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 Berkel
ey. 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.
The Last Man Who Knew Everything Page 15
