Precise measurement of the absolute fluorescence yield of the 337 nm band in atmospheric gases

A measurement of the absolute fluorescence yield of the 337nm nitrogen band, relevant to ultra-high energy cosmic ray (UHECR) detectors, is reported. Two independent calibrations of the fluorescence emission induced by a 120GeV proton beam were employed: Cherenkov light from the beam particle and calibrated light from a nitrogen laser. The fluorescence yield in air at a pressure of 1013hPa and temperature of 293K was found to be Y337=5.61±0.06stat±0.22systphotons/MeV. When compared to the fluorescence yield currently used by UHECR experiments, this measurement improves the uncertainty by a factor of three, and has a significant impact on the determination of the energy scale of the cosmic ray spectrum.