The Pope of Physics

by Gino Segrè

  Amaldi’s recent marriage to Ginestra Giovene gave the vacation an extra celebratory mood. Laura, in particular, was overjoyed to have Ginestra, only three years younger than she, become part of their circle. A close friendship between the two women complemented the one between their husbands. During the vacation, Ginestra realized she was pregnant, and the bonds between the women strengthened. Laura knew that she and Ginestra would soon also share the joys and complexities of motherhood. Years later, Amaldi would joke with his son Ugo, himself a physicist, that it had been an unbelievable vacation: finding out he was going to be a father and hearing about Fermi’s theory.

  After a hard day on the slopes, Fermi asked Amaldi and Segrè to come to his hotel room. Segrè, sore from falls on icy slopes, remembers thinking a hot bath might have been better than squeezing into a hotel room and listening to a new physics idea. Such thoughts vanished when Fermi said that what he was about to share with them was probably the best work he had ever done.

  In his small room, Fermi laid out for them his new idea. Gravity and electromagnetism were not alone. A third force was operative, one that could transform a neutron inside the nucleus into a proton, an electron, and a very light neutron like the one that Pauli, the Viennese-born wunderkind, had tentatively proposed three years earlier. Fermi stated that if that transformation were to occur, the proton would remain behind inside the nucleus. On the other hand, the electron and Pauli’s neutron would escape immediately from the nuclear confines. This explained the mystery of how the electrons observed in beta decay could have resided, contrary to principles, within the nucleus. The answer was that they hadn’t really been there. Created by the new force, they had been present only for an instant and then exited.

  Nobody had ever developed a theory that could change a particle’s identity, much less construct a theory with a third force. Fermi told his skiing friends that it was relatively easy to devise one that did that. What he didn’t say was that it was easy for him, not for others.

  Amaldi thought it was confusing to speak of Pauli’s neutron and Chadwick’s neutron. One had a mass that was at most a few times larger than the electron’s and the other was as massive as the proton, two thousand times greater than the electron. He suggested a way, at least for Italians, to make it immediately clear that the two neutrons were really very different. Amaldi thought Chadwick’s neutron should be referred to as neutrone (big neutral one) and Pauli’s as neutroncino or more simply, neutrino. The first name has continued only in Italy but neutrino has become the universal way of referring to the little neutral one.

  Fermi’s theory was not immediately acclaimed. In 1934, Nature’s editors rejected the letter describing it that he submitted to them for publication. They felt “it contained abstract speculations too remote from physical reality to be of interest to the reader.” Cited as a famous example of editors showing poor judgment, their response was not without its reasons. Two years earlier, physicists had been saying that the microscopic atomic world was made of electrons and protons held together by electric forces. Now they were being asked to take a leap of imagination into a world with new particles and new forces. They thought, as they said, that it was overly fanciful, “too remote from physical reality.”

  The editors were soon asked to accept for publication a letter hailing the correctness of that world. Bethe and Peierls, great admirers of the Pope’s approach to physics, had written this new letter. In it, and similarly in one independently submitted to an Italian journal by Giancarlo Wick, they pointed out how Fermi’s theory explained a recent sensational experiment Curie and Joliot had performed. Using the same type of alpha particle beam they had employed in the past to induce radioactivity, Curie and Joliot had observed nuclear decays in which positrons were produced.

  The discovery of positrons, the antiparticle partners of electrons, had been the most unexpected and most surprising experimental result of 1932. Positrons had the same mass and were identical in every respect to electrons except for having the opposite electric charge, positive instead of negative. The theory of antimatter predicted that if an electron encountered a positron, the two would vanish, leaving only electromagnetic radiation (photons) in their place.

