Bothe–Geiger coincidence experiment

In the history of quantum mechanics, the Bothe-Geiger coincidence experiment was conducted by Walther Bothe and Hans Geiger from 1924 to 1925. The experiment explored x-ray scattering from electrons to determine the nature of the conservation of energy at microscopic scales, which was contested at that time. The experiment confirmed existence of photons, the conservation of energy and the Compton scattering theory.

At that time, quantum mechanics was still under development in what was known as the old quantum theory. Under this framework, the BKS theory by Niels Bohr, Hendrik Kramers, and John C. Slater proposed the possibility that energy conservation is only true for large statistical ensembles and could be violated for small quantum systems. BKS theory also argued against the quantum nature of light. The Bothe-Geiger experiments helped disprove BKS theory, marking an end to old quantum theory, and inspiring the re-intepretation of the theory in terms of matrix mechanics by Werner Heisenberg.

The experiment used for the first time a coincidence method, thanks to the coincidence circuit developed by Bothe. Bothe received the Nobel Prize in Physics in 1954 for this development and successive experiments using this method.