THE DISCOVERY OF FISSION
Berlin, Germany (1938-1939)
Events: Atomic Discoveries, 1890s-1939

The English word "atom" derives from the Greek word "atomon" ("ατομον"), which means "that which cannot be divided."  In 1938, the scientific community proved the Greek philosophers wrong by dividing the atom.  

Fission, the basis of the atomic bomb, was discovered in Nazi Germany less than a year before the beginning of the Second World War.  It was December 1938 when the radiochemists Otto Hahn (right, with Lise Meitner) and Fritz Strassmann, while bombarding elements with neutrons in their Berlin laboratory, made their unexpected discovery.  They found that while the nuclei of most elements changed somewhat during neutron bombardment, uranium nuclei changed greatly and broke into two roughly equal pieces.  They split and became not the new transuranic elements that some thought Enrico Fermi had discovered but radioactive barium isotopes (barium has the atomic number 56) and other fragments of the uranium itself.  The substances Fermi had created in his experiments, that is, did more than resemble lighter elements -- they were lighter elements.  The products of the Hahn-Strassmann experiment weighed less than that of the original uranium nucleus, and herein lay the primary significance of their findings.  It folIowed from Albert Einstein's E=mc² equation that the loss of mass resulting from the splitting process must have been converted into energy in the form of kinetic energy that could in turn be converted into heat.  

Calculations made by Hahn's former colleague, Lise Meitner (above, with Otto Hahn), a refugee from Nazism then staying in Sweden, and her nephew, Otto Frisch, led to the conclusion that so much energy had been released that a previously undiscovered kind of process was at work.  Frisch, borrowing the term for cell division in biology -- binary fission -- named the process "fission."  Fermi had produced fission in 1934; he had just not recognized it.  

It soon became clear that the process of fission discovered by Hahn and Strassmann had another important characteristic besides the immediate release of enormous amounts of energy.  This was the emission of neutrons.  The energy released when fission occurred in uranium caused several neutrons to "boil off" the two main fragments as they flew apart.  Given the right set of circumstances, perhaps these secondary neutrons might collide with other atoms and release more neutrons, in turn smashing into other atoms and, at the same time, continuously emitting energy.  Beginning with a single uranium nucleus, fission could not only produce substantial amounts of energy but could also lead to a reaction creating ever-increasing amounts of energy.  The possibility of such a "chain reaction" (right) completely altered the prospects for releasing the energy stored in the nucleus.  A controlled self-sustaining reaction could make it possible to generate a large amount of energy for heat and power, while an unchecked reaction could create an explosion of huge force.