Johannes Wilhelm "Hans" Geiger was a German physicist. He is best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discovered the atomic nucleus. He also carried the Bothe–Geiger coincidence experiment that confirmed the conservation of energy in light-particle interactions.

A Geiger counter is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry.