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 entra
nce, 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.*
The Last Man Who Knew Everything Page 14