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 Kolhörster coupled the coincidence technique with the new Geiger–Müller 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.
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7.
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8.
For an in-depth discussion on Bothe’s contributions on these topics, see
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H.
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15
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Bothe recalled that “Film consumption however was so enormous that our laboratory with the film strips strung up for drying sometimes resembled an industrial laundry.”10
13.
Well aware that some coincidences could result from uncorrelated signals (chance coincidences), Bothe and Geiger also devised a simple method to evaluate their number. The evaluation was based on the number of events in which the two signals were at a time distance larger than 1 ms, for example, between 5 and 6 ms.
14.
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A. H.
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,” ibid
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The name “photons” for light quanta was coined by Gilbert Lewis only in the 18 December 1926 issue of Nature:
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The Nobel Prize in Physics 1927 was divided equally between
A. H.
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Wilson
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The Nobel Prize in Physics, 10 December
1927
,K. M. G.
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V.
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,” Phys. Z.
13
, 1084
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(1912
), p. 1090. On 7 August 1912, Hess’ free balloon reached an altitude of 5300 m with Hess and three Wulf electrometers on board to measure the rate of ion production inside a hermetically sealed container. The Wulf bifilar electrometer had been specifically constructed by the Jesuit priest, father Theodor Wulf of Aachen to answer the question of how the intensity of the recognized strongly penetrating radiation changes as a function of altitude. This early version of the bifilar electrometer was later improved by Wulf and by Hess, and especially by Werner Kolhörster. It was easily transportable and could be sealed to be air-tight and withstand high and low pressures. It was thus very suitable for measurements aboard balloons and aircraft.22.
D.
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For a discussion on Pacini’s forgotten contributions see A. De Angelis
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A most important advance in the ionization chamber technique as applied to cosmic-ray research occurred in the 1920s with the development of self-recording instruments, by Millikan and by Erich Regener. To carry out high-altitude observations, scientists, such as Hess and Kolhörster, previously had to personally “accompany” their ionization chamber in balloon flight. Now the measurements could be carried out with unmanned balloons, which could reach much greater altitudes, thus eliminating risks and high costs. For the first time it also became possible to perform measurements down to great depths under water.
26.
R. A.
Millikan
, “High frequency rays of cosmic origin
,” Proc. Natl. Acad. Sci. U.S.A.
12
, 48
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(1926
), p. 52. His lecture had such an immediate and incredible official echo that an article entitled “Dr. Millikan discovers strange new rays, 10 miles above the Earth, coming from the void” appeared in the 11 November 1925 issue of The New York Times, p. 25: “Scientists attending the National Academy of Sciences here are seeking a name for the powerful new rays discovered by Dr. R. A. Millikan […]. It is probable that the rays […] will become known as Millikan rays, in honor of the man who first found authentic evidence of their existence.” Available at <www.nytimes.com/ref/membercenter/nytarchive.html>. In the U.S. Millikan was considered the discoverer of the new rays. As a result European scientists in the field felt excluded from the priority of their pioneering discoveries (see Ref. 24).27.
For an accurate reconstruction of Millikan’s strategy for measuring cosmic ray energy, and his theories on the origin of cosmic rays see P. Galison, “
The discovery of the muon and the failed revolution against quantum electrodynamics,”
Centaurus 26
, 262–316 (1983). See also A. Russo, Le reti dei fisici. Forme dell’esperimento e modalità della scoperta nella fisica del Novecento (La Goliardica Pavese, Pavia, 2000
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R. A.
Millikan
and G. H.
Cameron
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Proc. Natl. Acad. Sci. U.S.A.
14
(6
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R. A.
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Phys. Rev.
32
, 533
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J. H.
Jeans
, Problems of Cosmogony and Stellar Dynamics (U.P., Cambridge, 1919
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R. A.
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Proc. Natl. Acad. Sci. U.S.A.
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[PubMed]
R. A.
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R. A.
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. After introducing the concept of entropy, Clausius argued that there would come a time when the entire universe would be a state of maximum entropy. See33.
Reference 27, p.
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For the origin of the Geiger-Müller counter, see
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39.
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D. V.
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52
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Bothe
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56
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Bothe and Kolhörster remarked that the double Compton collision involving the same photon, is a rare phenomenon, so that the rate of double coincidences could not be ascribed to this occurrence (see Ref. 46, pp.
764
–765
).48.
This conclusion is not surprising, considering that in the 1920s electrons and ionized hydrogen were the only known elementary particles to serve as building blocks for atoms.
49.
50.
51.
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56.
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58.
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For a review on the early impact of the introduction of the tube counter in Italy, see
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Leone
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,” Z. Phys.
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, “Eine neue Methode zur Messung der Elementarstrahlen
,” Z. Phys.
36
, 364
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(1926
).H.
Greinacher
, Greinacher showed it was possible to amplify the current due to an α-particle sufficiently to register it as a click in a headphone: “Über die Registrierung von α- und H-Strahlen nach der neuen elektrischen Zählmethode
,” Z. Phys.
. 44
, 319
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(1927
);H.
Greinacher
, “Eine neue elektrische Zählmethode für α-und H-Strahlen
,” Die Naturwiss.
16
, 1102
–1102
(1928
).Greinacher’s method of “purely electronic amplification” spread all over Europe: in Wien and in England, where particularly appreciated by Rutherford and his collaborators in France and was adopted in Maurice de Broglie’s laboratory, thanks to the young Louis Leprince-Ringuet.
63.
B.
Rossi
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Nature
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H.
Becker
and W.
Bothe
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Z. Phys.
