Cosmic Rays: A Way to Introduce Modern Physics Concepts Steve Schnetzer Rutgers CR Workshop May 19, 2007
Concepts Astrophysics Particle Physics Radiation Relativity (time dilation) Solar Physics Particle Detection Forefront Research Rutgers CR Workshop May 19, 2007 1
Ionizing Radiation electroscope We are surrounded by ionizing radiation What causes ionization? Is it natural radioactivity? Becquerel Curie Roentgen charged particles have electromagnetic interactions with charges stored on electroscope also ionize atoms Rutgers CR Workshop May 19, 2007 2
Discovery of the Nature of Cosmic Rays In 1912 Victor Hess road a balloon up to 17,500 feet (without oxygen) found increasing radiation at higher altitudes Radiation is coming from the cosmos! Nobel prize for Hess in 1936 Rutgers CR Workshop May 19, 2007 3
Cosmic Ray Spectrum log-log plot Powers of 10 flux/decade 1 E 2 flux falls by 30 orders of magnitude over 12 decades of energy What are the cosmic rays? Where are they coming from? Rutgers CR Workshop May 19, 2007 4
Astrophysics Rutgers CR Workshop May 19, 2007 5
The Sun Can the Sun be the source? Core temperature: 15, 000, 000 o C = 15, 000, 000 K Temperature proportional to average particle energy At room temperature, 300 K, average particle energy is 1/40 ev. Average particle energy in core of Sun 1 kev Not energetic enough by far! Solar wind 1 to 100 kev particles Rutgers CR Workshop May 19, 2007 6
Galactic Cosmic Rays Cosmic rays of energy > 1 GeV must be of galactic origin Sun is not energetic enough. Where do they come from? Prime suspect the most energetic thing in the galaxy Rutgers CR Workshop May 19, 2007 7
Supernovae Crab Nebula Gravitational collapse of a massive star A galactic supernova occurs about once every 50 years A few percent of the 10 46 Joules of energy released can account for the energy density of cosmic rays Why do cosmic rays arrive continuously? Why do cosmic rays arrive isotropically? Rutgers CR Workshop May 19, 2007 8
Fermi Acceleration Mechanism Cosmic rays do not reach full energy in the direct SN explosion SN remnants of clouds of gas and magnetic fields last for several thousand years Cosmic rays are accelerated by bouncing back and forth between magnetic clouds across the shock front Rutgers CR Workshop May 19, 2007 9
What are the Cosmic Ray Particles? Must be stable in order to get here. protons neutrons neutrinos photons nuclei (He Fe) Not neutrons neutron lifetime 15 minutes What about antiparticles anti-protons positrons anti-nuclei } not seen We live in a matter universe Rutgers CR Workshop May 19, 2007 10
Neutron Lifetime Neutron has mean lifetime τ = 15 minutes. cτ = 15 light minutes Relativistic time dilation factor γ = E mc 2 For 10 15 ev γ = 10 6 neutron travels only 30 light years Rutgers CR Workshop May 19, 2007 11
Galactic Magnetic Field Cosmic rays are contained in the galaxy through curvature in the galactic magnetic field magnetic force = centripetal force qvb = mv2 R qb = mv R = p R p = 3 10 20 qbr p = qbr p[gev/c] q[proton charge] B[Gauss] R[light years] If galactic magnetic field is 5 10 10 Gauss, what is highest energy cosmic ray that is contained? Rutgers CR Workshop May 19, 2007 12
Knee in Spectrum What is the source of the knee in the cosmic ray spectrum? Hint: Note that the contained momentum depends on the charge. Rutgers CR Workshop May 19, 2007 13
Extragalactic Cosmic Rays For energies > 10 18 ev no known energy source in galaxy AGN most likely source: Active Galactic Nuclei galaxies with super massive black holes at their center spewing out large amounts of energies in jets of particles very high energies plus weak extragalactic magnetic field may point back to source This is the energy frontier Rutgers CR Workshop May 19, 2007 14
Particle Physics Rutgers CR Workshop May 19, 2007 15
Interaction in Atmosphere Primary cosmic rays don t reach the surface of the Earth they interact with atomic nuclei in the upper atmosphere producing a shower of particles interaction in an emulsion Rutgers CR Workshop May 19, 2007 16
High Energy Particle Beam Discovery of positron 1933 anti-electron Discovery of muon 1937 heavy electron Discovery of pion 1947 quark/anti-quark bound state positron in cloud chamber Discovery of kaon 1947 particle containing a strange quark Rutgers CR Workshop May 19, 2007 17
