The discovery of nuclear fission came as a sensational surprise to scientists everywhere, not least to the discoverers themselves. That much is undisputed, but almost everything else about the history of this discovery has been controversial, at the time and ever since. Most often the discovery is portrayed as a purely chemical achievement: Fission is discovered in Berlin in December 1938 at the Kaiser Wilhelm Institute for Chemistry, when the chemists Otto Hahn and Fritz Strassmann identify barium as a product of the neutron irradiation of uranium. This corresponds to Hahn's own account of the discovery, for which he was awarded the 1944 Nobel Prize for Chemistry. Over the years, however, the chemistry narrative has been criticized for omitting physics and the contributions of Hahn's closest colleague, the physicist Lise Meitner (Fig. 1). In a more comprehensive narrative, the team of Hahn, Meitner, and Strassmann (Fig. 2) conducts a four-year investigation that involves physics and chemistry at every stage, and even after Meitner is forced to emigrate from Germany in July 1938 she and physics play an essential role in the discovery. In this narrative Meitner is a co-discoverer of nuclear fission who is unjustly denied the recognition she is due.
Which of these narratives more truly represents how the science was done and who was responsible for it? In this article I will focus on the science, keeping in mind that this discovery cannot be fully understood without also considering its historical context. For it took place at a time of racial persecution, forced emigration, political pressure, and fear – conditions that affected the scientists involved, the authorship of publications, the recognition for their work, and ultimately the ways in which the discovery was remembered and received.
We begin in 1934, when Enrico Fermi and his group in Rome irradiated uranium with neutrons and detected several new beta-emitting species that were not uranium or the elements just preceding it. Fermi proposed that the reaction was neutron capture, followed by beta decay to elements beyond uranium. At the Kaiser Wilhelm Institute for Chemistry in Berlin Lise Meitner quickly repeated Fermi's experiments – they were exactly in her area of expertise – and then asked her colleague and friend Otto Hahn to join her for their first collaboration in many years (Fig. 1). By the end of 1934 they began working together, joined soon after by one of Hahn's assistants, Fritz Strassmann. They made a formidable team. Hahn was a renowned radiochemist, Meitner was internationally recognized as a pioneer in nuclear physics, and Strassmann was an outstanding young analytical chemist. And they were politically compatible: Meitner was of Jewish origin; Hahn detested the Nazis; and Strassmann refused to join Nazi-affiliated professional organizations and was unemployable outside the institute.1
The Berlin group tested Fermi's beta activities, ruled out all elements down to mercury, and began to disentangle the new species they believed were transuranium elements. It is important to note that Fermi, the Berlin team, and all others in the field were guided by two basic assumptions that turned out to be false. From physics, it had always been observed that only small changes take place in nuclear reactions; this was consistent with theory, such as George Gamow's quantum approach to alpha decay in which only small particles can tunnel through the nuclear barrier. No physicist predicted or imagined that a neutron could cause a large nucleus to split in two. As to the chemists, they had classified uranium and the elements just before it as transition elements, so they assumed that the elements beyond uranium would chemically resemble rhenium, osmium, and so on (see Fig. 3). It was only after the first true transuranium elements were identified that this assumption was shown to be false.
The experiments were extremely difficult. Because their neutron sources were weak and the amounts of beta activities miniscule, the chemists devised a method for cleanly separating the presumed transuranics from the reaction mixture. Over the next two years the Berlin group identified two parallel chains of beta decays extending to element 96 and perhaps further (processes 1 and 2, Fig. 4). (After the fission discovery it was clear that these were fission processes, with neutron-rich fission fragments initiating the long sequences of beta decays. The separation of “transuranics” had precipitated fission fragments that were lighter elements with transition-like chemistry.)
