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Cosmic Rays -- Overview

Cosmic Rays -- Overview. Outside earth’s atmosphere, these are charged particles, 86% protons These primary cosmic ray particles interact with air high in the atmosphere (15 km), creating showers of secondary particles By time the secondaries reach sea level, the muons dominate the flux

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Cosmic Rays -- Overview

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  1. Cosmic Rays -- Overview • Outside earth’s atmosphere, these are charged particles, 86% protons • These primary cosmic ray particles interact with air high in the atmosphere (15 km), creating showers of secondary particles • By time the secondaries reach sea level, the muons dominate the flux • The detectable (vertical) rate at sea level is 1/cm2/min (e.g. in CRDs) • Outside the solar system, CRs have an energy density of 1 eV/cm3 • Starlight: 0.6 eV/cm3 • 1 eV = 1.6  10-19 J • 30% of natural radiation • Provide charge and seeds for lightning from QuarkNet CRD manual QuarkNet, R Frey

  2. composition of primaries • 90% protons (not anti-protons) • The remainder mostly follow solar system abundances (eg meteorites and solar photosphere) • Spallation of O and C nuclei, for example, create more Li, Be, B than is typical of solar system cosmic rays solar system QuarkNet, R Frey

  3. energy and origin of primaries • Steeply falling, power law energy spectrum • For E1014 eV, spectrum and flux are consistent with acceleration by shock waves from supernovae (“Fermi acceleration”) • For larger energy, mechanism is unknown (black hole at galactic center??) • For E1019 eV, protons would not be captured by galactic magnetic field (310-10 T) • So higher energy CRs must be extra-galactic... but GZK cutoff... a mystery

  4. cosmic rays at earth’s surface (secondaries) • Primaries interact at z15 km, producing a shower of (mostly) short-lived particles. • e.g. pion () lifetime is 2.610-8 s • The long-lived secondaries are: • e, photons: mostly absorbed • neutrinos (): practically invisible • muons (): =lifetime is 2.210-6 s • Without time dilation, muons would travel dc=660 m, with a survival fraction e-0.66/15 10-10 • Instead, for a 10 GeV muon, =10/0,1=100, then mean distance is 66 km. (OK) • Detectable (vertical) flux is 1/cm2/min • Simulated event: start...end • Movie • Sim tool

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