The Making of the Atomic Bomb

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 Pickel-haube 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 radio-chemistry. 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. 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-H” 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-Muller 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.” 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 indi-visibly 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.”

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. “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.” 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. They continued the actinium-lanthanum experiment even 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. 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.”

She was traveling on Friday to the village of Kungalv 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.”

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.”

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