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From spontaneous ionization to subatomic physics: Some vignettes from cosmic ray history
Astroparticle Physics 53 (2014) 6–18
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From spontaneous ionization to subatomic physics: Some vignettes from cosmic ray history James W. Cronin ⇑ Department of Astronomy and Astrophysics, Enrico Fermi Institute, University of Chicago, 5640 South Ellis Ave., Chicago, IL 60637, USA
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Article history: Available online 19 April 2013 Keywords: History of cosmic rays Early particle physics
a b s t r a c t In 1879 Crookes discovered that air seemed to ionize spontaneously. With the discovery in 1896 of radioactivity by Henri Becquerel, it appeared that the mystery was solved. However a number of physicists sought a quantitative agreement between the ’’spontaneous ionization’’ and the radioactivity in the earth. The persistence of these physicists led to the discovery of another source of radiation which appeared to come from the heavens. The nature of this ’’cosmic radiation’’ involved phenomena that were completely unknown. Coming to an understanding of the nature of this cosmic radiation took about 40 years between 1912 and 1953. This history involves extraordinary scientists and the invention of dramatic new detection techniques. This story finishes with a remarkable conference organized by Patrick Blackett and Louis Leprince–Ringuet (1953) in the Pyrenees town of Bagnères de Bigorre. Following 1953 the cosmic ray researchers divided into two groups, those who continued the investigation of the new particles with the accelerators and those who continued with the search for the origin and the astrophysics of the cosmic rays. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction It was known for a long time that an object charged electrostatically could not retain its charge. In 1879 Crookes [1] performed some experiments which characterized the phenomenon in a scientific manner. He showed that a charged object enclosed in a sealed container filled with air gradually lost its charge. When the container was evacuated the object retained its charge. The conclusion was that air itself ionized spontaneously. The phenomenon could be made quantitative with a gold leaf electrometer as shown in Fig. 1. The nature of this process remained a mystery until the discovery of radioactivity by Becquerel in 1896. With that discovery it was thought that the ionization of the air by residual radioactivity could account for the loss of charge. A number of physicists sought to confirm this conjecture and if possible give it a quantitative foundation. One of the earliest of such investigations was by Wilson [2]. Sections excerpted from his paper are shown in Figs. 2 and 3. His conclusions were that the ionization as measured by the discharge rate was a function of the air pressure, confirming Crookes result that the discharge rate goes to zero in a vacuum. He also measured the ionization rate to be about 20 ion pairs/cm3/s. In a second paper the following year Wilson [3] showed for several gasses that the discharge rate was very similar to that ⇑ Tel.: +1 312 240 0181. E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.04.003
produced by radioactive substances. These results, shown in Fig. 4, supported the hypothesis that the air was indeed ionized by residual radioactivity. During the decade 1901–1910 a number of physicists investigated surface radioactivity as the source of the ‘‘spontaneous ionization’’. Among these were Elster and Geitel [4], Rutherford and Cook [5], Eve [6], Schrödinger [7], and Pacini [8]. The famous Schrödinger calculated the effect of radon gas in the atmosphere. Pacini, a meteorologist, made extensive measurements of radioactivity over the ground and over oceans. Fig. 5 shows a portion of his paper. He concluded that his data and the early investigations of ionization at higher altitudes ‘‘seem to show that an appreciable part of the penetrating radiation, particularly that which is subject to some significant variations, has an origin independent of the direct action of (radio) active substances contained in the upper layers of the earth’s crust. Pacini’s estimate of the excess ionization was 2 ion pairs/cm3/s. This is just the amount of the ionization due to cosmic rays at sea level that we know today. The hypothesis that the ‘‘spontaneous ionization’’ was surface radioactivity suggested that if one carried an electrometer to high altitude that the ionization should decrease. The first to carry out such a measurement was Father Theodor Wulf, a Jesuit priest [9]. He carried an electrometer of his own design, shown in Fig. 6, to the summit of the Eiffel tower. This style of electrometer was used by a number of subsequent researchers. Wulf’s data is presented in Fig. 7. His laboratory was in Valkenburg, Netherlands. One can estimate the error of the measurement by the variation of the four
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Fig. 1. A typical gold leaf electrometer.
Fig. 5. Page from Pacini’s paper presenting his conclusions concerning surface radiation as the source of the spontaneous ionization [8].
Fig. 2. An exerpt from C.T.R. Wilson’s paper [2].
Fig. 3. Conclusions from C.T.R. Wilson’s paper [2].
Fig. 6. Schematic view of the Wulf electrometer.
Fig. 4. Comparison of relative ionization of air and other gasses with and without a radioactive source [3].
measurements on successive days at the top of the tower. The RMS error is about 1.25 ions/cm3/s so there was a small but hardly
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Fig. 7. Ionization data taken by Wulf in his climbing of the Eiffel Tower [9].
significant decrease as the electrometer was placed at the summit of the tower. A more significant altitude could be attained with balloons. The first to attempt a flight with an electrometer was the Swiss physicist Albert Gockel [10]. In two flights in 1910 he flew an electrometer to heights of 2800 m and 2500 m respectively. There was no significant decrease in the ionization recorded. Gockel’s work was followed up by Victor Hess who systematically began to address the spontaneous ionization problem. He first measured
Fig. 9. First page of Hess paper reporting on the seven balloon flights [13].
Fig. 10. Readings of the three Wulf electrometers on the seventh balloon flight of Hess [13].
Fig. 8. One of the balloons of the type used by Hess in his 1912 flights. Figure courtesy Museum of Military History, Vienna.
directly in air the absorption coefficient of gamma rays, then the most penetrating radiation known [11]. Using the 1.5 MeV gamma rays from Radium C he measured the absorption coefficient to be 0.447 10 4 cm 1. With this number one could estimate the decrease of ionization as a function of altitude if the radiation consisted of gamma rays from the ground. To these estimates have to be added the effect of radon in the atmosphere as pointed out by Schrödinger. In 1911 Hess made two balloon flights with inconclusive results [12]. In 1912 he made seven flights [13]. The balloons were provided by the Royal Imperial Austrian Aeronautical Club an example of which is shown in Fig. 8. On each flight he carried three Wulf electrometers, two sealed at atmospheric pressure and one opened to the atmosphere. The two sealed electrometers had rather thick walls (3 mm zinc) while the third had thin walls (.188 mm zinc). The third electrometer was specially designed to be sensitive to less penetrating radiation such as beta rays. The thin wall necessitated the equilibrium with the external atmosphere. On the first flight there was a solar eclipse! The second flight was taken at night to assure that the sun was not a source. All
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the flights until the last were of an altitude of about 2000 m. None of these showed any significant variation in the ionization rate. This fact was perhaps surprising if the source of the radiation was on the earth’s surface. Normally the balloons were filled with illuminating gas. On the seventh flight Hess chose a larger balloon, provided by the German Aero Club in Bohemia, filled with hydrogen gas for greater lifting power. The route of this flight was from Aussig, now in the north of the Czech Republic, to Pieskow, near Berlin. The flight achieved an altitude of 5300 m. A description of all seven flights was published [13] in a paper entitled: ‘‘On the Observation of Penetrating Radiation by Seven Balloon Flights’’, shown in Fig. 9. Fig. 10 shows the readings of the three Wulf electrometers on the Hess’s seventh flight. All three show a clear rise at the highest altitude. It was necessary to correct the third electrometer for the lowered pressure as a function of altitude to observe the rise. The fluctuations of the several readings at ground gives an estimate of the RMS error of an individual reading to be 1.0 ion pairs/cc/s. There is clear evidence of the rise of ionization with altitude. However one might be concerned about the discrepancy between q1 and q2 with q3 (corrected). One notes that the descent was very rapid, falling from 5000 m to near ground in little more than one hour. It is said that Hess suffered from altitude sickness requiring the rapid descent. No data during the descent was taken from the electrometer open to the air. In coming to his conclusions Hess considers the possible size of the penetration of gamma rays from the earth as well as the effect of radon gas in the atmosphere. In his conclusion Hess writes [14]: The results of the present observations seem to be most readily explained by the assumption that a radiation of a very high penetrating power enters our atmosphere from above, and still produces in the lowest layers a part of the ionization observed in closed vessels. The intensity of this radiation appears to be subject to transient variations, recognizable in hourly readings. The results of Hess were quickly and decisively confirmed by flights to even higher altitude by a young German physicist, Werner Kolhörster, from the Kaiser Wilhelm Institute, Charlottenberg. In 1913 Kolhörster [15,16] made three balloon flights, the third
Fig. 11. Kolhörster’s paper reporting on ionization measurements up to an altitude of 9300 m [17].
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Fig. 12. Kolhörster’s measurements of ionization vs altitude [17].
Fig. 13. Excerpt from a paper by Otis and Millikan concluding that penetrating radiation from above does not exist [18].
achieving a height of 6200 m. This flight required the use of oxygen for breathing. On June 28, 1914 with a 2200 m3 balloon filled with hydrogen Kolhörster achieved an altitude of 9300 m [17]. The first page of his paper is shown in Fig. 11. At this altitude he found that the ionization rose to about nine times its value on the ground! Kolhörster’s results from1913 and 1914 are shown in Fig. 12. The persistence of few researchers to understand the discharge rates of electrometers had led to the serendipitous discovery of radiation coming from above. It was to take about 40 years to understand the nature of this radiation called ’’Höhenstrahlung’’ by Hess. The subsequent research made many discoveries of new particles and ultimately led to the field of sub-atomic physics. One must remember that the only fundamental particles at the time were ionized hydrogen and electrons. Kolhörster’s paper was published one month before the outbreak of World War I and all research on hohenstrahlung ceased. Research on the nature and sources of Höhenstrahlung resumed in the early twenties. The prewar researchers were joined by new persons including the world famous physicist Robert A. Millikan. Millikan was a very self confident individual who tended to dismiss prior results if they did not agree with his own findings. Among the results of a number of these physicists including Millikan was the conclusion that the Höhenstrahlung did not exist. Discussion of all this work in detail is worth a paper in itself. I will give only one
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Fig. 14. Hess argues against Millikan taking credit for the discovery of Höhenstrahlung [19].
example, a paper by Otis and Millikan [18], an excerpt of which is shown in Fig. 13. Millikan persisted in continuing studies of the penetrating radiation and by 1926 reversed himself concluding that there was penetrating radiation coming from above. Given his prestige many assumed that it was he and not Hess and Kolhörster who discovered the penetrating radiation. This prompted Hess [19] in a paper shown in Fig. 14, commenting on some work of Millikan to conclude: As concerns the publication of Millikan cited above, I would like to remark that he tells a story of the discovery of hohenstrahlung that could be easily misunderstood’’ In the third footnote Hess adds: The recent determination by Millikan and his colleagues of the high penetrating power of hohenstrahlung has been the occasion for American scientific journals such as ‘‘Science’’ and ‘‘Scientific Monthly’’ to introduce the term ‘‘ Millikan Rays’’. Millikan’s work is only a confirmation and extension of the results obtained by Gockel, by myself, and by Kolhörster from 1910 to 1913 using balloon borne measurements of the rays. To refuse to acknowledge our work is an error and unjustified.
Fig. 15. Apparatus of Bothe and Kolhörster that demonstrated that the cosmic rays at sea level are mostly charged [23].
Hess was of course successful in retaining for himself and his colleagues the priority for the discovery of the hohenstrahlung but in one sense Millikan put his stamp on the discovery. In a lecture Millikan gave at Leeds University [20] in 1928, describing his experiments, he states . . .. . .., all this constitutes pretty unambiguous evidence that the high altitude rays do not originate in our atmosphere, very certainly not in the lower nine-tenths of it, and justifies the designation ‘cosmic rays’. Millikan was to continue cosmic ray research for the rest of his life and he never gave up his conviction that the cosmic rays were gamma rays. Millikan and Cameron [21] were the authors of a paper suggesting that out in the cosmos heavy nuclei were formed from the fusion of lighter nuclei and protons, the release of the binding energy being the cosmic ray photons. According to Millikan the cosmic rays were the ‘‘birth cries of atoms’’. But new researchers doing new experiments with new detection techniques showed that for the most part the cosmic rays consisted of charged particles. The advent of Geiger-Müller counters and the coincidence technique, pioneered by Bothe, allowed experiments where one could test the notion that the cosmic rays were gamma rays. Knowledge of the Compton effect [22] played an important role in the analyses. A seminal experiment with Geiger counters was carried out by Bothe and Kolhörster [23] in a paper entitled ‘‘Das Wesen der Hohenstrahlung’’. They were influenced by the Russian physicist, Skobelzyn [25], who, using a cloud chamber to study the Compton effect, occasionally observed very energetic charged particles unrelated to his experiments. Bothe and Kolhorster reasoned that Skobeltzyn was observing cosmic radiation and were then motivated to investigate whether the cosmic rays consisted of charged particles. With the apparatus shown in Fig. 15, Bothe and Kolhörster demonstrated that the bulk of the ionizing radiation at sea level consisted of single charged particles of great penetrating power. The insertion of a 4.1 cm thick gold absorber between the Geiger counters hardly decreased the rate of coincidences. The paper is very detailed, concerned with accidentals, geometric effects and possible effects of Compton electrons. The fact that the cosmic rays
Fig. 16. Rossi’s apparatus to demonstrate that half of the charged cosmic ray particles could penetrate 1 m of lead reference [27].
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at sea level were extraordinarily penetrating charged particles attracted a talented new scientist to the field, Bruno Rossi. Rossi had just received his PhD from the University of Bologna and was appointed to work at the University of Florence at Arcetri, located in the hills above the city. He came to Arcetri searching for a new line of research, distinct from his thesis research. Rossi describes the development of his interest in cosmic ray research in a memoir [24]: For me, the turning point in the search came in the fall of 1929, with the appearance, in Zeitschrift für Physik, of the historical paper ‘‘Das Wesen der Höhenstrahlung’’ by W. Bothe and W. Kolhörster. Until then I had not been particularly interested in the phenomenon of the ‘‘hohenstrahlung’’ or ‘‘cosmic radiation’’, using the suggestive expression introduced by Robert Millikan. I had not thought it would offer, to me at least, a profitable field of research. I had not been seduced by Millikan’s well publicized theory maintaining that cosmic rays were the ‘‘birth cry of atoms’’ in cosmic space, being born, in the form of gamma rays when hydrogen atoms ‘‘fused’’ to form heavier elements. To my skeptical mind this was a romantic idea, lacking sound experimental support. On the other hand, I had accepted, uncritically, the prevailing view that the cosmic rays were high-energy gamma-rays. Therefore I read with particular keen interest the paper by Bothe and Kolhörster relating the first attempt to submit this assumption to a direct test. Rossi began his cosmic ray research with superb experimental ability and was not hampered by any preconceived ideas about the nature of cosmic rays. His work was greatly aided by his invention of a coincidence circuit for Geiger counters [26] that allowed any number of counters to be placed in coincidence. Rossi’s contributions were enormous. We will just mention two of his experiments. The first, exploited his coincidence circuit with a dramatic result. With the apparatus shown in Fig. 16, Rossi [27] demonstrated that more than half of the charged cosmic ray particles at sea level could penetrate one meter of lead. A third coincidence counter in the middle of the lead brick stack was essential to eliminate accidental coincidences. Rossi [28] demonstrated that the cosmic rays at sea level produced interactions as illustrated in Fig. 17. On the left panel is a sketch of his apparatus. A triple coincidence indicates at least two charged particles. On the right are the results. Curve I is the triples rate for a lead screen of variable thickness placed 14.6 cm above the top counter. Curve II is for the lead screen placed 1.2 cm above the top counter. Curve III is for an iron screen placed 1.2 cm above the top counter. From this simple apparatus Rossi learned that the sea level cosmic rays produced secondary interactions, that the maximum rate occurred for a lead thickness of 10– 20 g/cm2, and that the attenuation of the secondaries was much greater than the known attenuation of the penetrating particles. The interactions were produced by a different and softer
component than the penetrating particles whose existence was heretofore unknown. The sea level radiation had two components of distinctly different character. Curve II showed that the production and absorption of the secondaries depended not only on the thickness of the absorber in g/cm2 but also on the atomic number of the absorbing material. Overall Rossi’s contributions to cosmic ray research were extensive. In a brief and remarkable letter to the Physical Review [29] Rossi wrote of a number of ideas and experiments. These included proposing the east–west effect using the earth’s magnetic field, which could determine the sign of the charge of the cosmic ray particles. In a subsequent paper with Fermi, Rossi [30] showed that the east–west experiment required both high altitude and low magnetic latitude. The east–west measurements were subsequently made by Alvarez and Compton [31], and by Johnson [32]. Neither of these authors gave credit to Rossi’s original idea. A few months later Rossi [33] presented his own measurement of the east–west effect with an apparatus located in Asmara, Ethiopia. All experiments gave the same answer – the primaries were positive! This result was a great surprise as one had expected the primaries to be negative electrons. As soon as the result of Bothe and Kolhörster was announced many researchers realized that the earth’s magnetic field could be used to determine if the cosmic rays that entered the earth were also charged. As noted above Rossi was among the first. It was a Dutch physicist J. Clay who measured the dependence of the rate of cosmic rays as a function of magnetic latitude. It was expected that near the magnetic equator more charged cosmic ray particles would be deflected away from the earth than at the magnetic pole. In 1932 Clay [34] reported on the measurement of the cosmic ray intensity on a voyage from Genoa to Batavia (Indonesia). The results of this measurement showed convincingly an effect
Fig. 17. Left: Rossi’s apparatus to detect secondary interactions of cosmic rays; Right: Triple coincidences as a function of lead or iron absorbers [28].
Fig. 18. Arthur Compton checking one of his ionization chambers used in his world survey of cosmic ray intensity.
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Fig. 19. Location of the intensity measurements in Compton’s world survey [35].
amounting to a 17% at latitudes less than 400 and almost no effect at higher latitudes. The fact that the effect was constrained to low latitudes led a number of researchers, including Millikan, to conclude there was no latitude effect. At the 1931 Conference on Nuclear Physics in Rome, Fermi had asked the very young Rossi to review the cosmic ray results. In attendance was Arthur Compton who was there influenced by Rossi to enter the field of cosmic ray research. In early 1932 Compton organized a world wide survey of the intensity of cosmic rays as a function of latitude and altitude. Compton designed a high pressure ionization chamber which could easily be reproduced and calibrated by a standard radioactive source. Fig. 18 shows Compton in front of one of his ionization chambers. Fig. 19 shows the locations of the measurements of cosmic ray intensity. Compton had quickly organized many colleagues around the world to participate in the survey. In Fig. 20 the results of the survey are plotted as a function of latitude for three different altitudes. The results were published in a single author paper [35] with acknowledgement to his collaborators in the text. The work of Clay and Compton with his colleagues showed without question that in addition to producing charged particles at sea level the primary cosmic rays were largely charged. This did not convince Millikan. Compton and Millikan participated in a debate at a meeting of the American Association for the Advancement of Science at Atlantic City on Dec 31, 1932. Reporting on this conference the science reporter, William Lawrence, wrote on the front page of the New York times: In an atmosphere surcharged with drama, in which the human element was by no means lacking, the two protagonists presented their views with the vehemence and fervor of those theoretical debates of bigone days when learned men clashed over the number of angels that could dance on the point of a needle. Dr. Millikan particularly sprinkled his talk with remarks directly aimed at his antagonist’s scientific acumen. There was obvious coolness between the two men when they met after the debate was over.
In the late 20’s another powerful technique, the cloud chamber invented by C.T.R. Wilson was introduced through serendipity into cosmic ray research. We mentioned above that the accidental appearance of charged tracks in Skobelzyn’s cloud chamber was the direct stimulus for the pioneering experiment of Bothe and Kolhörster. Skobelzyn [36] published one of his early photos in 1927, shown in Fig. 21, and began to study these events which occurred rarely and randomly. By 1932 Skobelzyn [37] reported on 1700 expansions of his chamber and found cases where there were multiple tracks, even one with four tracks. Skobelzyn was showing visually the complexity of the cosmic rays as Rossi had shown with his counters. Following the work of Skobelzyn a number of groups began cloud chamber investigations of cosmic radiation. Among these were Robert Millikan and Carl D. Anderson at Cal Tech and P.M.S. Blackett at Manchester. The yield of pictures with cosmic rays in them was quite low when the chamber was expanded randomly. A number of researchers realized that there should be a Geiger counter signal accompanying a cloud chamber expansion. The most successful in using Geiger counters with cloud chambers was Blackett. In 1930 he had invited Guiseppi Occhialini to arrange the expansion of the cloud chamber to be triggered by arrays of Geiger counters. Occhialini was a student of Rossi’s at Arcetri and an expert on counters and electronics. The triggered expansions were very rich in photos containing cosmic ray tracks, many of them a shower of tracks in a single photo with both positive and negative tracks. An example of such a shower is shown in Fig. 22. Blackett and Occhialini wrote a masterful paper on their research [38] which was received by the Proceedings of the Royal Society on Feb 7, 1933. A copy of their summary is shown in Fig. 23. In Section 3 of the summary they confirm the conclusion of Anderson that positive electrons exist. In Section 6 they interpret the positive electrons in terms of Dirac’s theory of holes [39]. Their paper was submitted 3 weeks ahead of Anderson’s paper on the positron, but they graciously acknowledged Anderson’s priority.
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Fig. 20. Results of Compton’s world survey of cosmic ray intensities [35].
Simultaneously, Anderson was observing cosmic ray events as well, but without the enrichment provided by a Geiger counter trigger. In one of these events was a single particle judged to be a positively charged particle moving upward as it passed through
a plate losing energy. The upper limit on its mass was 20 times an electron mass and its charge at most twice that of an electron. But the charge and mass was totally consistent with a positive electron. In 1932 Anderson [40] published a brief report on this
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Fig. 21. Cosmic ray track with momentum 7 MeV/c randomly appearing in Skobelzyn’s experiment on Compton scattering [36].
Fig. 23. Blackett and Occhialini summary of their triggered cloud chamber research [38].
Fig. 24. Famous photo of Anderson’s discovery of the positron [41].
Fig. 22. Photo of a cosmic ray shower taken with the counter controlled cloud chamber of Blackett and Occhialini [38].
event. The famous event is shown in Fig. 24. A paper entitled The Positive Electron was published in the Physical Review [41]. In the acknowledgements Anderson writes: While this paper was in preparation, press reports have announced that P.M.S. Blackett and G. Occhialini in an extensive study of cosmic ray tracks have also obtained evidence for the existence of light positive particles confirming our earlier report. Interestingly Anderson did not mention Dirac’s theory of holes. The positron was the first of many new particles to be discovered in the cosmic radiation. This discovery was significant in that it emphasized that the cosmic rays were the path to fundamental physics discoveries. The discovery of the positron led to the award of the 1936 Nobel prize in physics to Carl Anderson and Victor
Fig. 25. Photograph of Victor Hess and Carl Anderson at the time of their reception of the Nobel prize in 1936 (photograph courtesy Nobel Foundation).
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Fig. 26. Paper of Auger and Maze showing coherence of cosmic ray particles out to 20 m in the Paris laboratory [45].
Fig. 27. Coincidence curves measured by Auger and colleagues at sea level and high altitude [45,48].
Hess. It was clear that the motivation for the prize was the positron discovery but it was realized that the cosmic rays were the means for the discovery. Hess also had been nominated and the Swedish committee led by Erik Hulthen concluded that recognition of the positron discovery required that the prize also be awarded to Hess. Photographs of Hess and Anderson at the time of the Prize are shown in Fig. 25. In 1937 a particle now called the mu meson was discovered by Neddermeyer and Anderson [42] and Street and Stevenson [43]. It had a mass intermediate between the electron and the proton and was the principal charged particle contained in the cosmic rays at sea level. The muon was the particle that traversed 1 m of lead in Rossi’s early experiments. This particle was at the time assumed to be the particle proposed by Yukawa [44] as the carrier of the nuclear force. In 1938 Pierre and Auger [45] and colleagues showed that some cosmic rays produced very extensive showers in the atmosphere. Fig. 26 shows a page of their short paper. With Geiger counters they showed that cosmic ray particles could make coincidences at large distances and hence had a common origin. To suppress accidental coincidences the resolving times of the coincidence circuits had to be significantly reduced as well as the dead time of the Geiger counters. Maze [47] clipped the length of the pulses with an inductance and electronically returned the full voltage on the counter more rapidly than the normal RC time constant. In this
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specific experiment the counting rate changed from 1.7/ to 0.9/ events per hour for counter separations of 2–20 m respectively. Auger and collaborators moved their equipment to high altitudes at the Jungfraujoch and extended the coincidence measurements to 300 m [48]. The Paris results and the Jungfrau results are plotted in Fig. 27. Auger estimated that the coincidences at 300 m required primary cosmic ray particles of 1015 eV. Among historians there has been extensive discussion as to who discovered the extensive air showers. Rossi mentioned in his memoirs [24] that when checking for accidentals as he laid out his counters on a flat plane the counts did exceed his expectation for accidentals. However he did not follow up his observation. In 1938 Kolhörster [46] made measurements similar to Auger but he did not pursue this observation. And no doubt there were others. From this author’s perspective it was Auger and his colleagues who continued his original measurements with many additional studies of the shower components. There was an international conference on the subject of cosmic rays held at the University of Chicago in June 1939. The proceedings of this conference were published in the October issue of Reviews of Modern Physics [49]. Cosmic Ray research was at the time a leading topic in physics as indicated by the prominent physicists who attended. Fig. 28 shows the group photo. In the center are Hess and Anderson. Just to the right are Bothe and Heisenberg. Also seen in the photo are Bethe, Compton, Teller, Schein, Oppenheimer, Rossi, Auger (with bow tie), and Serber. Auger gave a long report on his work following up on his pioneering work on extensive air showers [50]. This conference held just three months before the outbreak of World War II on September 1 with the Nazi invasion of Poland. For a second time with some notable exceptions research on cosmic rays ceased. During the war in Italy the physicists Conversi, Pancini, and Piccioni began the study of muon stopping in materials. If the muon was the Yukawa particle one expected that the positive muons would decay but all the negative ones would be captured. In 1947 the Italian team used a magnetic arrangement invented by Rossi [29] to select either positive or negative muons to come to rest on the absorber. For a heavy absorber only positive muons decayed as expected. However for a light carbon absorber both signs of muons decayed. This result [51] showed that the muon was weakly interacting and could not be the proposed Yukawa particle. This was an astounding result. But even before this result the Japanese physicists Sakata and Inoue [52], noting that the lifetime of the muon was longer than expected and that its interaction cross section was much smaller than expected, postulated that the muon was the decay product of the Yukawa particle. Following World War II cosmic ray physics flourished producing many discoveries of new particles. Instrumentation included cloud chambers, Geiger counters and a new technique employing photographic emulsions. In 1947 using the latter technique Lattes et al. [53] discovered the p meson (the Yukawa particle) and showed that it decayed to the muon as Sakata and Inoue had predicted. In addition a myriad of new particles were discovered in the cosmic rays in the late 40’s and early 50’s. Not all of them were convincingly established and there was much confusion. A summary of the data on the new unstable particles presented at the third Rochester Conference is shown in Fig. 29. By the early 1950’s the science of subatomic physics was initiated by cosmic ray research. The positron, muons, charged pions, K mesons, lambda, sigma, and cascade hyperons were discovered. The Rochester conference spent only half a day discussing these unstable particles. The bulk of the conference was devoted to pions and nuclear forces. Pions could be produced artificially and their interactions with protons could be studied directly. An international conference solely dedicated to the new unstable particles found in the cosmic rays was organized by Patrick
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Fig. 28. Photo of attendees of July 1995 conference on cosmic rays at the University of Chicago.
