Detection of Extensive Air Showers with HiRes
Cosmic ray experiments must overcome tremendous obstacles. The flux of particles above 1019 eV is extremely low (about 0.5 km-2yr-1sr-1), so detectors need to probe a large effective area to detect sufficient flux. This requires earthbound observatories. Consequently, the high energy particle is detected indirectly, as cosmic ray primaries entering the Earth's atmosphere interact with atmospheric nuclei to produce large cascades of relativistic secondary particles known as extensive air showers.
The Air Fluorescence Technique
Cosmic ray experiments are typically large, sparsely instrumented arrays of counters which measure the secondary particles reaching the ground. Alternatively, air fluorescence detectors like HiRes make use of the fact that the particle cascade dissipates much of its energy exciting and ionizing air molecules. The excited Nitrogen molecules fluoresce in the near UV with an emission line spectrum, where roughly 80% of the light is emitted between 300 and 450 nm.
The fluorescence light is emitted isotropically and its intensity is proportional to the number of charged particles in the shower. Air fluorescence detectors consist of arrays of telescopes that image fluorescence light from distant air showers onto arrays of photomultiplier tubes.
Both techniques have disadvantages. Detector arrays only sample the shower at the ground, at a late stage of the shower development well past the shower maximum, with uncertainties in energy scale, resolution and tails that result from the numerically challenging simulations based on unverified assumptions of interaction details.
Air fluorescence detectors image the actual shower development in the atmosphere rather than sampling the shower at ground level only, but require detailed knowledge of atmospheric transmission of light, and are presently restricted to operation on clear dark nights, or about 10% of the "on time" of a ground array.
The HiRes Detector
The HiRes experiment, a successor to the Fly's Eye experiment (which pioneered the air fluorescence technique and operated at Dugway, Utah between 1981 and 1992), consists of two sites. Comprising each of the sites are wide-angle telescopes with a mirror for light collection and a cluster of photomultiplier tubes for signal detection. Each tube has a field of view of 1°×1°.
With 22 (42) telescopes with 256 photomultiplier tubes each at the first (second) site, the full detector covers about 360° (336°) in azimuth and 3°-16.5° (3°-30°) in elevation above horizon. The two sites are separated by a distance of 12.6 km, which allows for "stereoscopic" observation of air showers, leading to a more precise geometrical reconstruction than "monocular" mode. HiRes is sensitive to showers with energies above 1018 eV.
The general scheme for monocular shower reconstruction is shown in the figure. The fluorescence light generated by the passage of an air shower is viewed by a succession of photomultipliers. Each photomultiplier has a fixed field of view and detects light from a small part of the shower trajectory (shaded). The arrival direction of the primary cosmic ray can then be determined using the photomultiplier pointing directions and triggering times.
In addition, the behavior of the light intensity along the shower trajectory allows the determination of Xmax, the atmospheric depth of the shower maximum. In stereo mode, the shower trajectory is either given by the line of intersection of the two monocular shower-detector planes, or as the result of a global fit combining the timing information of both detectors.
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