CHAPTER NINE

GOLDFISH

WELL BEFORE HE BEGAN THINKING ABOUT BETA RAYS, FERMI knew that the next big thing would be nuclear physics.

Until about 1930, cutting-edge physics focused at the atomic level. Most of the work at Rome, like that throughout Europe and the rest of the world, tried to understand the structure of atoms and how they behaved. Recognizing the importance of atomic physics, Fermi wrote a comprehensive textbook on the subject in 1928, Introduzione alla fisica atomica, for use as a basic introduction for college students throughout Italy. It was yet another way to supplement his university income and provided a much-needed way for Italian physics and engineering students to gain exposure to the new physics.

In late 1929/early 1930, Fermi began to focus his research at the next level down—the nucleus of the atom. At that time, the nucleus was still a bit of a mystery. Physicists knew that it was suspended deep within the inner space of the atom. If a typical carbon atom were magnified to the size of a football field, the nucleus would be a penny in the center of the fifty-yard line and the nearest electrons would be at the goal lines, with empty space between them most of the time. Physicists also knew that the nucleus was positively charged. They knew that it contained the bulk of the mass of an atom, but its inner constituents and structure remained a mystery. One great puzzle was that the mass of the nucleus tended to be about twice as large as it should be, given the charge of the nucleus. Before Fermi’s beta decay paper, the emission of beta rays suggested that at least some electrons were also inside the nucleus. No one knew of the existence of the neutron, a neutral particle of almost the same mass as the proton. Furthermore, though many speculated, no one knew how protons could coexist in such close proximity to each other, overcoming the electrostatic force that causes like charges to repel each other.

To Fermi, the nucleus presented an attractive new frontier, so he conspired with Corbino to map out a plan of action. There were a number of major steps in the plan, steps that enabled the Rome School, a few years later, to pounce on a new discovery at exactly the right moment.

First was Corbino’s decision to publicly stake out a new direction by making a high-profile speech in September 1929 to a gathering of the Italian Society for the Progression of Science. In this presentation, Corbino baldly stated the new goals of experimental physics: “Italy will regain with honor its lost eminence… the only possibility of great discoveries lies in the chance that one might be able to identify the internal nucleus of the atom. This will be the worthy task of physics of the future.” With these words he launched a decade-long effort to persuade the fascist state to finance and support Fermi’s nuclear work in Rome. With envy, Corbino and Fermi looked across the Atlantic to places like Berkeley, where Ernest Lawrence had built an eleven-inch cyclotron designed to explore the inner workings of the nucleus at high energies. Soon Lawrence would be building even bigger cyclotrons. Equipment like this was expensive, and Corbino concluded that only a national commitment could provide the funding for the expensive equipment required to put Italy at the forefront of this new field.

Unfortunately, Corbino was never able to get the kind of support from the Mussolini regime that he believed the Rome School deserved. The physics program in Rome would be funded for teaching and more modest equipment, but only after World War II would Italy build its own high-energy cyclotron. Mussolini’s reluctance to commit the necessary financial resources may well have influenced Fermi’s eventual decision to leave Italy for good.

The second step was to get up to speed on the most recent research in the field of nuclear physics. Fermi knew that Rutherford and his team at Cambridge led the field, at least as far as experimental work went, and instructed Amaldi to study the most recent book on radioactivity by the Cambridge physicists and lead a small group, including Fermi, Rasetti, Segrè, and Majorana, through a colloquium on the subject. The massive 575-page book, published in 1930 by Rutherford and his colleagues John Chadwick and Charles Drummond (C. D.) Ellis, summarized all experimental data on the various forms of radiation, with many photos and diagrams. It clearly influenced Fermi’s thinking about beta radiation when he turned his mind to the subject in late 1933. The book was essentially an experimenter’s treatise and, characteristically for the Cambridge group, it contained little in the way of theoretical speculation, relying heavily on empirical data and experimental technique, a reflection of Rutherford’s instinctive distrust of theory. It was exactly the introduction that Fermi wanted.

