approximately 0.7 degrees for showers with the median number of particles, improving to 0.25 degrees at higher energies. The muon array provided important capability to reject background cosmic ray events; at the median energy of 115 TeV, the fraction of cosmic ray events passing the muon selection criteria for gamma rays was 0.06 (i.e. approximately 17 cosmic ray events were rejected for each one accepted). At higher energies, the background rejection power was significantly increased; for example, at a median energy of 5,000 TeV, the fraction of cosmic rays passing muon selection criteria was reduced to approximately 0.0001. History The scientific motivation for CASA came from intriguing results from several experiments in the 1980s. These experiments reported excess air shower events from the direction of two well-known Galactic X-ray binary sources: Cygnus X-3 and Hercules X-1. In 1983, the Kiel and Haverah Park experiments reported an excess of events from the direction of Cygnus X-3, where the arrival times of the events appeared to be modulated by the 4.8-hour orbital periodicity of the binary source. The statistical significance of each signal was weak (around four standard deviations above background), but the results implied that Cygnus X-3 was a luminous emitter of ultra high energy gamma rays and that, in order to do so, it must be a very efficient accelerator of high energy cosmic rays and hence it could provide a large fraction of the pervading flux of cosmic ray particles in our Galaxy. After these results, a number of groups around the world began designing, or improving, air shower arrays to make follow-up studies. One of these groups was from the University of Chicago,led by James Cronin. Cronin's idea was to build a definitive experiment that could easily verify, or refute, the results on Cygnus X-3. The experiment would be much larger (and much more sensitive) than the Kiel or Haverah Park experiments and it would use a large array of muon detectors to reject the background of hadronic cosmic ray events (i.e. protons and nuclei). (Showers initiated by gamma-ray primaries are expected to have far fewer muons than those initiated by cosmic ray primaries). Cronin assembled a team of scientists (discussed in Collaboration) to develop and construct CASA. The University of Chicago group was partnered with groups from the University of Michigan and the University of Utah, who had already constructed a muon array and smaller air shower