She was traveling on Friday to the village of Kungälv in the west of Sweden for a week’s vacation, she told Hahn; “if you write me in the meantime please address your letter there.” She sent him and his family “warmest greetings . . . and much love and the very best for the New Year.”968
That day Hahn and Strassmann had finished the actinium-lanthanum experiment—and confirmed lanthanum from barium decay. In the late evening, after they turned off their counters, Hahn wrote his exiled colleague again. The paper was not yet finished; a phrase from the letter would be reworked to more cautious language for the final draft: “Our radium proofs convince us that as chemists we must come to the conclusion that the three carefully-studied isotopes are not radium, but, from the standpoint of the chemist, barium.”969
Hahn had hoped Meitner might quickly find some physical explanation for his unprecedented chemistry. That would strengthen his conclusion and also put Meitner’s name on the paper, the best possible Christmas gift. With the lanthanum confirmation at hand he could no longer delay. As it was he had withheld the news from physicists on his own staff and at the new physics institute nearby. Someone else—Curie and Savitch, for example—might very well have made the same discovery. And whatever the explanation, the discovery was clearly of major importance, a reaction unlike any other yet found. “We cannot hush up the results,” Hahn wrote Meitner, “even though they may be absurd in physical terms. You can see that you will be performing a good deed if you find an alternative [explanation]. When we finish tomorrow or the day after I will send you a copy of the manuscript. . . . The whole thing is not very well suited for Naturwissenschaften. But they will publish it quickly.”
Hahn mailed the letter to Stockholm. He did not yet know about Meitner’s Kungälv vacation.
* * *
Leo Szilard’s work at the University of Rochester confirmed that no neutrons came out when indium was irradiated. On December 21, as Hahn and Meitner exchanged their excited letters, Szilard advised the British Admiralty by letter:
Further experiments . . . have definitely cleared up the anomalies which I have observed in 1936. . . . In view of this new work it does not now seem necessary to maintain [my] patent . . . nor would the waiving of the secrecy of this patent serve any useful purpose. I beg therefore to suggest that the patent be withdrawn altogether.970
Szilard’s faith in the possibility of a chain reaction, as he said later, had “just about reached the vanishing point.”971
* * *
Hahn and Strassmann had originally titled their paper “On the radium isotopes produced by the neutron bombardment of uranium and their behavior.”972 With their new data they realized “radium” would no longer do. They considered changing “radium” to “barium” throughout the paper. But most of it had been written before the lanthanum experiment firmed their convictions. They would have had to rewrite from beginning to end, “especially,” says Hahn in retrospect, “since in view of this result its major portion was not especially interesting any more.”973 Christmas and the journal deadline were upon them and they had no time. They decided to juryrig what was on hand. The results would be no less effective for being inelegant. They substituted the noncommittal phrase “alkaline-earth metals” for “radium isotopes” in the title—both barium and radium are alkalineearth metals, as are beryllium, magnesium, calcium and strontium. They went through the draft putting equivocal quotation marks around their many references to radium and actinium. Then they attached seven cautious paragraphs at the end.
“Now we still have to discuss some newer experiments,” this final section began, “which we publish rather hesitantly due to their peculiar results.” They then summarized their series of experiments:
We wanted to identify beyond any doubt the chemical properties of the parent members of the radioactive series which were separated with the barium and which have been designated as “radium isotopes.” We have carried out fractional crystallizations and fractional precipitations, a method which is well-known for concentrating (or diluting) radium in barium salt solutions. . . .
When we made appropriate tests with radioactive barium samples which were free of any later decay products, the results were always negative. The activity was distributed evenly among all the barium fractions. . . . We come to the conclusion that our “radium isotopes” have the properties of barium. As chemists we should actually state that the new products are not radium, but rather barium itself. Other elements besides radium or barium are out of the question.
