The Story of Nuclear Energy, Volume 3 (of 3): The Discovery of Fission

Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume III, NUCLEAR FISSION: The Discovery of Fission

The Discovery of Fission

But let us get back to the bombardment of uranium with neutrons research that Fermi had begun. After he had reported his work, other physicists repeated it and also got a variety of beta particles and were also unable to decide what was going on.

Lise Meitner and Otto Hahn in their laboratory in the 1930s.

One way to tackle the problem was to add to the system some stable element that was chemically similar to the tiny traces of radioactive isotopes that might be produced through the bombardment of uranium. Afterwards the stable element could probably be separated out of the mixture and the trace of radioactivity would, it was hoped, be carried along with it. The stable element would be a “carrier”.

Among those working on the problem were Otto Hahn and his Austrian co-worker, the physicist Lise Meitner 124(1878-1968). Among the potential carriers they added to the system was the element, barium, which has an atomic number of 56. They found that a considerable quantity of the radioactivity did indeed accompany the barium when they separated that element out of the system.

A natural conclusion was that the isotopes producing the radioactivity belonged to an element that was chemically very similar to barium. Suspicion fell at once on radium (atomic number 88), which was very like barium indeed as far as chemical properties were concerned.

Lise Meitner, who was Jewish, found it difficult to work in Germany, however, for it was then under the rule of the strongly anti-Semitic Nazi regime. In March 1938 Germany occupied Austria, which became part of the German realm. Meitner was no longer protected by her Austrian citizenship and had to flee the country and go to Stockholm, Sweden. Hahn remained in Germany and continued working on the problem with the German physical chemist Fritz Strassman (1902- ).

Although the supposed radium, which possessed the radioactivity, was very like barium in chemical properties, the two were not entirely identical. There were ways of separating them, and Hahn and Strassman busied themselves in trying to accomplish this in order to isolate the radioactive isotopes, concentrate them, and study them in detail. Over and over again, however, they failed to separate the barium and the supposed radium.

Slowly, it began to seem to Hahn that the failure to separate the barium and the radioactivity meant that the isotopes to which the radioactivity belonged had to be so much like barium as to be nothing else but barium. He hesitated to say so, however, because it seemed unbelievable.

If the radioactive isotopes included radium, that was conceivable. Radium had an atomic number of 88, only four less than uranium’s 92. You could imagine that a neutron being absorbed by a uranium nucleus might make the latter 125so unstable as to cause it to emit 2 alpha particles and become radium. Barium, however, had an atomic number of 56, only a little over half that of uranium. How could a uranium nucleus be made to turn into a barium nucleus unless it more or less broke in half? Nothing like that had ever been observed before and Hahn hesitated to suggest it.

While he was nerving himself to do so, however, Lise Meitner, in Stockholm, receiving reports of what was being done in Hahn’s laboratory and thinking about it, decided that unheard-of or not, there was only one explanation. The uranium nucleus was breaking in half.

Actually, when one stopped to think of it (after getting over the initial shock) it wasn’t so unbelievable at that. The nuclear force is so short-range, it barely reaches from end to end of a large nucleus like that of uranium. Left to itself, it holds together most of the time, but with the added energy of an entering neutron, we might imagine shock waves going through it and turning the nucleus into something like a quivering drop of liquid. Sometimes the uranium nucleus recovers, keeps the neutron, and then goes on to beta-particle emission. And sometimes the nucleus stretches to the point where the nuclear force doesn’t quite hold it together. It becomes a dumbbell shape and then the electromagnetic repulsion of the two halves (both positively charged) breaks it apart altogether.

It doesn’t break into equal halves. Nor does it always break at exactly the same place, so that there were a number of different fragments possible (which was why there was so much confusion). Still, one of the more common ways in which it might break would be into barium and krypton. (Their respective atomic numbers, 56 and 36, would add up to 92.)

Meitner and her nephew, Otto Robert Frisch (1904- ), who was in Copenhagen, Denmark, prepared a paper suggesting that this was what was happening. It was published in January 1939. Frisch passed it on to the Danish physicist 126Niels Bohr (1885-1962) with whom he was working. The American biologist William Archibald Arnold (1904- ), who was also working in Copenhagen at the time, suggested that the splitting of the uranium nucleus into halves be called “fission”, the term used for the division-in-two of living cells. The name stuck.

In January 1939, just about the time Meitner and Frisch’s paper was published, Bohr had arrived in the United States to attend a conference of physicists. He carried the news of fission with him. The other physicists attending the conference heard the news and in a high state of excitement at once set about studying the problem. Within a matter of weeks, the fact of uranium fission was confirmed over and over.

One striking fact about uranium fission was the large amount of energy it released. In general, when a very massive nucleus is converted to a less massive one, energy is released because of the change in the mass defect, as Aston had shown in the 1920s. When the uranium nucleus breaks down through the ordinary radioactive processes to become a less massive lead nucleus, energy is given off accordingly. When, however, it breaks in two to become the much less massive nuclei of barium and krypton (or others in that neighborhood) much more energy is given off.

It quickly turned out that uranium fission gave off something like ten times as much nuclear energy per nucleus than did any other nuclear reaction known at the time.

Even so, the quantity of energy released by uranium fission was only a tiny fraction of the energy that went into the preparation of the neutrons used to bring about the fission, if each neutron that struck a uranium atom brought about a single fission of that 1 atom.

Under those conditions, Rutherford’s suspicion that mankind would never be able to tap nuclear energy probably still remained true. (He had been dead for 2 years at the time of the discovery of fission.)

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However, those were not the conditions.

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49821/49821-h/49821-h.htm#c31

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