Fig. 29. Data on the new unstable ‘‘elementary’’ particles presented at the third Rochester Conference, December 1952.
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Fig. 30. Photograph of the attendees of the July 1953 conference at Bagnères de Bigorre (proceedings unpublished). A scanned version can be accessed at the Ecole Polytechnique web site, www.polytechnique.edu/accueil/l-ecole-polytechnique/histoire-et-patrimoine/les-polytechniciens-illustres.
Blackett and Louis Leprince–Ringuet. It took place in the French Pyrenees at Bagnères de Bigorre during 6 days, July 6–11. While this conference was intended to be one of a biannual series of conferences on cosmic rays, the strong will of Leprince–Ringuet forced the conference exclusively to a discussion of the new unstable particles found in the cosmic rays. Attending were nearly all of the experimenters who had produced the data. With their concentration on the narrow subject, the conferees were able to resolve many of the disagreements in the data, consolidate the solid results, and provide a list of questions to be answered by future experiments. It was a seminal conference to be compared in importance with the famous Shelter Island Conference of 1948. A detailed article on the planning and results of this conference has been published [54]. A photograph of the conferees is shown in Fig. 30. Alone in the first row is Blackett. Immediately to the left of Blackett is Leprince–Ringuet and to his left is Rossi. On Saturday afternoon Rossi presented a summary of the conference. Almost all the conclusions concerning the unstable particles turned out to be correct. The cosmic ray results defined in detail the early experiments to be carried out with the new multi-GeV accelerators – the Cosmotron at Brookhaven National Laboratory and the Bevatron at Berkeley. Already the properties of these particles were sufficiently known that Pais [55], Gell-Mann [56] and Nakano and Nishijima [57], in advance of the accelerator experiments, could predict their production in pairs and the
strangeness scheme which defined permitted and forbidden modes of production and decay. At the end of his summary Rossi added the following comment: Before concluding my remarks there is one point I would like to make which was already made in the course of this conference: it is the very close similarity between the masses of the two best established particles, I mean the charged tau particle with a mass of 970 and the h0 with a mass of 971. This looks hardly like an accident, and on the other hand it is very difficult to see how the h0 -particle could be the neutral counterpart of the tau-particle. The famous s – h puzzle which led to the discovery of parity violation was defined. At end the conference Leprince–Ringuet made some incisive comments which I quote in loose translation: If we want to draw certain lessons from this congress let’s point out first that in the future we must use the particle accelerators. Let’s point out for example the possibility they will permit the measurement of certain fundamental curves (scattering, ionization, range) which will permit us to differentiate effects such as the existence of pi mesons among the secondaries of K mesons. Finally I would like to finish with some words on a subject that is dear to my heart and is equally so to all the ‘‘old timers’’. Gradually one becomes an ‘‘old timer’’. One does not perceive the exact moment, but one day one notices that the hair is turning grey and that the youngsters that were your students are catching up with you and in some cases passing you. I can see in the audience some of the ‘‘old
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timers’’: the professors Regener, Powell, Blackett, Vallarta, Bothe, Fretter . . .. . .. . .We have to face the grave question: what is the future of cosmic rays? Should we continue to struggle for a few new results or would it be better to turn to the machines? It was precisely at this moment that the cosmic ray community divided into two groups: those who were concerned with the rich particle physics offered by the accelerators and those concerned with the origin and astrophysics of the cosmic rays. Leprince–Ringuet made his choice and moved with his students to the accelerators. Many of those concerned with the sources and astrophysics of the cosmic rays moved into space, the exception being at the energies P 1015 eV where fluxes became so low that the cosmic rays could be detected only by extensive air showers. For the past 60 years accelerators have reached progressively higher energies culminating in the LHC with collision energies of 8 TeV. A very comprehensive understanding of the basic elements of matter has been achieved including the recent discovery of a possible Higgs particle. The particle beams had shifted from the sky to the accelerators but those who made the discoveries with the vertical beams were impressive experimentalists who deciphered the complex nature of the radiation from space observed serendipitously in 1912. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Crookes, Proc. Roy. Soc. 38 (1879) 347. C.T.R. Wilson, Proc. Cambridge Philos. Soc. 26 (1900) 32. C.T.R. Wilson, Proc. Roy. Soc. A68 (1901) 151. J. Elster, H. Geitel, Phys. Z. 2 (1901) 560. E. Rutherford, H.L. Cooke, Phys. Rev. 16 (1903) 183. A.S. Eve, Philos. Mag. 21 (1911) 26. E. Schrödinger, Sitz.-Ber. Akad. Wiss. Wien 121 (1912) 2391. D. Pacini, Le Radium 8 (1911) 307. Th. Wulf, Phys. Z. 11 (1910) 811. A. Gockel, Phys. Z. 12 (1911) 595. V. Hess, Phys. Z. 12 (1911) 998. V. Hess, Sitz.-Ber. Akad. Wiss. Wien 120 (1911) 1575. V. Hess, Phys. Z. 13 (1912) 1084. A.M. Hillas (Ed.), An English Translation of the Principal Parts of the Hess Paper can be Found in the Book Cosmic Rays, Pergamon Press, London, 1972, p. 139.