They discussed actinium then, distinguished their work from that of Curie and Savitch and pointed out that all so-called transuranics would have to be reexamined. Not quite prepared to usurp the prerogative of the physicists, they closed on a tentative note:
As chemists we really ought to revise the decay scheme given above and insert the symbols Ba, La, Ce [cerium], in place of Ra, Ac, Th [thorium]. However as “nuclear chemists,” working very close to the field of physics, we cannot bring ourselves yet to take such a drastic step which goes against all previous laws of nuclear physics. There could perhaps be a series of unusual coincidences which has given us false indications.
Promising further experiments, they prepared to release their news to the world. Hahn mailed the paper and then felt the whole thing to be so improbable “that I wished I could get the document back out of the mail box”; or Paul Rosbaud came around to the KWI the same evening to pick it up.974 Both stories survive Hahn’s later recollection. Since Rosbaud knew the paper’s importance and dated its receipt December 22, 1938, he probably picked it up. But Hahn also visited the mailbox that night, to send a carbon copy of the seminal paper to Lise Meitner in Stockholm. His misgivings at publishing without her—or some dawning glimmer of the fateful consequences that might follow his discovery—may have accounted for his remembered apprehension.
* * *
The Swedish village of Kungälv—the name means King’s River—is located some ten miles above the dominant western harbor city of Goteborg and six miles inland from the Kattegat coast.975 The river, now called North River, descends from Lake Vanern, the largest freshwater lake in Western Europe; at Kungälv it has cut a sheer granite southward-facing bluff, the precipice of Fontin, 335 feet high. The modern village is built along a single cobblestone lane on the narrow talus between the bluff and the river, its back to the wall.
As Norwegian Kongahalla the village was founded at a less constricted place downstream around A.D. 800. But an island hill rises from the river at Kungälv and is thus guarded by a natural moat, a defensive geography which the precipice of Fontin reinforces. In 1308, to mark the border there between Norway and Sweden, the Norwegians began to build on that island hill a monumental granite fortress, Bohus’ Fäste (i.e., King Bohus’ Fort), sod-ridged block walls mazing inward and upward to a cylindrical tower of thick stone with a conical roof that dominates the entire coastal valley. An accident of placement of three of the deep windows that penetrate the tower—two open above, one centered below—transforms it into a face staring with hollow eyes toward the Fontin bluff. To soften the grimness of that face the people of the valley named the tower Fars Hatt, Father’s Hat, as if it evoked a workman in a cap. Through four hundred years of occupation Bohus’ Fäste was besieged fourteen times while the settlements in the valley were put to the torch and the graveyard filled on the island below its hard walls.
The village was ordered moved upriver onto the island in 1612. The Danes ruled Norway from the fifteenth century to the early nineteenth century; they ceded the Kungälv region, Bohuslän, to Sweden by the Treaty of Roskilde in 1658. Fire in 1676 burned the island village and its burghers shifted for safety to the narrow shore. They laid out their lane and strip of houses extending west and east from a cobblestone marketplace where the talus widened to make room. Despite its fortress Kungälv is peaceful, especially in winter with the river frozen and a depth of clean snow on the ground. Its snug wooden houses, painted pastel, enclose rooms cozy w
ith ships’ chests and china cabinets and lace curtains, warmed by corner fireplaces faced with decorative tile, aromatic with coffee and baking. Eva von Bahr-Bergius and her husband Niklas built a house there in 1927, larger than most Kungälv houses but constructed in the same style. In 1938 Lise Meitner was alone in Stockholm. Otto Frisch was alone in Copenhagen, his mother, Meitner’s sister, beyond reach in Vienna, his father incarcerated at Dachau, a victim of Kristallnacht. The Bergiuses therefore considerately invited aunt and nephew to Kungälv for Christmas dinner.
