thumb|left|Former Kaiser Wilhelm Institute for Chemistry building in Berlin. Badly damaged by bombing during World War II, it was restored and became part of the Free University of Berlin in 1948. It was renamed the Otto Hahn Building in 1956, and the Hahn-Meitner Building in 2010. Meitner returned to the KWI for Chemistry and her research in October. In January 1917, she was appointed as head of her own physics section. The Hahn-Meitner Laboratory was divided into separate Hahn and Meitner Laboratories, and her pay was increased to 4,000 marks (). Hahn returned to Berlin on leave, and they discussed another loose end from their pre-war work: the search for the mother isotope of actinium (element 89). According to the radioactive displacement law of Fajans and Soddy, this had to be an isotope of the undiscovered element 91 on the periodic table that lay between thorium (element 90) and uranium (element 92). Kasimir Fajans and Oswald Helmuth Göhring discovered the missing element in 1913, and named it brevium after its short half-life. However, the isotope they had found was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope of the same element.
thumb|left|Former Kaiser Wilhelm Institute for Chemistry building in Berlin. Badly damaged by bombing during World War II, it was restored and became part of the Free University of Berlin in 1948. It was renamed the Otto Hahn Building in 1956, and the Hahn-Meitner Building in 2010. Meitner returned to the KWI for Chemistry and her research in October. In January 1917, she was appointed as head of her own physics section. The Hahn-Meitner Laboratory was divided into separate Hahn and Meitner Laboratories, and her pay was increased to 4,000 marks (). Hahn returned to Berlin on leave, and they discussed another loose end from their pre-war work: the search for the mother isotope of actinium (element 89). According to the radioactive displacement law of Fajans and Soddy, this had to be an isotope of the undiscovered element 91 on the periodic table that lay between thorium (element 90) and uranium (element 92). Kasimir Fajans and Oswald Helmuth Göhring discovered the missing element in 1913, and named it brevium after its short half-life. However, the isotope they had found was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope of the same element.