Diana Preston - Before the Fallout

Over the next few weeks they worked relentlessly, trying to discover more about the material they had created. Yet whatever they did, as Otto Hahn recalled, it behaved like barium—an element in the middle of the periodic table and much lighter than uranium. They could not understand how bombarding uranium could possibly have produced such a result. A few days before Christmas Hahn mailed his findings off to Meitner, hoping that “perhaps you can put forward some fantastic explanation” for results that were “physically absurd,” adding, “You will do a good deed if you can find a way out of this.” He knew he needed her expertise; as he later wrote, “We poor chemists… we are so afraid of these physics people.” His letter reached her on 21 December, and she immediately replied that his findings were “very odd. A process in which slows neutrons are used and the product seems to be barium…. but we’ve had so many surprises in nuclear physics that one can’t very well just say it’s impossible.”

Two days later she left Stockholm to spend Christmas with a friend in the small, windswept seaside resort of Kungelv near Gothenburg. Otto Frisch arrived from Copenhagen to join the “short, dark, and bossy” aunt he was so fond of. With his father still in Dachau and his mother trapped in increasingly desperate conditions in Vienna, he was deeply anxious and, like Lise Meitner, glad to immerse himself in Hahn’s scientific dilemma as a distraction. Frisch’s initial reaction was that Hahn must have made a mistake, but Meitner insisted that she “knew the extraordinary chemical knowledge and ability of Hahn and Strassmann too well to doubt for one second the correctness of their unexpected results.”

They “sort of kept rolling this thing around,” as Frisch later recalled, and went out into the snow to think, Frisch on skis and his aunt walking rapidly on foot. The problem confronting them was that until then no one had thought it possible to chip more than tiny pieces off nuclei. Yet the barium nucleus was roughly half the size of a uranium nucleus—suggesting that Hahn had split the uranium in two. Searching her mind for an explanation, Meitner recalled an idea of Niels Bohr’s: that the nucleus was like a drop of liquid with nuclear forces playing the part of surface tension and keeping the nuclear “drop” spherical. She described how, “in the course of our discussions we evolved the following picture: if, in the highly-charged uranium nucleus—in which the surface tension is greatly reduced owing to the mutual repulsion of the protons—the collective motion of the nucleus is rendered violent enough by the captured neutron, the nucleus may become drawn out length-wise, forming a sort of ‘waist,’ and finally splitting into two more or less equal-sized, lighter nuclei which, because of their mutual repulsion, then fly apart with great force.”

They sat down in the cold on a tree trunk and started to calculate on scraps of paper. “The charge of a uranium nucleus,” they found, “was indeed large enough to overcome the effect of surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly, unstable drop, ready to divide itself at the slightest provocation, (such as the impact of a single neutron) “Yet, as Frisch later recalled, there was a problem: “After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and hence a very large energy, about 200 MeV [mega-election-volts] in all; where could that energy come from? Fortunately Meitner remembered the empirical formula for computing the masses of nuclei and worked out that the two nuclei formed by the division of a uranium nucleus together would be lighter than the original uranium nucleus, by about one-fifth the mass of a proton. [This was because] whenever mass disappears, energy is created, according to Einstein’s formula E = mc 2, and one-fifth of a proton mass was just equivalent to 200 MeV. So here was the source for that energy; it all fitted! “Yet the moment of realization was not entirely comfortable. Frisch felt as if he had “caught an elephant by the tail” without meaning to and now did not know what to do with it.

On 28 December Hahn wrote again. Prompted by Meitner’s note of 21 December, he too was wondering whether the uranium might have split. He did not understand the full picture but recognized that, if true, it meant that the transuranics they had studied for four years did not exist. They were, instead, smaller, lighter nuclei—like barium—which formed when uranium was split. He asked her to look at a note that he and Fritz Strassmann proposed to publish in Naturwissenschaften. On New Year’s Eve Meitner replied cautiously that perhaps it is energetically possible for such a heavy nucleus to break up.” The next day she returned to Stockholm, where she immediately began reviewing the evidence for the transuranics and realized that they could, indeed, be light nuclei. It was a bittersweet discovery. She had wasted years of study on transuranics, but on the other hand she had provided the first theoretical interpretation of fission of uranium, showing how it produced radioactive elements and liberated large amounts of energy. On 3 January 1939 she wrote to Hahn, “I am now almost certain that you really do have a splitting to barium and I find that to be a really beautiful result.” They were, she said, “wonderful findings.”

The most “wonderful” aspect of all, as Edward Teller wrote, was that “the secret of fission had eluded everybody for all those years.” As Hahn put it, “None of us realized that we had done it.” Fermi had failed to see it. So had Irene Joliot-Curie; her lanthanumlike substance had, in fact, been lanthanum, created by nuclear fission. Even the great Rutherford had been deceived. When an excited Otto Frisch returned to Copenhagen and broke the news to Bohr, the Dane “smote his forehead with his hand,” exclaiming, “We were all fools.”

Bohr urged Frisch to write a paper with Lise Meitner as soon as possible and promised to say nothing until it was published. By dint of long-distance telephone calls, aunt and nephew drafted a short note to the editor of the British journal Nature, describing the splitting of a nucleus and the theory underlying it. They also found a name for their new phenomenon. Frisch asked an American biologist, William A. Arnold, working in Bohr’s institute, what he called the process by which single cells divide into two. He replied, “Fission.”

However, before submitting the paper, Frisch wanted to be absolutely certain their conclusions were right. He conducted some experiments and became the first to provide experimental proof of the fission of a uranium atom when hit by a neutron. Having finally achieved the results he wanted, he went to bed at 3 a.m. on 13 January but four hours later “was knocked out of bed by the postman who brought a telegram to say that my father had been released from concentration camp.” His parents had been granted visas to emigrate to Sweden. His happiness was complete.

Meanwhile, on 6 January 1939, Hahn and Strassmann’s report appeared in Naturwissenschaften. It made no mention of Meitner’s and Frisch’s contribution, and could hardly have done so in the political climate in Germany. Nor was there any acknowledgment of Ida Noddack’s earlier work. Piqued, she wrote a short article in Naturwissenschaften, pointing out that five years earlier she had suggested the splitting of the uranium atom. Paul Rosbaud, as editor, asked Hahn to comment, but he refused. Rosbaud therefore added a terse note beneath her article, stating that “Otto Hahn and Fritz Strassmann have informed us that they have neither the time nor the desire to answer the preceding note.”