FLASH - FLuorescence in Air SHowers

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This proposal requests funds to carry out a spectrally resolved measurement of air fluorescence produced by electromagnetic showers at Stanford Linear Accelerator Center (SLAC). The group expects that this experiment will shed light on the apparent differences between the results reported by two ultra high energy cosmic ray (UHECR) experiments, HiRes and AGASA, and will provide reliable and much needed tests of shower-induced fluorescence for future UHECR experiments.The group aims to achieve an accuracy of 8% in the total fluorescence yield and in the individual spectral lines. The experiment will also verify that our understanding of the energy dependence of the yield is correct down to the lowest energies effective in showers, ~100 keV. The Final Focus Test Beam (FFTB) electron line at SLAC will be used at intensities of 10^7 to 10^9 per pulse. They propose to use both thin target and thick target approaches. A thin target run will allow them to measure the total fluorescence yield per beam track, and the spectrum, to 8%. It will also allow a precise measurement of the dependences of the yield and spectrum on pressure and on atmospheric impurities. The interaction of the beam in a thick target mimics the distribution of electron energies found deep in cosmic ray induced air showers. A beam with 10^8 electrons (~28 GeV /electron) has an energy per bunch equal to that of a 2.8 x10^18 eV shower. The superposition of the showers can be modeled by software such as EGS and GEANT. In the thick target experiment the fluorescent yield will be measured at various depths in the shower development, allowing detailed comparison with the Monte Carlo simulations. Measurements on air or nitrogen mixed with various concentrations of CO2, Ar and water vapor will also be performed.