  In their letter to Nature, Bethe and Peierls pointed out that the Curie-Joliot experiment offered convincing evidence for the correctness of Fermi’s theory of beta decay. The positrons Curie and Joliot had observed had to be due to a proton inside an unstable nucleus being transformed into a neutron, a positron, and a neutrino. Before then it had still been possible to reject Fermi’s theory. It was conceivable that a nucleus could harbor electrons, but as Bethe and Peierls wrote, “One can scarcely assume the existence of positive electrons inside the nucleus.” If any had been present they would have quickly sensed the presence of a nearby electron: the two would have annihilated each other.

  Bethe and Peierls also calculated the probability that, according to Fermi’s theory, a neutrino could produce an electron or a positron when colliding with a nucleus. They did this to see if the technique could be used to detect neutrinos. Their conclusion was “There is no practically possible way of observing the neutrino.”

  But science marches on and what was impossible in 1934 had become possible by 1956. Two decades later, neutrino beams became a standard feature of experiments at large accelerator laboratories. The detection of neutrinos from the core of the sun had also been reported.

  Yet the most amazing observation of these fleeting particles was to come.

  When a massive star collapses, an enormous number of neutrinos are emitted in a few seconds and a supernova appears in the star’s place. By the mid-1980s, three underground laboratories capable of detecting neutrinos from stellar collapse had been constructed—one in the United States, another in Japan, and a third in the Soviet Union. There was little hope for such a sighting, since the last supernova formation close enough to Earth for neutrino detection had occurred hundreds of years earlier. Luck intervened. A ten-second neutrino burst, followed hours later by the appearance of a supernova, was observed on February 24, 1987, at 7:35 a.m. Universal Time by all three laboratories. It had taken those neutrinos 170,000 years to reach Earth. The unlikely, but possible, had occurred.

  15

  THE NEUTRON COMES TO ROME

  Fermi’s explanation of beta decay remains arguably his greatest contribution to theoretical physics. Its design was entirely his: nobody had considered anything like it and the idea was of enormous import to the world of physics. The possibility of a particle changing its identity would become central. So would the idea of forces beyond electromagnetism and gravity. Several other major contributions had established him as one of the world’s most versatile physicists, but credit for his discoveries had usually been shared with others working on the same problem. Beta decay was Fermi’s alone.

  Despite the acclaim his theory received, Fermi never regarded his work on beta decay as other than an interlude. His goal did not veer from the one he had already set: to establish a solid and innovative experimental nuclear physics program in Rome.

  In 1931, when he and Amaldi had tried establishing such a program, they had been hampered by the lack of equipment and by Via Panisperna’s inadequate shop facilities. The failure had made them well aware of the need for improvement on this count. A year later, on September 30, 1932, Fermi wrote Segrè, “The problem of equipping the Institute for nuclear work is becoming ever more urgent if we do not want to be reduced to a state of intellectual torpor.”

  By 1934, the Boys had more experience and more training, Rasetti’s stay at Lise Meitner’s Berlin laboratory proving especially valuable. They also had funds for farming out the construction of equipment. They were ready. The remaining question was what problem in nuclear physics they should attack. Inspiration came when they heard of Curie and Joliot’s success in inducing radioactivity where it had previously been unknown.

  Only a handful of radioactive elements had been identified before then, radi
um and polonium being the prime examples. The recent Curie and Joliot experiment showed that radioactive isotopes could be created. Natural radioactivity had been studied for over thirty years. Artificial or induced radioactivity was a total novelty. The structure of nuclei could now be studied in a way never before available; countless applications to medicine and biology were certain to follow.

  Two years earlier, Curie and Joliot, erroneously interpreting an experiment, had lost the credit for detecting the neutron. This time they understood perfectly not only what they had measured, but to what it was due. Marie Curie, near death from radioactive poisoning due to her pioneering work with radioactive materials, was enormously pleased to see her daughter and son-in-law’s achievement recognized by a Nobel Prize in Chemistry in 1935, the same year Chadwick was awarded the Nobel Prize in Physics for discovering the neutron. Rutherford had been right: they were “so clever.” As someone who was a laureate in both physics and chemistry, Madame Curie understood the vagaries distinguishing each discipline and valued the contribution of each.