Third, the team continued to publish and maintain a public profile as it made the transition from atomic to nuclear physics, using spectroscopic analysis to study nuclear spin rather than electron energy shifts. The Rome group may not have had the most up-to-date cyclotrons, but they did have beautiful and precise spectrographs, including one that measured some five feet in length, nicknamed the “crocodile.” Rasetti was a master spectroscopic physicist who taught his skills to Amaldi, Segrè, and others who joined the team, most importantly, a young Pisan named Bruno Pontecorvo, who arrived at Via Panisperna in 1933.

FIGURE 9.1. Rasetti’s “crocodile” spectrograph. Photo by Susan Schwartz. Courtesy of the Department of Physics museum, University of Rome, La Sapienza.

As part of the continuing publication effort, Fermi alone published twenty-six papers between the time of Corbino’s speech and the beta radiation paper in 1933, covering subjects as varied as the magnetic moment of the nucleus and the Raman effect, in which the frequency of light changes when bounced off certain molecules. They were all solid, interesting papers, but they were incremental contributions to the field, nothing as significant either as the 1926 paper on statistics or the 1933 beta decay paper.

In another carefully considered step, Fermi sent each member of the team to a different, major foreign lab to learn new experimental techniques and gain insights of researchers who were themselves further along in the transition process. Earlier on, Rasetti went to Caltech to study the Raman effect with the esteemed American physicist Robert Millikan, who won a Nobel Prize in Physics in 1923 for his work measuring the electric charge of a single electron. Segrè visited Pieter Zeeman in Holland to study—not surprisingly—the Zeeman effect. In 1931, Rasetti went to Berlin to study techniques relating to the construction of cloud chambers—the standard particle detector at the time—with experimental physicist Lise Meitner. He also learned how to isolate and prepare radioactive samples for further study. Segrè went to Hamburg, where he studied experimental techniques with Otto Stern, a brilliant experimentalist who would go on to win a Nobel Prize for his measurement of the proton’s magnetic moment. Amaldi traveled north to Leipzig, where he spent time with Peter Debye, who won the 1936 Nobel Prize in Chemistry for his work in X-ray diffraction of gases. It is clear that Fermi chose widely differing labs for his team to visit to learn the breadth of skills that he thought would be valuable in future work.

Yet another step, calculated not only to bring the Rome team up to speed on matters nuclear but also to raise Italy’s profile in the field, was the convening in October 1931 of an international conference on nuclear physics, sponsored by the Reale Accademia. Instead of Como, the site of the 1927 conference, this one was held in Rome, with most of the activities centered on Via Panisperna. Like its 1927 predecessor, it attracted a wide range of impressive scientific names, including Niels Bohr, Marie Curie, Arthur Compton, Hans Geiger, Werner Heisenberg, Lise Meitner, Robert Millikan, Wolfgang Pauli, and Arnold Sommerfeld, among others. Representing the Italians, Corbino and Guglielmo Marconi were the copresidents of the conference. In contrast to the Como conference, Fermi now had a formal role as the secretary general of the meeting, responsible for all invitations and organization. It was, in many ways, Fermi’s conference. Garbasso from Florence was also there, as was Persico, at this point a professor at the University of Turin. Rasetti attended, as did Tullio Levi-Civita and the young Bruno Rossi.

FIGURE 9.2. Group photo, Rome conference, 1931. Marconi stands front, center. To his left on successive steps are Bohr, Corbino, and Fermi, who is enjoying a laugh with his friend Ehrenfest. Persico is standing at the back by the left side of the entrance, under the brass plaque. Arthur Compton can be seen, head down, on the first step directly to Marconi’s right. The woman who appears to be dressed in black standing over Marconi’s right shoulder is Madame Curie. From “Convegno di Fisicia Nucleare,” Rome: Reale Accademia D’Italia, 1932.

