In 1904, Otto Hahn went to University College London and became an associate of Sir William Ramsay, the discoverer of noble gases. Here Hahn worked in the field of radiochemistry, which was still young at that time. While working with salts of the element radium, Hahn discovered the so-called radiothorium (thorium 228) in 1905.
In 1905, Hahn moved to McGill University in Montreal, Canada, to deepen his knowledge under Sir Ernest Rutherford. Here Hahn discovered the radioactive chemical elements thorium C, radium D, and radioactinium, according to the terminology of the time. In 1910 Hahn was appointed professor at the University of Berlin. During World War I, he worked at the Kaiser Wilhelm Institute of Chemistry on military operations of the poison gas unit.
Hahn followed his mentor Fritz Haber as head of the Institute of Physical Chemistry and Electrochemistry. Here he followed the research results of Curie and Fermi and tried to split a uranium nucleus by neutrons with Lise Meitner and later with Friedrich Strassmann.