  However, by the time Curie and Joliot received the Nobel Prize, they were following Fermi’s lead. His ascent to prominence as an experimentalist had been precipitous. At the beginning of 1934, Fermi had been regarded almost exclusively as a theorist. By the end of the year he was well on his way to becoming one of the twentieth century’s greatest experimentalists.

  Fermi the experimentalist had made a major contribution by using neutrons rather than alpha particles to study induced radioactivity. Success came very quickly with two crucial breakthroughs, the first in March and the second in October 1934. Recognition of them was immediate and was highlighted in his Nobel Prize citation four years later: “To Professor Enrico Fermi of Rome for his identification of new radioactive elements produced by neutron bombardment and his discovery, made in connection with this work, of nuclear reactions brought about by slow neutrons.”

  Using their beam of alpha particles, Joliot and Curie had not been able to induce radioactivity in any element past aluminum, number 13 in the periodic table of elements. Fermi, assisted by the Boys, was able to reach all the way to the table’s end, uranium, number 92. And with slow neutrons he introduced a surprising and altogether new way of probing nuclear structure, one that would become extraordinarily important in future years.

  Trying to use neutrons instead of alpha particles had seemed impossible. In order to obtain neutrons, one had to start with a beam of alpha particles and bombard a beryllium target with them: one neutron was produced for approximately every ten thousand alpha particles, a dauntingly small number. But Fermi realized he might have a chance to make good use of those comparatively few neutrons. They could travel straight to the heart of a nucleus, whereas positively charged alpha particles would be repelled by the nucleus’s positive electric charge. Greater efficacy might compensate for smaller numbers; Fermi’s estimate was correct.

  Fermi was also lucky because the experiment he planned required a strong polonium source to produce the needed alpha particles, and he had one. This was not a foregone conclusion since such a source was both rare and expensive. Fortuitously, Corbino had introduced a bill in the Italian Senate in 1923 to create a special facility for handling and stockpiling radium and polonium; they were intended for use in research and medical therapeutic purposes. The Radium Institute was established in 1925 at Via Panisperna 29A and Corbino’s assistant, Giulio Cesare Trabacchi, was chosen as its head.

  Trabacchi, soon nicknamed Divina Provvidenza (Divine Providence) by the Boys, was an invaluable resource. His one-gram supply of radium had a market value of over a million Italian lire, an enormous sum at a time when university research budgets were generally only a few thousand lire. Its primary use was for medical treatments, but Divina Provvidenza would always supply the Boys with what they needed.

  In early March 1934, Fermi began trying to induce radioactivity by using neutrons as projectiles. At first he and Rasetti worked together, but Rasetti soon departed for a prolonged vacation in Morocco, leaving Fermi to proceed on his own. The setup he adopted was simplicity itself. A small radioactive neutron source was introduced into a hollow cylinder coated with the material to be irradiated. After a time that varied from minutes to hours, according to needs, Fermi removed the source and ran down the hall with the cylinder to another room. He there inserted a thin Geiger counter into the cylinder to check if radioactivity had been induced in the cylinder’s coating. Running down the hall was necessary to ensure that the counter was not measuring background radioactivity from the source.

  Success came quickly. It was first reported in a one-page paper dated March 25, 1934, that Fermi wrote for La Ricerca Scientifica, an Italian journal often used for quick and short publications. This paper was the first of ten on the subject of induced radioactivity appearing in La Ricerca over the next fifteen months. They were pivotal in Fermi’s career. His work over the next dozen years focused on neutrons. Beforehand, he had followed his whims, working on problems he found intriguing. He was now mining a rich vein that demanded his full attention. It was an unimaginable future. Eight years after Ricerca I was published, Fermi would prove that neutrons could initiate chain reactions, and three years after that he would be standing in the New Mexico desert watching what a chain reaction could yield.

