* * *
Otto Hahn opened the September 1938 issue of the Comptes Rendus to a shock. Part two of the Curie-Savitch study of the elusive 3.5-hour activity of uranium appeared there; amid much conjecture its most challenging conclusion was: “Taken altogether, the properties of R3.5h are those of lanthanum, from which it is not possible to separate it except by fractionation.”9421
Curie and Savitch believed that their R3.5h activity could be at least partly separated from lanthanum. It apparently did not occur to them that what was crystallizing out of solution might be another activity with a similar half-life, leaving a 3.5-hour lanthanum activity behind. They still could not believe—nor could anyone else—that uranium bombardment might produce an element thirty-five steps away down the periodic table. A Canadian radiochemist then visiting Dahlem records their German critic’s response: “You can readily imagine Hahn’s astonishment. . . . His reaction was that it just could not be, and that Curie and Savitch were very muddled up.”943
Despite his threat to Joliot in May, Hahn had not yet repeated the Curie-Savitch work. Now he passed the Comptes Rendus along to Fritz Strassmann.944 Strassmann studied the French paper and speculated that the muddle might have a physical cause—two similar radioactivities mixed together in the same solution. He told Hahn. Hahn laughed; the conclusion seemed improbable. On second thought, it was worth examining. As the Czechoslovakian crisis broke across Europe the two men bombarded uranium in peaceful Dahlem. They used a lanthanum carrier to precipitate rare-earth elements such as actinium (if any), a barium carrier to precipitate alkaline-earth elements such as radium (if any). (Carrier chemicals made it possible to separate from the parent solution the few thousand atoms of daughter substances produced by neutron bombardment. A chemically similar daughter substance, traceable by its unique half-life, would lodge in the spaces of the carrier’s crystals as those regular solids formed from solution by chemical precipitation and would thus be carried away. Which carrier accomplished the carrying gave a clue to the part of the periodic table to which the unknown daughter substance belonged. Then it became a matter of further separating the daughter substance from the carrier by fractional crystallization, following it as before by tracing its characteristic radioactivity.)
After a hard week’s work Hahn and Strassmann succeeded in identifying no fewer than sixteen different activities. Their barium separations gave them their most startling results: three previously unknown isotopes which they believed to be radium. They reported their findings in November in Naturwissenschaften. The creation of radium, element 88, from uranium, they pointed out, “must be due to the emission of two successive alpha particles.”945
If the physicists had found it hard to swallow that slow-neutron bombardment might produce thorium (90) or actinium (89), they found it even harder to swallow that it might produce radium. Lise Meitner wrote in warning from Stockholm suggesting pointedly that the two chemists check and recheck their results.946 Bohr invited Hahn to Copenhagen to lecture on the strange findings and tried to concoct a sufficiently crazy explanation:
Bohr was skeptical and asked me if it was not highly improbable. . . . I had to reply that there was no other explanation, for our artificial radium could be separated only with weighable quantities of barium as carrier-substance. So apart from the radium only barium was present, and it was out of the question that it was anything but radium. Bohr suggested that these new radium isotopes of ours might perhaps in the end turn out to be strange transuranic elements.947
Of the sixteen activities they had identified in neutron-bombarded uranium Hahn and Strassmann therefore now turned their full attention to the three controversial activities carried out of solution by barium.
* * *
Laura Fermi woke to the telephone early on the morning of November 10. A call would be placed from Stockholm, the operator advised her. Professor Fermi could expect it that evening at six.
Instantly awake to his wife’s message, Fermi estimated the probability at 90 percent that the call would announce his Nobel Prize. As always he had planned conservatively, not counting on the award. The Fermis had prepared to leave for the United States from Italy shortly after the first of the year. Ostensibly Fermi was to lecture at Columbia for seven months and then return. For stays of longer than six months the United States required immigrant rather than tourist visas, and because Fermi was an academic he and his family could be granted such visas outside the Italian quota list. The ruse of a lecture series was devised to evade a drastic penalty: citizens leaving Italy permanently could take only the equivalent of fifty dollars with them out of the country. But the plan required circumspection. The Fermis could not sell their household goods or entirely empty their savings account without risking discovery. So the money from the Nobel Prize would be a godsend.
In the meantime they invested surreptitiously in what Fermi called “the refugee’s trousseau.” Laura’s new coat was beaver and they distracted themselves on the day of the Stockholm call shopping for expensive watches. Diamonds, which had to be registered, they chose not to risk.
Near six o’clock the phone rang. It was Ginestra Amaldi wondering if they had heard. Everyone had gathered at the Amaldis to wait for the call, she reported. The Fermis turned on the six o’clock news. Laura long remembered the news:
Hard, emphatic, pitiless, the commentator’s voice read the second set of racial laws. The laws issued that day limited the activities and the civil status of the Jews. Their children were excluded from public schools. Jewish teachers were dismissed. Jewish lawyers, physicians, and other professionals could practice for Jewish clients only. Many Jewish firms were dissolved. “Aryan” servants were not allowed to work for Jews or to live in their homes. Jews were to be deprived of full citizenship rights, and their passports would be withdrawn.948
The passports of Jews had already been marked. Fermi had contrived to keep his wife’s passport clear.
