That was a mystery worth exploring. On October 18 they started a systematic investigation, a series of measurements made inside and outside a lead housing. By October 22 they were prepared to measure what might happen when only a lead wedge separated the neutron source from its target. But the experimenters had to give student examinations that morning and Fermi decided to go ahead on his own. He described the historic moment late in life to a colleague curious about the process of discovery in physics:
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 on the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came into 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 piece of lead was to have been.818
The extraordinary result of substituting paraffin wax for a heavy element like lead was a dramatic increase in the intensity of the activation. “About noon,” Segrè remembers, “everybody was summoned to watch the miraculous effects of the filtration by paraffin. At first I thought a counter had gone wrong, because such strong activities had not appeared before, but it was immediately demonstrated that the strong activation resulted from the filtering by the paraffin of the radiation that produced the radioactivity.”819 Laura Fermi says “the halls of the physics building resounded with loud exclamations: ‘Fantastic! Incredible! Black magic!’ ”820
Not even his most important discovery kept Fermi from going home for lunch. He was alone; his wife and daughter would not return from a visit to the country until the following morning. He pondered in solitude and may have considered the difference between wood and marble tables as well as between paraffin and lead. When he returned in midafternoon he proposed an answer: the neutrons were colliding with the hydrogen nuclei in the paraffin and the wood. That slowed them down. Everyone had assumed that faster neutrons were better for nuclear bombardment because faster protons and alpha particles always had been better. But the analogy ignored the neutron’s distinctive neutrality. A charged particle needed energy to push through the nucleus’ electrical barrier. A neutron did not. Slowing down a neutron gave it more time in the vicinity of the nucleus, and that gave it more time to be captured.
The simple way to test Fermi’s theory was to try some other material besides paraffin that contained hydrogen (other light nuclei would also work to slow neutrons down, but hydrogen would work best: its nuclei are protons, about the same size and mass as neutrons, and they therefore bounce hardest and soak up the most energy per collision). Down to the first floor and out the back door they marched with their silver cylinder and their neutron source extended in its long glass tube, to the pond in Corbino’s garden where Rasetti had experimented with raising salamanders, where they had all caught the fad one summer of sailing candle-powered toy boats, where the dark, curving leaves and leathery gray drupes of an almond tree shaded the lively goldfish.
The hydrogen in water (and in goldfish) worked as well as paraffin.821 Back in the lab they quickly tested whatever they could lay hands on to irradiate: silicon, zinc, phosphorus, which did not seem to be affected by the slow neutrons; copper, iodine, aluminum, which did. They tried radon without beryllium to make sure the paraffin was affecting neutrons and not gamma rays. They replaced the paraffin with an oxygen compound and found much less increase in induced radioactivity.
They went home to dinner but met afterward at Amaldi’s, whose wife had a typewriter, to prepare a first report. “Fermi dictated while I wrote,” Segrè remembers. “He stood by me; Rasetti, Amaldi, and Pontecorvo paced the room excitedly, all making comments at the same time.”822 Laura Fermi recreates the scene: “They shouted their suggestions so loudly, they argued so heatedly about what to say and how to say it, they paced the floor in such audible agitation, they left the Amaldis’ house in such a state, that the Amaldis’ maid timidly inquired whether the guests had all been drunk.”823
Ginestra Amaldi delivered the typed paper, “Influence of hydrogenous substances on the radioactivity produced by neutrons—I,” to the director of the Ricerca Scientifica the next morning.824 Tucked away in its historic paragraphs was a quiet justification for the confusion over aluminum: “The case of aluminum is noteworthy. In water it acquires an activity showing a period slightly shorter than 3 minutes. . . . This activity under normal conditions is so weak that it almost disappears compared to other activities generated in the same element.”825
Amaldi and Segrè had not been wrong about aluminum. They had simply irradiated different samples of the element on different tables. The hydrogen in the wooden table had slowed down some of the neutrons and enhanced the almost-three-minute activity. As Hans Bethe once noted wittily, the efficiency of slow neutrons “might never have been discovered if Italy were not rich in marble. . . . A marble table gave different results from a wooden table.826 If it had been done [in America], it all would have been done on a wooden table and people would never have found out.”
The discovery of slow-neutron radioactivity meant that Fermi’s group had to work its way through the elements again looking for different and enhanced half-lives—which is to say, different isotopes and decay products.
While that work proceeded a paper appeared in the Physical Review criticizing the group’s earlier study of uranium.827 The paper’s primary author was Aristide von Grosse, who had been one of Otto Hahn’s assistants at the KWI and who had purified the first substantial sample of protactinium, the element Hahn and Meitner had discovered in 1917. Von Grosse argued that when Fermi irradiated uranium he had created protactinium, atomic number 91, not a new transuranic element. The Rome group took the paper as a challenge to further experiment. At the same time Hahn and Meitner decided proprietarily to repeat Fermi’s previous uranium work. “It was a logical decision,” Hahn explains in his scientific autobiography; “having been the discoverers of protactinium, we knew its chemical characteristics.”828 The increasing number of different half-lives that investigators in Berlin and Paris found when they irradiated uranium were puzzling; Hahn correctly felt that he was better qualified than anyone else in the world to accomplish the subtle radiochemistry necessary to sort everything out.
