An Experiment to Measure the Air Fluorescence Yield in Electromagnetic Showers

Fluorescence in Air from Showers (FLASH) or E-165 is an experiment to be carried out at the Stanford Linear Accelerator Center (SLAC). It aims to measure the total and the spectrally resolved air fluorescence yield of electromagnetic showers with an accuracy of better than 10%. The experiment explores the energy dependence of the yield down to the lowest energies effective in air showers, ∼ 100 keV. For this experiment, the SLAC linac will deliver a 28.5 GeV electron beam at intensities of 107 to 109 particles per pulse. A thin target run will allow us to measure the fluorescence yield per beam track depending on pressure and atmospheric impurities. Later, the interaction of the beam in a thick target will mimic the distribution of electron energies found deep in cosmic ray induced air showers. In June 2002, a test experiment at SLAC measuring the total fluorescence of air and nitrogen between 300 and 400 nm in a thin target mode has proven the feasibility of such an experiment. Results of this test run will be presented