Teller's surprise Mici invited their countryman to stay with them:
We drove to our home, and I showed Szilard to his room. He felt the bed suspiciously, then turned to me suddenly and said: “Is there a hotel nearby?” There was, and he continued: “Good! I have just remembered sleeping in this bed before. It is much too hard.”
But before he left, he sat on the edge of the hard bed and talked excitedly: “You heard Bohr on fission?”
“Yes,” I replied.
Szilard continued: “You know what that means!”
What it meant to Szilard, Teller remembers, was that “Hitler's success could depend on it.”
The next day Szilard discussed his plan for voluntary secrecy with Teller, then entrained for Princeton to pursue the same subject with Eugene Wigner, who was still drydocked in the infirmary with jaundice. Szilard was thus present in Princeton when yet another momentous insight struck Niels Bohr.
Bohr and Leon Rosenfeld were staying at the Nassau Club, the Princeton faculty center. On Sunday, February 5, George Placzek joined them at breakfast in the club dining room. The Bohemian theoretician had arrived in Princeton from Copenhagen the night before, another refugee from Nazi persecution. Talk turned to fission. “It is a relief that we are now rid of those transuranians,” Rosenfeld remembers Bohr saying, referring to the confusing radioactivities Hahn, Meitner and Strassmann had found in the late 1930s that Bohr assumed could now be attributed to existing lighter elements — barium, lanthanum and the many other fission products researchers were beginning to identify.
Placzek was skeptical. “The situation is more confused than ever,” he told Bohr. He began then to specify the sources of confusion. He was directly challenging the relevance of Bohr's liquid-drop model of the nucleus. The Danish laureate paid attention.
Physicists use a convenient measurement they call a “cross section” to indicate the probability that a particular nuclear reaction will or will not happen. The theoretical physicist Rudolf Peierls once explained the measurement with this analogy:
For example, if I throw a ball at a glass window one square foot in area, there may be one chance in ten that the window will break, and nine chances in ten that the ball will just bounce. In the physicists' language, this particular window, for a ball thrown in this particular way, has a “disintegration cross-section” of Vio square foot and an “elastic cross-section” of 9/10 square foot.
Cross sections can be measured for many different nuclear reactions, and they are expressed not in square feet but in minute fractions of square centimeters, customarily 10–24, because the diminutive nucleus is the target window of Peierls' analogy. The cross section that concerned Placzek in his discussion with Bohr was the capture cross section: the probability that a nucleus will capture an approaching neutron. In terms of Peierls' analogy, the capture cross section measures the chance that the window might be open when the ball arrives and might therefore admit the ball into the living room.
Nuclei capture neutrons of certain energies more frequently than they capture neutrons of other energies. They are naturally tuned, so to speak, to certain specific energy levels — as if Peierls' window opened more easily to balls thrown at only certain speeds. This phenomenon is known as resonance. The confusion Placzek delighted in reporting concerned a resonance in the capture cross sections of uranium and thorium.
Placzek pointed out that uranium and thorium both exhibit a capture resonance for neutrons with medium- range energies of about 25 electron volts. That meant, first of all, that although fission was one behavior uranium could exhibit under neutron bombardment, capture and subsequent transmutation continued to be another. Bohr was not ever to be rid of those inconvenient “transuranians.” Some of them were real.
If a neutron penetrated a uranium nucleus, for example, the result might be fission. But if the neutron happened to be traveling at the appropriate energy when it penetrated — somewhere around 25 eV — the nucleus would probably capture it without fissioning. Beta decay would follow, increasing the nuclear charge by one unit; the result should be a new, as-yet-unnamed transuranic element of atomic number 93. That was one of Plac-zek's points. It would prove in time to be crucial.
The other source of confusion was more straightforward. It was also more immediately relevant to the question of how to harness nuclear energy. It concerned differences between uranium and thorium.
Thorium, element 90, a soft, heavy, lustrous, silver-white metal, was first isolated by the celebrated Swedish chemist Jons Jakob Berzelius in 1828. Berzelius named the new element after Thor, the Norse god of thunder. Its oxide found commercial use beginning in the late nineteenth century as the primary component of the fragile woven mantles of gas lanterns: heat incandesces it a brilliant white. Because it is mildly radioactive, and radioactivity was once considered tonic, thorium was also for some years incorporated into a popular German toothpaste, Doramad. Auer, the company that made German gas mantles, also made the toothpaste. Hahn, Meitner and Strassmann, the Joliot-Curies and others had regularly studied thorium alongside uranium. Its behavior was often similar. Otto Frisch had first demonstrated that it fissioned. He bombarded it next after uranium in the course of his January experiment in Copenhagen, the experiment he had discussed with Bohr after he returned from Kungalv and Bohr had worked so hard in the United States to protect.
Frisch was then also the first to notice that the fission characteristics of thorium differed from those of uranium. Thorium did not respond to the magic of paraffin; it was unaffected by slow neutrons. Richard B. Roberts and his colleagues at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington had just independently confirmed and extended Frisch's findings. With their 5 million volt Van de Graaffthey could generate neutrons of several different, known energies. Continuing their experiments after their Saturday-night show for the Washington Conference group, they had compared uranium and thorium fission responses at varying energies as Frisch with his single neutron source could not. They found to their surprise (Frisch's paper had not yet appeared in
Why, Placzek now prodded Bohr, should both uranium and thorium have similar capture resonances and similar fast-neutron thresholds but different responses to slow neutrons? If the liquid-drop model had any validity at all, the difference made no sense.
Bohr abruptly saw why and was struck dumb. Not to lose what he had only barely grasped, oblivious to courtesy, he pushed back his chair and strode from the room and from the club. Rosenfeld hurried to follow. “Taking a hasty leave of Placzek, I joined Bohr, who was walking silently, lost in deep meditation, which I was careful not to disturb.” The two men tramped speechless through the snow across the Princeton campus to Fine Hall, the Neo- Gothic brick building where the Institute for Advanced Study was then lodged. They went in to Bohr's office, borrowed from Albert Einstein. It was spacious, with leaded windows, a fireplace, a large blackboard, an Oriental rug to warm the floor. No peripatetic like Bohr, Einstein had judged it too large and moved into a small secretarial annex nearby.
“As soon as we entered the office,” Rosenfeld remembers, “[Bohr] rushed to the blackboard, telling me: ‘Now listen: I have it all.’ And he started — again without uttering a word — drawing graphs on the blackboard.”
The first graph Bohr drew looked like this:
The horizontal axis plotted neutron energy left to right — low to high, slow to fast. The vertical axis charted cross sections — the probability of a particular nuclear reaction — and served a double purpose. The lazy S that filled most of the frame represented thorium's cross section for capture at different neutron energies, the steep central peak demonstrating the 25 eV resonance in the middle range. The tail that waved from the horizontal axis on the right side represented a different thorium cross section: its cross section for fission beginning at that high 1