The papers delivered at the conference covered a wide range of topics in nuclear physics. Ellis from the Cambridge group delivered a paper on beta and gamma rays, summarizing and extending what Fermi and the team learned from the treatise the Cambridge group published in 1930. The problems associated with beta decay were on everyone’s mind and Pauli spent much of the session chatting with Fermi about them. As we have seen, Bohr’s paper ventured the notion that energy was not conserved in beta decay. George Gamow, an ebullient and gregarious Russian theorist who defected to the West two years later at the 1933 Solvay conference, and Cambridge theorist Ralph H. Fowler presented papers proposing theories of nuclear structure. It was a productive meeting, although, as Segrè suggests, it came just a few months too early. In early 1932 American chemist Harold Urey would discover an isotope of hydrogen, deuterium. Even more important, a month later, in February 1932 Rutherford’s colleague James Chadwick would announce the discovery of a neutral particle in the nucleus with just a bit more mass than the proton, which he dubbed the neutron. Its existence explained the weird discrepancy between the mass and the charge of the nucleus and also explained the existence of Urey’s heavy hydrogen isotope. Later that year, in August, Carl Anderson, an American physicist working with Robert Millikan at Caltech, made a further experimental discovery while studying cosmic rays: the positron, the antimatter counterpart of the electron, predicted by Dirac in 1927.

FIGURE 9.3. Signatures procured by Persico of several attendees, including Ellis, Aston,* Richardson,* Pauli,* Brouillin, Goudsmit, Millikan,* Sommerfeld, A. Compton,* Bohr,* Debye,* Blackett,* Geiger, Heisenberg,* Perrin,* Meitner, Bothe,* Mott,* Ehrenfest, ?, Beck.* The asterisks mark those who had won or were to win the Nobel Prize. Photo by Giovanni Battimelli. Courtesy of the Enrico Persico Archives, Department of Physics, University of Rome, La Sapienza.

Two further conferences, one in Paris in 1932 and the Solvay conference of 1933, pushed nuclear physics even further along. Fermi attended both and, immediately after returning from Solvay in 1933, put together his beta decay paper. In February 1934, however, he received the startling news, published in Nature and in the French physics journal Comptes Rendus, that the French husband and wife team of Irène and Frédéric Joliot-Curie, the former being the daughter of Nobel Prize winner Marie Curie, had made nonradioactive elements like aluminum, boron, and magnesium radioactive by bombarding them with alpha particles from a polonium source. To Fermi, as for the rest of the physics world, this was astonishing news. Scientists had been bombarding elements with alpha particles for some time and had been noting the breakdown into a variety of new isotopes and elements, none of which were radioactive. Though radioactivity was well understood experimentally, the theory behind it was not well developed, and it came as a complete surprise that, with experiments like the ones the Joliot-Curies conducted, radioactivity could be induced in nonradioactive elements. With their experiments, the Joliot-Curies created new radioactive versions of otherwise stable elements.

When Fermi read the Joliot-Curie papers, his critical intuition began to twitch. Because alpha particles are positively charged, he reasoned that they are not particularly efficient as “bullets” for striking the positively charged nuclei of atoms. Positive charges repel each other and, he figured, it would be a lucky alpha particle indeed that would make its way into the nucleus of a target atom. Most would be repelled long before nearing the target. That the Joliot-Curies got any results at all was due to the intensity of the alpha radiation created by the polonium source. Polonium emitted an enormous number of alpha particles per second. Some would be bound to get through. However, if instead of alpha particles, neutrons were aimed at the nucleus of an atom, they would have a much better chance of striking the nucleus directly, causing similar radioactive transmutations, because, being neutrally charged, they were not repelled by the positively charged nucleus. True, the available neutron sources were nowhere nearly as intense as alpha ray sources, but they would not have to be. The better odds any given neutron would have in striking the nucleus would offset the relatively low numbers available.

