Air Fluorescence
The term "fluorescence" refers to the process by which atoms absorb photons of one wavelength and emits photons at a longer wavelength. A common application of this effect is in nearly every household around the world: fluorescent lights. Fluorescent lights were first introduced in 1939 at the World's Fair in New York City by Interelectric corporation and offered to the public in 1940. An electric current passes through an elongated bulb, colliding with mercury atoms. The collision process excites the mercury atoms, which then emits ultra-violet (UV) light. This emission is actually referred to as "luminescence" or "scintillation". These UV photons are then absorbed by the phosphor coating of the bulbs, which re-emits in the visible. It is of course the re-emission process which is properly called "fluorescence".
The passage of charged particles in an extensive air shower through the atmosphere results in the ionization and excitation of the gas molecules (mostly nitrogen). Some of this excitation energy is emitted in the form of visible and UV radiation.
Rigorously speaking, this is a "luminescence" process analogous to the emission by mercury in a fluorescent light. Much to the horror of the optical physicists, the name "Air Fluorescence" has been adopted by the astrophysics community to describe the scintillation light from extensive air showers. This misuse of the term is in part due to the apparent similarity to the workings of a fluorescent light. On the positive side, this usage makes it easy to distinguish between a fluorescence detector from a scintillation detector (the latter is the name commonly used for desk-top particle detection devices made from inorganic salts or organic plastics).
The Origin of the Fluorescence Technique
Air fluorescence was studied in the early 1960's by the Los Alamos National Laboratory (LANL), then called the Los Alamos Scientific Laboratory. The technique was investigated as a method for detecting the yield of nuclear explosions in atmospheric tests.
Many charged particles are expelled from a nuclear explosion, and these particles will also produce scintillation light as they pass through air. The amount of light collected can then be use to estimate the total amount of energy released from the device.
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.
University of Utah: 1976
In 1976, Physicists from University of Utah were the first to detect fluorescence light from cosmic ray air showers. Three prototype modules were used in a test at Volcano Ranch near Albuquerque, New Mexico.
Each prototype module contained a 1.8m diameter mirror for light collection, with 14 PMT's at the focal plane. Each PMT covers a solid angle about 0.008 steradians (~5° by 5°) in the sky. The large mirrors provided a 20-fold increase in the light collection area over that of the lenses used in the Cornell detector. The clear desert air also provided much improved visibility over the Cornell experiment.
Volcano Ranch was chosen for the test because the site also hosted a large ground array which had been operated since 1958 by a group from MIT under the leadership of John Linsley and Livio Scarsi. On Thanksgiving Night in 1976, the prototype fluorescence detectors were able to observe air showers in coincidence with the ground array.
It should be noted that the Volcano Ranch Ground Array itself was one of the great pioneering efforts in UHE cosmic ray physics. It was known as the "Desert Queen" within MIT. In 1961, the Volcano Ranch Array reported the detection of a cosmic ray air shower with a measured energy of 1020 eV. The significance of such a cosmic ray shower was not realized until after the discovery of the Cosmic Microwave Background Radiation (CMBR) in 1965 by Penzias and Wilson.
Selected References
- Bunner, A. N., Cosmic Ray Detection by Atmospheric Fluorescence, Ph. D. Thesis, Cornell University (1967).
