Lise MeitnerSplit Happens

Split happens. She did the arithmetic.

December 1938. Lise Meitner is in Sweden, months after leaving Nazi Germany. She was Jewish, and had to get out. A letter comes from Otto Hahn. He and Fritz Strassmann have fired neutrons into uranium and found barium, an element about half uranium's size. Adding a neutron should not produce half an atom. Hahn asks what it means.

Over the Christmas holidays Meitner and her nephew, Otto Frisch, work it out. Treat the uranium nucleus as a charged liquid drop. Its surface tension only just holds against the repulsion of 92 protons; they estimate that for nuclei near charge 100 it would give out entirely. One extra neutron, and the drop wobbles, pinches, and divides. The two halves are both positive, so they fly apart. From the charges and sizes of the fragments, Meitner and Frisch get about 200 MeV of kinetic energy. Then the check: the fragments together weigh less than the uranium did, and that missing mass, times \(c^{2}\), comes to about the same number. Two separate routes, one answer.

Their letter appeared in Nature in February 1939. It gave the process its name. Frisch borrowed the word from biology after asking the biophysicist William Arnold what you call it when a cell divides: fission.

For scale: 200 MeV from one nucleus is about twenty million times the energy released per molecule when methane burns.

The 1944 Nobel Prize in Chemistry went to Hahn alone, "for his discovery of the fission of heavy nuclei." Many have argued since that Meitner deserved a share. She was invited to work on the Manhattan Project and refused. Element 109, meitnerium, is named for her.

This sticker is the reply to that December letter.