Hans Geiger — German physicist and co‑developer of the Geiger counter

Johannes Wilhelm "Hans" Geiger (30 September 1882 – 24 September 1945) was a German experimental physicist known for fundamental work in early nuclear physics and for developing the particle detector commonly called the Geiger counter. He is widely remembered for his role in the Geiger–Marsden experiment, which provided the experimental basis for the discovery of the atomic nucleus. As a practicing physicist, Geiger combined meticulous measurement with instrument design, producing tools that remained central to radiation detection for decades.

Geiger came from an academic family: his father, Wilhelm Ludwig Geiger, was a scholar of Indic studies. Hans began university studies in 1902, taking courses in physics and mathematics at the University of Erlangen, where he completed a doctorate in 1906. Early in his career he moved to work with Ernest Rutherford, joining Rutherford’s laboratory and experimental program that explored the structure of the atom.

In Manchester, under Rutherford’s direction and together with Ernest Marsden, Geiger carried out scattering experiments on thin metal foils. The results of that program, commonly called the gold foil or Geiger–Marsden experiment, showed that a small, dense, positively charged nucleus sits at the center of the atom — a finding that transformed atomic models and launched a new phase of nuclear research. Working directly with Rutherford, Geiger helped to refine early scintillation counters and the predecessor devices to the modern Geiger–Müller detector.

Geiger’s skill in instrument design also led to quantitative relations and practical devices. In 1911 he and J. M. Nuttall formulated the Geiger–Nuttall law, an empirical relation linking the range or decay rate of alpha particles to their energy. Later, in 1928, Geiger and his student Walther Müller produced an improved gas-filled detector, the Geiger–Müller tube, that combined simplicity with sensitivity and became a standard tool for detecting ionizing radiation. Geiger collaborated with other leading experimentalists of his era, including James Chadwick, who later discovered the neutron.

Geiger's academic appointments reflected his standing in German physics. In 1912 he took leadership of portions of the Physical-Technical Reichsanstalt in Berlin. He later held professorships in Kiel (Kiel) and Tübingen (Tübingen) and returned to a Berlin post from 1936 onward. During the 1930s and World War II he was associated with Germany’s nuclear research community, sometimes referred to as the Uranium Club; historians note that his public statements about the Nazi regime were minimal and accounts differ on his private actions toward colleagues of Jewish origin.