- Slides: 19
History of Cosmic Rays • Cosmic rays were “born” when Victor F. Hess measured radiation intensity as it changed with altitude. • He determined that radiation is entering the atmosphere from above.
What are Cosmic Rays?
• Cosmic rays are not rays, they are particles. • Cosmic rays originate from outer space and impinge on Earth’s atmosphere. • About 90% are protons, ~9% alpha particles, and <1% heavier nuclei, electron, or gamma ray photons. • Cosmic ray energies range from less then 107 e. V to at least 3 x 1020 e. V
Where do they come from?
The Sun • Solar winds, flairs, and coronal mass ejections constantly emit charged particles • Relatively low-energy particles E ~ 107 e. V
Supernovae • Source of higher energy cosmic rays E ≤ 1016 e. V • At such high energies, speed of particles is essentially c • Particles are accelerated by bouncing back and forth in the magnetic field left by supernova gaining energy and becoming cosmic rays • Eventually some gain enough speed to escape
Larmor Radius • the radius of the circular motion of a charged particle in the presence of a uniform magnetic field. • Magnetic Force • Centripetal Lorentz Force
Example 15. 5. 1. (p. 552) If the Larmor radius of the “orbit” significantly exceeds the size scale for the magnetic field, the particle cannot be considered to be bound to the associated system. In interstellar space, magnetic field strengths of 10 -10 T are typical. For a proton with an energy of 1015 e. V, find the Larmor radius. r = 3 x 1016 m = 1 pc. This radius is characteristic of the size of a supernova remnant, suggesting that for energies much larger than 1015 e. V, cosmicray particles are not likely to be bound to a supernova remnant.
15. 7) The highest-energy cosmic-ray particle that had been recorded at the time this text was written was measured by the Fly’s Eye Hi. Res experiment in the Utah desert in 1991. The energy of the particle was 3 x 1020 e. V. a) Convert the energy of the particle to Joules. 1 e. V = 1. 602 x 10 -19 J 3 x 1020 e. V = 48 J
b) If the particle was a baseball of mass 0. 143 kg, calculate the speed of the ball. KE = ½ mv 2 v = √(672) = 25. 9 m/s
C) Convert your answer to miles per hour and compare your answer to the speed of a fast ball of the fastest major league pitchers (approximately 100 mph, or 45 m/s) 25. 9 m/s = 58 mph
What about higher energies? • Impossible to trace back to their origin • Cosmic-ray particles collide with interstellar gas and produce gamma rays • Neutron stars or black holes • Outside our galaxy, intergalactic shocks, supermassive black holes