Walther Bothe and Bruno Rossi: the birth and development of coincidence methods in cosmic-ray physics

TL;DR

This paper reviews the historical development of coincidence methods in cosmic-ray physics, highlighting key experiments, technological innovations, and their impact on particle physics from the 1920s to 1940s.

Contribution

It provides a comprehensive historical analysis of the invention and evolution of coincidence techniques and their crucial role in early cosmic-ray and particle physics research.

Findings

Coincidence methods confirmed radiation quanta and conservation principles.

Development of Rossi's coincidence circuit improved time resolution tenfold.

Coincidence techniques were fundamental in discovering particle properties and muon decay.

Abstract

Theoretical and experimental developments in the 1920s that accompanied the birth of coincidence methods, as well as later crucial applications during the 1930s and 1940s are presented. In 1924 Walther Bothe and Hans Geiger applied a coincidence method to the study of Compton scattering with Geiger needle counters. Their experiment confirmed the existence of radiation quanta and established the validity of conservation principles in elementary processes. At the end of the 1920s, Bothe and Werner Kolh\"orster coupled the coincidence technique with the new Geiger-M\"uller counter to study cosmic rays, marking the start of cosmic-ray research as a branch of physics. The coincidence method was further refined by Bruno Rossi, who developed a vacuum-tube device capable of registering the simultaneous occurrence of electrical pulses from any number of counters with a tenfold improvement in time resolution. The electronic coincidence circuit bearing Rossi's name was instrumental in his research on the corpuscular nature and the properties of cosmic radiation during the early 1930s, a period characterized by a lively debate between Millikan and followers of the corpuscular interpretation. The Rossi coincidence circuit was also at the core of the counter-controlled cloud chamber developed by Patrick Blackett and Giuseppe Occhialini, and became one of the important ingredients of particle and nuclear physics. During the late 1930s and 1940s, coincidences, anti-coincidences and delayed coincidences played a crucial role in a series of experiments on the decay of the muon, which inaugurated the current era of particle physics. PACS numbers: 96.50.S-, 84.30.-r, 96.50.S-, 95.85.Ry, 29.40.-n, 13.35.Bv, 45.20.dh, 12.20.-m, 91.25.-r, 29.40.Cs, 13.20.-v, 14.60.Ef, 14.60.Cd, 78.70.Bj, 20.00.00, 95.00.00, 01.60.+q, 01.85.+f, 01.65.+g