At this moment, however, the news from the Paris team was public knowledge. Rutherford and his team in Cambridge had been bombarding elements with neutrons for the past year but had not developed sources of sufficient intensity to compete with alpha particles and had so far been unable to do the types of studies that were so successful in Paris. However, they were expert in the experimental techniques being used in Paris to pursue this work. Fermi knew that his real competition would be Rutherford and Chadwick, who would find a way to use neutrons instead of alpha particles to induce radioactivity sooner or later on their own. If Fermi wanted to establish priority in the use of neutrons to bombard nuclei—and the ever-competitive Fermi certainly did—the Rome team would have to work fast.

A new colleague of Fermi’s, Gian-Carlo Wick, provided additional stimulus. Wick, a former student of the Ukrainian-Italian physicist Gleb Wataghin in Turin, came to Via Panisperna in 1932 as an assistant to Corbino, when Rasetti, who previously held the post, was promoted to a professorship. Wick was an insightful theorist and observed that the positron emissions seen by the Joliot-Curies were the result of “reverse” beta decay, as Fermi’s paper predicted. The idea delighted Fermi.

Fortunately, Fermi knew of a technique to produce high-intensity neutron sources. He and Rasetti had been working on an earlier project that required neutron sources—a spectroscopic study of gamma-ray scattering—and had located a small sample of radium in the bowels of Via Panisperna. The radium belonged to the Institute of Public Health, located in the basement of the building and headed by a prominent public health official named Giulio Cesare Trabacchi, and was being used by the institute for preparations related to cancer treatment. In an act of extraordinary generosity, Trabacchi allowed Rasetti and Fermi to draw off radon gas produced by the radium for the gamma-ray studies. After they pumped the radon gas off the radium and into a glass tube, they dipped the tube in liquid nitrogen, condensing the gas into a liquid and giving them a short time to seal the glass tube before all the radon evaporated. It was a finicky process and often resulted in the liquid nitrogen cracking the glass tube. By November 1933, however, they had more or less perfected the technique. In the wake of the news from Paris, they decided that a mixture of radon gas and beryllium would provide exactly the intense neutron source they required to see whether neutrons could induce radioactivity in otherwise stable elements.

FIGURE 9.4. Corbino’s boys. From left to right: D’Agostino, Segrè, Amaldi, Rasetti, and Fermi. Probably taken in the spring of 1934, during the first neutron bombardment experiments. Courtesy of the Amaldi Archives, Department of Physics, University of Rome, La Sapienza.

The team included Fermi, Rasetti, Segrè, and Amaldi and a new member, a radiochemist named Oscar D’Agostino, who had been working with Trabacchi in the basement of the building and who was at that moment studying radiochemistry separation at the Joliot-Curies’ lab in Paris. Fermi arranged for a division of labor. He and Rasetti would prepare the neutron source. Fermi and Amaldi would expose the target elements to the neutron source and would measure the resulting radiation with Geiger counters they built by hand. Segrè would help out as needed in either of these processes and would also use his considerable entrepreneurial skills to scour Rome and procure target elements to expose. D’Agostino would analyze the by-products of the bombardment using newly developing techniques of radiochemistry. Trabacchi was also considered an honorary member of the group, having loaned the team the radium from which they obtained the radon gas.

A seventh would be added to the team during the year. Bruno Pontecorvo, from a wealthy Jewish family long associated with the textile trade in Pisa, arrived at Via Panisperna in 1933 and participated in Fermi’s gamma-ray studies that year. He was strikingly handsome and a fine athlete. He was also decidedly left-wing, verging on communist. At this point, though, his family’s social standing and his own involvement with Fermi’s team inoculated him against attack by the aggressively anticommunist fascist regime. He was, it turned out, a gifted researcher, destined to make a singular contribution to the story of neutron bombardment.

