ONLY – Oxygen Nitrogen Light Yield

Since 1998 IASF-Palermo groups are deeply involved in experimental activities of support for Astroparticle Research from space and in particular in those activities related to the EUSO mission. EUSO will explore the Extreme Energy Cosmic Rays (EECR), using the Earth’s atmosphere as a scintillating giant absorber. EUSO will detect EECRs from space by looking, in the Earth’s night sky, at the streak of UV fluorescence light produced when the particles interact with the Earth’s atmosphere.

In this framework, laboratory experiments had been identified as propaedeutic experimental supporting activities to know fundamental information concerning the UV fluorescence yield. Laboratory measurements of UV fluorescence yield at low energy (22 KeV), using the X-ray facility LAX located in Palermo, have been performed in 1998-2000 with the ONLY (Oxygen Nitrogen Light Yield) experiment, whereas the possibility of measurements at higher energies, of basic importance for the project, is presently under discussion within the EUSO collaboration. 

The ONLY experiment referred to measurements of UV light yield obtained in pure dry air and, for comparison, in pure Nitrogen, using an intense flux of X-ray photons at the LAX facility in Palermo. The UV fluorescence radiation from air results, almost entirely, from electronic transition from the first excited levels of the Nitrogen molecule or molecular ion to the ground level. The radioactive emission from these electronic transitions is detected as very narrow lines in the near UV band (mainly lines 337, 357 for Nitrogen molecule and 391 nm for molecular ion).

To measure the UV fluorescence yield we used the technique of the counting of events seen by two photomultipliers operated in coincidence mode. The apparatus of measurement consisted of a cylindrical gas cell of ceramic closed at top and bottom by quartz windows 5 mm thick. Two photomultipliers (EMI D319Q operated at 2 kV) of 8 cm of diameters detected the UV light produced inside the cell. To reduce the variations of the solid angle subtended by the photomultipliers at the point production of the UV light, the X-ray photons were collimated inside the cell in an absorption region of only 4 cm of length centered on the Pmts.

Due to the low photoelectric cross section of air and the small absorption region, it had been used the intense X-ray source of the X-ray Beam Facility at LAX in Palermo. The X-ray generator was a “Seyfert” tube, covering the energy range from few keV to 25 keV depending of the target material used as anode in the tube. It had been chosen the 22.1 keV fluorescence line of the Silver target with a trade-off between as much UV photons as possible inside the cell and a minimum X-ray Compton diffusion. As the fluorescence decay time of the excited Nitrogen molecules is about 50 nsec, it had been chosen in the electronic chain a coincidence gate time of 500 nsec. For the photomultipliers and the electronic chain, it had been used high gain settings and low discrimination thresholds. Following a statistical approach and measuring the coincidence rate of the X-ray events detected via UV fluorescence photons, it had obtained the UV light yield. The UV light yield measured with ONLY The UV yield followed a proportional inverse law with the pressure (expected from kinetic theory) both for air and for pure nitrogen. The value, for pure-air, ranged from 0.2% to 0.02% below 300 mbar of air pressure. 

 With different commitments and tasks, the following IASF-Palermo personnel was part of the ONLY group: Salvo Giarrusso, Gaetano Agnetta, Benedetto Biondo, Filippo Celi, Osvaldo Catalano, Renato Di Raffaele, Gino Gugliotta, Giovanni La Rosa, Angelo Mangano, Francesco Russo, Bruno Sacco, Paolo Scarsi.