Meitner left Stockholm Friday morning, two days before Christmas. Frisch took the train ferry across from Denmark.976 His aunt arrived before him and registered at a quiet inn on Västra gatan, West Street, where they both would stay, a pale green building much like its modest neighbors but with a café on the ground floor.977 It faced a shadowed strip of garden north across the lane; above the stunted garden trees the dark bluff loomed. The other way, behind the inn, the flat, snow-covered flood plain of the river extended into open woods. The Bergiuses’ house was a short walk eastward past the marketplace and the white church. Tired from travel, Frisch and Meitner met only briefly in the evening when Frisch came in.978
In Copenhagen that winter he had been studying the magnetic behavior of neutrons. To further his work he needed a strong, uniform magnetic field, and on his way to Kungälv he had sketched out a large magnet he meant to design and build.979 He came downstairs on the morning before Christmas prepared to interest his aunt in his plans. She was already at breakfast and had no intention of discussing magnets: she had brought Hahn’s December 19 letter downstairs with her and insisted Frisch read it.980 He did. “Barium,” he told her, “I don’t believe it. There’s some mistake.”981 He tried to change the subject to his magnet; she changed it back to barium. “Finally,” says Meitner, “ . . . we both became absorbed in my problem.”982 They decided to go for a walk to see what they could puzzle out.
Frisch had brought cross-country skis and wanted to use them. He was concerned that his aunt would be unable to keep up. She could walk as fast as he could ski on level ground, she told him. She could and did. He fetched his skis and they went out, probably eastward to the Kungälv marketplace, which gave onto the flood plain of the river, then across the frozen river and into the open woods beyond.
“But it’s impossible,” Frisch remembers them saying in their collective effort to understand. “You couldn’t chip a hundred particles off a nucleus in one blow.983 You couldn’t even cut it across. If you tried to estimate the nuclear forces, all the bonds you’d have to cut all at once—it’s fantastic. It’s quite impossible that a nucleus could do that.” Thirty years afterward Frisch summarized their thinking in more formal terms:
But how could barium be formed from uranium? No larger fragments than protons or helium nuclei (alpha particles) had ever been chipped away from nuclei, and the thought that a large number of them should be chipped off at once could be dismissed; not enough energy was available to do that. Nor was it possible that the uranium nucleus could have been cleaved right across. Indeed a nucleus was not like a brittle solid that could be cleaved or broken; Bohr had stressed that a nucleus was much more like a liquid drop.984
The liquid-drop model made a division of the nucleus seem possible. They sat down on a log. Meitner found a scrap of paper and a pencil in her purse. She drew circles. “Couldn’t it be this sort of thing?”985
Frisch: “Now, she always rather suffered from an inability to visualize things in three dimensions, whereas I had that ability quite well. I had, in fact, apparently come around to the same idea, and I drew a shape like a circle squashed in at two opposite points.”986
“Well, yes,” Meitner said, “that is what I mean.”987 She had meant to draw what Frisch had drawn, a liquid drop elongated like a dumbbell, but had drawn it end-on, indicating with a smaller dashed circle inside a larger solid circle the dumbbell’s waist.
Frisch: “I remember that I immediately at that instant thought of the fact that electric charge diminishes surface tension.”988 The liquid drop is held together by surface tension, the nucleus by the analogous strong force. But the electrical repulsion of the protons in the nucleus works against the strong force, and the heavier the element, the more intense the repulsion. Frisch continues:
And so I promptly started to work out by how much the surface tension of a nucleus would be reduced. I don’t know where we got all our numbers from, but I think I must have had a certain feeling for the binding energies and could make an estimate of the surface tension. Of course we knew the charge and the size reasonably well. And so, as an order of magnitude, the result was that at a charge [i.e., an atomic number] of approximately 100 the surface tension of the nucleus disappears; and therefore uranium at 92 must be pretty close to that instability.
They had discovered the reason no elements beyond uranium exist naturally in the world: the two forces working against each other in the nucleus eventually cancel each other out.
They pictured the uranium nucleus as a liquid drop gone wobbly with the looseness of its confinement and imagined it hit by even a barely energetic slow neutron. The neutron would add its energy to the whole. The nucleus would oscillate. In one of its many random modes of oscillation it might elongate. Since the strong force operates only over extremely short distances, the electric force repelling the two bulbs of an elongated drop would gain advantage. The two bulbs would push farther apart. A waist would form between them. The strong force would begin to regain the advantage within each of the two bulbs. It would work like surface tension to pull them into spheres. The electric repulsion would work at the same time to push the two separating spheres even farther apart.
