After World War II, due to the ravages of war and the Allied occupation policies, Bothe's Institute for Physics fell on hard times. Maier-Leibnitz, Kurt Starke, and other younger colleagues of Bothe left for employment in North America. Maier-Leibnitz left in the spring of 1947. When his contract expired in the spring of 1948, he returned to again work for Bothe. Maier-Leibnitz continued to work on nuclear spectroscopy and radioactive tracers in biochemistry and medicine. He also took up the study of positron annihilation in solids, which became a new tool for measuring the momentum distribution of bound electrons.