Computer Science
Scientific paper
Aug 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........10g&link_type=abstract
Thesis (PHD). THE UNIVERSITY OF MICHIGAN , Source DAI-B 59/02, p. 693, Aug 1998, 197 pages.
Computer Science
2
Scientific paper
The energy spectrum and composition of cosmic rays with primary energies between 1014 eV and 1016 eV have been studied with the CASA-MIA detector in order to provide experimental evidence to compare with competing models of cosmic ray acceleration. New measurements in this energy region are needed to understand previous contradictory experimental results. The CASA-MIA detector is a ground based air shower array, with 1089 surface detectors spaced 15 meters apart and 1024 buried muon detectors in 16 patches, used to measure the lateral distributions of the electromagnetic and muonic portions of air showers. The CASA-MIA measured differential energy spectrum is a power law with spectral indices of 2.68/pm.05 below approximately 1015 eV and 2.97/pm.05 above. A new method for measuring primary energy is derived from ground based data in a compositionally insensitive way. The 'knee' is shown to be a feature of this energy spectrum, not merely one of the size spectrum. In contrast with some previous reports, the 'knee' does not appear sharp, but rather a smooth transition over the energies from 1015 eV to 3.0×1015 eV. The composition of cosmic rays is roughly half light elements (e.g. protons and Helium) and half heavier at 1014 eV, and becomes increasingly heavy beginning near 5.0×1014 eV and extending to 1016 eV. At 1016 eV, the average composition is roughly 90% heavier than Helium. This result supports the model of cosmic ray acceleration by supernovae, possibly multiple classes of them, and the model of cosmic rays escaping from the galaxy in a charge dependent way.
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