  Ricerca I announced to the physics world the creation of a vital and comparatively inexpensive way to study nuclear phenomena. Ricerca II delineated how radioactivity had been induced in elements of the periodic table all the way up to barium, number 56. This was more than four times as far along the table as Curie and Joliot’s limit of aluminum, number 13. Appearing only two months after the French couple’s groundbreaking paper, Fermi’s achievement was sensational.

  Fermi anticipated that groups around the world would rapidly follow his lead. If Rome was to play a major role, he had to work quickly and he would need help. Fermi began by asking Amaldi and Segrè to join him on the experiment. They agreed at once: the Pope had spoken. He then sent a telegram to Rasetti in Morocco. The Cardinal (Rasetti), summoned back to Rome, returned immediately. The Boys also needed an expert chemist who specialized in radioactive materials. Trabacchi’s assistant, Oscar D’Agostino, joined their team. It was now a five-man group.

  While there was no strict division of labor, Fermi was clearly the commander in chief. He made the ultimate decision of whether a measurement was valid or not. D’Agostino took the lead in chemical analyses, Amaldi oversaw electronics, and Segrè was in charge of acquiring the materials that would be irradiated. Since additional funds were needed, Fermi applied to the Consiglio for a supplement of twenty thousand lire, approximately $1,000 at the time. It was immediately granted.

  Physics experiments had traditionally been carried out by only one or two individuals working together. By contrast, Fermi asked the entire group to cooperate. Primary assignments were specified, but all participated in every stage of the labor. Having worked at first on his own, Fermi was the sole author of I and II in the sequence of the ten Ricerca papers, but all five members of the group were coauthors of the subsequent ones, their names appearing in alphabetical order. These were probably the first papers to appear in the physics literature with five authors.

  Word about what Fermi had achieved spread rapidly. On April 23, only a month after Ricerca I’s publication, Rutherford sent Fermi a letter. The tone he adopted reflects his somewhat tongue-in-cheek opinion that only experimentalists did real work. It started with “I congratulate you on your successful escape from the sphere of theoretical physics” and ended by asking Fermi to send him future publications on the subject.

  Rutherford was not the only one to want those publications: copies of each of them were subsequently sent on a routine basis to the most prominent scientists studying nuclear physics. As Columbia University’s young star physicist Isidor Rabi reportedly joked, “Now we will all have to learn Italian.”

  The atmosphere of camaraderie and playfulness that characte
rized the Boys’ interactions was heightened by their awareness of working together on a potentially paradigm-shifting project in nuclear physics. The waggish mood relieved the strain of making sure each measurement was done correctly. The precise Cardinal, Rasetti, would accuse the others of being clumsy. They would reciprocate by threatening to defrock the Cardinal. The Pope, who could be childish in wanting to win every competition, would claim he was the speediest in carrying samples from the end of the second-floor corridor, where they were irradiated, to the other end, where the Geiger counter was housed. Amaldi would challenge him. The title of fastest remained contested.

  Laura Fermi recounts the story of a distinguished Spanish visitor coming to the Physics Institute looking for Sua Eccellenza Fermi. Meeting Segrè on the ground floor, he was curtly told, “The Pope is upstairs.” After learning that Sua Eccellenza and the Pope were one and the same, the visitor climbed the stairs, but he soon returned, shaken. He had almost been knocked down by two young men in lab coats running up and down the corridor. Further explanations revealed to him who the two were: Fermi and Amaldi.

  The first paper coauthored by all five members of the new Via Panisperna group concluded with a resounding message. The periodic table of elements had to be changed. They had apparently succeeded in producing number 93, an element with 93 protons. Up until then the table had ranged from hydrogen, number 1, to uranium, number 92. It now had a transuranic. The group was not altogether surprised. Unlike the Curie and Joliot findings, the radioactive isotopes produced by neutron bombardment typically decayed by electron emission. Therefore, when an electron was seen being emitted after uranium had been bombarded, the suspicion of having produced a transuranic was natural. The subsequent check that the resultant nucleus did not lie below uranium on the periodic table apparently confirmed the suspicion. The atomic number had increased by one unit, 92 turning into 93.