They probably heard the news from Germany as well: of a vast pogrom the previous night—Kristallnacht, the night of glass. A seventeen-year-old Polish Jewish student had attempted to assassinate Ernst vom Rath, third secretary in the German Embassy in Paris, on November 7, in reprisal for German mistreatment of the student’s parents. Vom Rath died on November 9 and the assassination served as an excuse for general antiSemitic riot. Mobs torched synagogues, destroyed businesses and stores, dragged Jewish families from their homes and beat them in the streets. At least one hundred people died. A volume of plate glass was shattered that night across the Third Reich equal to half the annual production of its original Belgian sources. The SS arrested some thirty thousand Jewish men—“especially rich ones,” its order had specified—and packed them into the concentration camps at Buchenwald, Dachau and Sachsenhausen, from which they could be ransomed only at the price of immediate pauperized emigration.949
Fermi took the Stockholm call. The Nobel Prize, undivided, would be awarded for “your discovery of new radioactive substances belonging to the entire race of elements and for the discovery you made in the course of this work of the selective power of slow neutrons.”950 In security the Fermis could leave the madness behind.
* * *
Lise Meitner had written Otto Hahn of her worries a few days before the Fermis arrived. “Most of the time I feel like a wind-up doll running on automatic,” she told her old friend, “smiling along happily and empty of real life. From that you can judge for yourself how productive my efforts are at work. And still in the end I’m thankful for it because it forces me to keep my thoughts together, which isn’t always easy.” She was sorry Hahn’s rheumatism had returned and was afraid he wasn’t taking care of himself; she asked after Planck and von Laue by their private Hahn-Meitner nicknames, Max Sr. and Max Jr.; she greeted Hahn’s wife, Edith, and wondered what Christmas plans he had for his son. His uranium work was “really very interesting.”951 She hoped he would write again soon.
She was living in a small hotel room—there was hardly spac
e to unpack—and having trouble sleeping. People told her she was too thin. Worse, conditions at the Physical Institute were not what she had expected them to be.952 A Swedish friend, Eva von Bahr-Bergius, a physicist she knew from Berlin who had been a lecturer at the University of Uppsala, had helped with arrangements and was gradually breaking the bad news.953 Manne Siegbahn had not wanted to take Meitner on. He had no money for her, he had complained; he could give her a place to work but no more. Von Bahr-Bergius had pursued the Nobel Foundation grant. But it provided nothing for equipment or assistance. Meitner blamed herself: “Of course it’s my fault; I should have prepared much better and much earlier for my leaving, should at least have had drawings made of the most important apparatus [she would need].”954
She was a strong woman, but she was miserable and alone. Hahn responded with sympathy. At midmonth she thanked him for that “dear letter,” then changed moods and charged him with indifference: “Concerning myself I sometimes suspect you don’t understand my way of thinking. . . . Right now I really don’t know if anyone cares about my affairs at all or if they will ever be taken care of.”955
Hahn was pursuing Meitner’s affairs as well as his own. With her moody letter at hand he stormed down to the revenue office, which was responsible for inventorying her furniture and other property before allowing its release, and laid on what he called “a little seizure of my ‘ecstasy,’ ” after which “the matter went somewhat better.”956, 957 That news he wrote to Meitner on Monday evening, December 19, from the KWI.958 Only then did he report why he had not yet left the laboratory:959
As much as I can through all of this I am working, and Strassmann is working untiringly, on the uranium activities. . . . It’s almost 11 at night; Strassmann will return at 11:30 so that I can see about going home. The fact is, there’s something so strange about the “radium isotopes” that for the time being we are mentioning it only to you. The half-lives of the three isotopes are quite precisely determined; they can be separated from all elements except barium; all the processes are in tune. Just one is not—unless there are extremely unusual coincidences: the fractionation doesn’t work. Our radium isotopes act like barium.
Hahn and Strassmann worked in three rooms on the ground floor of the Kaiser Wilhelm Institute for Chemistry, the building with the Pickelhaube dome: Hahn’s large personal chemistry laboratory north off the main lobby, a measurement room across the hall at the near end of the wing that extended northwest along Faradayweg and an irradiation room at the far end of the wing. They separated the three functions of irradiation, measurement and chemistry to avoid contaminating one with radiation from another. All the rooms were fitted with worktables of unfinished raw pine roughed out by a careful carpenter who took the trouble to add a graceful taper to the legs. On the table in the irradiation room rested cylinders of beeswax-colored paraffin like angelfood cakes drilled for the neutron sources, which were gram-strength radium salts mixed with beryllium powder. Handmade Geiger counters, fixed in hinged, hollowed-out bricks of lead shielding on the table in the measurement room, connected through thin coiling wires back to breadboard amplifiers worked by silvered vacuum tubes like inverted bud vases. The amplifiers actuated gleaming brass clockwork counters with numbers showing black through angled miniature windows on their spines. Kraftboard-covered 90-volt Pertrix dry batteries that powered the system packed a shelf below the table. Hahn’s laboratory table held the brackets, beakers, flasks, funnels and filters of radiochemistry. The two men moved in their work from room to room on a regular schedule determined by the duration of the half-lives they were studying. There would have been a pungency of nitrates in the air, mingled with the aroma of Hahn’s inevitable cigar.