78
, 421
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Baeyer
, “Anwendung der Koinzindenzmethode auf die Untersuchung von Kernprozessen
,” ibid
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, 417
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and H.
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,” ibid
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, “Zur Struktur der kosmischen Ultrastrahlung
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The Nobel Prize in Physics 1954 was divided equally between Max Born and Walther Bothe <nobelprize.org/nobel_prizes/physics/laureates/1954/>.
67.
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Rossi
, “Magnetic experiments on the cosmic rays,”
Nature
128
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), 300
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(1931
);the “Puccianti magnetic lenses” later became part of the heritage left by Rossi to the Italian physicists working in cosmic rays, and after about 15 years would be applied in the study of positive and negative mesons, playing a central role in the historical experiment performed in Rome by Marcello Conversi, Ettore Pancini, and Oreste Piccioni during World War II:
M.
Conversi
, E.
Pancini
, and O.
Piccioni
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3
, 372
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(1946
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, “On the disintegration of negative mesons
,” Phys. Rev.
71
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(1947
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J.
Clay
, “Penetrating radiation
,” Proc. R. Acad. Sci. Amsterdam
30
, 1115
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(1927
), http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf;J.
Clay
, “Penetrating radiation, II
,” Proc. R. Acad. Sci. Amsterdam
. 31
, 1091
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(1928
), http://www.dwc.knaw.nl/DL/publications/PU00015672.pdf;J.
Clay
, “Ultra radiation (penetrating radiation), III. Annual variation and variation with the geographical latitude
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and W.
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, “Vergleichende Höhenstrahlungsmessungen auf nördlichen Meeren
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26
, 450
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(1930
).70.
R.
Millikan
and G.
Cameron
, “High altitude tests on the geographical, directional, and spectral distribution of cosmic rays
,” Phys. Rev.
31
, 163
–173
(1928
);R.
Millikan
and G.
Cameron
, “New results on cosmic rays
,” Nature
121
(3036
), 19
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(1928
).71.
B.
Rossi
, “On the magnetic deflection of cosmic rays
,” Phys. Rev.
36
, 606
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(1930
).72.
E.
Fermi
and B.
Rossi
, “Azione del campo magnetico terrestre sulla radiazione penetrante
,” Atti Acc. Naz. Lincei Rend.
17
, 346
–350
(1933
);see also Rossi’s introduction to the article reprinted in Enrico Fermi
, Collected Papers (Note e Memorie)
, Italy 1921–1938, Vol. 1
, edited by E.
Amaldi
et al. (The University of Chicago and Accademia Nazionale dei Lincei
, Chicago and Rome
, 1962
), p
–509
.73.
L.
Alvarez
and A. H.
Compton
, “A positively charged component of cosmic rays,”
Phys. Rev.
33
, 835
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(1933
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T. H.
Johnson
, “The azimuthal asymmetry of the cosmic Radiation
,” Phys. Rev.
43
, 834
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(1933
).75.
B.
Rossi
, “I risultati della missione scientifica in Eritrea per lo studio dei raggi cosmici
,” La Ric. Sc.
4
, 365
–368
(1933
)B.
Rossi
, and “Directional measurement on the cosmic rays near the geomagnetic equator
,” Phys. Rev.
45
, 212
–214
(1934
).At the beginning of 1930s Geiger, too, noted that counters placed in separate rooms periodically registered simultaneous bursts of radiation, T. J. Trenn, “Geiger, Hans (Johannes) Wilhelm
,” in Dictionary of Scientific Biography
, edited by C. C.
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, on p. 332.76.
B.
Rossi
, “I risultati della Missione scientifica in Eritrea per lo studio della radiazione penetrante (Raggi cosmici). Misure sulla distribuzione angolare di intensità della radiazione penetrante all’Asmara
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1
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P.
Auger
, “Les grandes gerbes cosmiques de l’atmosphère
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, 228
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Ehrenfest
, R.
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, J.
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, and R.
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Toward the end of 1932, when Compton presented the results of the world survey supporting the existence of the latitude effect, an acrimonious debate with Millikan was played out in several tense scientific meetings and in the press. See
M.
De Maria
and A.
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, “Cosmic ray romancing: the discovery of the latitude effect and the Compton–Millikan controversy
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B.
Rossi
, “Absorptionsmessungen der durchdringenden Korpuskularstrahlung in einem Meter Blei,”
Die Naturwiss.
20
, 65
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(1932
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B.
Rossi
, “Nachweis einer Sekundärstrahlung der durchdringenden Korpuskularstrahlung,
” Phys. Z.
33
, 304
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(1932
).83.
B.
Rossi
, “Über die Eigenschaften der durchdringenden Korpuskularstrahlung in Meeresniveau
,” Z. Phys.
82
, 151
–178
(1933
).For an undergraduate project on the reproduction of Rossi’s experiment see
D. P.
Jackson
and M. T.
Welker
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, 896
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Fermi invited Rossi to give an introductory talk on the problems of cosmic rays. In discussing the most popular issue of the day, the problem of the origin and nature of the penetrating radiation, Rossi gave a detailed and cogent report on why he thought that Millikan’s assumption could not be correct. Rossi, who at the time was only 26 years old, later commented that “such a brash behavior on the part of a mere youngster […] clearly did not please Millikan who, for a number of years thereafter, chose to ignore my work altogether.” His talk stimulated Compton who, some time later, told him that it “had provided the initial motivation for his research program in cosmic rays.” See Ref. 52, p. 39.
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In 1932 Rossi left Florence and moved to a chair of experimental physics in Padua University. In 1938, when the anti-semitic laws passed in Italy, Rossi was dismissed, and had to emigrate from his country. See
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