Fission
- Albert Einstein
By the time Frank Delano Roosevelt said we have nothing to fear except fear itself at his first inaugural address the structure of the atom was well understood and just six months later the brilliant young Hungarian physicist Leo Szilard had conceived the idea of a nuclear chain reaction driven by a catalyst of neutrons.
Outside the English speaking world alarming developments were occurring with a ferocity that accelerated as the decade elapsed. In Germany of 1938, and it’s important to remember NAZI controlled Germany, besides Kristallnacht (the only state sponsored pogrome of destruction of all things Jewish outside Czarist Russia) the Uranium atom, bombarded by neutrons, was first split by Otto Hahn and Fritz Strassmann.
To understand why Hahn and Strassmann were shooting neutrons at Uranium we have to go back a little, in 1932 Chadwick had discovered the neutron. Shortly after Irène Curie and Frédéric Joliot took to bombarding aluminium foil with alpha particles and found they had induced short lived radioactivity in the foil (they had in fact transmuted aluminium into phosphorus, the first time, I'm aware, a human had successfully transmutated anything). Building on Curie and Joliot, Enrico Fermi, in Rome, started bombarding all sorts of random elements with neutrons.
It's an interesting aside to consider, Fermi did his work in Italy where the humble workbench was made of marble, what if he had done his work anywhere else, on a wooden workbench. The wood would have moderated the neutrons and then interesting things would be far more likely to happen.
[Correction,
January 3, 2021, In fact Fermi's team worked on both wood and marble table
tops. They noticed they had better results on wood. Fermi recalled that both
Joliot-Curie as well as Chadwick had observed that paraffin wax slowed neutrons.
Fermi guessed the hydrogen atoms in the paraffin, or wood, were slowing the
neutrons, Fermi did some experiments with water and confirmed his hypothesis.
Fermi realized as well the lower the atomic number of the 'moderating atom' the
more energy the neutron loses in a collision. Fermi also noticed that the lower
the energy of the neutron, the more likely the neutron was to interact with an
atomic nucleus and induce radioactivity. For this work, Fermi received the 1938
Nobel Prize in Physics.]
The Jewish physicist (and only the second female doctorate in physics) Lise Meitner, reached out to her old colleague Otto Hahn and proposed investigating Uranium neutron reactions, Hahn was initially uninterested, but questions around Fermi's results inspired greater attention. Between 1934 and 1938 Hahn, Meitner and Strassmann created many 'transuranic' elements, they had themselves transmutated.
In March of 1938 Meitner lost her Austrian citizenship when Austria was unified with Germany. Her citizenship had previously protected her from what was to that point, the worst of the NAZI's anti-Jewish laws. Ultimately Meitner fled to Sweden in July of 1938, in a story roughly as complex as that of many other Jewish refugees, including the maternal grandparents of the author of this little article.
In November Meitner, Niels Bohr (one of the founders of modern quantum theory), Meitner's nephew Otto Robert Frisch (another great physicist on his own), and Hahn met in Copenhagen. In this meeting Hahn described his most recent findings involving Uranium research, Hahn had found traces of radium, which the other physicists were convinced must be in error, the experiments would have to be redone.
Just before Christmas, which Meitner normally celebrated with her nephew Frisch, Hahn and Strassmann sent a letter to their colleague letting her know they observed Barium as a product of neutron bombardment of Uranium, this was unexpected, Barium has less than half the mass of Uranium.
Writing in 'What Little I Remember' in 1979, Otto Frisch said:
Was it a mistake? No, said Lise Meitner; Hahn was too good a chemist for that. But how could barium be formed from uranium? No larger fragments than protons or helium nuclei (alpha particles) had ever been chipped away from nuclei, and to chip off a large number not nearly enough energy was available. Nor was it possible that the uranium nucleus could have been cleaved right across. A nucleus was not like a brittle solid that can be cleaved or broken; George Gamow had suggested early on, and Bohr had given good arguments that a nucleus was much more like a liquid drop. Perhaps a drop could divide itself into two smaller drops in a more gradual manner, by first becoming elongated, then constricted, and finally being torn rather than broken in two? We knew that there were strong forces that would resist such a process, just as the surface tension of an ordinary liquid drop tends to resist its division into two smaller ones. But nuclei differed from ordinary drops in one important way: they were electrically charged, and that was known to counteract the surface tension.
At that point we both sat down on a tree trunk (all that discussion had taken place while we walked through the wood in the snow, I with my skis on, Lise Meitner making good her claim that she could walk just as fast without), and started to calculate on scraps of paper. The charge of a uranium nucleus, we found, was indeed large enough to overcome the effect of the 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.
But there was another 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 in all; where could that energy come from? Fortunately Lise 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. Now whenever mass disappears energy is created, according to Einstein's formula E = mc2, and one-fifth of a proton mass was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!
In early 1939 Frisch was speaking with the American biologist William A. Arnold about the discovery he had participated in, Frisch asked what biologists called the process by which living cells divided, Arnold told Frisch the process was called fission. Frisch mailed two papers to the English language science magazine Nature describing the process of fission on January 16, 1939. In 1940 Frisch and Rudolf Peierls wrote the Frisch-Peierls memorandum, a warning to the government of England that an atomic explosion was at least theoretically possible.
Hahn won the 1944 Nobel Prize in Chemistry for the discovery of nuclear fission, none of the other brilliant minds involved were ever recognized by the Nobel committee for their contributions.
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