100 years since the discovery of Cosmic Rays. A brief history

1. 100 years since the discovery of Cosmic Rays. A brief history Andrea Chiavassa Universita` degliStudidi Torino Carpathian Summer School of Physics 2012 June 24 – July 7, Sinaia, Romania

2. Most of the material comes from the book:“L’enigmadeiraggicosmici” byAlessandro De Angelis. Ed. Springer (in italian)

3. The key instrument that led to the discovery of cosmic rays was the electroscope • 1785 Coulomb observed that, even if insulated, an electroscope spontaneously discharge • 1879 Crookes measured that the speed of discharge decreased if the air pressure was reduced  the cause is the ionized air

4. 1986 Becquerel discovered the spontaneous decay of radioactive elements • In the presence of a radioactive material, a charged electroscope promptly discharges • The electroscope discharge can be attributed to charged particles emitted during radioactive decays • The discharge rate of an electroscope was then used to measure the level of radioactivity • Where does these element come from?  most natural answer  from the ground

5. Experimental situation in the beginning of the XX century • 1900: Wilson and Elster & Geitel improve the technique for a careful insulation of electroscopes in a closed vessel, improving the sensitivity • 1901: Wilson’s measurements in tunnels with solid rock overhead (to check if the radiation was coming from outside) show no reduction in ionization • 1903-06: Rutherford & Cooke and McLennan & Burton show that ionization is marginally reduced when the electroscope was surrounded by metal shields. McL&B put also the electroscope in a box, and they fill it with water. Mache compares the variations of the radioactivity when the electroscope is surrounded by shields of metal with the diurnal variations; he finds no significant reduction • 1907: Strong studies radioactivity in a variety of places including (I) his lab (2) the center of a cistern filled with rain water and (3) the open air; results dominated by statistical & systematic errors • 1907-08: Eve makes measurements over the Atlantic Ocean, which indicate as much radioactivity over the centre of the ocean as he had observed in England and in Montreal • 1908: Elster & Geitel observe a fall of 28% when the apparatus is taken from the surface down to the bottom of a salt mine. They conclude that, in agreement with the literature, the Earth is the source of the penetrating radiation and that certain waters, soils and salt deposits, are comparatively free from radioactive substances, and can therefore act as efficient screens

6. Technological Improvements: Father Wulf contributions • Father Theodor Wulf, German scientist and Jesuit priest • Electroscope replacing the metallic leaves with two thin plate made by silicon glass • Sensibilty 1 Volt • He measured the radiation level on top of the Eiffel tower  the radiation was much higher than expected  anyhow he concluded that the most plausible explanation was “radiation emission from the ground”

7. These results were summarized by Kurz in a review article in 1909. • He discussed three sources of this penetrating radiation • Extraterrestrial radiation, probably originated by the sun • Earth crust radioactivity • Radioactivity coming from the atmosphere • Kurz concludes that the extraterrestrial origin was disfavored • The prevailing data interpretation was that radiation was of terrestrial origin

8. E.g. in 1911 Schrödinger (working in the Wien group) wrote that : “the third source […] is completely hypothetical and should be introduced , only if suitably justified, in case the first two hypothesis were absolutely insufficient to explain the observations” • Calculation of the radiation decrease with height were performed (e.g. A.S. Eve 1907) • Two further experimental checks were proposed. Radiation measurements: • During balloon flights • Underwater

9. Underwater measurements • DomenicoPacini, Bari University, performed such studies. • Pacini began his studies starting from the problem of the air’s ionization and became familiar with electroscopes.

10. From 1907 to 1911 he measured the ionization in different places, from sea level to mountain altitudes, and their variations due to temperature, pressure and humidity • Since end 1908, Pacini can use the destroyer “Fulmine” from the Navy • Fluctuations appear similar, which raises in Pacini doubts about the Terrestrial origin • In 1909 presenting his results at “AccademiadeiLincei” • “….in the hypothesis that the origin of all penetrating radiation is in the soil, since we must admit that, at least when it is not covered be recent waterfalls, they are emitted from the ground at a constant rate, we cannot justify the results obtained so far.” These results were cited by Cline (1910) and by Marie Curie in “Traité de radioactivité

11. The measurement in 1910(quoted by Hess) • First, two electroscopes (A and B) with walls of different thickness are cross-calibrated • Simultaneous measurements are performed at ground and on the sea’s surface, and then the instruments are exchanged • “The number of ions due to penetrating radiation on the sea is estimated to be 2/3 of that on the ground” • ``the evolution of the phenomenon on the sea surface and on the land reveals for both the same trend of the penetrating radiation during the ten days of observation […] But it is clear that in order to show the existence of a possible correlation […] a period of time longer than that I dedicated to the experiment would be needed.’’ “such results seem to indicate that a substantial part of the penetrating radiation in the air […] has an origin independent of the direct action of active substances in the […] Earth’s crust.”

