Carroll Quigley - Tragedy and Hope - A History of the World in Our Time

As early as 1934, in Rome, Enrico Fermi (Nobel Prize, 1938) and Emilio Segre (Nobel Prize, 1959), without realizing what they had done, had split uranium atoms into lighter elements (chiefly barium and krypton) by shooting neutrons into the uranium nucleus. (Such neutrons had been isolated and identified in 1932, by Sir James Chadwick, Nobel Prize winner in 1935.) Although Ida Noddack at once suggested that Fermi had split the atom, the suggestion was generally ignored until Otto Hahn, Lise Meitner, and Fritz Strassmann in Germany, in 1937-1939, repeated Fermi’s experiments and sought to identify the bewildering assortment of lighter radioactive elements which emerged when uranium was bombarded with a stream of neutrons.

By February 1939, it was established that the heaviest element, 92 uranium, could be split in various ways into lighter elements nearer the middle of the atomic table and that large amounts of energy were released in the process. For example, 92 uranium might be split into 56 barium and 36 krypton. The reason for the release of energy was that the nuclear particles (protons and neutrons) had smaller masses in the nucleus of elements near the middle of the atomic table than they had in the nuclei of elements nearer the top or the bottom of the table or than the particles had alone outside any nucleus. This meant that the nuclear particles had the least mass in the elements near 26 iron and that energy would be released if heavier elements could be broken into lighter ones nearer iron or if lighter elements could be built up into heavier elements nearer iron. Now that scientists can do both of these things, at least at the very top (hydrogen) and the very bottom (uranium) of the table, we call the splitting process “fission” and the building-up process “fusion” of nuclei. As explosive forces, they are now represented by the “atomic” bomb and the “hydrogen,” thermonuclear, bomb. The amount of energy released by either process can be calculated by Einstein’s equation, E = mc2, where c is the speed of light (30 billion centimeters, or about 186,000 miles a second). By this equation, if only an ounce of matter is destroyed, 5,600,000 kilowatt hours of energy would be released. In 1939, of course, no one could conceive how lighter elements could be fused into heavier ones, as scientists had just revealed uranium could be fissured.

To the historian of these events, the months of January and February 1939 are of crucial significance. On January 2nd, Fermi, self-exiled from Mussolini’s Italy, reached New York with his wife and children, from Stockholm, where he had just received the Nobel Prize. Four days later the Hahn-Strassmann report on uranium fission was published in Germany, and Otto Frisch, sent by his aunt, Lise Meitner, from Sweden (where they were both refugees from Hitler’s Germany), dashed to Copenhagen to confer with Bohr on the real meaning of Hahn’s report. Bohr left the next day, January 7th, to join Einstein at the Institute for Advanced Study in Princeton, while Frisch and Meitner, in Sweden, repeated Hahn’s fissure of uranium and reported on the results in quantitative terms, in the English journal Nature on February 11 and 18, 1939. These reports, which first used the word “fission,” introduced the “Atomic Age,” and showed that, weight for weight, uranium fission would be twenty million times more explosive than TNT.

Such a burst of energy would, of course, not be noticed in nature if only a few atoms of uranium split; moreover, no large number would split unless the uranium was so pure that its atoms were massed together and unless the stream of splitting neutrons continued to hit their nuclei. Immediately, in February 1939, a number of scientists thought that these two conditions, which do not exist in nature, might be created in the laboratory. It took only a few minutes to realize that this process would become an almost instantaneous chain reaction if extra neutrons, to serve as fission bullets, were issued by the splitting process. Since the uranium nucleus has 146 neutrons, while barium and krypton together have only 82 plus 47, or 129, it is obvious that each split uranium atom must release 17 neutrons capable of splitting other uranium atoms if they hit their nuclei with the right momentum.

This idea was tested at once by Frédéric Joliot-Curie (Nobel Prize, 1935) in Paris, and by Fermi and another refugee, Leo Szilard, with their associates, at Columbia University, New York. The three teams submitted their reports to publication in March 1939. Bohr and others had already suggested that large-scale uranium fission does not occur in nature because natural uranium was widely dispersed atomically by being overwhelmingly diluted in chemical combination and mixture with other substances in its ores; they pointed out also that even pure natural uranium would probably not explode because it was a mixture of three different kinds, or isotopes, of uranium, all with the same atomic number 92 (and thus with the same chemical reactions, since these are based on the electrical charge of the nucleus as a whole) but with quite different atomic weights of 234, 235, and 238. These isotopes could not be separated by chemical means, since their identical atomic numbers (or nuclear electrical charges) meant that they had the same chemical reactions in joining to form different compounds. They could be separated only by physical methods based on their slightly different mass weights.

As uranium is extracted only with great difficulty, and in small amounts , from its ores, 99.28 percent of it is U-238, 0.71 percent of it is U-235, and only a trace is U-234. Thus, natural uranium has 140 times as much U-238 as U-235. It was soon discovered that U-235 was split by slow or very fast neutrons, but, when it split, it emitted very energetic neutrons traveling at high speeds. These fast neutrons would have to be slowed down to split any more U-235, but since U-238 gobbles up all neutrons which come by at intermediate speeds, chain-reaction fission in uranium cannot occur in nature, where each atom of U-235 is surrounded by atoms of U-238 as well as by other neutron-absorbing impurities.

From this it was clear that a chain reaction could be continued in either of two cases: (1) if very pure natural uranium could be mixed with a substance (called a “moderator”) which would slow down neutrons without absorbing them or (2) if a mass of U-235 alone could be obtained so large that the fast neutrons emitted by fission would slow down to splitting speed before they escaped from the mass. The former reaction could probably be controlled, but the latter mass of U-235 would almost certainly explode spontaneously, since there are always a few slow neutrons floating around in space to start the chain reaction. Even in 1939 scientists guessed that ordinary water, heavy water (made of hydrogen with a nucleus of a neutron and a proton instead of only one proton), or carbon would make good moderators for a controlled reaction, They also knew at least four ways in which, by physical methods, U-235 could be separated from U-238.

At the very end of 1939, scientists had worked out what happened when U-238 gobbled up intermediate speed neutrons. It would change from 92 U-238 to 92 U-239, but almost at once the U-239, which is unstable, would shoot out a negative charge (beta ray or electron) from one of the 147 neutrons in its nucleus, turning that neutron into a proton, and leaving the weight at 239 while raising its positive charges (atomic number) to 93. This would be a new element, one number beyond uranium, and therefore named neptunium after the planet Neptune, one planet beyond Uranus as we move outward in the solar system. Theory seemed to show that the new “transuraniac” element 93 Np-239 would not be stable, but would soon (it turned out to be about two days) shoot out another electron from a neutron along with energy in the form of gamma rays. This would give a new transuraniac element number 94 with mass of 239. This second transuraniac element was called plutonium, with symbol 94 Pu-239. At the very end of 1939 theory seemed to indicate that this plutonium, like U-235, would be fissured by slow neutrons, if a sufficiently large lump of it could be made. Moreover, since it would be a different element, with 94 positive charges, it could be separated from the 92 U-238, in which it was created, by chemical methods (usually much easier than the physical methods of separation required for isotopes of the same element).