SO MUCH HAS BEEN WRITTEN ABOUT THE PERIOD FROM MARCH TO October 1934 at Via Panisperna and so much of what has been written comes from the memories of participants well after the fact that historians must be cautious in accepting any particular participant’s narrative at face value. One example is the story, told by Laura Fermi, of how Rasetti was away in Morocco on an extended vacation when the work on neutron bombardment started and that Fermi sent a cable asking him to return so he could participate in the experiments. In fact, Rasetti was in Rome giving lectures on spectroscopy during the period of initial work, although he did not participate in these initial experiments. On March 20, 1934, when Fermi first induced radioactivity through neutron bombardment, Rasetti was delivering the final lecture of the course. He left for a conference in Morocco, not an extended vacation. Another story, told by Segrè and Rasetti, is that the project began by exposing elements to neutron bombardment, going systematically through the periodic table of elements starting from the beginning of the table. The lab notebook for this initial period, lost for decades and found in 2006 by professors Francesco Guerra and Nadia Robotti in the estate of D’Agostino, suggests that Fermi started with the element fluorine, number nine in the periodic table.

Fermi’s experimental design was complicated by several constraints. First, the measurement of radioactivity by Geiger counters had to take place in an area that was not affected by the intense radioactivity of the radon gas itself, so they placed the Geiger counters in a room at the farthest end of the lab’s corridor. Since the half-life of some irradiated targets was very short, measured in mere minutes, getting the target to the counters involved running up and down the corridor at high speed. For the next few years, while work on neutron bombardment continued, distinguished visitors arriving at Via Panisperna were astonished to find Fermi, Amaldi, and others in lab coats running back and forth along the second floor corridor of the institute, with Fermi, as was his nature, always in the lead, carrying irradiated samples.^*

Second, the geometry of the experiment needed careful consideration. Glass tubes of radon-beryllium radiated neutrons in all directions with equal intensity. Understanding this, Fermi decided to form the target materials into cylinders and drill holes vertically through them, into which glass tubes containing the neutron source were inserted. When the exposure was complete, the tube was removed and the sample quickly transported to the room with the Geiger counters.

On March 25, 1934, in the first of a series of ten short reports he sent to the journal Ricerca Scientifica, the journal of the Italian National Research Council, Fermi reported on the induced radiation in fluorine and in aluminum. For each of these reports, he sent preprints to physics labs throughout Europe and the United States to establish priority over the discovery of induced radiation through neutron bombardment and to provide data for others to replicate should they wish to do so. This time, in contrast to the muted reaction to the beta decay paper, the physics community paid immediate attention. In receipt of the first reports at Cambridge, Rutherford drafted a generous reply to Fermi:

Your results are of great interest, and no doubt later we shall be able to obtain more information as to the actual mechanism of such transformations. It is by no means clear that in all cases the process is as simple as it appears to be the case in the observations of the Joliots.

I congratulate you on your escape from theoretical physics! You seem to have struck a good line to start with. You may be interested to hear that Professor Dirac also is doing some experiments. This seems to be a good augury for the future of theoretical physics!

During the summer, Amaldi and Segrè traveled to England to deliver to Rutherford a paper on the work for publication in the Proceedings of the Royal Society. When they met in Cambridge with the legendary experimentalist, so much of whose work laid the foundation for the Rome team’s efforts, Segrè asked whether it would be possible to arrange for speedy publication. Rutherford replied, with characteristic wit, “What did you think I was president of the Royal Society for?”

Leading up to the summer academic holidays, Fermi and the team continued through the periodic table, finally coming to the heaviest of elements, thorium and uranium. With these heavy elements, the expectation, shared widely throughout the physics community, was that neutron bombardment would result in the creation of even heavier elements, so-called transuranic elements. Nevertheless, Fermi insisted that the team be thorough, and D’Agostino went through the process of trying to identify any lighter by-products, moving down the periodic table until he came to lead, assuming there would be nothing lighter. Finding none, he gave up. The tentative conclusion the team reached—somewhat reluctantly, since D’Agostino was unable to get a clean separation of by-products—was that new heavier elements might actually have been created. Only a German physicist, Ida Noddack, suggested that the transuranic hypothesis was wrong, that Fermi had actually caused the uranium nucleus to split into two much smaller pieces, lower than lead in the periodic table. Her suggestion was ignored, largely because neither she nor anyone else could come up with a possible mechanism for explaining such an event.

