The Discovery of Cosmic Rays

Transcription

1 The Discovery of Cosmic Rays Centenary Symposium 2012 Denver 27 June 2012 P. Carlson, Stockholm

2 Pre 1900 Discharge, ionization, fundamental discoveries in physics A mysterious penetrating radiation An extraterrestrial radiation finally discovered by Hess, confirmed by Kolhörster. Hess Nobel Prize World War I. Research stopped Millikan s measurements; extraterrestrial cosmic rays generally accepted Experiments using the Earth s magnetic field and absorption measurements. Cosmic rays are found to be mostly positive particles 1940s Discovery milestones Cosmic rays are mostly protons

3 Memoires de l Académie Royale 1785, p Charles-Augustin de Coulomb

4 18th and 19th centuries 1785 Coulomb: Spontaneous discharge 1835 Faraday: Confirmes discharge 1879 Crookes: Discharge rate is reduced with reduced pressure. Ionized air the cause.

5 1900: A stream of milestone discoveries set the scene Thomson: The electron Röntgen: X-rays Becquerel: Radioactivity Curie s: New radioactive elements Radiation produces ionization!

6 Contributors and experiments Wilson McLennan and Burton Rutherford and Cooke Elster and Geitel Eve Pacini Bergwitz Wulf McLennan and Macallam Gockel Mache Strong Electroscope improvements Experiments with shields Experiments on towers, in tunnels, on sea The instrument: an electroscope in a closed vessel

7 Elster and Geitel 1900 Experiments on electroscopes gradually losing charge. After experiments on conduction in air and gases they could conclude that free ions exist in the atmosphere

8 Wilson In air few ions per second and cm 3 are produced continually. What could produce that? Wilson 1901: Maybe a radiation from outside our atmosphere? To test this idea an electroscope was put in the Caledonian railway tunnel near Peebles. Wilson was a student of J.J. Thomson and received the 1927 Nobel Prize.

9 Wilson After experimenting with a gold leaf electroscope, Wilson concludes 1901 It is unlikely, therefore, that the ionization is due to radiation which has traversed our atmosphere; it seems, as Geitel concludes, a property of air itself It is assumed that the ionization is caused by the newly discovered X-rays or gamma-rays coming from outside the electroscope vessel

10 Status 1909, important review by Karl Kurz Three possible sources for the penetrating radiation: Radioactivity from the Earth s crust Radioactivity in the atmosphere An extraterrestrial radiation, possibly from the sun Kurz concludes from the ionization measurements in the lower part of the atmosphere that an extra-terrestrial radiation was unlikely. It was generally assumed that large part of the radiation came from radioactive material in the crust.

11 In action on penetrating radiation: Wulf, Gockel, Pacini, Hess, Kohlhörster Improvements and experiments: Electroscope improvements Experiments on sea, in sea, on Eiffel tower, with balloons

12 Wulf electroscope 1909 Zinc cylinder 17 cm dia Silicon glass wires Microscope Mirror Nacontainer Volume approx. 2.7 litres, cap. 1.3 pf. Ionization rate of 0.84 ionpairs/ml/s gave a potential reduction of 1 V/h.

13 Theodore Wulf ( ), German scientist and a Jesuit priest, active in Holland, visits friends in Paris easter He brings his electroscope and climbs the Eiffel tower... (Phys. Inst. Des Ignat.-Koll., Valkenburg, Holland) Expected with an 80 m absorption length was a few percent of the radiation at ground. Results requires another source for the gamma-radiation or a significantly weaker absortion of gamma-rays.. OR? Th. Wulf Phys. Zeitschr. 11, 811 (1910)

14 OR Is the radiation coming from the tower structure?

15 Albert Gockel First balloon flight 1910 Results inconclusive Ionization Altitude (m)

16 June 1911 With an electroscope 3 m deep in the sea at Livorno and Bracciano Pacini finds a significant 20% decrease in the radiation. He concludes in the Nuovo Cimento article (translated from italian): Domenico Pacini..a sizable cause of ionization exists in the atmosphere, originating from penetrating radiation, independent of the direct action of radioactive substances in the soil Pacini made important contributions that were not fully appreciated

17 Victor Hess, discoverer of cosmic rays Victor Hess PhD 1906, Graz Nobel Prize 1936 Austrian alps cosmic ray station Ultrastrahlenforschung

