responsible for slow-neutron fission in natural uranium:
If this isotope could be used for maintaining chain reactions, it would have to be separated from the bulk of uranium. This, no doubt, would be done if necessary, but it might take five to ten years before it can be done on a technical scale. Should small scale experiments show that the thorium and the bulk of uranium would not work, but the rare isotope of uranium would, we would have the task immediately to attack the question of concentrating the rare isotope of uranium.[4]
Strauss's surge-generator losses had inoculated him against further investment in the nuclear enterprise. He wanted to know, Szilard says, “just how sure I was that this would work.” Since Szilard could offer no guarantees, Strauss offered no support. Szilard turned then to Benjamin Liebowitz. “He was not poor but he was not exactly wealthy… I told him what this was all about, and he said, ‘How much money do you need?’ I said, ‘Well, I'd like to borrow $2,000.’ He took out his checkbook, he wrote out a check, I cashed the check, I rented the… radium, and in the meantime the beryllium block arrived from England.”
The cylinder of beryllium, which Walter Zinn thought “a strange and unique object” and took for proof of Szilard's magic ways, arrived on February 18. The same day Szilard heard from Teller about significant work in Washington at the DTM. Richard Roberts and R. C. Meyer were preparing a letter to the
The general excitement Teller found at the busy DTM laboratories impressed him more:
As soon as I began taking interest in uranium, sharp discussion started on the practical significance. Tuve, Hafstad, and Roberts are entirely aware of what is involved. They also know of Fermi's experiments. Of course, I didn't say anything. The above-mentioned letter [to the
I do not know their detailed plans, but I believe that urgent action [to maintain secrecy] is required. Very many people have discovered already what is involved. Those in Washington would like to persuade the Carnegie Institution that it should provide more money for U-research in view of the practical significance of the matter… But right now this has no reality unless the [Carnegie] leadership becomes more interested than it has been so far…
I repeat that there is a chain-reaction mood in Washington. I only had to say “uranium” and then could listen for two hours to their thoughts.
The president of the Carnegie Institution was a New England Yankee, the grandson of two sea captains, an electrical engineer, inventor and former dean of the school of engineering at the Massachusetts Institute of Technology named Vannevar Bush. If Bush was initially less willing to invest in chain-reaction experiments than Teller would have liked him to be, he kept good company; neither Ernest Lawrence at Berkeley nor Otto Hahn in Dahlem nor Lise Meitner, visiting Copenhagen that February to work with Otto Frisch, chose to pursue moonshine. Only Columbia and Paris mounted early experiments, though the DTM would soon follow the Columbia lead.
Frdderic Joliot and two colleagues, a cultivated Austrian named Hans von Halban and a huge, keen Russian named Lew Kowarski, began an experiment similar to Fermi's the last week in February to identify secondary neutrons from fission. They also used a tank of water with a central neutron source but dissolved their uranium in the water rather than packing it around the source. More important to their priority of research, they had immediate access to the Radium Institute's ample radium supply.
Because Fermi's neutron source relied on radon rather than radium it induced an ambiguity into his experiment that Szilard caught and called to his attention: radon ejected much faster neutrons from beryllium than did radium; at least part of any increase in neutrons Fermi found in his tank might therefore result not from fission but from another, competing reaction in beryllium. Fermi thought the ambiguity trivial, but agreed, as Zinn had before, to repeat the experiment using a radium-beryllium source. Szilard generously offered his. But the radium to energize it was not yet in hand; Szilard was still negotiating its rental because his lack of official affiliation made the Radium Chemical Company nervous.
He got his radium, two grams sealed in a small brass capsule, early in March, after he arranged admission to the Columbia laboratories for three months as a guest researcher. He and Zinn immediately set up their experiment. They made an ingenious nest, like Chinese boxes, of its various components: a large cake of paraffin wax, the beryllium cylinder set at the bottom of a blind hole in the paraffin, the radium capsule fitted into the beryllium cylinder; resting on the beryllium, inside the paraffin, a box lined with neutron-absorbing cadmium filled with uranium oxide; pushed into that box, but shielded from the radium's gamma radiation by a lead plug, the ionization tube itself, which connected to an oscilloscope. With this arrangement, says Szilard, they could measure the flux of neutrons from the uranium with and without the cadmium shield:
Everything was ready and all we had to do was to turn a switch, lean back, and watch the screen of a television tube. If flashes of light appeared on the screen, that would mean that neutrons were emitted in the fission process of uranium and this in turn would mean that the large-scale liberation of atomic energy was just around the corner. We turned the switch and saw the flashes. We watched them for a little while and then we switched everything off and went home.
They had made a rough estimate of neutron production: “We find the number of neutrons emitted per fission to be about two.” With radium available merely by picking up the phone, the French team a week earlier had found “more than one neutron… produced for each neutron absorbed.” Fermi and Anderson estimated “a yield of about two neutrons per each neutron captured.” Szilard immediately alerted Wigner and Teller. Teller remembers the moment well:
I was at my piano, attempting with the collaboration of a friend and his violin to make Mozart sound like Mozart, when the telephone rang. It was Szilard, calling from New York. He spoke to me in Hungarian, and he said only one thing: “I have found the neutrons.”
Szilard also wired Lewis Strauss:
PERFORMED TODAY PROPOSED EXPERIMENT WITH BERYLLIUM BLOCK WITH STRIKING RESULT. VERY LARGE NEUTRON EMISSION FOUND. ESTIMATE CHANCES FOR REACTION NOW ABOVE 50 %.
Szilard had known what the neutrons would mean since the day he crossed the street in Bloomsbury: the shape of things to come. “That night,” he recalled later, “there was very little doubt in my mind that the world was headed for grief.”
Though he was still recovering from jaundice, Eugene Wigner responded vigorously to Szilard's disturbing news while a storm of betrayal broke over Central Europe. Hitler ordered the President and the Foreign Minister of Czechoslovakia to Berlin on March 14 and threatened to bomb Prague to rubble unless they surrendered their country. With the Nazi leader's encouragement the Slovaks formally seceded from the republic that day. Ruthenia, Czechoslovakia's narrow eastern extension along the Carpathians, also claimed independence as Carpatho-Ukraine, an exercise in grave-robbing abruptly terminated the following morning when the fascist Hungary of Admiral Horthy invaded the new nation with German endorsement. Hitler flew in triumph to Prague. On March 16 he decreed what was left of Czechoslovakia — Bohemia and Moravia — to be a German protectorate. The country that France and Great Britain had abandoned at Munich was partitioned without resistance.
Wigner caught the train to New York. On the morning of March 16 he met with Szilard, Fermi and George Pegram in Pegram's office. Since at least the end of January Szilard had been promoting a new version of his Bund — he called it the Association for Scientific Collaboration — to monitor research, collect and disburse funds and maintain secrecy, a civilian organization that might guide the development of atomic energy. He had discussed it with Lewis Strauss on the train to Washington, with Teller after the night of the hard bed, with Wigner in Princeton
