by Close, Frank
Guido’s insistence that Bruno go to Rome stemmed from his friendship with one of the Via Panisperna Boys, Franco Rasetti. He and Guido had been friends for years and had explored the Alps together as hiking companions. At that time, Bruno was a child, patronizingly known as “the cub.” Rasetti paid him little attention. Years later, when Bruno presented himself to Rasetti, announcing that he wished to complete his studies in Rome, Rasetti teased him: “Just out of your diapers and you want to become a physicist!”14
Although he was confident and spoke with ease—and was, in the words of Laura Fermi, “uncommonly good looking”—Bruno had a tendency to blush at the least provocation. In response to Rasetti’s joke about his youth, Bruno gave one of his familiar blushes, but Rasetti—well aware of the intellectual strength of the Pontecorvo family—encouraged Fermi to take a look at him.
Fermi gave him an informal exam. Years later, Bruno claimed that he showed only “mediocre knowledge.” Fermi explained to him that there were two categories of physicists: theoreticians and experimentalists. He then added: “If a theorist does not have exceptional ability, his work does not make sense. As for experimental physics, there exists the possibility of useful work, even if the person has only average intelligence.”15
Fermi was infamously slow to praise and blunt in his criticism. It’s unclear if Fermi was delicately giving his opinion, so as to guide Bruno toward experiment, or simply providing idiosyncratic commentary. In any case, in 1931 Bruno Pontecorvo entered the third year of physics at the University of Rome. This meant he had the good fortune to be studying physics in the annus mirabilis of 1932, when the atomic nucleus was discovered to have a labyrinthine structure of it own.16 By 1934 Bruno was ready to take part in genuine research as a member of Fermi’s team, right at the dawning of a new science: nuclear physics. At age twenty-one, he was destined to be at the epicenter of one of the greatest and most far-reaching discoveries of the twentieth century.
THE PREHISTORY OF NUCLEAR PHYSICS
At the end of the nineteenth century, atoms were believed to be the fundamental seeds of all matter. The standard model of that time asserted that all atoms of the same element were identical, that different elements consisted of different types of atoms, and that compounds formed from atoms of the constituent elements.17 Much of this remains true today. However, the scientists of yesteryear also believed that atoms were indestructible and impenetrable objects, like miniature billiard balls. This is not the case.
In 1911, working in Manchester, Ernest Rutherford discovered that an atom is mostly empty space, with a massive, dense kernel at its center carrying a positive electric charge—a kernel that he called the nucleus. In 1913, the year of Bruno’s birth, Rutherford’s colleague, Danish theorist Niels Bohr, proposed that atoms are held together by the electrical attraction of opposite charges. In this model, negatively charged electrons orbit the positively charged nucleus.
At that stage, no one knew what an atomic nucleus consisted of. By the time Bruno started school, Rutherford had shown that the nucleus of a hydrogen atom is the simplest of all, consisting of a single positively charged particle, which he called a proton. Rutherford had deduced that the proton was fundamental to the nuclei of all atomic elements. As a student, Bruno would have learned that atomic nuclei are lumps of positive charge, made up of protons, and that the more protons there are in the lump, the greater the charge. It is the amount of this positive charge that determines how many negatively charged electrons can be ensnared in the outer regions of the atom. The chemical elements are distinguished by the complexity of their atoms—hydrogen, the simplest, consists of a single electron encircling a single proton, while helium has two protons in its nucleus, carbon has six—onward to uranium with ninety-two. The chemical identity of an element is a result of its electrons, and chemical reactions occur when electrons move from one atom to another.
This simple picture first started to change in 1932, when James Chadwick of Cambridge University discovered a third basic seed of matter, the neutron. Neutrons are similar to protons but carry no electric charge; they cluster in atomic nuclei and add to the nuclear mass without changing the total charge. We now know that neutrons are an essential component of all atomic nuclei, except for that of hydrogen, which normally consists of just a single proton.