[15] [16] [17] [18] [19] [20] [21] [22] [23]
[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]
W. Kolhörster, Phys. Z. 14 (1913) 1153. W. Kolhörster, Verh. Dtsch. Phys. Ges. 15 (1913) 1111. W. Kolhörster, Verh. Dtsch. Phys. Ges. 16 (1914) 719. R.M. Otis, R.A. Millikan, Phys. Rev. 23 (1924) 778. V. Hess, Phys. Z. 27 (1926) 159. R.A. Millikan, Nature 121 (suppl) (1928) 19. R.A. Millikan, G.H. Cameron, Phys. Rev. 32 (1928) 533. A.H. Compton, Phys. Rev. 21 (1923) 484. W. Bothe, W. Kolhörster, Z. Phys. 56 (1929) 751; A.M. Hillas (Ed.), An English Translation of the Principal Parts of This Paper can be Found in the Book Cosmic Rays, Pergamon Press, London, 1972, p. 148. Y. Sekido, H. Elliot (Eds.), Early History of Cosmic Ray Studies, D. Reidel Publishing Company, London, 1985, p. p. 53. D. Skobelzyn, Z. Phys. 43 (1927) 354. B. Rossi, Nature 125 (1930) 636. B. Rossi, Nature 20 (1932) 65. B. Rossi, Ziet. fur Phys. 82 (1933) 151. B. Rossi, Phys. Rev. 36 (1930) 606. B. Rossi, E. Fermi, Accad. Lincei 17 (1933) 346. L. Alvarez, A.H. Compton, Phys. Rev. 43 (1933) 835. T.H. Johnson, Phys. Rev. 43 (1933) 834. B. Rossi, Phys. Rev. 45 (1934) 212. J. Clay, Proc. Roy. Acad. Amsterdam 35 (1932) 1282. A.H. Compton, Phys. Rev. 43 (1933) 387. D. Skobelzyn, Zeit. fur Phys. 43 (1927) 354. D. Skobelzyn, Comp. R. 194 (1932) 315. P.M.S. Blackett, G. Occhialini, Proc. Roy. Soc. 139 (1933) 699. P.A.M. Dirac, Proc. Roy. Soc. A 133 (1931) 60. C.D. Anderson, Science 76 (1932) 238. C.D. Anderson, Phys. Rev. 43 (1933) 491. S. Nedermeyer, C.D. Anderson, Phys. Rev. 51 (1937) 884. J.C. Street, E.C. Stevenson, Phys. Rev. 51 (1937) 1005. H. Yukawa, Proc. Phys. Math. Soc. Japan 17 (1935) 48. P. Auger, R. Maze, Comp. R. 207 (1938) 228. W. Kolhörster, Naturwiss. 26 (1938) 576. R. Maze, J. Phys. 9 (1938) 162. P. Auger, R. Maze, P. Ehrenfest Jr., A. Freon, J. Phys. 10 (1939) 39. Cosmic Ray Conference, University of Chicago, Rev. Mod. Phys. 11 (1939). P. Auger, P. Ehrenfest Jr., R. Maze, J. Daudin, Robley, A. Freon, Rev. Mod. Phys. 11 (1939) 288. M. Conversi, E. Pancini, O. Piccioni, Phys. Rev. 71 (1947) 209. S. Sakata, T. Inoue, Prog. Theory Phys. (Japan) 1 (1946) 143. C.M.G. Lattes, G. Occhialini, C.F. Powell, Nature 160 (1947) 453. J.W. Cronin, Eur. J. Phy. H 36 (2011) 183. A. Pais, Phys. Rev. 86 (1952) 663. M. Gell-Mann, Phys. Rev. 92 (1953) 833. T. Nakano, K. Nishijima, Prog. Theory Phys. 10 (1953) 581.
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Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart
From spontaneous ionization to subatomic physics: Some vignettes from cosmic ray history James W. Cronin ⇑ Department of Astronomy and Astrophysics, Enrico Fermi Institute, University of Chicago, 5640 South Ellis Ave., Chicago, IL 60637, USA
a r t i c l e
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Article history: Available online 19 April 2013 Keywords: History of cosmic rays Early particle physics
a b s t r a c t In 1879 Crookes discovered that air seemed to ionize spontaneously. With the discovery in 1896 of radioactivity by Henri Becquerel, it appeared that the mystery was solved. However a number of physicists sought a quantitative agreement between the ’’spontaneous ionization’’ and the radioactivity in the earth. The persistence of these physicists led to the discovery of another source of radiation which appeared to come from the heavens. The nature of this ’’cosmic radiation’’ involved phenomena that were completely unknown. Coming to an understanding of the nature of this cosmic radiation took about 40 years between 1912 and 1953. This history involves extraordinary scientists and the invention of dramatic new detection techniques. This story finishes with a remarkable conference organized by Patrick Blackett and Louis Leprince–Ringuet (1953) in the Pyrenees town of Bagnères de Bigorre. Following 1953 the cosmic ray researchers divided into two groups, those who continued the investigation of the new particles with the accelerators and those who continued with the search for the origin and the astrophysics of the cosmic rays. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction It was known for a long time that an object charged electrostatically could not retain its charge. In 1879 Crookes [1] performed some experiments which characterized the phenomenon in a scientific manner. He showed that a charged object enclosed in a sealed container filled with air gradually lost its charge. When the container was evacuated the object retained its charge. The conclusion was that air itself ionized spontaneously. The phenomenon could be made quantitative with a gold leaf electrometer as shown in Fig. 1. The nature of this process remained a mystery until the discovery of radioactivity by Becquerel in 1896. With that discovery it was thought that the ionization of the air by residual radioactivity could account for the loss of charge. A number of physicists sought to confirm this conjecture and if possible give it a quantitative foundation. One of the earliest of such investigations was by Wilson [2]. Sections excerpted from his paper are shown in Figs. 2 and 3. His conclusions were that the ionization as measured by the discharge rate was a function of the air pressure, confirming Crookes result that the discharge rate goes to zero in a vacuum. He also measured the ionization rate to be about 20 ion pairs/cm3/s. In a second paper the following year Wilson [3] showed for several gasses that the discharge rate was very similar to that ⇑ Tel.: +1 312 240 0181. E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.04.003
produced by radioactive substances. These results, shown in Fig. 4, supported the hypothesis that the air was indeed ionized by residual radioactivity. During the decade 1901–1910 a number of physicists investigated surface radioactivity as the source of the ‘‘spontaneous ionization’’. Among these were Elster and Geitel [4], Rutherford and Cook [5], Eve [6], Schrödinger [7], and Pacini [8]. The famous Schrödinger calculated the effect of radon gas in the atmosphere. Pacini, a meteorologist, made extensive measurements of radioactivity over the ground and over oceans. Fig. 5 shows a portion of his paper. He concluded that his data and the early investigations of ionization at higher altitudes ‘‘seem to show that an appreciable part of the penetrating radiation, particularly that which is subject to some significant variations, has an origin independent of the direct action of (radio) active substances contained in the upper layers of the earth’s crust. Pacini’s estimate of the excess ionization was 2 ion pairs/cm3/s. This is just the amount of the ionization due to cosmic rays at sea level that we know today. The hypothesis that the ‘‘spontaneous ionization’’ was surface radioactivity suggested that if one carried an electrometer to high altitude that the ionization should decrease. The first to carry out such a measurement was Father Theodor Wulf, a Jesuit priest [9]. He carried an electrometer of his own design, shown in Fig. 6, to the summit of the Eiffel tower. This style of electrometer was used by a number of subsequent researchers. Wulf’s data is presented in Fig. 7. His laboratory was in Valkenburg, Netherlands. One can estimate the error of the measurement by the variation of the four
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Fig. 1. A typical gold leaf electrometer.
Fig. 5. Page from Pacini’s paper presenting his conclusions concerning surface radiation as the source of the spontaneous ionization [8].
Fig. 2. An exerpt from C.T.R. Wilson’s paper [2].
Fig. 3. Conclusions from C.T.R. Wilson’s paper [2].
Fig. 6. Schematic view of the Wulf electrometer.
Fig. 4. Comparison of relative ionization of air and other gasses with and without a radioactive source [3].
measurements on successive days at the top of the tower. The RMS error is about 1.25 ions/cm3/s so there was a small but hardly
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Fig. 7. Ionization data taken by Wulf in his climbing of the Eiffel Tower [9].
significant decrease as the electrometer was placed at the summit of the tower. A more significant altitude could be attained with balloons. The first to attempt a flight with an electrometer was the Swiss physicist Albert Gockel [10]. In two flights in 1910 he flew an electrometer to heights of 2800 m and 2500 m respectively. There was no significant decrease in the ionization recorded. Gockel’s work was followed up by Victor Hess who systematically began to address the spontaneous ionization problem. He first measured
Fig. 9. First page of Hess paper reporting on the seven balloon flights [13].
Fig. 10. Readings of the three Wulf electrometers on the seventh balloon flight of Hess [13].
