The Cornell Experiment
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The first attempts to observe extensive air showers by the fluorescent (more correctly: luminescent) emissions were made by a group led by Kenneth Greisen at Cornell University in the middle 1960's. This group included Dr. Alan Bunner, working then as a graduate student under Greisen. Greisen himself was the first graduate student of Bruno Rossi, one of the foremost cosmic ray physicists of the 20th Century. Rossi and Greisen both worked on the Manhattan Project in Los Alamos during World War II. Greisen was in fact an eyewitness at the Trinity test and filed an official report of his observations.
The Cornell detector images the nigh-sky using 500 photo-multiplier tubes (PMT). Each PMT corresponds to a pixel covering a solid angle of 0.01 steradian (~6° by 6°). The 500 PMT's are divided into 10 modules. Each module is equipped with a 0.1 m2 Fresnel lens seen in the foreground of the photograph above (Note: the "s" in "Fresnel" is silent, and the stress is on the second syllable).
The drawing on the right, taken from Dr. Bunner's thesis, shows a cut-away section of a detector module. The Fresnel lens is shown on the left, and the PMT's are arranged at the focal surface (roughly spherical). An optical filter is placed before the lens at the entrace aperture to reduce night-sky background and eliminate contamination from filament lamps visible near the horizon.
The Cornell detector is triggered by requiring a coincidence between any two adjoining pixels. The signals are piped to a bank of 3" cathode ray tube displays, and recorded on 70 mm film. This detector operated for several years but was not sensitive enough to detect UHE cosmic rays reliably. In particular, the 0.1 m2 lenses are too small to collect sufficient light, and the atmosphere in Up-state New York is too contaminated with water vapor and aerosols.
