Cosmic ray composition studies with CASA-MIA.

Abstract

The energy spectrum and composition of cosmic rays with primary energies between $10\sp{14}$ eV and $10\sp{16}$ 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 10$\sp{15}$ 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 10$\sp{15}$ eV to $3.0\times10\sp{15}$ eV. The composition of cosmic rays is roughly half light elements (e.g. protons and Helium) and half heavier at 10$\sp{14}$ eV, and becomes increasingly heavy beginning near $5.0\times10\sp{14}$ eV and extending to 10$\sp{16}$ eV. At $10\sp{16}$ 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.