18 HESS flight Solar eclipse 1912 HESS research Discrepancy between Eve s absorption estimates and Wulf s results Early g radium, 2.6x10 10 per second (!) Measurements up to 90 m from source Confirmed earlier results if ionization comes from the crust it should decrease fast with hight October 1911 Balloon flight during night. At m altitude no difference in ionization day-night 12 April 1912: Solar eclipse No reduction in ionization during eclipse 7 August 1912 Discovery flight

19 Hess 7 August th flight

20 Hess 7th flight 7 August 1912 Following Elbe in the Bohemian (Böhmen) countryside. Diplomarbeit Georg Federmann Institut für Radiumforschung und Kernphysik Wien, 2003

21 Data 7th flight Diplomarbeit Georg Federmann Institut für Radiumforschung und Kernphysik Wien, 2003

22 Ionization as function of altitude Hess August 1912 Kolhörster

23 V.F. Hess Phys. Zeit. 13(1912)1804 Reported at a meeting in Münster, September 1912 The results of the present observations seem to be most readily explained by the assumption that a radiation of 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. (Transl. A.M Hillas, Cosmic Rays, Pergamon 1972)

24 Hess 1952

25 In action on penetrating radiation: Wulf, Gockel, Pacini, Hess, Kolhörster Improvements and experiments: Electroscope improvements On sea, in sea, on Eiffel tower, with balloons Common view : There is a radiation coming from outside the earth But: Not everybody believed Per an Carlson external Denver 2012 source for the radiation

26 A difficult time for Europe and for Science: World War I Also on 28th June 1914: Kolhörster measured the ionization at 9300 m!

27 World War I Scientific research almost stopped More nationalism Less communication

28 The 1920s Few measurements in Europe, focus moved to the US. An extra-terrestrial nature of the radiation still questioned. Millikan (Nobel Prize 1923) 1924 APS: The whole of the penetrating radiation is of local origin. Compton was of another opinion.

29 Robert Millikan Nobel Prize in Physics 1923 "for his work on the elementary charge of electricity and on the photoelectric effect".

30 Millikan vs Europe Millikan and Otis, PR 23,778(1924)

31 Millikan and Cameron, experiments in lakes Millikan changes mind

32 Millikan and Cameron 1926 Experiments in two Californian lakes, Muir Lake (3600 m) and Arrowhead (1500 m) The underwater rate in the lower lake corresponded to the rate about 2 m deeper in the higher lake, corresponding to the 2100 m difference in altitude Per Carlson Figure Denver adapted 2012from Hillas: Cosmic rays (1972)

33 The 1920s Few measurements in Europe, focus moved to the US. Radiation was uniform, not depending on e.g. thunderstorms or on the time of the day. An extraterrestrial nature of the radiation was still questioned. Millikan (Nobel Prize 1923) at the 1924 APS: The whole of the penetrating radiation is of local origin. Compton was of another opinion. Millikan changed mind in 1926 and coined the name cosmic rays. He suggested that the penetrating gamma-rays were birth cries of atoms in our galaxy. Focus went to properties of the cosmic rays: Radiation or particles? Source?

34 : Millikan vs. Austria/Germany Phys. Zeit. 29(1928)705

35 Uncovering the nature of the radiation Clay 1927, 1928: Ionization increased with latitude Clay s work disputed by Millikan Skobeltzyn 1927: Observes a high energy particle Bothe, Kolhörster 1929: Particles, confirmed by Rossi Compton 1932: Latitude survey showed that cosmic rays were charged particles. Millikan attacked Compton. Millikan 1933: Admitted that there was a latitud effect 1933: Three independent experiments, Alvarez and Compton, Johnson and Rossi, showed a significant eastwest effect, cosmic ray particles are positive Schein 1941: Cosmic rays mostly protons

36 Nature of the penetrating radiation Until the experment by Bothe and Kolhörster 1929 it was assumed that the radiation was gamma-radiation. Why? The gamma-rays from radioactive elements were the most penetrating emisssion Pair production was not known and the Compton cross-section decreases with energy, giving an increased penetrating power with energy!

