A physicist.[ref]
https://www.tu.berlin/en/about/history/people-portraits/hans-geiger. Independent biographical identity and professional specialization
Portrait of Hans Geiger
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Hans Geiger was a German physicist, best known for his work in nuclear physics and for co-developing the Geiger counter, an instrument used for detecting and measuring ionizing radiation. Born on September 30, 1882, Geiger made significant contributions to the understanding of atomic structure and radiation.