Fig. 8. One of the balloons of the type used by Hess in his 1912 flights. Figure courtesy Museum of Military History, Vienna.
directly in air the absorption coefficient of gamma rays, then the most penetrating radiation known [11]. Using the 1.5 MeV gamma rays from Radium C he measured the absorption coefficient to be 0.447 10 4 cm 1. With this number one could estimate the decrease of ionization as a function of altitude if the radiation consisted of gamma rays from the ground. To these estimates have to be added the effect of radon in the atmosphere as pointed out by Schrödinger. In 1911 Hess made two balloon flights with inconclusive results [12]. In 1912 he made seven flights [13]. The balloons were provided by the Royal Imperial Austrian Aeronautical Club an example of which is shown in Fig. 8. On each flight he carried three Wulf electrometers, two sealed at atmospheric pressure and one opened to the atmosphere. The two sealed electrometers had rather thick walls (3 mm zinc) while the third had thin walls (.188 mm zinc). The third electrometer was specially designed to be sensitive to less penetrating radiation such as beta rays. The thin wall necessitated the equilibrium with the external atmosphere. On the first flight there was a solar eclipse! The second flight was taken at night to assure that the sun was not a source. All
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the flights until the last were of an altitude of about 2000 m. None of these showed any significant variation in the ionization rate. This fact was perhaps surprising if the source of the radiation was on the earth’s surface. Normally the balloons were filled with illuminating gas. On the seventh flight Hess chose a larger balloon, provided by the German Aero Club in Bohemia, filled with hydrogen gas for greater lifting power. The route of this flight was from Aussig, now in the north of the Czech Republic, to Pieskow, near Berlin. The flight achieved an altitude of 5300 m. A description of all seven flights was published [13] in a paper entitled: ‘‘On the Observation of Penetrating Radiation by Seven Balloon Flights’’, shown in Fig. 9. Fig. 10 shows the readings of the three Wulf electrometers on the Hess’s seventh flight. All three show a clear rise at the highest altitude. It was necessary to correct the third electrometer for the lowered pressure as a function of altitude to observe the rise. The fluctuations of the several readings at ground gives an estimate of the RMS error of an individual reading to be 1.0 ion pairs/cc/s. There is clear evidence of the rise of ionization with altitude. However one might be concerned about the discrepancy between q1 and q2 with q3 (corrected). One notes that the descent was very rapid, falling from 5000 m to near ground in little more than one hour. It is said that Hess suffered from altitude sickness requiring the rapid descent. No data during the descent was taken from the electrometer open to the air. In coming to his conclusions Hess considers the possible size of the penetration of gamma rays from the earth as well as the effect of radon gas in the atmosphere. In his conclusion Hess writes [14]: The results of the present observations seem to be most readily explained by the assumption that a radiation of a very high penetrating power enters our atmosphere from above, and still produces in the lowest layers a part of the ionization observed in closed vessels. The intensity of this radiation appears to be subject to transient variations, recognizable in hourly readings. The results of Hess were quickly and decisively confirmed by flights to even higher altitude by a young German physicist, Werner Kolhörster, from the Kaiser Wilhelm Institute, Charlottenberg. In 1913 Kolhörster [15,16] made three balloon flights, the third
Fig. 11. Kolhörster’s paper reporting on ionization measurements up to an altitude of 9300 m [17].
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Fig. 12. Kolhörster’s measurements of ionization vs altitude [17].
Fig. 13. Excerpt from a paper by Otis and Millikan concluding that penetrating radiation from above does not exist [18].
achieving a height of 6200 m. This flight required the use of oxygen for breathing. On June 28, 1914 with a 2200 m3 balloon filled with hydrogen Kolhörster achieved an altitude of 9300 m [17]. The first page of his paper is shown in Fig. 11. At this altitude he found that the ionization rose to about nine times its value on the ground! Kolhörster’s results from1913 and 1914 are shown in Fig. 12. The persistence of few researchers to understand the discharge rates of electrometers had led to the serendipitous discovery of radiation coming from above. It was to take about 40 years to understand the nature of this radiation called ’’Höhenstrahlung’’ by Hess. The subsequent research made many discoveries of new particles and ultimately led to the field of sub-atomic physics. One must remember that the only fundamental particles at the time were ionized hydrogen and electrons. Kolhörster’s paper was published one month before the outbreak of World War I and all research on hohenstrahlung ceased. Research on the nature and sources of Höhenstrahlung resumed in the early twenties. The prewar researchers were joined by new persons including the world famous physicist Robert A. Millikan. Millikan was a very self confident individual who tended to dismiss prior results if they did not agree with his own findings. Among the results of a number of these physicists including Millikan was the conclusion that the Höhenstrahlung did not exist. Discussion of all this work in detail is worth a paper in itself. I will give only one
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Fig. 14. Hess argues against Millikan taking credit for the discovery of Höhenstrahlung [19].
example, a paper by Otis and Millikan [18], an excerpt of which is shown in Fig. 13. Millikan persisted in continuing studies of the penetrating radiation and by 1926 reversed himself concluding that there was penetrating radiation coming from above. Given his prestige many assumed that it was he and not Hess and Kolhörster who discovered the penetrating radiation. This prompted Hess [19] in a paper shown in Fig. 14, commenting on some work of Millikan to conclude: As concerns the publication of Millikan cited above, I would like to remark that he tells a story of the discovery of hohenstrahlung that could be easily misunderstood’’ In the third footnote Hess adds: The recent determination by Millikan and his colleagues of the high penetrating power of hohenstrahlung has been the occasion for American scientific journals such as ‘‘Science’’ and ‘‘Scientific Monthly’’ to introduce the term ‘‘ Millikan Rays’’. Millikan’s work is only a confirmation and extension of the results obtained by Gockel, by myself, and by Kolhörster from 1910 to 1913 using balloon borne measurements of the rays. To refuse to acknowledge our work is an error and unjustified.
Fig. 15. Apparatus of Bothe and Kolhörster that demonstrated that the cosmic rays at sea level are mostly charged [23].
Hess was of course successful in retaining for himself and his colleagues the priority for the discovery of the hohenstrahlung but in one sense Millikan put his stamp on the discovery. In a lecture Millikan gave at Leeds University [20] in 1928, describing his experiments, he states . . .. . .., all this constitutes pretty unambiguous evidence that the high altitude rays do not originate in our atmosphere, very certainly not in the lower nine-tenths of it, and justifies the designation ‘cosmic rays’. Millikan was to continue cosmic ray research for the rest of his life and he never gave up his conviction that the cosmic rays were gamma rays. Millikan and Cameron [21] were the authors of a paper suggesting that out in the cosmos heavy nuclei were formed from the fusion of lighter nuclei and protons, the release of the binding energy being the cosmic ray photons. According to Millikan the cosmic rays were the ‘‘birth cries of atoms’’. But new researchers doing new experiments with new detection techniques showed that for the most part the cosmic rays consisted of charged particles. The advent of Geiger-Müller counters and the coincidence technique, pioneered by Bothe, allowed experiments where one could test the notion that the cosmic rays were gamma rays. Knowledge of the Compton effect [22] played an important role in the analyses. A seminal experiment with Geiger counters was carried out by Bothe and Kolhörster [23] in a paper entitled ‘‘Das Wesen der Hohenstrahlung’’. They were influenced by the Russian physicist, Skobelzyn [25], who, using a cloud chamber to study the Compton effect, occasionally observed very energetic charged particles unrelated to his experiments. Bothe and Kolhorster reasoned that Skobeltzyn was observing cosmic radiation and were then motivated to investigate whether the cosmic rays consisted of charged particles. With the apparatus shown in Fig. 15, Bothe and Kolhörster demonstrated that the bulk of the ionizing radiation at sea level consisted of single charged particles of great penetrating power. The insertion of a 4.1 cm thick gold absorber between the Geiger counters hardly decreased the rate of coincidences. The paper is very detailed, concerned with accidentals, geometric effects and possible effects of Compton electrons. The fact that the cosmic rays
Fig. 16. Rossi’s apparatus to demonstrate that half of the charged cosmic ray particles could penetrate 1 m of lead reference [27].
