The Chicago Air Shower Array (CASA) was a significant ultra high high-energy astrophysics experiment operating in the 1990s. It consisted of a very large array of scintillation detectors located at Dugway Proving Grounds in Utah, USA, approximately 80 kilometers southwest of Salt Lake City. The full CASA detector, consisting of 1089 detectors began operating in 1992 in conjunction with a second instrument, the Michigan Muon Array (MIA), under the name CASA-MIA. MIA was made of 2500 square meters of buried muon detectors. At the time of its operation, CASA-MIA was the most sensitive experiment built to date in the study of gamma ray and cosmic ray interactions at energies above 100 TeV (10<sup>14</sup> electronvolts). Research topics on data from this experiment covered a wide variety of physics issues, including the search for gamma rays from Galactic sources (especially the Crab Nebula and the X-ray binaries Cygnus X-3 and Hercules X-1) and extragalactic sources (active Galactic nuclei and gamma-ray bursts), the study of diffuse gamma-ray emission (an isotropic component or from the Galactic plane), and measurements of the cosmic ray composition in the region from 100 to 100,000 TeV. For the topic of composition, CASA-MIA worked in conjunction with several other experiments at the same site: the Broad Laterial Non-imaging Cherenkov Array (BLANCA), the Dual Imaging Cherenkov Experiment (DICE) and the Fly's Eye HiRes prototype experiment. CASA-MIA operated continuously between 1992 and 1999. In summer 1999, it was decommissioned.

Specifications and design

thumb|Plan view of the CASA-MIA detectors at Dugway Proving Grounds in Utah, USA. CASA consisted of 1089 scintillation detectors on a square 15 m × 15 m grid. MIA consisted of 1024 scintillation counters arranged in 16 patches. Five small Cherenkov telescopes were co-located at the site and used for angular alignment.

CASA was built to study the possibility of astrophysical sources of ultra high energy (UHE, E > 100 TeV) gamma-ray emission (see Science below). Gamma rays at these energies interact in the Earth's atmosphere to create an extensive air shower that propagates to the Earth's surface. At the surface, the shower consists predominantly of electrons/positrons, low-energy gamma rays, muons, and some hadrons, with a typical footprint on the ground of 50–100 m. (There is also a component of Cherenkov radiation reaching the ground that can be recorded by imaging atmospheric Cherenkov telescopes). An air shower array is a distributed set of particle detectors (scintillation detector, water Cherenkov detectors, etc.) spread out on the ground to record the passage of the shower particles. The primary particle direction is estimated from the relative arrival time of the shower hitting each detector; the primary particle energy is estimated from the number of particles recorded in each detector and from the lateral distribution of those measurements.

thumb|Aerial view of the Chicago Air Shower Array (CASA) and associated detectors at Dugway Proving Grounds, Utah, USA. The CASA scintillation detectors are the white square boxes laid out on a 15-meter grid spacing. At the center of the array (left of center in this image) is the Fly's Eye II detector.

Prior to CASA, air shower arrays were typically modest in size, typically consisting of 50-100 detectors covering an area of around 50,000 square meters. The plan for CASA was to build a much more sensitive experiment that would be much larger in size, use state-of-the-art electronics, and be coupled with a large array of muon detectors (MIA). The expectation was that showers initiated by gamma rays would contain far fewer muons compared to showers initiated by cosmic rays. The original plan was for an array of 1064 detectors, but the number was subsequently increased to 1089.

Some of the key design features CASA-MIA were the following:

  • 1089 scintillation detectors, spread out on a square grid of 33 × 33 detectors, with a detector spacing of 15 m, covering a total area of 230,000 square meters.
  • A CASA detector consisted of four separate scintillation counters; each counter consisted of a piece of acrylic scintillator 61&nbsp;cm × 61&nbsp;cm × 1.27&nbsp;cm in size and read out by a single photomultiplier tube (PMT, either Amperex 2212 or EMI 9256).
  • Each CASA detector contained a local high voltage module and a custom-made electronics board that allowed each detector to take data independently of other detectors.
  • The CASA detectors were connected to a central controller via a rib-spine network consisting of coaxial cables with three functions: trigger request, trigger acknowledge and Ethernet.
  • The muon array (MIA) consisted of 1024 scintillation counters, each of size 1.9 m × 1.3 m. The muon counters were arranged in 16 patches of 64 counters each and were buried beneath 3 m below the surface. Signals from the MIA counters were run under the ground to a central trailer where relative arrival times were measured by conventional LeCroy 4290 time-to-digital converters (TDCs).

The trigger and data-acquisition sequence for CASA was complex because of the distributed electronics; it worked as follows: 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 Crab Nebula, and known high-energy active galactic nuclei. For these sources, the CASA-MIA limits were typically two to three orders of magnitude lower than the flux levels reported by the previous instruments. Searches were also made for transient and period emission from point sources and a general survey of the overhead sky was also carried out.

  • Diffuse gamma-ray sources: the rejection power of the large muon array allowed CASA-MIA to study diffuse gamma-ray sources with great sensitivity. The most significant result came from a search of diffuse isotropic emission, which provided a limit on the electromagnetic fraction of the cosmic rays at a level less than 2 × 10<sup>−5</sup> at the highest energies. Another significant result came from a study of diffuse emission from the Galactic plane. A separate study searched for bursts from arbitrary directions in the sky to constrain short timescale cosmic events, such as the explosions of primordial black holes.
  • Cosmic ray physics: with its large and uniform air shower array, couple with a large muon detector, CASA-MIA had good capability to making measurements of the properties of the ultra high energy cosmic rays. The electron and muon shower size distributions (determined from CASA and MIA, respectively) were used to measure the cosmic ray energy spectrum between 100 and 10,000 TeV. The CASA-MIA results showed a smooth steepening of the spectrum, in contrast with some earlier experiments that reported a sharper feature (known as the "knee"). CASA-MIA measurements of the cosmic ray composition were made from a combined fit to the surface and muon detector data and indicated a mixed composition at lower energies (below 1,000 TeV) that evolved smoothly to a heavier composition at energies approaching 10,000 TeV. A separate, and complementary, measurement of the cosmic ray composition was made by the BLANCA instrument that operated in conjunction with CASA-MIA and used the lateral distribution of the Cherenkov radiation in air showers.

Scientific collaborations

The CASA project was conceived by James W. Cronin and the design and construction were carried out by a team of scientists, engineers and technicians in the Enrico Fermi Institute of the University of Chicago (see