The tentative conclusion became a definitive one, however, at the beginning of the summer break when Corbino, in a premature but enthusiastic speech before the Accademia dei Lincei, publicly announced the discovery of transuranic elements by Fermi and the rest of the team. The speech made headlines in Italy and around the world. Corbino had not consulted with anyone beforehand, and Fermi was devastated that an uncertain conclusion had been presented as final. Meticulous in his conservative approach of announcing results only when he was absolutely sure of them, he worried that his reputation might be ruined, particularly if it was proven not to be true. He spent a sleepless night wondering what he should do. Corbino was his mentor and key supporter in Italy, and there were limits to how openly Fermi could differ with him. The next morning Fermi approached Corbino directly with his concerns. Corbino understood his mistake immediately and tried to downplay the story, but the damage had been done. The story was just too exciting to go away of its own accord. Perhaps because he wanted to believe it or perhaps because he did not want to embarrass Corbino, Fermi himself never absolutely repudiated it. Five years later, the Nobel committee awarded Fermi the physics prize for this work, citing slow neutrons and the discovery of transuranic elements. At that same moment, the work of the brilliant team of Lise Meitner, Otto Hahn, and Fritz Strassmann, laboring away in Berlin, uncovered the truth of what Fermi and his team had actually done. They hadn’t discovered transuranic elements at all. They had split the uranium atom.

WORK ON NEUTRON BOMBARDMENT PAUSED DURING THE SUMMER, while team members went their separate ways. Fermi spent much of that summer lecturing in South America and came back through London, where he attended a conference and delivered a full report on the neutron work. In the fall, work continued under his direction. At this point, he invited Pontecorvo, who had been at the institute for a year, to join the team.

One of the problems Fermi was trying to solve was the difficulty of getting reproducible results from specific irradiations. The level and type of radioactivity induced seemed to differ from experiment to experiment. The best the team could do was to categorize levels of induced radioactivity as strong, medium, or weak. Fermi wanted to see whether a more quantitative standard could be developed, and he asked Amaldi and Pontecorvo to give it a try.

Very occasionally, Mother Nature decides to give us a peek behind the curtain at what is really happening. On October 18, 1934, Amaldi and Pontecorvo were given just such a peek.

They began by irradiating silver, with a known half-life of 2.3 minutes. They wanted to establish this as a standard against which other quantitative measurements would proceed. The problem they encountered, however, was that the effect of the neutron source on the silver target depended not only on the distance from the source to the target but also on the table on which the source and the target were placed. When they placed the silver on a marble table, the level of radioactivity was markedly lower than when placed on a wooden table. This was perplexing, to say the least. Why should the level of induced radioactivity change depending on the table on which the source and target were placed? Amaldi and Pontecorvo continued measurements throughout the next day. By Saturday, October 20, 1934, this strange phenomenon refused to go away, and they approached Fermi with the puzzle.

Fermi had been preparing a wedge made of lead to place between the neutron source and the silver target. What he did now is best described in his own words, related to his good friend Subrahmanyan Chandrasekhar after World War II:

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 in the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came to 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 lead was to have been.

Fermi’s memory may not be reliable. His notebooks suggest that the lead wedge was actually used before he decided to use paraffin. In any case, Fermi conducted the paraffin experiment with Amaldi and Persico in the morning, while Segrè and Rasetti were engaged in supervising exams in another part of the building. At about noon, he repeated the experiment in front of the whole team, along with Persico and Bruno Rossi, both of whom were visiting. The results were astonishing. The level of induced radioactivity in the silver was much higher than it had been without the paraffin—indeed, much higher than any levels the team had yet measured.

Having established the effect of the paraffin, Fermi decided—for the first, but not the last time, in the midst of a crucial experiment—that the team should break for lunch. He was, as always, a man of habit, but this break also gave him time to mull over the extraordinary effect they had witnessed. By three o’clock, when the team returned to Via Panisperna refreshed and ready to pick up where they had left off, Fermi understood the phenomenon and was ready to share his insights.