Every atom of the element uranium has ninety-two protons in its nucleus. The number of neutrons may vary, however. A rare form of uranium known as U-235 contains 143 neutrons, while the most common form, known as U-238, has 146. Adding the neutrons to the ninety-two protons in each case gives a total of 235 or 238 constituents, respectively. These varying forms are called isotopes, from the Greek isos and topos—meaning “the same place” (in the periodic table of atomic elements). Although all the isotopes of a particular element have the same chemical identity, the behavior of their atomic nuclei can vary dramatically. Indeed, the neutron number is the key to extracting energy from the nucleus, either gradually in nuclear reactors or explosively in weapons. For example, U-235 forms the raw material for both nuclear power plants and atomic bombs.18
An atomic nucleus, then, is more than just a core: it is a new level of reality. Within its labyrinthine structures, powerful forces are at work, which are unfamiliar in the wider world. The presence of these forces is suggested by the otherwise paradoxical fact that nuclei exist. Why do the protons, which all have the same electric charge, not repel each other and cause the nucleus to disintegrate? The answer is that there is a strong attractive force that grips protons and neutrons when they are in contact with one another. Within the nucleus this strong attraction between a pair of protons is over a hundred times more powerful than the electrical repulsion.
There is a limit, however, to the number of protons that can coexist like this. For any individual proton, the attractive glue acts only between it and its immediate neighbors. The electrical disruption, however, acts across the entire volume of the group. In a large nucleus, the total amount of electrical repulsion can exceed the localized attraction, in which case the nucleus cannot survive. The neutron, being electrically neutral, helps counteract this disruption and stabilize the nucleus. Even so, many neutrons are needed to do this, especially in larger nuclei. Uranium, with ninety-two protons, is the largest stable example in practice, requiring 140 neutrons to stabilize its protons. Yet the counterbalance is so delicate that the slightest disturbance can split a uranium atom in two, in the phenomenon known as fission.
It turns out that the strong attractive force acts most efficiently when the constituent neutrons and protons pair off exactly. Thus U-238, which has an even number of neutrons, is more stable than U-235, which has one odd neutron without a partner.
Even the simplest element of all, hydrogen, has isotopes. A proton accompanied by one neutron forms a stable isotope, known as a deuteron, the nucleus of deuterium. Water (H2O) contains hydrogen; the analogous molecule consisting of deuterium—D2O—forms “heavy water.” A proton accompanied by two neutrons forms the nucleus of tritium. Tritium is mildly unstable, however, with a half-life of about twelve years.19 These are all the possible isotopes of hydrogen; “quadium” or “H4,” in which a proton is joined by three neutrons, does not exist.
Why don’t clusters of tens or hundreds of neutrons exist on their own, without any protons mixed in? In short, it is because neutrons are inherently unstable. A neutron is slightly heavier than a proton. Given Einstein’s equivalence between mass and energy, this implies that a neutron has slightly more energy locked within it than a proton does. This extra energy leads to instability—so much so that an isolated neutron cannot survive for more than a few minutes on average, whereas a single proton can exist for eons, possibly even forever. As a general rule: if there are too many neutrons in a nucleus, the assembly becomes unstable.20
The result of all this is that only a limited number of stable isotopes exist, namely those where the number of neutrons is close to, or larger than, the number of protons. As we go furth
er up the periodic table of the elements, the atomic nuclei become larger, and the number of excess neutrons expands as well.
It is difficult to predict in advance which isotopes will be stable and which will not. Much of the work on atomic weapons and nuclear reactors in the 1940s and 1950s would rely on experimental tests or rules of thumb. The different isotopes of uranium, plutonium, and hydrogen would become central players in this saga, all as a result of the discovery of the neutron. Indeed, the whole course of history was changed as a result of this discovery, causing the astrophysicist Hans Bethe to describe the years leading up to 1932 as the “prehistory of nuclear physics.” Everything that came after was history.21
It was the perfect moment for an ambitious young scientist to start a career—a moment when he could investigate the mysteries of the atomic nucleus. And Enrico Fermi’s group in Rome was ideal. Bruno Pontecorvo was about to become an expert in neutron physics.
