A search for the galactic origins of ultrahigh-energy cosmic rays

After many decades of cosmic-ray research, the origins of cosmic-ray nuclei are still unknown. Because interstellar magnetic fields deflect charged particles, measurements of the incident directions of cosmic-ray nuclei do not reveal the sites of their production. Therefore, direct evidence of the sites of cosmic-ray acceleration requires detection of neutrally-charged particles, such as gamma rays, produced by the interactions of accelerated cosmic rays with other nuclei near the cosmic-ray accelerators.; It is generally accepted that supernova remnants predominantly accelerate Galactic cosmic rays at least up to energies {dollar}sim{dollar}10{dollar}sp{lcub}14{rcub}{dollar} eV. No other single known class of objects can explain the properties of the cosmic rays observed at Earth.; Pulsars may accelerate some fraction of the galactic cosmic-ray electrons and nuclei. Pulsars are observed to accelerate particles to energies of at least {dollar}sim{dollar}10{dollar}sp{lcub}12{rcub}{dollar} eV and are the strongest galactic gamma-ray sources at these energies and at energies {dollar}sim{dollar}10{dollar}sp8{dollar} eV.; This thesis describes searches for evidence of gamma-ray emission from five supernova remnants and from the Geminga pulsar at energies {dollar}sim{dollar}10{dollar}sp{lcub}14{rcub}{dollar} eV using the data set of the CYGNUS-I extensive air-shower array.; The analyses of the five supernova remnants, which are positionally coincident with gamma-ray sources observed with the EGRET detector at energies {dollar}sim{dollar}10{dollar}sp8{dollar} eV, reveal no evidence of gamma-ray emission at energies {dollar}sim{dollar}10{dollar}sp{lcub}14{rcub}{dollar} eV. A comparison of the gamma-ray flux upper limits from the CYGNUS-I data to the fluxes measured with the EGRET instrument suggests that the integral cosmic-ray spectra of the five supernova remnants may soften by about 10{dollar}sp{lcub}14{rcub}{dollar} eV or may be steeper than an {dollar}Esp{lcub}-1.3{rcub}{dollar} power-law distribution.; The analysis of the Geminga pulsar reveals no evidence of either pulsed or unpulsed gamma-ray emission from this object at energies {dollar}sim{dollar}10{dollar}sp{lcub}14{rcub}{dollar} eV. This result is consistent with present models of particle acceleration by pulsars.