The first observation Fermi made was that paraffin has a high proportion of hydrocarbons in its makeup, which means that much of paraffin consists of hydrogen. The second observation was that a hydrogen nucleus has very nearly the same mass as a neutron, in contrast to heavier nuclei, where the mass is two, three, fifty, or even a hundred times the mass of a neutron. The effect of a neutron bouncing around against hydrogen atoms was to slow down the neutron considerably. As an analogy, it is helpful to think of balls on a billiard table. When a cue ball hits another ball, the kinetic energy is redistributed between the two balls and the cue ball bounces away from the target ball at a slower speed because much of the cue ball’s kinetic energy is transferred to the target ball, which also travels considerably as a result of the impact. Now imagine that instead of a cue ball, a ping-pong ball is driven into a bowling ball on that same billiard table. The bowling ball will hardly move; little of the kinetic energy of the ping-pong ball is transferred because the ping-pong ball is so much lighter than the bowling ball. The ping-pong ball will hardly slow down upon impact but instead will carom around the billiard table at about the same speed as it had before hitting the bowling ball.

In this way, hydrogenous substances could slow down neutrons in a way substances rich in heavier elements could not. The next question was: Why would slowing down neutrons boost the radioactivity of the target elements? This was the final part of the puzzle that Fermi figured out during the lunch break. At high speeds the neutron is likely to spend less time inside a nucleus it enters. A slower neutron has a higher probability of entering the nucleus, bouncing around inside, and coming to rest there, thereby causing the instability that gives rise to radioactivity. It was, in fact, just the opposite of the conventional wisdom, which suggested that higher energy neutrons would induce greater radioactivity in the target.

Fermi’s ideas explained the results of the paraffin experiment. A wooden table has more hydrogen atoms than does a marble table, leading to the anomalies seen by Amaldi and Pontecorvo. When a hydrogenous filter like paraffin was placed between the source and the target, far more of the neutrons were slowed down.

What was needed to verify the observation was an experiment with a substance that has an even higher concentration of hydrogen at room temperature. Fortunately, there was water on the premises: a goldfish pond in the rear garden of Via Panisperna. In what may well be an apocryphal tale, the group supposedly traipsed outside and watched as Fermi repeated the experiment in the pond, using water as the medium to slow down the neutrons. The effect of the water was even stronger than that of the paraffin. History does not record how the goldfish were affected.^*

That evening the team repaired to Amaldi’s apartment. Amaldi’s wife, Ginestra, had a job at the National Research Council and could bring a report of the discovery to work with her on Monday morning and hand it to the editors at Ricerca Scientifica. That evening her job was to type up the report as it was dictated by Fermi, with vigorous interruptions and arguments from the rest of the team. At times the group was so noisy that the Amaldis’ maid later asked Ginestra whether the group had had too much to drink. The report is dated October 22, 1934, the date on which Ginestra submitted it for the team.

FERMI’S EXPLANATION OF HIS DECISION THAT SATURDAY TO USE paraffin illuminates as much about him and his thinking as it does about the problem itself. The actual mechanism by which neutrons induced radioactivity was still being understood and Fermi was himself seeking a better grasp. He had been thinking about nuclear reactions for several years, the first result of which was his theory of beta radiation. When he considered a problem, he thought about it continually, using the early morning hours of quiet to set out his thoughts and continuing in spare moments at the office and in his private seminars. He had thought about the problem in such depth that it had burrowed into his subconscious. In this way, he was perhaps the best prepared person in the world to react when confronted with the anomalies that Amaldi and Pontecorvo had discovered accidentally on October 18 and 19, 1934. The team’s inability to standardize the impact of neutron bombardment had baffled him, but now everything fit into place. Somehow the accumulated data knocked something loose in his subconscious and he knew instinctively to reach for the block of paraffin. When the results were so dramatic, he was in a position to withdraw for a few hours and put together a definitive explanation of the phenomenon. Perhaps others might have been able to do this and we certainly owe much to Amaldi and Pontecorvo, who took note of the anomalies and brought them to Fermi’s attention. Yet, in the end, it was Fermi who created the definitive experiment and explained its results to the satisfaction of his team and to the physics community at large.

