Absolute Fluorescence Spectrum and Yield Measurements for a wide range of experimental conditions

The fluorescence yield is the key parameter that allows fluorescence telescope experiments to estimate the air shower energies from the number of UV-fluorescence photons detected. This fluorescence emission is induced by the energy loss in the air medium of the secondary charged particles dominated by the electron/positron component. The fluorescence emission is a line spectrum, of which the line at 337 nm is the strongest. Actual fluorescence telescopes measure the integral of this spectrum in the UV range. The process of photo-deexcitation of the Nitrogen involved in the UV emission is dependent on the pressure, temperature and humidity and requires dedicated measurements at various thermodynamic states of air representative of the atmospheric conditions. Most of the previous experiments have performed measurements on a subset of the dominant spectrum peaks and at standard pressure and temperature conditions with the statistical/systematic errors being at best at the 10% level. For practical fluorescence telescope, these yield measurements have to be extrapolated to account for the whole spectrum and for any atmospheric conditions with poorly known systematic errors. We propose an experiment based on a few MeV electron beam produced by an accelerator, targeting a fiducial volume collecting all the fluorescence light emitted. The goal is to measure the yield absolutely for each spectrum-line and also the integral spectrum by varying the thermodynamic conditions with a statistical accuracy better than 5%. This accuracy can be achieved through control of the electron source intensity, knowledge of primary electron geometrical path and the fiducial volume where the fluorescence light is generated and collected containing more than 95% of the deposited energy and the calibrated detection without relying on any Monte Carlo simulation.