TWO
SLOW NEUTRONS AND FAST REACTIONS
1934–1936
IT WAS IN 1934, JUST AS BRUNO WAS ABOUT TO JOIN THE TEAM, THAT Enrico Fermi’s genius began to bear fruit. The circumstances that inspired him resulted from a setback in his attempt to explain one of the fundamental natural processes: a form of radioactivity known as beta decay.
Ernest Rutherford had identified three different varieties of radioactivity in 1899, and named them after the first three letters of the Greek alphabet: alpha, beta, and gamma. Alpha radiation consists of massive, positively charged particles, which emerge in a staccato burst when spontaneously emitted by substances such as radium. (Despite its name, the “alpha particle” is not a fundamental particle, since it is built from protons and neutrons—two of each, or four particles in all. However, the quartet is so commonly produced in radioactive decays that it was identified before its nuclear structure was recognized, and the name stuck.) Beta radiation consists of electrons, not those preexisting in the atom but ones created when an unstable isotope changes into a more durable form. Gamma radiation involves high-energy photons, with much shorter wavelengths than visible light. Although these three forms of radioactivity were known at the start of the twentieth century, no one understood how they arose, or what effects they had on the nucleus, for another two decades.
Beta decay was especially tantalizing, since energy seemed to be disappearing without a trace. As an explanation, Austrian theorist Wolfgang Pauli proposed in 1930 that the electron—the beta particle—is accompanied by an unseen, electrically neutral particle, which carries away the missing energy. This is the neutrino, literally the “little neutral.”1 Unfortunately, Pauli’s hypothetical neutrino was so ghostly that he feared it might never be detected. The idea of the neutrino excited Fermi, however, who used it in a theory of beta decay in 1933.2
Fermi’s inspiration came when he visualized a nucleus made from neutrons and protons. He realized that a neutron behaves much like a proton with its electric charge removed, and he guessed that the neutrino might be similarly related to the electron. He proposed that beta decay occurs when a neutron, within a nucleus, spontaneously changes into a proton, conservation of electric charge is maintained due to the appearance of an electron, and the overall energy balances due to the creation of a neutrino.
Today we know that Fermi was fundamentally correct. However, the editor of Nature, the journal to which Fermi submitted his paper, “Tentative Theory of Beta Rays,” for publication, rejected it on the grounds that it contained “speculations too remote from reality to be of interest to the reader.”3 Fermi’s paper was eventually published in another journal, but the arguments with the editor of Nature had exhausted Fermi to such an extent that he decided to switch from theory to experiments “for a short while.”4 In fact, this change of focus lasted for the rest of his life.
In January 1934, Fermi went on a skiing trip to the Alps. It was on his return that he saw the way forward, thanks to a discovery made in France by Irène Joliot-Curie (the daughter of Marie Curie) and her husband, Frédéric. The Joliot-Curies had been exploring the uncharted inner space of the atomic nucleus since at least 1930. Four years later, after a series of misadventures, they made a discovery that would inspire Fermi and his group—including its new member Bruno Pontecorvo.
This discovery was made possible through the investigation of radioactivity, which enabled scientists to unravel the deep structure of atomic nuclei. Radioactivity intrigued many physicists, but for Irène and Frédéric there was a special motivation. Irène’s mother, Marie Curie, had discovered that the element radium is so highly radioactive that it is warm—one can literally feel it pour out energy spontaneously. This energy is carried off by alpha particles. A few grams of radium can therefore function as a practical source of large numbers of alpha particles, which are like atomic bullets, able to smash into atoms of other elements. In this respect, Irène and Frédéric were in a privileged position. Thanks to Marie Curie, their laboratory in Paris had access to more radium than anywhere else in the world. This inspired the Joliot-Curies to use this invaluable element as a source of alpha particles, which they used to bombard atoms of other elements. The result was a memorable series of experiments in the early 1930s. In one such experiment, the Joliot-Curies bombarded a sample of aluminum with alpha particles. A Geiger counter near the target sample started crackling when the irradiation began; when the barrage ended, the crackling continued, decreasing to half its original intensity after about three minutes.
