CosmicCosmic RaysRays II Cosmic rays continually bombard the Earth. In fact, about 100 000 cosmic rays pass through a person every hour! AstroparticleCourse 1 CosmicCosmic RaysRays II Cosmic rays continually bombard the Earth. In fact, about 100 000 cosmic rays pass through a person every hour! AstroparticleCourse 2 CosmicCosmic RaysRays II Cosmic rays continually bombardWhere the Earth. do they come from? In fact,How about are 100 they 000 accelerated to cosmic rayssuch highpass energies? through a person every hour! AstroparticleCourse 3 CosmicCosmic RaysRays II TheThe discoverydiscovery ofof cosmiccosmic raysrays CosmicCosmic rayray andand particleparticle physicsphysics CRCR deflectionsdeflections inin magneticmagnetic fieldfield CRCR fromfrom thethe SunSun ShowerShower theorytheory AstroparticleCourse 4 SomeessentialSomeessential bibliographybibliography • Cosmic rays: A dramatic and authoritative account by Bruno Rossi • Cosmic Rays and Particle Physics , Thomas K. Gaisser • Origin and propagation of Extremely High Energy Cosmic Rays , P. Bhattacharjee & G. Sigl, Phys. Rept. 327 (2000) 109. • Observation and implications of the ultrahigh-energy cosmic rays , M. Nagano & A.A. Watson, Rev. Mod. Phys. 72 (2000) 689. AstroparticleCourse 5 JustJust beforebefore …… When scientists first started studying radiation in the early 1900s, they found 3 different types of rays: • α rays: turned out to be Helium nuclei • β rays: turned out to be electrons and positrons • γ rays: turned out to be e.m. radiation Of the known radiation, the one emitted by radioactive substances had the highest energies (MeV). Cosmic ray physics had to involve much greater energies, till 10 20 eV! AstroparticleCourse 6 ThediscoveryThediscovery “At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 7 ThediscoveryThediscovery “At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 8 ThediscoveryThediscovery “At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 9 AtmosphericAtmospheric depthdepth When comparing radiation absorbers of different substances, it becomes necessary to consider the density as well as the thickness of the absorber. Thus, it is customary to define an ab- sorber not by its geometrical thickness, but by the mass of a column of unit cross sectional area. This quantity – the mass per unit area – is usually measured in grams per square centimeters (g/cm 2). For an absorber of constant density, the mass per unit area is just the product of its thickness and its density: so, it’s like a length which takes into account the density . The mass per unit area of the atmosphere above a given level is known as atmospheric depth . AstroparticleCourse 10 NewNew particlesparticles Colombo, searching for a new route to India, discovered America. In the same way physicists, searching for a solution to the cosmic ray puzzle, discovered a zoo of new particles, opening an entirely new field of research: at the beginning, cosmic ray physics and elementary particle physics were strictly connected. The instrument which made possible these discovers is the cloud (or expansion ) chamber , invented by Wilson in 1911. Photon conversions γ→e+ e− e+ e- Photo of α-particles emitted by radioactive source AstroparticleCourse 11 CloudCloud chamberchamber The cloud (or expansion ) chamber was invented by Wilson in 1911. The expansion of the gas in the chamber causes condensation around the ions present, producing a visible track along the trajectory of a charged particle. However, to be detected, the particle must traverse the chamber at some time during the so-called expansion phase: so the chamber, in its early version, was sensitive for a period of about 0.01 second at each expansion. A major technical achievement was the counter- controlled chamber, which was triggered by Geiger- Müller counters when they were hitted by a CR particle (Blackett & Occhialini, 1932). For a given velocity, the density of ions per unit length increases with increasing charge of the initial particle. For a given charge, it decreases with increasing velocity. The ion trail of smallest possible density is one left by a singly charged particle moving at nearly the velocity of light ( minimum-ionizing particle ). AstroparticleCourse 12 AnAn elementaryelementary zoo:zoo: thepositronthepositron Anderson,Anderson, 19321932 The positron in the figure is identified as the particle that enters the cloud chamber from below and curves sharply to the left after traversing the lead plate. At first Anderson thought the positive particles were protons. But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one. AstroparticleCourse 13 AnAn elementaryelementary zoo:zoo: thepositronthepositron Ion density in multiples Anderson,Anderson, of the 19321932 density of a minimum-ionizing The positron in the figure is particle identified as the particle that enters the cloud chamber from below and curves sharply to the left after traversing the lead plate. At first Anderson thought the positive particles were protons. But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one. Magnetic rigidity AstroparticleCourse 14 AnAn elementaryelementary zoo:zoo: thethe muonmuon AndersonAnderson && NeddermayerNeddermayer ,, 19371937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays. AstroparticleCourse 15 AnAn elementaryelementary zoo:zoo: thethe muonmuon AndersonAnderson && NeddermayerNeddermayer ,, 19371937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays. AstroparticleCourse 16 AnAn elementaryelementary zoo:zoo: thethe muonmuon AndersonAnderson && NeddermayerNeddermayer ,, 19371937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays. Electron energy losses AstroparticleCourse 17 AnAn elementaryelementary zoo:zoo: thethe muonmuon AndersonAnderson && NeddermayerNeddermayer ,, 19371937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the Photograph by Blackett interaction of primary cosmic rays. and Occhialini AstroparticleCourse 18 AnAn elementaryelementary zoo:zoo: thethe muonmuon AndersonAnderson && NeddermayerNeddermayer ,, 19371937 Physicists observed that cosmic rays contained a soft and hard component; the particlesThecircleis of the the latter could penetrate asresult much of theas 1 m of lead. They couldmeasurement not be e+- e-, since their estimatedrelativeto energy thetrack should have been absurd,in figure. and their energy losses did