AIRWATCH: the Fast Detector Anna Gregorioa, Roberto Stalioa,), Ezio jjppjC, Giovanni Bonanno', Luciano 0jj0e, Pietro Brun&', Rosario Cosentino", Rosario Di Benedettod, Flavio Fontanelli', Gianrossano Gianninje, Valerio Gracco, Anna Lenti', Alessandro Petro1ini', Mario Sannino, Livio Salvatore Scuderid, Paolo Trampus', Andrea Vacchi' a Center for Advanced Research in Space Optics, Area Science Park, Trieste b Dipartimento di Astronomia, Università di Trieste C Laben S.p.A., Milano d Osservatorio Astrofisico di Catania e Dipartimento di Fisica, Università di Trieste f Dipartimento di Fisica, Universitã di Genova & 1NFN, Sezione di Genova g Istituto di Fisica Cosmica e Informatica - CNR, Palermo h Sezione di Trieste ABSTRACT The discovery of the Extreme Energy Cosmic Rays (EECR) with energy greater than 1020 eV has opened a new research branch of astrophysics on both observational and interpretative point of views. Together with the EECR one has also to consider the neutrino component which, independently on its primary or secondary origin, can reach comparable energies. These particles can be detected through the giant showers (EAS) produced in the Earth atmosphere and the induced fluorescent molecular nitrogen emission. Observing the EECR "signals" is very difficult; we need forefront technology or new developments. The main reason is that their flux is very weak, typically ofthe order ofa few events/year/l000 km2 per EECR of E 1020 eV. The proposed AirWatch mission, based on a single orbiting telescope which can measure both intensity and direction of the EAS, impose new concepts for the detectors: single photon sensitivity, fast response of the order of few microseconds with sampling times of tenths of nanoseconds, low noise and good S/N ratio, large area (linear dimension of the order of several tenths of cm), adaptability to a curved surface. Fortunately the spatial resolution requirements are somehow relaxed. The peculiar characteristics of this application are such that no available detectors satisfies completely the requirements. Therefore the fmal detector has to be the result of a R&D program dedicated to the specific problem. In this paper we survey a number of possible detectors and identify their characteristics versus the AirWatch mission requirements. Key words: AirWatch mission, Extreme Energy Cosmic Rays (EECR), Earth Atmospheric Showers (EAS), segmented- type detectors, projective detectors 1. INTRODUCTION The discovery of cosmic rays with energy larger than 1020 eV (Extreme Energy Cosmic Rays - "EECR") opened a new chapter in Astrophysics, both from observational and theoretical points of view. Their origin, currently one of the most interesting and controversial problems of High Energy Astrophysics 2 seems directly connected to extremely efficient accelerating mechanisms which are still unidentified. These cosmic rays can be observed by means of the giant showers produced by their interaction with Earth atmosphere. About twenty events with energy larger than 1020 eV have been Part of the SPIE Conference on EUV. X-Ray, and Gamma-Ray 486 Instrumentation for Astronomy IX • San Diego, California • July 1998 SPIE Vol. 3445 . 0277-786X1981$lO.OO