In January and February 1935, in the midst of other projects, Amaldi set to work looking for alpha-emitting reactions in uranium in addition to the beta reactions the group had originally found. If uranium emitted alpha particles when it captured neutrons it would be transmuting down the periodic table rather than up, which might indeed produce protactinium along the way. Amaldi chose to use an ionization chamber connected to a linear amplifier to capture and measure the radiation. “I began to irradiate some foil[s] of uranium,” he writes, “ . . . and put them immediately after irradiation in front of the thin-window ionization chamber.”829 Nothing happened. Conceivably the half-lives were too brief for the run down the hall from the irradiation area to the ionization chamber. Amaldi decided to try irradiating his samples directly in front of the chamber. That required screening out unwanted radiation. The gamma rays from his neutron source, which would have disturbed the ionization chamber, he blocked by setting a piece of lead between the source and the chamber: the desirable neutrons would find the lead no obstacle.
He also wanted to filter out uranium’s natural alpha background. To do that he took advantage of the basic law of radioactivity that shorter half-lives mean more energetic radiation. The half-life of natural uranium is about 4.5 billion years; its alphas are
proportionately mild, mild enough to be blocked by a layer of aluminum foil. On the other hand, if there really were half-lives in his experiment so short that he had to irradiate directly in front of the ionization chamber to catch them, their alphas should be energetic enough to breeze easily through the aluminum and the chamber window and enter the chamber for counting. So Amaldi wrapped his uranium samples with aluminum foil. It did not occur to him that his shielding might also screen out other reaction products. In 1935, alpha, beta and gamma radiation were the only reaction products anyone knew. “The experiments,” Amaldi concludes, “gave negative results.”830 He found no artificially induced alphas from uranium.
The Italians thought it even more probable then that by irradiating uranium they were creating new, man-made elements. Hahn and Meitner reported they thought so too. Fermi’s group rounded up its work in the Proceedings of the Royal Society in a paper Rutherford approvingly passed along to that journal on February 15:
Through these experiments our hypothesis that the 13-minute and 100-minute induced activities of uranium are due to transuranic elements seems to receive further support. The simplest interpretation consistent with the known facts is to assume that the 15-second, 13-minute and 100-minute activities are chain products [i.e., one decays into the next], probably with atomic number 92, 93 and 94 respectively and atomic weight 239.831
But the truth was, uranium was a confusion, and no one yet knew.
What else besides beryllium? Leo Szilard asked himself in London. Beryllium looked suspicious. What other elements might chain-react? He answered with an amended patent specification on April 9, 1935: “Other examples for elements from which neutrons can liberate multiple neutrons are uranium and bromine.”832 He was guessing, and without research funds he saw no way to experiment. The physicists he talked to remained profoundly skeptical of his ideas. “So I thought, there is after all something called ‘chain reaction’ in chemistry. It doesn’t resemble a nuclear chain reaction, but still it’s a chain reaction. So I thought I would talk to a chemist.”833 The chemist he thought he would talk to was someone even more skillful than Leo Szilard at raising funds: Chaim Weizmann, who now lived and worked in London. Weizmann received Szilard and “understood what I told him.”834 He asked Szilard how much money he needed. Szilard said £2,000—about $10,000. Though he was certainly hard-pressed for funding himself, Weizmann said he would see what he could do. Szilard recalls:
I didn’t hear from him for several weeks, but then I ran into Michael Polanyi, who by that time had arrived in Manchester and was head of the chemistry department there. Polanyi told me that Weizmann had come to talk to him about my ideas for the possibility of a chain reaction, and he wanted Polanyi’s advice on whether he should get me this money. Polanyi thought that this experiment should be done.
A decade passed before Szilard and Weizmann met again, a gulf of history. Weizmann had not neglected Szilard’s request, he explained then in apology in late 1945; he had only not succeeded in raising the funds.
Since the beginning of his rescue work in England Szilard had been in occasional contact with the physicist Frederick Alexander Lindemann, who was professor of experimental philosophy at Oxford and director of the Clarendon Laboratory there.835 It was Lindemann, wealthy and well-connected, who was arranging a fellowship for Szilard, part of his continuing campaign to arm the decrepit Oxford science laboratory against its splendid Cambridge rival. Lindemann had made effective use in that campaign of the Nazi expulsion of the Jewish academics but had given as good as he got: immediately upon hearing of the civil service law he had gone to Imperial Chemical Industries and convinced its directors to establish a grant program, arguing that such an investment would be not charity but money well spent. ICI had already begun paying out its first grant on May 1, 1933, while Beveridge and Szilard were still laying plans. It was an ICI grant that Szilard missed winning the following August, perhaps because he had not yet accomplished his summer of impressive experiment at St. Bart’s, but Lindemann was paying attention now.