As the team's physicist, Meitner's responsibility was to integrate the data from chemistry and her own physical experiments into a coherent explanation of the nuclear processes. She found that very slow (thermal) neutrons greatly enhanced the yield of the beta chains, and reasoned that the process must be neutron capture (n, ϒ): according to theory, slow neutrons could not eject an alpha particle (n, α) or a proton (n, p). Since fast neutrons produced exactly the same species, Meitner calculated reaction cross-sections and concluded that the starting nuclide must be the most abundant uranium isotope, 238U. In 1935 she identified a third process, a typical resonance capture reaction of 25 eV neutrons by 238U (Fig. 4). (After the fission discovery Niels Bohr used Meitner's data to propose that 235U is fissile with thermal neutrons while 238U, which captures 25 eV neutrons, will only undergo fission with fast neutrons.)
As a rule the Berlin group published together, with Hahn as senior author for chemistry and Meitner for physics. In a 1937 report Hahn was certain that the chemistry of the transuranics left “no doubt about their position in the periodic system. Above all, their chemical distinction from all previously known elements needs no further discussion.”2 But Meitner was troubled by the physics. Neutron capture by 238U initiated three different processes, but theory could not account for the triple isomerism of 239U or the parallel beta chains with their inherited isomerism, nor could theory explain how the capture of just one neutron could initiate a long chain of beta decays. Her 1937 report to Zeitschrift für Physik ended abruptly: the results were “very difficult to reconcile with current concepts of the nucleus.”3 The Berlin team kept going.
The breakthrough came from Paris, where Irène Curie and her co-worker Pavel Savitch devised a method for searching the reaction mixture without separating the transuranics, as was done in Berlin. Late in 1937 they reported a strong new beta activity that was neither uranium nor a transuranic, but they were unable to identify it. By the time Hahn and Strassmann looked into it, it was October 1938 and Meitner was gone from the institute. That summer, knowing she was about to be dismissed and fearing she would be forbidden to leave Germany, she fled and took a position in Stockholm.4 From there she and Hahn corresponded constantly: she was still very much a member of their team.5
Strassmann found that the Curie activity followed a barium carrier and decided it was radium; within a week he and Hahn identified several radium isomers and their actinium decay products, all beta emitters. Although the chemists were working feverishly to get ahead of Curie, Hahn kept Meitner informed – mail between Berlin and Stockholm was delivered in a day or two – and she peppered him with questions, particularly about the reaction conditions. Hahn confirmed what Curie and Savitch had already reported: the radium isomers, like the transuranics, were formed by fast neutrons but greatly enhanced by thermal neutrons, so they concluded that the effective nucleus, as before, was 238U. But the reaction process was different. To form radium, the most likely reaction was 238U(n,α)235Th, followed immediately by alpha decay to 231Ra. This was something new. Not only had an (n,α) reaction with slow neutrons never before been observed but according to theory it could not happen, because only highly energetic neutrons could expel alpha particles from uranium.
Hahn and Strassmann, now on the familiar ground of radium and actinium chemistry, spent weeks elucidating the multiple radium isomers and their decay products. But following the radium decays was not enough. At this point Meitner and other physicists provided a crucial check: they did not believe that radium could be formed by the impact of slow neutrons on uranium.
In November 1938 Meitner and Hahn met at Niels Bohr's institute in Copenhagen. Outside the city their meeting was a secret, to avoid political difficulties for Hahn, and he never mentioned it in his remembrances later on. But Hahn's own pocket diary shows that they met on November 13, talked for hours, and then joined other physicists for more discussions. Later Hahn remembered only that Bohr and other institute physicists were skeptical of the radium isomers, but the strongest objections must have come from Meitner, because that was the message Hahn brought back to Berlin.6 According to Strassmann, she “urgently requested” that they scrutinize their radium more rigorously. “Fortunately L. Meitner's opinion and judgment carried so much weight with us in Berlin that the necessary control experiments were immediately undertaken.”7 These were the experiments that led directly to the fission discovery a few weeks later.
