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 Cor-bino'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. 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,” Segre remembers. “He stood by me; Rasetti, Amaldi, and Pontecorvo paced the room excitedly, all making comments at the same time.” 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.”
Ginestra Amaldi delivered the typed paper, “Influence of hydrogenous substances on the radioactivity produced by neutrons — I,” to the director of the
Amaldi and Segre 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. 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
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.” 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.” 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
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.
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.” 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.” 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.” 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. It was Lindemann, wealthy and well-connected, who was arranging a fellowship for
