Otto Hahn
Discovery of nuclear fission

After James Chadwick discovered the neutron in 1932, Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles, they found that this results in a short-lived radioactive isotope of phosphorus. They noted that positron emission continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table. Chadwick noted that being electrically neutral, neutrons could penetrate the atomic nucleus more easily than protons or alpha particles. Enrico Fermi and his colleagues in Rome picked up on this idea, and began irradiating elements with neutrons.

The radioactive displacement law of Fajans and Soddy said that beta decay causes isotopes to move one element up on the periodic table, and alpha decay causes them to move two down. When Fermi's group bombarded uranium atoms with neutrons, they found a complex mix of half lives. Fermi therefore concluded that the new elements with atomic numbers greater than 92 (known as transuranium elements) had been created. Meitner and Hahn had not collaborated for many years, but Meitner was eager to investigate Fermi's results. Hahn, initially, was not, but he changed his mind when Aristid von Grosse suggested that what Fermi had found was an isotope of protactinium. "The only question", Hahn later wrote, "seemed to be whether Fermi had found isotopes of transuranian elements, or isotopes of the next-lower element, protactinium. At that time Lise Meitner and I decided to repeat Fermi's experiments in order to find out whether the 13-minute isotope was a protactinium isotope or not. It was a logical decision, having been the discoverers of protactinium."

Between 1934 and 1938, Hahn, Meitner and Strassmann found a great number of radioactive transmutation products, all of which they regarded as transuranic. At that time, the existence of actinides was not yet established, and uranium was wrongly believed to be a group 6 element similar to tungsten. It followed that first transuranic elements would be similar to group 7 to 10 elements, i.e. rhenium and platinoids. They established the presence of multiple isotopes of at least four such elements, and (mistakenly) identified them as elements with atomic numbers 93 through 96. They were the first scientists to measure the 23-minute half life of uranium-239 and to establish chemically that it was an isotope of uranium, but were unable to continue this work to its logical conclusion and identify the real element 93. They identified ten different half lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. Hahn and Strassmann refined their chemical procedures, while Meitner devised new experiments to shine more light on the reaction processes.

In May 1937, they issued parallel reports, one in Zeitschrift für Physik with Meitner as the principal author, and one in Chemische Berichte with Hahn as the principal author. Hahn concluded his by stating emphatically: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion ("Above all, their chemical distinction from all previously known elements needs no further discussion"); Meitner was increasingly uncertain. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the neutron cross section, it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be another case of the nuclear isomerism that Hahn had discovered in protactinium. She therefore ended her report on a very different note to Hahn, reporting that: "The process must be neutron capture by uranium-238, which leads to three isomeric nuclei of uranium-239. This result is very difficult to reconcile with current concepts of the nucleus."

With the Anschluss, Germany's unification with Austria on 12 March 1938, Meitner lost her Austrian citizenship, and fled to Sweden. She carried only a little money, but before she left, Hahn gave her a diamond ring he had inherited from his mother. Meitner continued to correspond with Hahn by mail. In late 1938 Hahn and Strassmann found evidence of isotopes of an alkaline earth metal in their sample. Finding a group 2 alkaline earth metal was problematic, because it did not logically fit with the other elements found thus far. Hahn initially suspected it to be radium, produced by splitting off two alpha-particles from the uranium nucleus, but chipping off two alpha particles via this process was unlikely. The idea of turning uranium into barium (by removing around 100 nucleons) was seen as preposterous.

During a visit to Copenhagen on 10 November, Hahn discussed these results with Niels Bohr, Lise Meitner, and Otto Robert Frisch. Further refinements of the technique, leading to the decisive experiment on 16–17 December 1938, produced puzzling results: the three isotopes consistently behaved not as radium, but as barium. Hahn, who did not inform the physicists in his Institute, described the results exclusively in a letter to Meitner on 19 December:

We are more and more coming to the awful conclusion that our Ra isotopes behave not like Ra, but like Ba... Perhaps you can come up with some fantastic explanation. We ourselves realize that it can't actually burst apart into Ba. Now we want to test whether the Ac-isotopes derived from the "Ra" behave not like Ac but like La.

In her reply, Meitner concurred. "At the moment, the interpretation 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'." On 22 December 1938, Hahn sent a manuscript to Naturwissenschaften reporting their radiochemical results, which were published on 6 January 1939. On 27 December, Hahn telephoned the editor of Naturwissenschaften and requested an addition to the article, speculating that some platinum group elements previously observed in irradiated uranium, which were originally interpreted as transuranium elements, could in fact be technetium (then called "masurium"), mistakenly believing that the atomic masses had to add up rather than the atomic numbers. By January 1939, he was sufficiently convinced of the formation of light elements that he published a new revision of the article, retracting former claims of observing transuranic elements and neighbours of uranium.

As a chemist, Hahn was reluctant to propose a revolutionary discovery in physics, but Meitner and Frisch worked out a theoretical interpretation of nuclear fission, a term appropriated by Frisch from biology. In January and February they published two articles discussing and experimentally confirming their theory. In their second publication on nuclear fission, Hahn and Strassmann used the term Uranspaltung (uranium fission) for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction. This was proved to be the case by Frédéric Joliot and his team in March 1939. Edwin McMillan and Philip Abelson used the cyclotron at the Berkeley Radiation Laboratory to bombard uranium with neutrons, were able to identify an isotope with a 23-minute half life that was the daughter of uranium-239, and therefore the real element 93, which they named neptunium. "There goes a Nobel Prize", Hahn remarked.

At the Kaiser Wilhelm Institute for Chemistry, Kurt Starke independently produced element 93, using only the weak neutron sources available there. Hahn and Strassmann then began researching its chemical properties. They knew that it should decay into the real element 94, which according to the latest version of the liquid drop model of the nucleus propounded by Bohr and John Archibald Wheeler, would be even more fissile than uranium-235, but were unable to detect its radioactive decay. They concluded that it must have an extremely long half life, perhaps millions of years. Part of the problem was that they still believed that element 94 was a platinoid, which confounded their attempts at chemical separation.
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