Their first experiment demonstrated a brilliantly simple method for separating isotopes of iodine by bombarding an iodine compound with neutrons. They then used this Szilard-Chalmers effect (as it came to be called), which was extremely sensitive, as a tool for measuring the production of neutrons in their second experiment: knocking neutrons out of beryllium using the gamma radiation from radium. “These experiments,” Szilard reminisces wryly, “established me as a nuclear physicist, not in the eyes of Cambridge, but in the eyes of Oxford. [Szilard had in fact applied to Rutherford that spring to work at the Cavendish and Rutherford had turned him down.] I had never done work in nuclear physics before, but Oxford considered me an expert… Cambridge… would never had made that mistake. For them I was just an upstart who might make all sorts of observations, but these observations could not be regarded as discoveries until they had been repeated at Cambridge and confirmed.”
If Szilard's summer work helped establish his Oxford reputation, it was also a personal disappointment: beryllium proved an unsatisfactory candidate for chain reaction. The problem, not settled until 1935, lay with the established mass of helium. The one stable isotope of beryllium consists of two helium nuclei lightly bound by a neutron. Its apparently high mass, which was calculated from Francis Aston's measurements of the mass of helium, seemed to indicate that it should be unstable. But the mass spectrograph was a skittish instrument even in the hands of its inventor, and as Bethe, Rutherford and others were about to demonstrate, Aston's measurements were inaccurate: he had set the mass of helium too high. One casualty of that error was beryllium's candidacy for chain reaction, for nuclear power and atomic bombs.
Emilio Segre and Edoardo Amaldi pilgrimaged to Cambridge early in July, short on English but carrying with them a comprehensive report on the Rome neutron-bombardment investigations. They met Chadwick, Kapitza and the other regulars at the Cavendish; observed the retired J. J. Thomson making his rounds; noted Aston, says Amaldi innocently, “going on improving the accuracy of his measurements of atomic masses”; and had a memorable meeting with Rutherford, whose “strong personality dominated the whole laboratory.”
The two young physicists had come to compare experiments with two of Rutherford's boys. An unanswered question hung over the neutron work, a question that called existing nuclear theory into doubt. The
Theory at the time treated the nucleus as if it was one large particle. As such, it had a definite diameter, which was modest enough that a speeding neutron could go in one side and exit out the other in about 10– 21 seconds, a billion times less than a trillionth of a second. Any capture process would have to work within that brief interval of time. Otherwise the neutron would be gone. Capturing a neutron means stopping it within a nucleus. To do that the nucleus has to absorb the neutron's energy of motion. The nucleus in turn has to get rid of the excess energy. Which it does: by emitting a gamma photon.
But the gamma-emission times Fermi's group had measured were different from what theory said they ought to be. The nuclei the Rome group had studied took at least 10–16 seconds to get around to gamma emission — one hundred thousand times too long. And that was unaccountable.
Definite proof of radiative capture would sharpen the challenge to theory. That required proving beyond doubt, by experiment, that a heavier isotope really forms when a heavy nucleus captures a neutron. The Cavendish team Segre and Amaldi came to visit in the summer of 1934 accomplished the first part of the proof, using sodium, while the Italians were on hand. They then returned to Rome and enlisted D'Agostino's help to perform the confirming chemistry. In the heat of Roman August they looked for additional clear-cut examples and won a double prize: “We also found a second case of ‘proven’ radiative capture,” Amaldi writes, “which was based on the discovery of a new radioisotope of [aluminum] with a lifetime of almost 3 minutes.”
Fermi planned to stop off in London for an international physics conference on his way home from South America. His young colleagues sent him word of their aluminum discovery. He reported to the conference on the neutron work. (Szilard also attended, happy to hear praise for his summer experiments and well launched toward a paying fellowship at Oxford.) Fermi said his group had studied sixty elements so far and had induced radioactivity in forty of them. Discussing the radiative-capture problem he cited the Cavendish results “and those of Amaldi and Segre on aluminium,” which were both, he said, “to be considered particularly important.” Segre describes the tempestuous aftermath:
Shortly afterwards I caught a cold and could not go to the laboratory for several days. Amaldi tried to repeat our experiments and found a different [half-life] for irradiated aluminum which showed that our so-called (11,7) reaction [i.e., neutron in, gamma photon out] did not occur. This was hurriedly relayed to Fermi who resented having communicated a result which now looked to be in error. He strongly criticized us and did not conceal his displeasure. The whole business was becoming very troublesome because we could not find any fault with the various experiments which gave inconsistent results.
The chastened junior members had their work cut out for them. A new recruit joined them, a tall, broad- shouldered, handsome tennis champion from Pisa named Bruno Pontecorvo, as they set about polishing their first rough work. Neutron bombardment activated some elements more intensely than others. They had previously categorized that activation only generally as strong, medium or weak. Now they proposed to establish a quantitative scale of activibility. They needed some standard intensity against which to measure the intensity of other activations. They chose the convenient 2.3-minute half-life period that neutron bombardment induced in silver.
Amaldi and Pontecorvo got the assignment. They immediately found, to their surprise, that their silver cylinders activated differently in different parts of the laboratory. “In particular,” writes Amaldi, “there were certain wooden tables near a spectroscope in a dark room which had miraculous properties, since silver irradiated on those tables gained much more activity than when it was irradiated on a marble table in the same room.”
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.
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,” Segre 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.” Laura Fermi says “the halls of the physics building resounded with loud exclamations: ‘Fantastic! Incredible! Black magic!’”
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
