Studies of the energy spectrum and composition of the primary cosmic rays at 100-1000 TeV from the GRAPES-3 experiment

The composition and energy spectrum of primary cosmic rays (PCRs) are the only observables at high energies to study the nature of sources accelerating PCRs to >= 1000 TeV. These observables have been directly measured up to similar to 300 TeV with detectors aboard balloons and satellites. But measurements at >1000 TeV have to be obtained indirectly from ground-based observations of extensive air showers. However, their interpretation relies on an inadequate knowledge of hadronic interactions at >= 1000 TeV. The GRAPES-3 experiment is designed to operate at >= 30 TeV providing a sizable overlap in energy with direct measurements, enabling the selection of a suitable model of hadronic interactions at similar to 1000 TeV. We present salient features of GRAPES-3 including details of muon multiplicity distributions observed with a 560 m(2) detector as a function of shower size from an analysis of data of 545 days. These distributions were compared with expectations from Monte Carlo simulations, using some of the hadronic interaction generators in CORSIKA, to deduce energy spectra of five nuclear groups in the 100-1000 TeV region. A comparison of GRAPES-3 results with direct measurements indicates that SIBYLL provides a good description of hadronic interactions for interpreting our data. These measurements extend energy spectra and composition of PCRs that is consistent with extrapolation of direct measurements.