CORBINO BECAME AWARE OF THE DISCOVERY THAT MONDAY AND immediately understood that it was important, although he, like the rest of them, could not anticipate just why it would become an historic milestone. He was thinking mainly that a method of enhancing induced radiation would have commercial value in the production of radioactive substances for use in cancer treatment and other medical applications. He insisted that the method of slow neutron–induced radiation be patented, and Fermi immediately began work filing a patent for the process. The Italian patent, which was granted a year or so later, names Fermi, Amaldi, Pontecorvo, Rasetti, and Segrè as the inventors of the slow-neutron process. An agreement was reached whereby D’Agostino and Trabacchi would share equally in any commercial benefit. Fermi’s former student Gabriello Giannini had moved to the United States and now offered to shepherd the idea through the US patent office and manage the process internationally. The issue of the patents for the slow-neutron process would arise again after the war, with results that would be disappointing to the team. For now, though, Fermi and the others members felt that they had a lock on the new concept and had certainly established priority over competing teams in Cambridge, Paris, and Berlin.

Immediately upon the discovery of the slow-neutron effect, the group commenced redoing all the work that had been done since March 1934, seeing how exposure to slow neutrons irradiated each element in different ways. By the end of 1934, Fermi was confident that he understood the effects. In February 1935, he submitted a rather lengthy paper to the Royal Society summarizing the work in slow neutrons. He also began to analyze neutron-nuclei collisions using a primitive, paper-based form of simulation he would later pursue at Los Alamos with the first generation of computers, in which the course of a neutron’s travel through a particular target would be simulated according to the probabilities of different outcomes at each stage of the journey. Using pencil and paper, he could repeat the simulations over and over again to analyze the distribution of outcomes given the underlying probabilities. This method was later christened the “Monte Carlo” method, reflecting the role of chance in the range of possible outcomes, much as it would be in a casino. It was one of Fermi’s lasting and most broadly useful analytical contributions. Oddly, Fermi did not seem to think that this new analytical technique was a significant development, perhaps because he did not anticipate the subsequent advent of electronic computers that made Monte Carlo simulations far easier to do. He only told Segrè about his attempts with this kind of calculation years later, during the war.

THE YEAR 1934 IS REGARDED AS A HIGH WATER MARK FOR THE ROME School. In a period of little more than seven months, Fermi and his team explored radioactivity and the atomic nucleus with poor financial backing and primitive equipment, certainly nothing of the sort available to Lawrence’s team at Berkeley. Yet they made the astonishing discovery that slowing down neutrons enhanced the radioactivity induced by neutron bombardment. They developed a detailed body of data that could be used by other teams throughout the physics world to replicate and study. By pushing hard on the experimental program, Fermi achieved results of lasting significance ahead of all his competitors, including the formidable teams in Berlin, Paris, Berkeley, and Cambridge. In the process he set the stage, quite unwittingly, for an historic drama some five years later, a drama in which he would find himself a major player. Fortunately for the world at large, neither Fermi, nor the team around him, realized at the time what they had actually accomplished.

* Those who speculate that Fermi’s later terminal stomach cancer might have been caused by radiation exposure point to these experiments as possible evidence. The entire team was involved, and only Fermi developed stomach cancer. Given Fermi’s hands-on experimental style, it is likely that he did most of the running up and down the corridors himself, holding the irradiated targets close to his chest. Throughout his life, whether running on the beach or in a lab corridor, Enrico Fermi was always out in front.

* There seems to be some question as to whether the goldfish pond incident really took place. Laura Fermi described it as above, but it is mentioned neither by Segrè nor by Amaldi. Ugo Amaldi reports that his father did not recall the incident and doubted that it occurred. Ugo Amaldi, interview with author, June 8, 2016.