by Close, Frank
Bruno’s first piece of research followed the discovery by Amaldi and Segrè that the spectra of the gaseous form of certain elements alter when other gases are present. Fermi theorized that this is because the electrons at the periphery of heavy elements are nearly free, move relatively slowly, and bounce off the surrounding atoms. This changes the energy of the electrons and, in turn, the spectrum of light they emit. To test Fermi’s theory, Bruno repeated the experiment, and measured the spectrum of mercury vapor in the presence of various gases. The measurements were delicate; their analysis complicated. Based on this work, Bruno published the first paper of his life, at the age of twenty-one. Fermi must have been impressed, for in the summer of 1934 he co-opted the young experimenter onto his team.
BY THIS TIME, FERMI AND THE VIA PANISPERNA BOYS HAD BEEN working on induced radioactivity for six months—ever since Fermi had returned from his ski vacation and learned of the Joliot-Curies’ discovery.7 Fermi had decided this phenomenon was ripe for his team to investigate—not using alpha particles, as the Joliot-Curies had, but neutrons.
In hindsight, this is an obvious idea, but at the time it was radical.8 The fact that others hadn’t immediately tried it came down to logistics: free neutrons are very rare. To create beams of neutrons, you first have to bombard atoms of the element beryllium with alpha particles. Because most of these fail to hit the beryllium nuclei, the process generates only one neutron per 100,000 alpha particles. This seemed so wasteful that most laboratories dismissed the project, if they considered it at all.
Nonetheless, Fermi persevered with neutrons because they had one huge advantage over alphas: neutrons are electrically neutral. Because alphas are electrically charged, like the atomic nuclei they are invading, getting an alpha particle into a nucleus is like forcing the north poles of two magnets to touch. When alphas (like those used by the Joliot-Curies) enter the dense forest of atoms in a bulk target, they are rejected by the positive nuclei and ensnared by the negative electrons, usually within a fraction of a millimeter of the sample’s surface; even a sheet of paper can absorb them. There is little chance of an alpha particle hitting an atomic nucleus in so short a journey. Neutrons, being electrically neutral, can enter a nucleus without this difficulty. On March 20, 1934, Fermi accomplished his goal, inducing radioactivity in aluminum by means of neutrons, before doing the same with fluorine. In each case the balance of neutrons and protons in the target atoms is delicate, and the invader disturbs it. The new grouping gives up some energy and attains equilibrium by readjusting the ratio of neutrons and protons, which it achieves by emitting an electron or a positron—the phenomenon of beta radioactivity.9 Fermi announced his discovery in a letter to La ricerca scientifica on March 25, 1934: “Radioactivity induced by neutron bombardment.”
Next, Fermi attacked heavier elements. Frédéric and Irène Joliot-Curie had successfully induced radioactivity only in elements that were relatively light, mainly because such elements had only a limited amount of charge with which to resist the alpha-particle invader. Fermi saw that neutrons had a huge advantage when it came to bombarding heavier atoms, so he decided to launch a systematic attack—firing neutrons at every element on the periodic table.
This would require a team effort, so Fermi co-opted Amaldi, Segrè, and Rasetti, as well as a young chemist named Oscar D’Agostino. By the summer of 1934, they had tested about sixty elements, and induced radioactivity in about forty of them. Some elements released more radioactivity than others—hydrogen gave none; fluorine a little; aluminum more. These qualitative differences were clearly real, but some means of quantifying the results was needed.
Fermi’s team developed a standard scale based on silver, which had been in the middle of the qualitative range. By this stage the team had mastered the techniques, meaning that the continuing work of recording these measurements was straightforward, ideally suited to a novice. So the task of building the scale fell to Bruno Pontecorvo, working with Amaldi.
BRUNO EXPERIENCES A MIRACLE
The team’s protocol called for samples of each element to be engineered into hollow cylinders, into which they placed the neutron source. To protect the surroundings from radiation, they placed the sample and the source inside a box of lead and left them, giving the neutrons time to activate the sample. After a while they removed the sample and measured its activity.
