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    ONLY - Oxygen Nitrogen Light Yield



    Introduction

    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) component in the region above 1019 eV, 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 have 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.

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    The ONLY Experiment

    The ONLY experiment refers 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).


    The apparatus of measurement

    To measure the UV fluorescence yield it has chosen to use of the technique of the counting of events seen by two photomultipliers operated in coincidence mode.
    The apparatus of measurement consists 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 detect 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 are collimated inside the cell in an absorption region of only 4 cm of length centered on the Pmts.


    Schematics of the entire experimental set-up

    Due to the low photoelectric cross section of air and the small absorption region, it has been used the intense X-ray source of the X-ray Beam Facility at LAX in Palermo. The X-ray generator is a "Seyfert" tube, with flux ranging from 1010 to 1012 photons sr-1 sec-1, covering the energy range from few keV to 25 keV depending of the target material used as anode in the tube.
    It has 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 has been chosen in the electronic chain a coincidence gate time of 500 nsec.
    For the photomultipliers and the electronic chain, it has 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 has obtained the UV light yield.


    The UV light yield measured with ONLY

    The UV yield follows a proportional inverse law with the pressure (expected from kinetic theory) both for air and for pure nitrogen. The value, for pure-air, range from 0.2% to 0.02% below 300 mbar of air pressure.

    Now the IASF-Palermo, in collaboration with the EUSO groups, is willing to spend its activity to develop and build an more complex experimental apparatus suitable to measure the fluorescence Yield in the atmosphere, as produced by a particle beam (mainly electrons) with energies in the range from about 1 MeV to several GeV, and to perform the connected measurements in the suitable accelerator sites.
    The scientific goal is to measure the fluorescence Yield in a gas simulating the Earth's atmosphere as a function of a gas composition, temperature, pressure and humidity. The measurements should mimic, in a controlled environmental situation, the fluorescence Yield of a cosmic ray induced shower in atmosphere.

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    The Team

    With different commitments and tasks, the following IASF-Palermo personnel is part of the ONLY group:

    * Salvo Giarrusso (ONLY Respons.)
    * Gaetano Agnetta
    * Benedetto Biondo
    * Filippo Celi
    * Osvaldo Catalano
    * Renato Di Raffaele
    * Gino Gugliotta
    * Giovanni La Rosa
    * Angelo Mangano
    * Francesco Russo
    * Bruno Sacco
    * Paolo Scarsi

    Contact person:

    • Salvo.Giarrusso at iasf-palermo.inaf.it (ONLY Responsible)

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    Selected publications

      "Laboratory measurements of Air UV fluorescence Light Yield induced by X-ray photons",
      S. Giarrusso, O. Catalano, F. Celi, G. Fazio, G. La Rosa, T. Schillaci,
      Proc. 26th ICRC, D. Kieda, M. Salamon, B. Dingus, (Eds.), Vol.5, 449, 1999

      "AIRWATCH/OWL: Preliminary Results from Lab Measurements and from Ballon Background Observations",
      S. Giarrusso, O. Catalano, F. Celi, G. La Rosa, J. Linsley, C. Maccarone, B. Sacco, L. Scarsi,
      Proc. 19th Texas Symposium on 'Relativistic Astrophysics and Cosmology', E. Aubourg, T. Montmerle, J. Paul, P. Peter, (Eds.), CD-ROM code 08/05, E627G, 1999

      "AIRWATCH: air induced fluorescence by radiation: laboratory experiments" ,
      S. Giarrusso, O. Catalano, F. Celi, G. Fazio, G. La Rosa, G. Richiusa, T. Schillaci, G. Bonanno, R. Cosentino, R. Di Benedetto, S. Scuderi,
      Proc. SPIE'98 Conf., 3445, 460-469, 1998

      "Air Fluorescence efficiency measurements for AIRWATCH based mission: experimental set-up" ,
      B. Biondo, O. Catalano, F. Celi, G. Fazio, S. Giarrusso, G. La Rosa, A. Mangano, G. Bonanno, R. Di Benedetto, S. Scuderi, G. Richiusa, A. Gregorio,
      Proc. Workshop on 'Observing Giant Cosmic Ray Air Showers from Space', J.F. Krizmanic, J. Ormes, R.E. Streitmatter, (Eds.), AIP-CP433, 361-366, 1998

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