12. The 1911 Underwater experiment • In June 1911 Pacini performed a series of measurements (lasting 7 days) in the Tirrenian Sea (in front of Livorno) • He measured the discharge time of an electroscope located • Sea level (300 m from the shore) • Mean decrease (8 measurements)  12.6 Volt/hour  equivalent to 11.0±0.5 ions/(sec cm3) • 3 m below sea level (water depth 7m) • Mean decrease (7 measurements)  10.3 Volt/hour  equivalent to 8.9±0.2 ions/(sec cm3) • The difference, 2.1 ions/(sec cm3), was attributed by Pacini to a radiation independent from that originated by the earth crust

13. These results were published in: “Il NuovoCimento” February 1912

14. “being the absorption coefficient of the water 0.034, we can easily deduce from the equation I/I0 = exp(-d/l), […], that, in the experimental conditions, the activity due to both the sea bed and the surface were negligible. The explanation being that, due to the absorption power of the water and to the minimal quantity of radioactive elements in the sea, an absorption of the radiation originated from the outside occurs when the instrument is located underwater. […..]it exists in the atmosphere an appreciable source of ionization, with penetrating radiation, independent from the direct action of soil radioactive substances. • Pacini thus showed, for the first time, that the experimental results cannot be explained with earth crust’s radioactivity. • Pacini was not able to exclude an atmospheric origin of the radiation (even citing Eve’s calculation showing that this contribution was negligible).

15. Scientific Balloon Flights • Balloon flights had been used for scientific purposes since their beginning. • Nov 30, 1784, 1st scientific flight in London with barometer , thermometer, hydrometer & electrometer by J.Jeffries (USA) & Blanchard • Aug. 24, 1804, Gay-Lussac & Biot (France) 4000 m hydrogen balloon • Sept. 16, 1804, Gay-Lussac 7016 m altitude, to study air properties at different p, T

16. Balloon measurements of the penetrating radiation • Bergwitz 1908 • 9 hour flight from Braunschweig. 1300 m. • 25% of the radiation at ground. 0% expected • After the flight an older university professor advised against it; he said that Bergwitz would lose his scientific reputation if he continues to pursue the idea of an extraterrestrial radiation

17. Albert Gockel (Freiburg, Switzerland) • 1908, 4 flights from 650 to 3000 m • 11 December 1909 • 15 October 1910 • 2 April 1911 • Measures with two Wulf electrometers 20 Ion pairs/(cm3s) 10 1 2 3 Altitude (km)

18. Gockel concluded that: penetrating radiation in the atmosphere independent on radioactive source 1) Gockel cites the results of Pacini

19. Discovery of Cosmic Rays: V. Hess • Victor Franz Hess: • 24.06.1883: born in castle Waldstein near to Peggau/Austria • 1901-1905: physics study Univ. Graz • 1906: PhD • 1907-1910: University Vienna at Exner • 1911-1920: Assistent of S. Meyer at the Vien Academy of Science • 1921-23: USA, director United States Radium Corp. • 1923 Returned to Graz University • 1925-1931: Professor in Graz • 1931-1937: Prof. in Innsbruck • 1936: Nobelprize • 1937-1938: Univ. Graz • 1938: emigration, USA • 1938-1956: Prof. at Fordham Univ. • 17.12.1964: died in Mt. Vernon, New York

20. Hess began his works studying Wulf’s results. • As the g absorption length was fundamental in the interpretation of the Wulf and Gockel results, Hess decided to accurately determine its value • Then he performed, in early 1912, careful calibration measurements with electroscopes improved by him using radium sources

21. Hess’s first balloon fligth • 28 August 1911 • 4 hours – up to 1070 m - 2nd flight in the night of October 12th 1911 - Both flights confirmed the Wulf and Gockel results

22. Six next flights around Wien Thin Zn foil to measure b 3 mm foil to measure g

23. 7th and conclusive flight • 7th August 1911 • ~60 km from Aussig to Pieslow • Maximum height 5200 m

24. Hess results Slightly decrease just above ground level Between 1000 and 2000m, slight increase Between 3000 and 4000m, 50% increase respect ground level Between 4000 and 5000m, radiation is more than 100% compared to ground level

25. “The results of present observations are more reliably explained assuming that an highly penetrating radiation enters our atmosphere from the top, and then produces in the lower layers part of the ionization observed in closed detectors.” • Possible reasons for the success: • Detailed knowledge of the electrometers from calibration • Improvement of the electrometers • Independent measurements with 3 electrometers: 2 g, 1b • Systematic studies at day and night • Hess published (1913) an article on PhysikalischeZeitschrift, calling this radiation “Höhenstralung” (radiation from above)

