Nuclear fission

A heavy nucleus like uranium is a tightly packed cluster of protons and neutrons held together against the mutual electric repulsion of its many protons. When such a nucleus absorbs an extra neutron it can wobble like a stretched drop of liquid and pinch in two, splitting into two middle-weight nuclei — for example barium and krypton — while flinging off two or three spare neutrons and about 200 million electron-volts of energy. The energy comes from the mass defect: the fragments together weigh slightly less than the original nucleus plus neutron, and that missing mass is released as energy according to E = mc². The spare neutrons matter enormously, because each can trigger the fission of another nucleus — the possibility of a chain reaction.

The energy per atom is staggering — tens of millions of times what any chemical reaction, including burning or explosives, releases per atom. Because fission also throws off free neutrons, that energy can be made self-sustaining: one split feeds the next. This is the physical basis of both nuclear reactors, which release the energy slowly and controllably, and nuclear weapons, which release it all at once. A chain reaction had been imagined in the abstract by Leó Szilárd in 1933, before anyone knew what reaction could supply the neutrons; fission was the reaction, and the nuclear age dates from it.

The history divides sharply, and the two halves belong to different people. In December 1938 the chemists Otto Hahn and Fritz Strassmann, in Berlin, performed the experiment: they bombarded uranium with neutrons, separated the products chemically, and found unmistakable barium. That was the discovery — but Hahn could not explain it and did not claim that uranium had split. The interpretation came over the Christmas holiday from Lise Meitner, who had fled to Sweden months earlier, and her nephew Otto Frisch. Using Bohr's liquid-drop model of the nucleus, they reasoned that the uranium drop could indeed divide, and Meitner calculated the energy: the fragments were lighter than the parent by about one-fifth the mass of a proton, which by E = mc² works out to roughly 200 MeV — exactly enough to account for the violent recoil. Frisch, needing a name, asked the biologist William Arnold for the term describing a cell dividing in two, and called it "fission." Hahn and Strassmann's chemistry appeared in *Naturwissenschaften* in January 1939; Meitner and Frisch's interpretation in *Nature* on 11 February 1939. Hahn alone received the 1944 Nobel Prize in Chemistry; Meitner, who supplied the physics, was passed over — one of the century's most cited Nobel omissions.

Within four years Enrico Fermi's team achieved the first controlled chain reaction under the stands of a Chicago squash court, in December 1942, and from that line came both nuclear power stations and, through the Manhattan Project, the atomic bomb — the technology that has shadowed geopolitics ever since. Fission also gave medicine and industry a flood of artificial radioisotopes and made possible the study of nuclei far from anything found in nature. It remains the only means by which humanity has released, at industrial scale, the energy bound inside the atom.