37 Klein-Nishina formula for Compton scattering Worked out in Copenhagen 1929 under Bohr. Klein and Nishina suggested that their formula could be used to determine cosmic ray energies The total cross-section can be written in the relativistic limit σ ~ log E γ /E γ The attenuation coefficient would decrease with photon energy E O. Klein and Y. Nishina: Zeitschr. Phys. 52(1929)853, Nature 122(1928)398

38 Photon interactions in Lead 1930: Pair production not known

39 The Geiger-Müller tube H. Geiger and W. Müller 1928 A break-through i cosmic ray research!

40 The Bothe Kolhörster experiment 1929 Starting point for Rossi Compton collisions: the only known process of γ-radiation interactions. If Compton e - caused the coincidences, a small absorber should stop them. However 4.1 cm gold only reduced the rate by 24%. The radiation is corpuscular! Conclusion not accepted by Millikan Coincidence resolution by photographic recording about 0.01 s. Lead very old, thus not very radioactive! W. Bothe and W. Kolhörster Z. Physik 56(1929)751, Die Naturwiss. 17(1929)271

41 Rossi: For me, the paper by Bothe and Kolhörster opened a window upon a new, unknown territory, with unlimited opportunities for exploration. B. Rossi: ARCETRI, , in Early History of Cosmic Ray Studies, Ed. Y. Sekido and H. Elliot, (Dreidel Publ. Comp., Dordrecht 1985) East-west effect proposal 1930 Coincidence circuit 1930 Measured the east-west effect 1933 Absorption measurements Air showers Bruno Rossi

42 Arthur H. Compton Nobel Prize in Physics 1927 "for his discovery of the effect named after him"

43 The latitude effect at different altitudes 4360 m The latitude effect is greater for softer radiation. Latitude 2000 m Sea level Phys. Rev. 43(1933)387

44 Rome conference 1931

45 The Rome conference October At the invitation of Fermi, I gave an introductory speech on the problem of cosmic rays. The main thrust of this talk was to present what, to my mind, were irrefutable arguments against Millikan s theory of the birth cry of atoms. Such a brash behavior on the part of a mere youngster (I was then 26 years old) clearly did not please Millikan, who for a number of years thereafter, chose to ignore my work altogether. Rossi Physics Today October 1981

46 Bruno Rossi, Robert A. Millikan, and Arthur H. Compton at the Rome conference, October 11 18, The Rome conference 1931 marked the beginning of the historical debate about the nature of cosmic rays. B. Rossi: ARCETRI, , in Early History of Cosmic Ray Studies, Ed. Y. Sekido and H. Elliot, (Dreidel Publ. Comp., Dordrecht 1985)

47 What are the cosmic rays? 1932

48 Rossi proposes the East-West experiments during a visit to Bothe s Berlin laboratory, summer 1930 B. Rossi: Phys. Rev. 36, 606(1930)

49 The East-West effect was established 1933 Alvarez and Compton Johnson Rossi Particles are positive but Positrons? Protons? Nuclei?

50 A cosmic ray telescope consisting of two coincidence counters mounted on pivots so measurements could be made in any direction desired.

51 Cosmic rays are protons Phys. Rev. 59(1941)615

52 A 1941 balloon flight gave evidence for protons as the main component of cosmic rays Total cosmic ray intensity Intensity hard component (muons) M. Schein et al., Phys. Rev. 59(1941)615

53 1936: The Nobel Prize to Hess

54 Compton s (Nobel Prize 1927) nomination 1936 The time has now arrived, it seems to me, when we can say that the so-called cosmic rays definitely have their origin at such remote distances from the Earth that they may properly be called cosmic, and that the use of the rays has by now led to results of such importance that they may be considered a discovery of the first magnitude.... It is, I believe, correct to say that Hess was the first to establish the increase of the ionization observed in electroscopes with increasing altitude; and he was certainly the first to ascribe with confidence this increased ionization to radiation coming from outside the Earth.

55 Compton s nomination cont d Before it was appropriate to award the Nobel Prize for the discovery of these rays, it was necessary to await more positive evidence regarding their unique characteristics and importance in various fields of physics.this has now been accomplished. Studies of the magnetic latitude effect on cosmic rays have shown that they include electrical particles of much higher energy than are available from artificial sources, further that these rays come from a source which may be properly called cosmic. The usefulness of the rays has been demonstrated by the experiment which have revealed the existence of the positron.

56 10 December 1936, Stockholm

57 Conclusions The discovery of cosmic rays came after detailed studies using electroscopes on land at sea level, on sea, in sea and at high altitudes Scientists in Europe and North America participated in the work characterized by lack of communication and by nationalism caused primarily by World War I. The nature of the radiation was established in steps: first particles, then positive particles, then primarily protons Cosmic rays opened the field of elementary particles. Today cosmic ray physics is a well estabished lively field of research.

58 Thank you!