To verify the radium, Hahn and Strassmann tried to partially separate it from its barium carrier, using fractional crystallization, a method first developed by Marie Curie. Finding that their radium did not separate at all, the chemists tested the fractionation procedure using known radium isotopes, which separated as expected. As a final test, they mixed their “radium” (by then they were using quotation marks) with a known radium isotope and fractionated the mixture. The known radium isotope separated; their “radium” did not. The radiochemistry was so beautifully done there could be no doubt: their “radium” was barium.
In retrospect the barium finding is usually considered to be the discovery of nuclear fission, but at the time it was not so obvious. Although Hahn was sure of the chemistry, the physics worried him. As he wrote to Meitner on December 19, it was a “frightening conclusion: Our Ra isotopes do not behave like Ra but like Ba … Perhaps you can come up with some sort of fantastic explanation. We know ourselves that it can't actually burst apart into Ba … If there is anything you could propose that you could publish, then it would still in a way be work by the three of us.”8 His letter shows the mutual dependence of physics and chemistry that characterized the entire investigation, and we see it in the report he was preparing for Naturwissenschaften. After pages of data on the many “radium” isomers, Hahn mentions the barium experiments only at the end and then “hesitantly, due to their peculiar results … As ‘nuclear chemists’ fairly close to physics we cannot yet bring ourselves to take this leap which contradicts all previous experience in nuclear physics.”9
Meitner received Hahn's letter on December 21 and replied by return mail. “A reaction with slow neutrons that supposedly leads to barium! … At the moment the assumption of such a thoroughgoing breakup seems very difficult to me, but in nuclear physics we have experienced so many surprises that one cannot unconditionally say: it is impossible.”10 This was Meitner's discovery moment, when she somehow set aside existing nuclear theory and began to consider a massive nuclear breakup. It was just what Hahn needed. On December 27, several days after receiving her letter, he added a paragraph to the Naturwissenschaften page proofs, suggesting that the “transuranics” (Hahn's quotation marks) might be lighter transition elements and that uranium could split in two.11 The paragraph strengthened the paper, showing that the chemists had not only found barium but understood its physical significance. The authors were Hahn and Strassmann only – it would have been politically impossible for them to include a Jew in exile. Yet the contributions of Meitner and physics are evident, and I would argue that the actual discovery of nuclear fission took place during these days in late December 1938 when the physics and the chemistry finally made sense together.
Just after Christmas Meitner and her nephew Otto Robert Frisch (Fig. 5), also a physicist and a refugee, devised the first theoretical interpretation of the fission process. Familiar with theory that treated the nucleus as a liquid drop, they began thinking of the uranium nucleus as a wobbly drop that was ready to split in two. They estimated that the surface tension of the uranium nucleus is vanishingly small, calculated the kinetic energy of the fission fragments as 200 MeV, and recognized that the source of that energy is the difference in mass between uranium and its fission fragments. Preparing a note for Nature, they suggested that the “transuranics” were fission fragments, pointed to the 23-minute 239U (see Fig. 4) as the precursor to the first true element 93, and proposed that the process be named “nuclear fission.”12
When the barium report appeared in Naturwissenschaften on 6 January 1939 it was immediately recognized as an exciting and fundamental new discovery.13 Meitner and Frisch's theoretical interpretation, which was published in Nature on February 11, was also regarded as a major discovery and was the starting point for further theoretical work by Bohr and others.14 Nevertheless Meitner's exclusion from the barium report was damaging, not only as an injustice to her and a violation of normal standards of scientific attribution, but also because it separated her and physics from the discovery itself. The separate publications created an artificial divide – between chemistry and physics, experiment and theory – and few understood that the division did not reflect how the science was done but was instead an artifact of Meitner's forced emigration and the political conditions of the time. Within weeks, Hahn exploited that division. Politically insecure, hoping that fission would protect him and his institute, he began to claim that the discovery belonged only to chemistry and that physics did not contribute to it.15 Hahn never wavered from this view; it influenced Nobel decisions16 and the received history of the fission discovery. Still, the historical record exists, and with it we can understand the remarkable interdependence of physics and chemistry that made this discovery possible.