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 uorescence 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 uorescence 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 uorescence detectors, respectively. Recent results from the two experiments indicate a signican t discrepancy in the ux of cosmic rays as a function of energy(1{3). This indicates that there may be a systematic oset in energy determination in the two techniques. The FLuorescence in Air from SHowers (FLASH) experiment is an eort to reduce the systematic uncertainty in energy determination for uorescence detectors by making an improved measurement of the uorescence yield. This work is intended 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. When a UHECR interacts with the earths atmo- sphere it creates an extensive air shower. This shower consists of, for the most part, electromagnetic sub- showers. The charged electromagnetic particles in- teract with the air molecules pushing them (primar- ily N2) into excited states. In turn, ultraviolet (UV) light is emitted upon deexcitation. In the FLASH experiment, we measure the number of UV photons produced per charged particle per meter. This u- orescence yield sets the energy scale for uorescence technique UHECR detectors. The FLASH experiment consists of three distinct experimental phases. First, in a proof in principle test beam experiment (T-461), we measured the total uorescence yield. Second, in the \thin target" run of the FLASH experiment (E-165) we remeasured the total uorescence yield and resolved the spectral shape of the uorescence light. Finally, during the \thick target" run, we measured the uorescence yield as a