Chicago Air Shower Array

Specifications and design

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 (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 cm × 61 cm × 1.27 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 PMT signals in each counter are each sampled by a low-level and high-level discriminator. The low discriminator level is set to around 0.1 of the signal from a typical minimum-ionizing particle; the high discriminator level is set to around three times that of the low level.
• A detector with two or more counters firing the high-level discriminator in 30 nsec is "alerted"; a detector with three or more counters firing the high-level discriminator if 30 nsec is "triggered.
• Alerted stations carry out local data acquisition tasks and inhibit further triggering of the station. Time-to-voltage converters on the local electronics board hold the four relative times of the four counters in a detector (determined by the crossing time of the low-level discriminator) and four times corresponding to the times of arrival of pulses sent by the neighboring four detectors, if they have been alerted. Sample and hold circuits record the four charges corresponding to the integrals of the PMT signals from each counter. Alerted stations wait for 10 μsecs for a trigger acknowledge signal from the central station; if no signal is received, their data are discarded.
• Triggered stations place a fast current pulse (5 mA, 10 μsec duration) onto the rib trigger request coaxial cable (RG-58, 50Ω); these signals are propagated to a central trigger box via a repeater at the rib/spine junction and a spine trigger request coaxial cable (RG-8, 50Ω).
• The entire array is triggered when three trigger request levels are received by the central trigger box. Then, a fast signal (12V, μsec duration) is placed on the trigger acknowledge coaxial line where it is propagated back to each station via the rib-spine network. Upon receipt of a trigger acknowledge signal, alerted stations digitize their eight times and four charges through a multiplexer and a 10-bit analog-to-digital converter (ADC). The digitized data are stored in a memory buffer under the control of a microprocessor (Intel 80186). The dominant deadtime for the array is when data are digitized (approximately 0.5 msec).
• Periodically (typically every 30 sec), the station electronic boards receive a command over Ethernet to transmit their data to a central computer (DEC μVAX III+). Each board switches their memory buffer and continues to accumulate data; the previous recorded data are transmitted over the rib-spine network to the center where they are recorded on disk.

CASA, and its associated muon array MIA, achieved excellent performance and was the state-of-the-art in air shower experiments in the ultra high energy band for a considerable period of time after its operational period in the 1990s. Only in the late 2010s have experiments such as the Tibet Air Shower Array and the High Altitude Water Cherenkov Experiment surpassed CASA-MIA in sensitivity at energies above 100 TeV. The median gamma-ray energy for a source passing near zenith was 115 TeV. The gamma-ray angular resolution varied with the size (number of particles) in the detected shower and was 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.