The experimenters require beam time to perform a precise measurement of air fluorescence yield from nitrogen excitation by charged particles. This measurement will allow for a substantial reduction of the systematic uncertainty on the energy determination of ultra-high energy cosmic rays (> 10¹⁸ eV) by the Pierre Auger Observatory.

The beam particles enter a dark box where an integrating sphere is placed. Fluorescence photons emitted along the beam axis within the integrating sphere are diffused by the interior coating of the sphere, and eventually reach the photon detector through a 90° exit port. Photomultipliers as well as Hybrid Photon Detectors (HPD) will be used. A 337 nm interference filter in front of the photon detector selects the main fluorescence emission band. The Cherenkov emission will also be measured in separate runs. The Cherenkov light emitted by the beam particles along their path in the integrating sphere will be diffused by closing the beam exit port with a port plug of the same material as the interior coating of the sphere. From the ratio of the measured fluorescence to the Cherenkov signal, the absolute fluorescence yield is determined. Since the same photon detector is used in both measurements, the systematic uncertainty due to the quantum efficiency is significantly reduced. Beam particles are triggered by 1 cm² scintillators placed at the entrance and exit ports of the integrating sphere. Additional veto scintillators counters are used to eliminate halo particles. An independent cross-check of the measurement will be provided by an in situ calibration of the photon detectors. Light from a 337 nm nitrogen laser beam is Rayleigh scattered into the integrating sphere. The laser power is measured by a NIST power meter with 5% accuracy. Since the number of photons 4 scattered away from the laser beam can be precisely calculated, a calibration of the photon detectors is achieved.

Particles: Protons
Energy: 120 GeV
Intensity Needed: 100,000 counts per spill