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at sea level were extraordinarily penetrating charged particles attracted a talented new scientist to the field, Bruno Rossi. Rossi had just received his PhD from the University of Bologna and was appointed to work at the University of Florence at Arcetri, located in the hills above the city. He came to Arcetri searching for a new line of research, distinct from his thesis research. Rossi describes the development of his interest in cosmic ray research in a memoir [24]: For me, the turning point in the search came in the fall of 1929, with the appearance, in Zeitschrift für Physik, of the historical paper ‘‘Das Wesen der Höhenstrahlung’’ by W. Bothe and W. Kolhörster. Until then I had not been particularly interested in the phenomenon of the ‘‘hohenstrahlung’’ or ‘‘cosmic radiation’’, using the suggestive expression introduced by Robert Millikan. I had not thought it would offer, to me at least, a profitable field of research. I had not been seduced by Millikan’s well publicized theory maintaining that cosmic rays were the ‘‘birth cry of atoms’’ in cosmic space, being born, in the form of gamma rays when hydrogen atoms ‘‘fused’’ to form heavier elements. To my skeptical mind this was a romantic idea, lacking sound experimental support. On the other hand, I had accepted, uncritically, the prevailing view that the cosmic rays were high-energy gamma-rays. Therefore I read with particular keen interest the paper by Bothe and Kolhörster relating the first attempt to submit this assumption to a direct test. Rossi began his cosmic ray research with superb experimental ability and was not hampered by any preconceived ideas about the nature of cosmic rays. His work was greatly aided by his invention of a coincidence circuit for Geiger counters [26] that allowed any number of counters to be placed in coincidence. Rossi’s contributions were enormous. We will just mention two of his experiments. The first, exploited his coincidence circuit with a dramatic result. With the apparatus shown in Fig. 16, Rossi [27] demonstrated that more than half of the charged cosmic ray particles at sea level could penetrate one meter of lead. A third coincidence counter in the middle of the lead brick stack was essential to eliminate accidental coincidences. Rossi [28] demonstrated that the cosmic rays at sea level produced interactions as illustrated in Fig. 17. On the left panel is a sketch of his apparatus. A triple coincidence indicates at least two charged particles. On the right are the results. Curve I is the triples rate for a lead screen of variable thickness placed 14.6 cm above the top counter. Curve II is for the lead screen placed 1.2 cm above the top counter. Curve III is for an iron screen placed 1.2 cm above the top counter. From this simple apparatus Rossi learned that the sea level cosmic rays produced secondary interactions, that the maximum rate occurred for a lead thickness of 10– 20 g/cm2, and that the attenuation of the secondaries was much greater than the known attenuation of the penetrating particles. The interactions were produced by a different and softer
component than the penetrating particles whose existence was heretofore unknown. The sea level radiation had two components of distinctly different character. Curve II showed that the production and absorption of the secondaries depended not only on the thickness of the absorber in g/cm2 but also on the atomic number of the absorbing material. Overall Rossi’s contributions to cosmic ray research were extensive. In a brief and remarkable letter to the Physical Review [29] Rossi wrote of a number of ideas and experiments. These included proposing the east–west effect using the earth’s magnetic field, which could determine the sign of the charge of the cosmic ray particles. In a subsequent paper with Fermi, Rossi [30] showed that the east–west experiment required both high altitude and low magnetic latitude. The east–west measurements were subsequently made by Alvarez and Compton [31], and by Johnson [32]. Neither of these authors gave credit to Rossi’s original idea. A few months later Rossi [33] presented his own measurement of the east–west effect with an apparatus located in Asmara, Ethiopia. All experiments gave the same answer – the primaries were positive! This result was a great surprise as one had expected the primaries to be negative electrons. As soon as the result of Bothe and Kolhörster was announced many researchers realized that the earth’s magnetic field could be used to determine if the cosmic rays that entered the earth were also charged. As noted above Rossi was among the first. It was a Dutch physicist J. Clay who measured the dependence of the rate of cosmic rays as a function of magnetic latitude. It was expected that near the magnetic equator more charged cosmic ray particles would be deflected away from the earth than at the magnetic pole. In 1932 Clay [34] reported on the measurement of the cosmic ray intensity on a voyage from Genoa to Batavia (Indonesia). The results of this measurement showed convincingly an effect
Fig. 17. Left: Rossi’s apparatus to detect secondary interactions of cosmic rays; Right: Triple coincidences as a function of lead or iron absorbers [28].
Fig. 18. Arthur Compton checking one of his ionization chambers used in his world survey of cosmic ray intensity.
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Fig. 19. Location of the intensity measurements in Compton’s world survey [35].
amounting to a 17% at latitudes less than 400 and almost no effect at higher latitudes. The fact that the effect was constrained to low latitudes led a number of researchers, including Millikan, to conclude there was no latitude effect. At the 1931 Conference on Nuclear Physics in Rome, Fermi had asked the very young Rossi to review the cosmic ray results. In attendance was Arthur Compton who was there influenced by Rossi to enter the field of cosmic ray research. In early 1932 Compton organized a world wide survey of the intensity of cosmic rays as a function of latitude and altitude. Compton designed a high pressure ionization chamber which could easily be reproduced and calibrated by a standard radioactive source. Fig. 18 shows Compton in front of one of his ionization chambers. Fig. 19 shows the locations of the measurements of cosmic ray intensity. Compton had quickly organized many colleagues around the world to participate in the survey. In Fig. 20 the results of the survey are plotted as a function of latitude for three different altitudes. The results were published in a single author paper [35] with acknowledgement to his collaborators in the text. The work of Clay and Compton with his colleagues showed without question that in addition to producing charged particles at sea level the primary cosmic rays were largely charged. This did not convince Millikan. Compton and Millikan participated in a debate at a meeting of the American Association for the Advancement of Science at Atlantic City on Dec 31, 1932. Reporting on this conference the science reporter, William Lawrence, wrote on the front page of the New York times: In an atmosphere surcharged with drama, in which the human element was by no means lacking, the two protagonists presented their views with the vehemence and fervor of those theoretical debates of bigone days when learned men clashed over the number of angels that could dance on the point of a needle. Dr. Millikan particularly sprinkled his talk with remarks directly aimed at his antagonist’s scientific acumen. There was obvious coolness between the two men when they met after the debate was over.
In the late 20’s another powerful technique, the cloud chamber invented by C.T.R. Wilson was introduced through serendipity into cosmic ray research. We mentioned above that the accidental appearance of charged tracks in Skobelzyn’s cloud chamber was the direct stimulus for the pioneering experiment of Bothe and Kolhörster. Skobelzyn [36] published one of his early photos in 1927, shown in Fig. 21, and began to study these events which occurred rarely and randomly. By 1932 Skobelzyn [37] reported on 1700 expansions of his chamber and found cases where there were multiple tracks, even one with four tracks. Skobelzyn was showing visually the complexity of the cosmic rays as Rossi had shown with his counters. Following the work of Skobelzyn a number of groups began cloud chamber investigations of cosmic radiation. Among these were Robert Millikan and Carl D. Anderson at Cal Tech and P.M.S. Blackett at Manchester. The yield of pictures with cosmic rays in them was quite low when the chamber was expanded randomly. A number of researchers realized that there should be a Geiger counter signal accompanying a cloud chamber expansion. The most successful in using Geiger counters with cloud chambers was Blackett. In 1930 he had invited Guiseppi Occhialini to arrange the expansion of the cloud chamber to be triggered by arrays of Geiger counters. Occhialini was a student of Rossi’s at Arcetri and an expert on counters and electronics. The triggered expansions were very rich in photos containing cosmic ray tracks, many of them a shower of tracks in a single photo with both positive and negative tracks. An example of such a shower is shown in Fig. 22. Blackett and Occhialini wrote a masterful paper on their research [38] which was received by the Proceedings of the Royal Society on Feb 7, 1933. A copy of their summary is shown in Fig. 23. In Section 3 of the summary they confirm the conclusion of Anderson that positive electrons exist. In Section 6 they interpret the positive electrons in terms of Dirac’s theory of holes [39]. Their paper was submitted 3 weeks ahead of Anderson’s paper on the positron, but they graciously acknowledged Anderson’s priority.
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Fig. 20. Results of Compton’s world survey of cosmic ray intensities [35].
Simultaneously, Anderson was observing cosmic ray events as well, but without the enrichment provided by a Geiger counter trigger. In one of these events was a single particle judged to be a positively charged particle moving upward as it passed through
a plate losing energy. The upper limit on its mass was 20 times an electron mass and its charge at most twice that of an electron. But the charge and mass was totally consistent with a positive electron. In 1932 Anderson [40] published a brief report on this
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Fig. 21. Cosmic ray track with momentum 7 MeV/c randomly appearing in Skobelzyn’s experiment on Compton scattering [36].
Fig. 23. Blackett and Occhialini summary of their triggered cloud chamber research [38].
Fig. 24. Famous photo of Anderson’s discovery of the positron [41].
Fig. 22. Photo of a cosmic ray shower taken with the counter controlled cloud chamber of Blackett and Occhialini [38].
event. The famous event is shown in Fig. 24. A paper entitled The Positive Electron was published in the Physical Review [41]. In the acknowledgements Anderson writes: While this paper was in preparation, press reports have announced that P.M.S. Blackett and G. Occhialini in an extensive study of cosmic ray tracks have also obtained evidence for the existence of light positive particles confirming our earlier report. Interestingly Anderson did not mention Dirac’s theory of holes. The positron was the first of many new particles to be discovered in the cosmic radiation. This discovery was significant in that it emphasized that the cosmic rays were the path to fundamental physics discoveries. The discovery of the positron led to the award of the 1936 Nobel prize in physics to Carl Anderson and Victor
Fig. 25. Photograph of Victor Hess and Carl Anderson at the time of their reception of the Nobel prize in 1936 (photograph courtesy Nobel Foundation).
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Fig. 26. Paper of Auger and Maze showing coherence of cosmic ray particles out to 20 m in the Paris laboratory [45].