Chain of events Primary proton interacts with atomic nucleus producing several particles mostly pions (π +, π, π 0 ) Neutral pion decays immediately (10 16 s) to two photons π 0 γγ Some of the charged pions interact with atomic nuclei. Others decay ( 10 8 s) to muons π ± µ ± ν µ Muon are relatively long-lived (2 µs) and don t interact strongly. These are the particles that reach the Earth s surface. The photons from π 0 decay produce electronpositron pairs which in turn radiate photons. This forms an electromagnetic shower that contains most of the energy. Rutgers CR Workshop May 19, 2007 18
At the Surface of the Earth Mostly muons (and neutrinos) Rule of thumb: 2 muons per second through outstretched hand Irreducible source of natural radioactivity 28 mrem/year at sea level twice as much if you live in Denver Rutgers CR Workshop May 19, 2007 19
Collision Energy Colliding beams: E = 2E beam Beam on fixed target: E = 2E beam m LHC: 7 TeV on 7 TeV E = 14 TeV (1 TeV = 10 12 ev) corresponds to a beam energy on fixed proton target of: E beam = E2 2m = 1017 ev The world is safe! Anything that might be produced in LHC collisions has already happened many times in the atmosphere. Rutgers CR Workshop May 19, 2007 20
What about the Future? If GZK cutoff, may have never occured in our galaxy before Watch out! Tevatron LHC PeV Collider RHIC Hunter College Colloquium, November 10 51 Rutgers CR Workshop May 19, 2007 21
Relativity Rutgers CR Workshop May 19, 2007 22
Time Dilation Lifetime of muon at rest: τ = 2 µs Maximum distance it can travel in 2 µs: cτ = 600 m Height at which muons created: L 20 km. Time needed to reach surface of Earth: T = L/c 50 µs Relativistic time dilation: for muon of energy E, lifetime is: T = γτ γ = E mc 2 Rutgers CR Workshop May 19, 2007 23
Muon Decay If you can stop some of the cosmic ray muons you can measure their decays. cosmic ray muon By quantum mechanics, a bunch of identical muons will not decay at the same time. Their decay times will follow an exponential distribution e t/τ where τ = 2.2 µs is the mean lifetime. Rutgers CR Workshop May 19, 2007 24
Solar Physics Rutgers CR Workshop May 19, 2007 25
Solar Wind Large steam of electrons and protons, E 1 kev emanating from the Sun. Earth s surface shielded by atmosphere and magnetic field. Particle s trapped and spiral along magnetic field lines. Ionization leads to Aurora s. Radiation from solar wind presents major dilemma for Mar s astronauts. Rutgers CR Workshop May 19, 2007 26
Heliopause Solar wind shields the solar system from the lower energy galactic cosmic rays. The heliopause is the boundary between the solar wind and the interstellar medium ( 200 AU) Also termination shock at 80 AU where solar wind becomes subsonic. Little understood region The voyager spacecraft have passed the termination shock and are on their way to the heliopause Rutgers CR Workshop May 19, 2007 27
Fun Facts Rutgers CR Workshop May 19, 2007 28
Cosmic Rays and Lightning Cosmic rays are the source of ionization that charges up the atmosphere Positive and negative ions have different mobilities leading to charge separation and an electric field (100 V/m). For reasons that are not completely understood, the charge separation and electric field are dramatically increased during a thunderstorm (10,000 V/m). Cosmic rays may also provide the trigger that causes the lightning discharge. Rutgers CR Workshop May 19, 2007 29
Cosmic Rays and Cloud Formation Some evidence that cosmic rays may play a role in cloud formation. Maybe the atmosphere is sort of like a large Wilson cloud chamber. Ionization by cosmic rays may help provide a seed for could nulceation. Relevance for global warming? Rutgers CR Workshop May 19, 2007 30
Cosmic Rays and Archeological Dating Cosmic rays produce radioactive carbon-14 in the atmosphere n + 14 N p + 14 C Carbon-14 half-life is 5,000 years Ratio of carbon-14 to carbon-12 tells how long ago animal or plant lived Rutgers CR Workshop May 19, 2007 31
Solar cycle Solar wind shields the Earth from galactic cosmic rays Anti-correlation between solar activity and flux of galactic cosmic rays. Variation in concentration of carbon-14. Also 11-year cycle Rutgers CR Workshop May 19, 2007 32
Cosmic Ray Detection Rutgers CR Workshop May 19, 2007 33
The QuarkNet Detector Package Rutgers CR Workshop May 19, 2007 34
Scintillator Paddles Rutgers CR Workshop May 19, 2007 35
Photomultiplier Rutgers CR Workshop May 19, 2007 36