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

📝 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

💡 Analysis

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

📄 Content

Walther Bothe and Bruno Rossi: The birth and development of coincidence methods in cosmic-ray physics Luisa Bonolis∗ Italian Association for the Teaching of Physics (A.I.F.) History of Physics Group [Via Cavalese 13, 00135 Rome, Italy] Abstract 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 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 1 arXiv:1106.1365v2 [physics.hist-ph] 29 Jul 2011 I. INTRODUCTION With the advent of Geiger-M¨uller (G-M) counter) in the late 1920s, cosmic ray research changed dramatically: for the first time, the physical nature of cosmic rays became accessible to experimentation. However, when used single, as cosmic-ray detectors, these devices did not have significant advantages over ionization chambers. They became a most powerful new tool for cosmic-ray experiments when used in coincidence arrangements. The coincidence technique, first used by Hans Geiger and Walther Bothe in 1924 to verify that Compton scattering produces a recoil electron simultaneously with the scattered γ-ray, achieved its full potentialities only in connection with the invention of electronic circuits at the beginning of 1930s. From then on, in conjunction with the invention of new sophisticated detectors, the coincidence method became one of the basic tools in the art of experimental physics. The following historical reconstruction, a scientific saga extending from the 1920s to late 1940s, will examine the scientific literature of the time in order to outline how the ar- rangement of complex arrays of counters, absorbers and electronic recording circuits became standard in cosmic-ray studies, as well as in nuclear and particle physics. II. WAVES AND CORPUSCLES IN THE 1920S At the beginning of the 20th century the classical, continuous-wave picture of radiation was challenged by Planck’s elementary quantum of action unexpectedly generated by his theory of black-body spectrum, and especially by Einstein’s light quantum interpretation of the photoelectric effect of 1905, whose validity was experimentally proved for the first time in 1916 by Robert Millikan.2 The question was taken over again in 1922 by Arthur Compton. Between 1916 and 1922, Compton pursued an experimental and theoretical research that culminated in the discovery that, contrary to what he had expected, the wavelength of X-rays increased due to scattering of the incident radiation by free electrons. The X-rays behaved as particles capable of exchanging their energy and momentum with another particle (the electron) during collisions. Peter Debye emphasized the importance of this discovery in support of light-quanta propagation.3 Most physicists at that time believed that light quanta did not represent physical reality, and was just a heuristic way of defining some quantity of energy related to a property 2 of electromagnetic fields. Bohr expressed his opposition to the concept during his Nobel Lecture of 1922: “In spite of its heuristic value the hypothesis of light quanta, which is quite irreconcilable with the so-called interference phenomena, is not able to throw light on the nature of radiation.”4 Notwithstanding the close agreement between theory and the actual wavelengths of the scattered rays observed by Compton, there was no direct evidence for the existence of the recoil electrons required by the theory of light-quantum scattering. Within a few months of Compton’s first official announcement, the cloud chamber gave strong suppor

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