Fig. 27. Coincidence curves measured by Auger and colleagues at sea level and high altitude [45,48].
Hess. It was clear that the motivation for the prize was the positron discovery but it was realized that the cosmic rays were the means for the discovery. Hess also had been nominated and the Swedish committee led by Erik Hulthen concluded that recognition of the positron discovery required that the prize also be awarded to Hess. Photographs of Hess and Anderson at the time of the Prize are shown in Fig. 25. In 1937 a particle now called the mu meson was discovered by Neddermeyer and Anderson [42] and Street and Stevenson [43]. It had a mass intermediate between the electron and the proton and was the principal charged particle contained in the cosmic rays at sea level. The muon was the particle that traversed 1 m of lead in Rossi’s early experiments. This particle was at the time assumed to be the particle proposed by Yukawa [44] as the carrier of the nuclear force. In 1938 Pierre and Auger [45] and colleagues showed that some cosmic rays produced very extensive showers in the atmosphere. Fig. 26 shows a page of their short paper. With Geiger counters they showed that cosmic ray particles could make coincidences at large distances and hence had a common origin. To suppress accidental coincidences the resolving times of the coincidence circuits had to be significantly reduced as well as the dead time of the Geiger counters. Maze [47] clipped the length of the pulses with an inductance and electronically returned the full voltage on the counter more rapidly than the normal RC time constant. In this
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specific experiment the counting rate changed from 1.7/ to 0.9/ events per hour for counter separations of 2–20 m respectively. Auger and collaborators moved their equipment to high altitudes at the Jungfraujoch and extended the coincidence measurements to 300 m [48]. The Paris results and the Jungfrau results are plotted in Fig. 27. Auger estimated that the coincidences at 300 m required primary cosmic ray particles of 1015 eV. Among historians there has been extensive discussion as to who discovered the extensive air showers. Rossi mentioned in his memoirs [24] that when checking for accidentals as he laid out his counters on a flat plane the counts did exceed his expectation for accidentals. However he did not follow up his observation. In 1938 Kolhörster [46] made measurements similar to Auger but he did not pursue this observation. And no doubt there were others. From this author’s perspective it was Auger and his colleagues who continued his original measurements with many additional studies of the shower components. There was an international conference on the subject of cosmic rays held at the University of Chicago in June 1939. The proceedings of this conference were published in the October issue of Reviews of Modern Physics [49]. Cosmic Ray research was at the time a leading topic in physics as indicated by the prominent physicists who attended. Fig. 28 shows the group photo. In the center are Hess and Anderson. Just to the right are Bothe and Heisenberg. Also seen in the photo are Bethe, Compton, Teller, Schein, Oppenheimer, Rossi, Auger (with bow tie), and Serber. Auger gave a long report on his work following up on his pioneering work on extensive air showers [50]. This conference held just three months before the outbreak of World War II on September 1 with the Nazi invasion of Poland. For a second time with some notable exceptions research on cosmic rays ceased. During the war in Italy the physicists Conversi, Pancini, and Piccioni began the study of muon stopping in materials. If the muon was the Yukawa particle one expected that the positive muons would decay but all the negative ones would be captured. In 1947 the Italian team used a magnetic arrangement invented by Rossi [29] to select either positive or negative muons to come to rest on the absorber. For a heavy absorber only positive muons decayed as expected. However for a light carbon absorber both signs of muons decayed. This result [51] showed that the muon was weakly interacting and could not be the proposed Yukawa particle. This was an astounding result. But even before this result the Japanese physicists Sakata and Inoue [52], noting that the lifetime of the muon was longer than expected and that its interaction cross section was much smaller than expected, postulated that the muon was the decay product of the Yukawa particle. Following World War II cosmic ray physics flourished producing many discoveries of new particles. Instrumentation included cloud chambers, Geiger counters and a new technique employing photographic emulsions. In 1947 using the latter technique Lattes et al. [53] discovered the p meson (the Yukawa particle) and showed that it decayed to the muon as Sakata and Inoue had predicted. In addition a myriad of new particles were discovered in the cosmic rays in the late 40’s and early 50’s. Not all of them were convincingly established and there was much confusion. A summary of the data on the new unstable particles presented at the third Rochester Conference is shown in Fig. 29. By the early 1950’s the science of subatomic physics was initiated by cosmic ray research. The positron, muons, charged pions, K mesons, lambda, sigma, and cascade hyperons were discovered. The Rochester conference spent only half a day discussing these unstable particles. The bulk of the conference was devoted to pions and nuclear forces. Pions could be produced artificially and their interactions with protons could be studied directly. An international conference solely dedicated to the new unstable particles found in the cosmic rays was organized by Patrick
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Fig. 28. Photo of attendees of July 1995 conference on cosmic rays at the University of Chicago.
Fig. 29. Data on the new unstable ‘‘elementary’’ particles presented at the third Rochester Conference, December 1952.
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Fig. 30. Photograph of the attendees of the July 1953 conference at Bagnères de Bigorre (proceedings unpublished). A scanned version can be accessed at the Ecole Polytechnique web site, www.polytechnique.edu/accueil/l-ecole-polytechnique/histoire-et-patrimoine/les-polytechniciens-illustres.
Blackett and Louis Leprince–Ringuet. It took place in the French Pyrenees at Bagnères de Bigorre during 6 days, July 6–11. While this conference was intended to be one of a biannual series of conferences on cosmic rays, the strong will of Leprince–Ringuet forced the conference exclusively to a discussion of the new unstable particles found in the cosmic rays. Attending were nearly all of the experimenters who had produced the data. With their concentration on the narrow subject, the conferees were able to resolve many of the disagreements in the data, consolidate the solid results, and provide a list of questions to be answered by future experiments. It was a seminal conference to be compared in importance with the famous Shelter Island Conference of 1948. A detailed article on the planning and results of this conference has been published [54]. A photograph of the conferees is shown in Fig. 30. Alone in the first row is Blackett. Immediately to the left of Blackett is Leprince–Ringuet and to his left is Rossi. On Saturday afternoon Rossi presented a summary of the conference. Almost all the conclusions concerning the unstable particles turned out to be correct. The cosmic ray results defined in detail the early experiments to be carried out with the new multi-GeV accelerators – the Cosmotron at Brookhaven National Laboratory and the Bevatron at Berkeley. Already the properties of these particles were sufficiently known that Pais [55], Gell-Mann [56] and Nakano and Nishijima [57], in advance of the accelerator experiments, could predict their production in pairs and the
strangeness scheme which defined permitted and forbidden modes of production and decay. At the end of his summary Rossi added the following comment: Before concluding my remarks there is one point I would like to make which was already made in the course of this conference: it is the very close similarity between the masses of the two best established particles, I mean the charged tau particle with a mass of 970 and the h0 with a mass of 971. This looks hardly like an accident, and on the other hand it is very difficult to see how the h0 -particle could be the neutral counterpart of the tau-particle. The famous s – h puzzle which led to the discovery of parity violation was defined. At end the conference Leprince–Ringuet made some incisive comments which I quote in loose translation: If we want to draw certain lessons from this congress let’s point out first that in the future we must use the particle accelerators. Let’s point out for example the possibility they will permit the measurement of certain fundamental curves (scattering, ionization, range) which will permit us to differentiate effects such as the existence of pi mesons among the secondaries of K mesons. Finally I would like to finish with some words on a subject that is dear to my heart and is equally so to all the ‘‘old timers’’. Gradually one becomes an ‘‘old timer’’. One does not perceive the exact moment, but one day one notices that the hair is turning grey and that the youngsters that were your students are catching up with you and in some cases passing you. I can see in the audience some of the ‘‘old
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timers’’: the professors Regener, Powell, Blackett, Vallarta, Bothe, Fretter . . .. . .. . .We have to face the grave question: what is the future of cosmic rays? Should we continue to struggle for a few new results or would it be better to turn to the machines? It was precisely at this moment that the cosmic ray community divided into two groups: those who were concerned with the rich particle physics offered by the accelerators and those concerned with the origin and astrophysics of the cosmic rays. Leprince–Ringuet made his choice and moved with his students to the accelerators. Many of those concerned with the sources and astrophysics of the cosmic rays moved into space, the exception being at the energies P 1015 eV where fluxes became so low that the cosmic rays could be detected only by extensive air showers. For the past 60 years accelerators have reached progressively higher energies culminating in the LHC with collision energies of 8 TeV. A very comprehensive understanding of the basic elements of matter has been achieved including the recent discovery of a possible Higgs particle. The particle beams had shifted from the sky to the accelerators but those who made the discoveries with the vertical beams were impressive experimentalists who deciphered the complex nature of the radiation from space observed serendipitously in 1912. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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