Measurements of the Fluorescence Light Yield in Electromagnetic Showers

The two most common methods of determining the energy of an ultra high energy cosmic ray (UHECR) are ground arrays and fluorescence telescopes. Ground array detectors determine energy by sampling the number of shower particles arriving at the surface of the earth. In general, the more particles, the higher the energy. Fluorescence telescopes, on the other hand, determine the energy by measuring the number of ultraviolet photons produced by the electromagnetic shower produced in the atmosphere. The number of photons is related to the number of particles in the shower by the fluorescence yield (measured in photons per meter per charged particle). The Akeno Giant Air Shower Array (AGASA) and the High Resolution Flys Eye (HiRes) are the current world leading ground array and fluorescence detectors, respectively. Recent results from the two experiments indicate a significant discrepancy in the flux of cosmic rays as a function of energy [1-3]. This indicates that there may be a systematic offset in energy determination in the two techniques. The Fluorescence in Air from Showers (FLASH) experiment is an effort to reduce the systematic uncertainty in energy determination for fluorescence detectors by making an improved measurement of the fluorescence yield. This work is intendedmore » to add to the prior work of Bunner, Kakimoto et al. and Nagano et al. [4-7]. We report on the current status of the experiment.« less