The very highest energy cosmic rays are so rare that a one square kilometer
area on the Earth gets hit by them less than one time per century! You need
a really big detector to measure very many of these events!
Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.