In his fifty-ninth year Hahn stooped slightly but looked younger than his age. His hairline had receded and his eyebrows had grown bushy; he had trimmed back to the edge of his upper lip the waxed Prussian mustache of his youth; his brown eyes still sparkled with warmth. By now he was unquestionably the ablest radiochemist in the world. He needed all his forty years’ experience to decode uranium.
He and Strassmann had begun their renewed examination of the three “radium” isotopes early in December by attempting a purer separation from uranium. Strassmann suggested using barium chloride as a carrier rather than the customary barium sulfate because the chloride, Hahn explains, “forms beautiful little crystals” of exceptional purity.960 They wanted to be sure their separations would be free of contamination from other bombardment products with similar half-lives, the difficulty that had muddled Curie and Savitch. The procedure for the 86-minute activity they were studying, which they called “Ra-III,” required them to irradiate about fifteen grams of purified uranium for twelve hours, wait several hours for their more intense 14-minute “Ra-II” to retreat from the foreground by decaying, then add barium chloride as a carrier and accomplish the separation. The Ra-III came out of the uranium solution with the barium, but it refused then to remain behind during fractionation when the barium crystallized away. Instead it crystallized with the barium.
“The attempts to separate our artificial ‘radium isotopes’ from barium in this way were unsuccessful,” Hahn would explain in his Nobel Prize lecture; “no enrichment of the ‘radium’ was obtained. It was natural to ascribe this lack of success to the exceptionally low intensity of our preparations. It was always a question of merely a few thousands of atoms, which could only be detected as individual particles by the Geiger-Müller counter. Such a small number of atoms could be carried away by the great excess of inactive barium without any increase or decrease being perceptible.”961 To check that possibility they retrieved from storage a known radium isotope they often worked with, the isotope they called “mesothorium.” They diluted it to match the pale radioactivity of their few thousand atoms of Ra-III, then ran it through barium precipitation and fractionation. It separated away cleanly from the barium. Their technique was not at fault.
On Saturday, December 17, the day after Hahn stormed the revenue office on behalf of Meitner’s furniture, he and Strassmann carried out a further heroic check. They mixed Ra-III with dilute mesothorium and precipitated and fractionated the two substances together. Then the chemical evidence was certain, whatever it might mean in physical terms: the mesothorium remained in solution when the barium carrier crystallized out but Ra-III went off with the barium, distributing itself uniformly and indivisibly throughout the small pure crystals. Hahn wrote an enthusiastic note in his pocket appointment book to mark the day: “Exciting fractionation of radium/barium/mesothorium.”962
It seemed their “radium” isotopes must be barium, element 56, slightly more than half as heavy as uranium and with just over half its charge. Hahn and Strassmann could hardly believe it. They conceived an even more convincing experiment. If their “radium” was really radium, then by beta decay it ought to transform itself one step up the periodic table to actinium (89). If, on the other hand, it was barium (56), then by beta decay it ought to transform itself one step up to lanthanum (57). And lanthanum could be separated from actinium by fractionation. They were carrying out this definitive project late Monday night, December 19, when Hahn sent Meitner the news.
“Perhaps you can suggest some fantastic explanation,” he wrote.963 “We understand that it really can’t break up into barium. . . . So try to think of some other possibility. Barium isotopes with much higher atomic weights than 137? If you can think of anything that might be publishable, then the three of us would be together in this work after all. We don’t believe this is foolishness or that contaminations are playing tricks on us.”
He closed by wishing his friend a “somewhat bearable” Christmas. Fritz Strassmann added “very warm greetings and best wishes.”964 Hahn posted the letter to Stockholm late at night on his way home.
The two men took time from their readings to attend the annual KWI Christmas party the next day, though Hahn had little joy of it with Meitner gone.965 They continued the actinium-lanthanum experiment eve
n as they worked up the radium-barium findings. After the party the institute would close for Christmas; they kept a typist busy until the end but were unable to finish their report. Hahn had called Paul Rosbaud at Naturwissenschaften, told him the news and asked him to make space in the next issue.966 Rosbaud was willing to pull a less urgent paper from the journal but cautioned that the manuscript must be delivered no later than Friday, December 23. Hahn arranged for a laboratory assistant to serve as typist on Thursday. In the meantime he and Strassmann would carry on alone.
Meitner received Hahn’s Monday-night letter in Stockholm on Wednesday, December 21. It was startling; if the results held she saw it meant the uranium nucleus must fracture and she immediately wrote him back:
Your radium results are very amazing. A process that works with slow neutrons and leads to barium! . . . To me for the time being the hypothesis of such an extensive burst seems very difficult to accept, but we have experienced so many surprises in nuclear physics that one cannot say without hesitation about anything: “It’s impossible.”967
Making of the Atomic Bomb Page 35
