Johannes "Hans" Wilhelm Geiger (30 September 1882 – 24 September 1945) was a German experimental physicist. He is known as the inventor of the Geiger counter, a device used to detect ionizing radiation, and for carrying out the Rutherford scattering experiments, which led to the discovery of the atomic nucleus. He also performed the Bothe–Geiger coincidence experiment, which confirmed the conservation of energy in light-particle interactions.
He was the brother of meteorologist and climatologist Rudolf Geiger.
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