she was ahead of her time. Bohr’s liquid-drop model of the nucleus had not yet been formulated, and so there was at hand no accepted way to calculate whether breaking up into several large fragments was energetically allowed.872
If Noddack’s physics was avant garde, her chemistry was sound. By 1938 her article was gathering dust on back shelves, but Bohr had promulgated the liquid-drop model of the nucleus and the confused chemistry of uranium increasingly preoccupied Lise Meitner and Otto Hahn.