Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction
Struck by a neutron, a uranium nucleus splits like a wobbling drop — and ~200 MeV pours out.
What happens if you split the core of an atom in two? Over Christmas 1938, an aunt and her nephew worked it out on a walk in the snow — and saw where the energy of the atomic age would come from.
The big idea
At the centre of every atom is a tiny, dense core — the nucleus — packed with protons and neutrons. A short-range nuclear “glue” holds them together, but the protons, all positively charged, also push each other apart. In a light atom the glue wins easily. In uranium, the heaviest natural element, there are so many protons that the core is only just holding together.
Meitner and Frisch realised that a single neutron could tip it over the edge. The crowded nucleus wobbles, stretches, pinches in the middle, and splits into two smaller nuclei. And because those two pieces are held together more efficiently than the bloated uranium was, the leftover energy — an astonishing 200 million electron-volts — is flung out. That splitting is what they named fission.
How it came about
In Berlin, the chemists Otto Hahn and Fritz Strassmann had fired neutrons at uranium and found, to their astonishment, the much lighter element barium among the debris. It made no sense: nuclei were not supposed to break roughly in half. Hahn wrote for help to his closest collaborator of thirty years, the physicist Lise Meitner — who was no longer in Berlin. A Jewish scientist, she had been forced to flee Nazi Germany months earlier and was living in exile in Sweden.
Over Christmas 1938, Meitner was visited by her nephew, the physicist Otto Frisch. On a walk in the snow near Kungälv they puzzled it through, sketching the energy sum on a scrap of paper: the nucleus, like an overloaded drop of liquid, had simply divided in two, and the numbers showed it would release a colossal amount of energy. Frisch, back in Copenhagen, named the process “fission” and confirmed it in the lab within days. Their explanation appeared in Nature in February 1939. Six years later the Nobel Prize for the discovery went to Hahn alone; Meitner's name was left off.
Why it mattered
This short letter revealed the source of nuclear energy — millions of times more concentrated than burning coal or any chemical fuel. Within three years it had led to the first controlled chain reaction, and within seven to the atomic bomb. Nuclear power stations, nuclear submarines, and the whole geopolitics of the nuclear age all trace back to the realisation, sketched on a walk in the snow, that a heavy nucleus can split.
A way to picture it
Think of a water balloon filled almost to bursting. Its skin (the nuclear “glue”) just barely holds the wobbling water in, while the water itself strains to push outward. Give the balloon the gentlest tap and it stretches, necks in the middle, and snaps into two smaller balloons that fly apart. The uranium nucleus is that overfilled balloon; the neutron is the tap; and the energy the two halves carry off is the burst of fission.
Where it sits
The pieces were all recently in place: Rutherford had found the nucleus (1911), Chadwick the neutron (1932), and Einstein had given the exchange rate between mass and energy, E = mc² (1905). Meitner and Frisch put them together. The story runs straight on from here to Enrico Fermi's first self-sustaining chain reaction in 1942 and the Manhattan Project — and, in the other direction, to the realisation that the very same binding-energy curve powers the stars by fusion.
… would be expected to move in a collective way which has some resemblance to the movement of a liquid drop. If the movement is made sufficiently violent by adding energy, such a drop may divide itself into two smaller drops.
The two nuclei will repel each other and should gain a total kinetic energy of c. 200 Mev., as calculated from nuclear radius and charge.