This is what had happened: An aluminum nucleus consists of thirteen protons and fourteen neutrons. The addition of an alpha particle to the mix temporarily supplies two more protons and two more neutrons; however, the collision of the particles chips off a single neutron from the nucleus, leaving a cluster of 15 neutrons and 15 protons. This group of thirty is a radioactive isotope of phosphorus, called phosphorus-30. It decays with a half-life of three minutes, which explains the behavior of the Joliot-Curies’ Geiger counter.
This was revolutionary work. In 1933, Ernest Rutherford had famously remarked that anyone who believed in extracting energy from the atomic nucleus was talking “moonshine.” If natural radioactivity had been the only possible kind, Rutherford would have been right. However, Frédéric and Irène had discovered that it is possible to alter the nucleus, and thereby induce radioactivity in otherwise inert material, such as ordinary aluminum. Their experiment showed that it is possible to liberate part of an atom’s latent nuclear energy at will, potentially in amounts far exceeding anything known to chemistry.
The vista the Joliot-Curies revealed included a wealth of opportunities for medicine, science, and technology. Frédéric and Irène received a Nobel Prize for their discovery in 1935. Upon receiving the award, Frédéric presciently remarked that by modifying atoms this way it might be possible to “bring about transmutations of an explosive type.” He went on: “If such transmutations do succeed in spreading in matter, the enormous liberation of useful energy can be imagined.”5 It was a chance observation by Bruno Pontecorvo that began the transformation of this idea from imagination to reality, and marked the start of a new age.
THE VIA PANISPERNA BOYS
Fermi’s group of young researchers, based at the laboratory on Rome’s Via Panisperna, had been working together for about a year when Bruno joined them. This team of brilliant individuals was the brainchild of Orso Corbino, the head of the physics department at the University of Rome. Combative and quick, the Sicilian Corbino was an astute politician with sound judgment, and he would tirelessly pursue any goal that enthused him. He saw Fermi’s talent, hired him, and provided the funds to build a research team.
Part of what enabled Corbino to accomplish this was his membership in Mussolini’s cabinet, despite never having joined the Fascist Party. Fermi, although barely in his thirties, had enough political acumen to appreciate the delicacy of the situation, realizing that the group’s resources were ultimately a gift of the government. He therefore insisted that physics and politics be kept se
parate within his team. This meant that Bruno’s first experience of scientific research was as an apolitical enterprise. Much would change later.
With Corbino’s support, Fermi attracted a handful of talented young people to the group. The eldest, Franco Rasetti, born in 1901, was the same age as Fermi. He would burst into high-pitched cackles of laughter at the least provocation. The two whom Bruno was closest to throughout his life were Emilio Segrè and Edoardo Amaldi. Segrè, four years younger than Fermi and Rasetti, was the most serious of the group, cautious and not inclined to go along with the tomfoolery of some of his colleagues. Amaldi, two years younger than Segrè, with a cherubic face and a mass of brown hair, was the baby until Bruno’s arrival. Fermi, with his infallible intellect, became known as the Pope; Corbino, holding the purse strings, was the Eternal Father; Rasetti, Fermi’s deputy, was the Cardinal Vicar. Segrè was called the Basilisk, reflecting what the others perceived as his rather irritable character, and Amaldi was called the Child. The final member of this holy caucus was Giulio Trabacchi, who provided them with a source of neutrons, which became key to their research; Trabacchi was thus known as the Divine Providence. When Bruno arrived, five years younger than Amaldi, he became known as the Puppy.6