Eventually Bruno noticed something odd: the position of the sample within the box, and the box within the room, influenced its ultimate degree of radioactivity, as if some strange telepathy linked it to surrounding objects. Bruno recalled his astonishment later: “There were wooden tables in the laboratory which had miraculous properties. Silver irradiated on these tables became much more radioactive than when an identical sample was irradiated on the marble tabletops in the room.”10 Bruno and his partner described this phenomenon to Rasetti—the Cardinal Vicar—who thought it was nonsense. Although he knew that Bruno had precocious abilities, Rasetti considered his laboratory work “extremely clumsy” and feared that Pontecorvo’s sloppiness had infected Amaldi. Hearing of their observations, he diplomatically suggested that their results were nothing more than evidence of “anomalies due to statistical error and inaccuracy of measurements.”
Fermi agreed that “the results did not make sense at all,” leaving Amaldi and Pontecorvo to suffer a terrible couple of weeks.11 However, as Fermi was always open-minded about the surprises nature might contain, he decided to investigate the phenomenon for himself, despite his misgivings. He later recalled, “It occurred to me to see what would happen if I put a piece of lead in front of the source of neutrons”—that is, between the source and the silver.12 He was preparing the lead on a lathe very carefully, when he noticed a piece of paraffin wax lying around. Then, “without any conscious reason,” he left the lathe and decided to use the paraffin instead of the lead. He confirmed that the radioactivity of the silver was much higher than it had been without the paraffin. Perhaps his criticism of Amaldi and Pontecorvo had been unfair.
It was the morning of Saturday, October 20.13 Amaldi, Rasetti, and Pontecorvo were in their offices. Fermi showed them his results, and then it was time for lunch.14 What happened next would become part of the folklore of physics.
During lunch, Fermi continued to ruminate. What do paraffin and wood have that marble does not?15 He visualized a neutron in flight, bumping into atoms in its surroundings and slowing down. A lightweight atom, such as hydrogen, would be especially good at reducing the neutron’s speed. Hydrogen is present in water, which is found in wood but not marble. It is also present in paraffin. Could slowed-down neutrons be the key to the riddle? Then he saw the answer: whereas alpha particles have a positive charge and need high speed to penetrate the repulsive electric fields that surround a nucleus, neutrons don’t need any such aid. For neutral neutrons, impervious to electrical impediment, the rule is: the slower, the better. Lumbering neutrons, slowed to the point that their motion is no more than thermal agitation, remain in the vicinity of the target atoms for longer than fast-moving ones, giving them a greater chance of being captured and activating the sample. Fermi had experienced an epiphany: slow neutrons are especially good at inducing nuclear reactions.16
This was a remarkably bold conclusion. Up to that time, the received wisdom had been that the harder you hit a nucleus, the more likely it is to fragment. If Fermi was correct, then this wisdom was wrong: nature is more subtle. In fact the radioactivity would become especially strong if there were some means of slowing the neutrons radically. His musings had already suggested a way to do this: use a substance containing plenty of hydrogen, such as water.
Hydrogen is the lightest element of all, its atomic nucleus consisting of a single proton. For our purposes, the key feature is that the proton has almost the same mass as a neutron. As can be seen in the analogy of two billiard balls colliding, it is when two particles of the same or similar masses collide that energy is most rapidly dissipated. Bounce a billiard ball against the edge o
f the massive table, and the ball bounces back at (almost) the same speed; in the case of a neutron, this is analogous to the neutron hitting a massive atom of lead and recoiling unslowed. However, if one billiard ball hits another ball, which was initially stationary, they both recoil, the first ball slowing in the process. As for billiard balls, so for neutrons and protons. It was the presence of hydrogen—each atom of which contains but a single proton—that slowed the neutrons most efficiently. The presence of hydrogen atoms in the wooden tabletop, and their absence in the marble, thus explained the difference in behavior that Pontecorvo and Amaldi had noticed. The hydrogen in the paraffin explained Fermi’s results too.
