The Highest Energy Cosmic Rays and Their Composition

Details The Highest Energy Cosmic Rays and Their Composition The Highest Energy Cosmic Rays and Their Composition

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aps..4cf..p101t&link_type=abstract

American Physical Society, Four Corners Section Fall Meeting, October 16-17, 1998 Brigham Young University Provo, Utah, abstract

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Taking advantage of the dark skies near Dugway, Utah, the University of Utah's Fly's Eye Detector has been observing cosmic rays with energies above 3× 10^16 eV by detecting light from the nitrogen fluorescence from the Extensive Air Showers produced by cosmic rays. The detection of an event measured to have 3× 10^20 eV raises the question of whether the cosmic ray spectrum continues above the energy where cosmic rays should lose energy to photopion production with the 3 K cosmic microwave background. This question will only be answered with detectors having a larger detection area (aperture). The University of Utah Cosmic Ray Group is currently constructing the High Resolution Fly's Eye Detector (HiRes) which will have at least an order of magnitude more aperture than the original Fly's Eye, enabling it to achieve higher statistics above 10^20 eV which will better characterize whether the spectrum continues past the photopion production threshold. Evidence will be presented that the composition of cosmic rays from 10^17 eV to 10^19 eV changes from primarily heavy nuclei such as iron at 10^17 eV, to more of a light nuclei dominated composition at 10^19 eV. A change in composition through this energy range could be evidence that the sources of cosmic rays shifts from galactic at 10^17 eV to extragalactic at 10^19 eV. The extragalactic origin of cosmic rays beyond 10^19 eV is also supported by the isotropic distribution of these energetic cosmic rays.

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