The tall, handsome Englishman, forty-nine years old in 1935, had been born in Germany, at Baden-Baden, because his mother chose not to allow advanced pregnancy to interfere with a visit to that fashionable spa. To provide their son with an outstanding education his English parents had sent him to the Gymnasium in Darmstadt. As a student before the Great War at the Darmstadt Technische Hochschule, where he was a protégé of the physical chemist Walther Nernst (the 1920 chemistry Nobelist), he had enjoyed such exceptional family connections that he found himself at times playing tennis with the Kaiser or the Czar. Inevitably the war made suspect such golden afternoons. Lindemann was chagrined and angered in 1915 to find that the British Army, noting his German birth certificate and German-sounding name, was unwilling to extend him a commission.
The Army’s decision injured him deeply and may have changed his life. He had served as a co-secretary to the 1911 Solvay Conference, standing up proudly with Nernst, Rutherford, Planck, Einstein, Mme. Curie, but even before that youthful apotheosis Nernst had predicted difficulty: “If your father were not such a rich man,” the blunt German had said, “you would become a great physicist.”836 When the Army questioned Lindemann’s patriotism, writes a refugee colleague, “he became withdrawn to avoid exposing himself to slights and insults. Secretiveness about his personal life developed into a mania and he discouraged personal approaches by a stand-offishness which was easily mistaken for arrogance.”837 Lindemann retreated from original work and became a talented administrator, “the Prof,” an “unbending Victorian gentleman,” always impeccable in bowler hat, summer gray suit, winter dark suit, rolled-up umbrella and long, dark coat.838 If he could not win a uniform he would adopt one of his own.
He worked for his country during the war at the Royal Aircraft Factory at Farnborough, designing what are now called avionics and doing aeronautical research. Tailspins were recognized maneuvers in air fighting by 1916, a good way to shake off an attacker. Lindemann was the first to study them scientifically. To do so he took flying lessons—only changing from civilian clothes to flying clothes on the runway beside the plane—then coolly flew spin after spin, memorizing his instrument readings as he plummeted and writing them down after he had recovered level flight.
After the war Lindemann accepted appointment to an Oxford still donnishly disdainful of science. He escaped from that further condescension, says his colleague, into “gracious living,” enjoying weekends with the nobility that were seldom vouchsafed to less well-born Oxford dons. By then a Rolls-Royce was part of his regalia. In June 1921, on a weekend at the country estate of the Duke and Duchess of Westminster, Lindemann met Winston Churchill, twelve years his senior. “The two men, so different in background and character, took to each other immediately and their acquaintance soon turned into a close friendship.”839 Churchill recalled that he “saw a great deal of Frederick Lindemann” during the 1930s. “Lindemann was already an old friend of mine. . . . We came much closer from 1932 onwards, and he frequently motored over from Oxford to stay with me at Chartwell. Here we had many talks into the small hours of the morning about the dangers which seemed to be gathering upon us. Lindemann . . . became my chief adviser on the scientific aspects of modern war.”840
To this illustrious personage, a vegetarian who daily consumed copious quantities of olive oil and Port Salut, Szilard turned in the early summer of 1935 to discuss “the question whether or not the liberation of nuclear energy . . . can be achieved in the immediate future.” If “double neutrons” could be produced, Szilard wrote Lindemann on June 3, “then it is certainly less bold to expect this achievement in the immediate future than to believe the opposite.” That meant trouble, Szilard thought, if Germany achieved a chain reaction first, and he argued for “an attempt, whatever small chance of success it may have . . . to control this development as long as possible.”841 Secrecy was the way to achieve such control: first, by winning agreement from the scientists involved to restr
ict publication, and second, by taking out patents.
Michael Polanyi had cautioned Szilard late in 1934 that “there is an opposition to you on account of taking patents.”842 The British scientific tradition that opposed patents assumed that those who filed them did so for mercenary purposes; Szilard explained his patents to Lindemann to clear his name:
Early in March last year it seemed advisable to envisage the possibility that . . . the release of large amounts of energy . . . might be imminent. Realising to what extent this hinges on the “double neutron,” I have applied for a patent along these lines. . . . Obviously it would be misplaced to consider patents in this field private property and pursue them with a view to commercial exploitation for private purposes. When the time is ripe some suitable body will have to be created to ensure their proper use.843
For the time being, Szilard proposed to work at Oxford on finding his “double neutrons,” possibly raising £1,000 on the side “from private persons” so that he could hire a helper or two. To bait Lindemann’s Clarendon ambitions, he argued in conclusion that “this type of work could greatly accelerate the building up of nuclear physics at Oxford.”844 As indeed, had it gone forward, it might have done.
When he learned, possibly from Lindemann, that he could keep his patents secret only by assigning them to some appropriate agency of the British government, Szilard offered them first to the War Office. Director of Artillery J. Coombes turned them down on October 8, noting that “there appears to be no reason to keep the specification secret so far as the War Department is concerned.”845 If Lindemann heard of the rejection he must have remembered his own rejection by the Army in 1915. The following February 1936, he intervened on Szilard’s behalf with the Admiralty, Churchill’s old bailiwick, writing the head of the Department of Scientific Research and Development cannily:
Making of the Atomic Bomb Page 31