This conclusion had not been obvious. The place to test it, however, was. Senator Corbino, who had founded Fermi’s laboratory, had a spacious apartment in the building, with access to a walled garden. Enclosed by the physics buildings and the church of San Lorenzo in Panisperna, it contained an almond tree, a classical water fountain, and a goldfish pond. The physicists rushed to Corbino’s pond, armed with their neutron source and silver sample.17 They put them underwater and watched expectantly. Corbino’s goldfish continued to swim unperturbed while the scientists leaned over the edge of the pond, full of eager anticipation. That historic afternoon—October 22, 1934—they found the answer. The activity in the silver rose dramatically. That same evening, highly excited, they drafted a paper for publication in a scientific journal.
Beyond supplying the pond, Corbino had not been involved, but he was always interested in the work of his “Boys.” Sensing their animation, he asked what was going on. Once he was told about the slow-neutron phenomenon, he became excited, and joined them in Amaldi’s small apartment as they drafted a paper. Corbino was initially relaxed, but when they started to write a second paper he erupted.18 He waved his hands and screamed, “Are you crazy?” This Sicilian man of the world had realized what the young scientists, living in an ivory tower, had not: their discovery could have industrial applications. Previously, the quantity of radioactive material that could be created using alpha particles or neutrons had been trifling. However, the slow-neutron technique could produce it a hundred times more abundantly, and the practical implications were tantalizing. “Take a patent before you give out more details on how to make radioactive substances,” he urged.19
SATURDAY NIGHT AND SUNDAY MORNING
The story just related was told by Laura Fermi in her biography of her husband, Enrico. The book was a best-seller, which helped turn the tale into folklore, and then into received wisdom. The story was then retold by Edoardo Amaldi and repeated by many, including Enrico Fermi himself. However some details are wrong, and reveal the tricks of false memory.
Enrico Fermi’s laboratory notebook shows that the first hint of the breakthrough came on Saturday, October 20, as stated above. However, Laura Fermi, Edoardo Amaldi, and Emilio Segrè, who wrote later from memory, placed it on the same day that they drafted the paper: October 22. Fermi’s logbook, which dates from the time in question, shows that two days elapsed between the epiphany and the paper. What really happened?20
Fermi’s own record shows that he performed tests, with and without paraffin, on Saturday, October 20. His insight about water matured during lunchtime. However, one cannot immediately rush to a pond, dunk samples in it, and see them spontaneously burst into radioactive life. First you have to irradiate the samples with neutrons, underwater, for some considerable time.
There seems to have been a bucket of water in the laboratory, which a cleaner had left.21 Fermi immersed samples of cesium and rubidium nitrate in this water, and irradiated them overnight, from Saturday night to Sunday morning. On Sunday, he measured the amount of induced activity in these two samples.
The results convinced him that he was on the right track, so he continued the exercise. He now prepared samples of sodium carbonate, lithium hydroxide, platinum, ruthenium, and strontium. Overnight, from Sunday to Monday, he irradiated them “in the water.”22 On Monday morning, October 22, he measured the amounts of induced radioactivity in each sample. He began with the sodium carbonate at 9:45 a.m., continued with lithium and platinum during the late morning, and completed the task with ruthenium and strontium after midday.
The two-day discrepancy with regard to the date is not important in itself, other than as proof that memory can be an unreliable guide. The story of Corbino’s pond is so delightful that I hope it really happened. By the afternoon of the twenty-second, Fermi was satisfied that the samples had become more active underwater. If the cleaner had removed the water bucket, as in some versions of the story, it is plausible that the excited youngsters would make a student demonstration in the goldfish pond. In any case, the fact that the paper was drafted on the evening of the twenty-second is certain. This took place at Edoardo Amaldi’s house. His son Ugo, who was then just a baby, recalls being told at several family gatherings that “I was asleep upstairs” on that fateful night, and also that the next day Ugo’s nanny asked his mother whether the “signori the night before had been tipsy.”23
IMAGE 2.1. The Via Panisperna Boys, from left to right: Oscar D’Agostino, Emilio Segrè, Edoardo Amaldi, Franco Rasetti, and Enrico Fermi. The photograph was taken by Bruno Pontecorvo. (COURTESY GIL PONTECORVO AND DEPARTMENT OF PHYSICS, SAPIENZA UNIVERSITY OF ROME.)
