Otto Hahn was born in Frankfurt, Germany, the son of a prosperous gla­zier and property owner. He led a sheltered childhood, and at 15 he became interested in chemistry, performing experiments in the laundry room of the family home. Although his father wanted him to study architecture, Otto convinced him that industrial chemistry would be a better occupa­tion. He began his studies in chemistry and mineralogy at the University of Marburg in 1879 and received his doctorate in chemistry in 1901. After completing a year of required military service, he returned to Marburg to work as a research assistant. In 1904, he took a job at University College London in England, working in the new field of radiochemistry. He moved

back to Germany in 1906 and by 1910 was head of the radioactivity depart­ment of the new Kaiser Wilhelm Institute of Chemistry in Berlin.

In 1907, Hahn met Dr. Lise Meitner at a University of Berlin physics colloquium. She was from Vienna, Austria, and had already published papers on alpha and beta radiation. They both needed collaborators, and as a physicist and a radiochemist they would make a strong team. They quickly became friends and worked closely together for 30 years.

When Chadwick announced the discovery of the neutron in 1932, radiochemistry assumed a heightened importance. It was suspected that neutron capture by a nucleus would cause transmutation to another ele­ment, one atom at a time, but neutron sources were weak and the effect could be small. A good polonium-210 source was desirable but unavailable to most labs. Researchers used radium salt, an alpha particle emitter and a workable substitute for polonium, mixed with beryllium powder to make neutrons, using the effect discovered by Chadwick. There was no way to detect a transmutation except by exceedingly careful chemistry that would detect tiny amounts of a new element mixed in with the original sample. The amount of transmutation to be detected could be as small as hundreds of atoms. Otto Hahn was the world’s leading radiochemist, and he used fractional crystallization as a method of finding minute contami­nants in a sample. It is a method that was pioneered by Marie Curie, and it uses the fact that different substances dissolved in water crystallize out of supersaturated solution at different temperatures.

In 1938, Hahn and Meitner were getting some interesting chemical results of bombarding uranium with neutrons, but in July the political conditions in Germany for Meitner, who was Jewish, were becoming dan­gerous, and she had to drop everything and escape to Holland. The unsafe atmosphere for Jewish people was quickly becoming critical, and Ger­many seemed to be preparing for war. Hahn’s work in radiochemistry was superb, but he needed Meitner’s knowledge of nuclear physics to help inter­pret experimental results. With Meitner in exile, the two scientists kept in touch by mail and with one secret meeting in Copenhagen, Denmark.

Hahn repeatedly performed chemical analysis of the neutron-exposed uranium with very puzzling results. It was obvious that the neutron expo­sure resulted in new, radioactive elements in the uranium sample, as the radiation-counting instrument showed increased radioactivity, but the product of this transmutation was not clear. Hahn guessed it was a new, previously undiscovered isotope of radium. To detect small contaminants

of radium, Hahn used barium as a carrier for the fractional crystallization. Meitner did not believe it was radium, as it would require a double alpha disintegration to go from uranium down to radium. To Hahns amaze­ment, all his tests for radium were negative, with nothing showing up in the barium carrier. In a burst of insight, Meitner figured out the problem, and she and Hahn exchanged excited letters on December 21,1938. Hahn had not detected any radium in his barium because there was no radium. The product of the neutrons hitting the uranium was radioactive barium.

A uranium atom is slightly less than twice the mass of a barium atom. The uranium nuclei had split roughly in half, resulting in one barium atom and one krypton atom per disintegration. Krypton is an inert gas, undetectable by chemical means. The barium atoms in Hahns experi­ment were neutron-heavy and therefore unstable and subject to radio­active decay, as was detected by his radiation counter. The next day, on