page_title: Chicago Air Shower Array
section_title: Specifications and design
hierarchical_section_title: Chicago Air Shower Array / Specifications and design
caption_attribution_description: English: Plan view of the CASA-MIA detectors at Dugway Proving Grounds, Utah, USA.
context_page_description: The Chicago Air Shower Array was 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, under the name CASA-MIA. MIA was made of 2500 square meters of buried muon detectors. At the time of its operation in the 1990s, 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. Research topics on data from this experiment covered a wide variety of physics issues, including the search for gamma rays from Galactic sources and extragalactic sources, the study of diffuse gamma-ray emission, 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, the Dual Imaging Cherenkov Experiment and the Fly's Eye HiRes prototype experiment. CASA-MIA operated continuously between 1992 and 1999.
context_section_description: 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.
  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. 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 specifications for CASA-MIA were the following: 
  1089 scintillation detectors, spread out on a square grid of 33 x 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 acrylic scintillators; each scintillator is 61 cm x 61 cm x 1.27 cm in size and read out by a single photomultiplier tube (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 x 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).
caption: 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 x 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.
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