PAPERS AND PATENTS
Fermi’s name appeared as the first author on the paper. This reflected his leadership in the discovery. His collaborators then appeared in alphabetical order: Amaldi, Pontecorvo, Rasetti and Segrè. Four days later, the discovery became the subject of a patent: “To increase the production of artificial radioactivity with neutron bombardment.” The patent owners are the above quintet, along with chemist Oscar D’Agostino and Giulio Trabacchi, who had provided the neutron sources.
The scientists knew they had stumbled upon something with potentially immense importance. To record the moment they took a photograph, which has since become iconic. It shows the young men—Fermi, Rasetti, Amaldi, and Segrè—and D’Agostino the chemist. Years later, Edoardo Amaldi’s son Ugo asked Bruno why he too was not in the famous picture. The answer: “I was on the other side of the camera.” As the youngest member of the team, Bruno was given the responsibility of taking the photograph.24
Although Bruno was the last person to join the team, his role in the discovery had been honored by his inclusion on the patent. On November 1, he received more formal recognition, receiving an appointment as a temporary assistant at the Royal Institute of Physics and the University of Rome.25 On November 7, his significance was further highlighted when a second paper about slow neutrons was sent to La ricerca scientifica for publication. This one had just three authors: Fermi, Pontecorvo, and Rasetti. This was an outstanding achievement: Pontecorvo’s name stood alone between those of the two senior professors on the team.
This paper provided experimental confirmation of Fermi’s conjecture: it is indeed the presence of hydrogen that causes neutrons to slow. It also reported that, in addition to being activated, the targets absorb the slow neutrons. Furthermore, the team discovered that there is an enormous range in the ability of various substances to absorb slow neutrons. This would become important later in selecting materials for use in nuclear reactors.
Soon afterward, Pontecorvo performed a series of experiments using substances that contained no hydrogen. He measured how effectively they slowed neutrons, and published a paper as sole author in April 1935. In less than a year he had become an expert in a new field of huge importance.26
The key to nuclear power is to slow neutrons efficiently, and the most effective way to do so is to use either heavy water or graphite.27 At the time, however, it didn’t occur to Fermi’s team that this could be the key to practical nuclear power—further discoveries would be needed before that route opened.
Even so, others were already anticipating the future. Hungarian physicist Leo Szilard believed that energy could be liberated from the atomic nucleus so abundantly and cheaply that
an “industrial revolution could be expected.”28 Corbino remarked that nuclear physics could become a new “super-chemistry,” producing more energy than conventional chemical reactions, with potential benefits for national electricity production. In 1934, however, these were little more than well-considered speculations.
Bruno Pontecorvo’s first real steps into physics had led to a patent for a means of inducing radioactivity through the use of slow neutrons. Years later he recalled how the process was sold to the US Government, leading to payments for many years—to “everyone except me.”29 The US patent, which was filed on October 3, 1935, includes the assertion “To obtain radioactive substances in quantities of practical importance.” Uranium is explicitly mentioned. The implications of this discovery, and the corresponding patents, were to prove far-reaching. They would affect both the world at large and Bruno Pontecorvo’s personal destiny. He had been a midwife at the birth of the nuclear age.
NIELS BOHR EXPLAINS THE NUCLEUS
Despite their success, Fermi’s team was still exploring in the dark. They had stumbled on a phenomenon—the efficacy of slow neutrons—and exploited it, but the breakthrough had given them no real understanding of what was going on deep in the atomic nucleus.
In Copenhagen, Niels Bohr was puzzled by the Italian team’s discovery that slow neutrons affected the nuclei of some elements more than others. Years before, he had published his model of the atom, which treated electrons like planets orbiting a central nuclear sun. He turned now to the nature of the nucleus itself.
Given the miniscule diameter of an atomic nucleus, a speeding neutron would pass through one in a billionth of a trillionth of a second. To capture a neutron, the nucleus first has to stop it, which involves absorbing its kinetic energy somehow. Because overall energy must be conserved, this kinetic energy has to be transferred somewhere, and there was no obvious way of getting rid of it in such a short time span. Fermi’s measurements unambiguously showed that the neutrons were captured. Bohr took it upon himself to find where the energy went.