Eventually the waist would give way. Two smaller nuclei would appear where one large nucleus had been before—barium and krypton, for example:
“Then,” Frisch recalls, “Lise Meitner was saying that if you really do form two such fragments they would be pushed apart with great energy.”989 They would be pushed apart by the mutual repulsion of their gathered protons at one-thirtieth the speed of light. Meitner or Frisch calculated that energy to be about 200 MeV: 200 million electron volts. An electron volt is the energy necessary to accelerate an electron through a potential difference of one volt. Two hundred million electron volts is not a large amount of energy, but it is an extremely large amount of energy from one atom. The most energetic chemical reactions release about 5 eV per atom. Ernest Lawrence was that year building a cyclotron with a nearly 200-ton magnet with which he hoped to accelerate particles by as much as 25 MeV. Frisch would calculate later that the energy from each bursting uranium nucleus would be sufficient to make a visible grain of sand visibly jump. In each mere gram of uranium there are about 2.5 × 1021 atoms, an absurdly large number, 25 followed by twenty zeros: 2,500,000,000,000,000,000,000.
They asked themselves what the source of all that energy could be. That was the crux of the problem and the reason no one had credited the possibility before. Neutron captures that had been observed before had involved much smaller energy releases.
When she was thirty-one, in 1909, Meitner had met Albert Einstein for the first time at a scientific conference in Salzburg. He “gave a lecture on the development of our views regarding the nature of radiation.990 At that time I certainly did not yet realize the full implications of his theory of relativity.” She listened eagerly. In the course of the lecture Einstein used the theory of relativity to derive his equation E = mc2, with which Meitner was then unfamiliar. Einstein showed thereby how to calculate the conversion of mass into energy. “These two facts,” she reminisced in 1964, “were so overwhelmingly new and surprising that, to this day, I remember the lecture very well.”
She remembered it in 1938, on the day before Christmas. She also “had the packing fractions in her head,” says Frisch—she had memorized Francis Aston’s numbers for the mass defects of nuclei.991 If the large uranium nucleus split into two smaller nuclei, the smaller nuclei wou
ld weigh less in total than their common parent. How much less? That was a calculation she could easily work: about one-fifth the mass of a proton less. Process one-fifth of the mass of a proton through E = mc2. “One fifth of a proton mass,” Frisch exclaims, “was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!”992
They converted not quite so suddenly as that. They may have been excited, but Meitner at least was profoundly wary. This new work called her previous four years’ work with Hahn and Strassmann into doubt; if she was right about the one she was wrong about the other, just when she had escaped from Germany into the indifferent world of exile and needed most to confirm her reputation. “Lise Meitner sort of kept saying, ‘We couldn’t have seen it. This was so totally unexpected. Hahn is a good chemist and I trusted his chemistry to correspond to the elements he said they corresponded to. Who could have thought that it would be something so much lighter?’ ”993
Christmas dinner at the Bergiuses’ came and went. Frisch skied and Meitner walked. Nineteen thirty-eight was ticking to its end. With a week to pass in a small village they would certainly have visited the fortress and looked down from its ramparts onto the snow-covered valley, onto centuries of violent graves. Though they understood its energetics now, the discovery was still only physics to them; they did not yet imagine a chain reaction.
Hahn’s letter of December 21, confirming lanthanum, was still not forwarded from Stockholm, nor was the carbon copy of the Naturwissenschaften paper.994 Hahn was eager to win Meitner’s support and wrote Kungälv directly on the Wednesday after Christmas to woo her. Careful not to seem to usurp her place, he called the discovery his “barium fantasy” and questioned everything except the presence of barium and the absence of actinium, taking the humble chemist’s part. “Naturally, I would be very interested to hear your frank opinion.995 Perhaps you could compute and publish something.” He had continued to hold off telling other physicists, though he itched for physical confirmation of his chemistry. It was as though a maker of hand axes had discovered fire by striking flints while the sorcerers pondered how to harness lightning. He might hardly believe his luck and urgently seek their authentication even though he knew what burned his hand was real.
Making of the Atomic Bomb Page 36