26. Confirmation by Kolhörster

28. After the World War I the main center of activities moved to US • Millikan and Bowen built a light (~200 gr) electrometer that flew in Texas up to 15000m, using data transimission technologies developed during the war. • They measured a radiation level one fourth of the one reported by Hess and Kolhörster. The difference was attributed to turnover at very high altitudes. In fact the effect is due to geomagnetic cutoff. • Millikan concluded, at the American Physical Society in 1925, that “the whole penetrating radiation is of local origin”

29. Nevertheless, in 1926, Millikan and Cameron performed radiation absorption measurements in lakes, at different depths, and at high altitudes. Concluding that the radiation was made of high energy g rays and that “these rays propagate uniformly through space in all directions” • This radiation was named Cosmic Rays • Neither Pacini, Hess and Kolhörsterresults were cited • Bergwitz, Hess and Kolhörsterwrote an article emphasizing their priority on the balloon results (Zeit. Phys. 1928)

30. Are cosmic rays charged or neutral? • Due to the penetrating power this radiation was supposed to be made by g rays • Cosmic rays intensity was measured at different geomagnetic latitudes. • Clay 1927/28. • Boat tripes: • Giava – Genova • Giava – Amsterdam • Giava - Southampton Increasing ionization with latitudes. Cannot be explained by g rays

31. Millikan argued against these results • In 1932 Compton conducted an experiment coordinating more than 70 scientists that measured the radiation independently all over the world. • In 1933 also Millikan accepted the dependence of the radiation level on latitude and that cosmic rays are charged particles. • Now the question became: are cosmic rays positive or negative particles?

32. Bruno Rossi idea: if cosmic rays are mainly positive particles interacting with the earth magnetic field they should appear mainly from west • He performed (1930) the measurement in Firenze: no conclusive results • He moved to Asmara (nearer to equator) in 1933, showing that cosmic rays are mainly composed by positively charged particles • He published the result in 1934 • Few months before Alvarez and Compton obtained the same results (citing Rossi as the first to propose the method)

33. Rossi also developed a coincidence system. During his Asmara measurements he detected a number of coincident events, between distant geiger counters, well above the one expected by accidentals. • The result was not stastically significant • These events were deeply investigated (1937) by Pierre Auger. Concluding that extensive particle showers are generated in atmosphere by the interaction of cosmic rays with air nuclei. • Extensive Air Showers

34. Early days of Particle Physics • 1933, Anderson detects antimatter using cloud chamber in the presence of a magnetic field • A cloud chamber contains a gas supersaturated with water vapor. In the presence of a charged particle, vapor condenses into droplets • The band across the middle is a lead plate, which slows down the particles. The radius of curvature of the track above the plate is smaller than that below => it must be travelling upwards (having lost energy) • From the direction in which the path curves one can deduce that the particle is positively charged • Mass can be deduced from the long range of the upper track - a proton would have come to rest in a shorter distance positron

35. 1936 Nobel prize: Hess & Anderson Hess and Anderson were awarded with Nobel Prize in 1936. Hess was nominated by Clay, Compton: • The time has now arrived […] when we can say that the so-called cosmic rays have their origin at remote distances from the Earth […] 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

36. 1933 – Blankett & Occhialini • Pair production • Yukawa predicted the existence of a particle of mass ~200 me mediating strong interactions. • 1935 – Anderson & Neddermayer • Detection of a particle with penetrating power higher than those already known and lighter than protons. “Mesotron” identified with Yukawa’s particle. • Rossi measures the decay time of this particle (~2ms) and that it decays in an electron + a neutral particle (n) • The high penetrating power was contraddicitting the Yukawa hypothesis. • We were in presence of two particles: p and m

37. 1947. Powell, Occhialini and Lattes p detection • Nuclear emulsions exposed at high altitudes (Chacaltaya, 5500 m) 1950  Powell awarded with Nobel Prize

38. 1947 • Rochester & Butler • Neutral Kaons • Mass ~500 MeV • So called “Strange particles”

39. Particle physics moved to accelerators • Cosmic rays research moved to “astrophysical” studies. • Spectrum • Anisotropies • Chemical Composition

40. 1990-2000 n oscillations • Solar n • Homestake experiment • (380 m3Perchloroethylene) • Super-kamiokande • (50000 T water, 11000 pmt) • Atmospheric n • Super-kamiokande • 2002 Nobel Prize to Davies, Koshiba and Giacconi

41. Conclusions • Cosmic Rays discovery • Victor HESS, 1912 • Conclusion of a series of studies conducted by different physicists • Birth of particle physics • Antimatter • m, p detection • n oscillations • Spectrum, Anisotropies, Chemical composition • Energies up to 1020eV