John Linsley detected first cosmic ray with an energy higher than 1020 electronvolts (eV) in New Mexico, US, in 1962. This was the highest energy cosmic ray particle ever detected at the time. He made this discovery using a ground-based array of detectors. His observations suggested that not all cosmic rays are confined to the galaxy and gave evidence for a flattening of the cosmic ray spectrum at energies above 1018 eV. This discovery clarified the structure of air showers and provided the first evidence of ultra-high energy cosmic ray composition and arrival directions.
John Linsley detects the first 10^20 eV cosmic ray