Bothe–Geiger coincidence experiment

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.

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.

In 1923, Arthur Compton had shown experimentally that x-rays were scattered elastically by free electrons, in accordance to the conservation of energy.

Bohr, Kramers and Slater published their BKS theory in February 1924 in Zeitschift fur Physik, arguing against energy conservation in individual atomic scattering events.

[3] After finishing his doctoral degree under the supervision of Max Planck in 1913, Walther Bothe joined the radioactivity group in the Physikalisch-Technische Reichsanstalt in Charlottenburg, Berlin, to work with Hans Geiger, at that time head of the lab.

[1] In April 1925,[7][8] Bothe and Geiger reported that the photon and electron counters responded simultaneously, with a time resolution of 1 millisecond.

"[5] Published in September of the same year, an experiment carried in parallel by Compton and Alfred W. Simon using a different technique, reached similar conclusions.

[5] Compton and Simon write: "the results do not appear to be reconcilable with the view of the statistical production of recoil and photo-electrons by Bohr, Kramers and Slater.

In a letter to Bohr, Kramers said "I can unfortunately not survey how convincing the experiments of Bothe and Geiger actually are for the case of the Compton effect".

[5] Bohr however finished by accepting the results, in a letter to Ralph H. Fowler he wrote: "there is nothing else to do than to give our revolutionary efforts as honourable a funeral as possible".

[11] Heisenberg writes to Pauli: "I argue with Bohr over the extent to which the relation p1q1~h has its origin in the wave-or the discontinuity aspect of quantum mechanics.

"[11] Almost a decade later, Robert S. Shankland performed an experiment that allegedly showed some inconsistencies with photon scattering, resurfacing the idea of BKS theory.

Geiger and Walther Müller further developed the Geiger–Müller tubes, that were used by Bothe and Werner Kolhörster experiment in 1929 to show that fast electrons detected in cloud chambers came from cosmic rays.