The ULTRA (UV Light Transmission and Reflection in the Atmosphere) experiment had been designed to provide quantitative measurements of the reflection/diffusion signal produced by Extensive Air Showers (EAS) impacting on the Earth surface, overcoming the lack of information in this specific field. The knowledge of such information for different surfaces of the EAS impact tested the possibility to detect from Space the Cherenkov light produced by the Ultra High Energy Cosmic Rays (UHECR).
The ULTRA apparatus was composed by two types of detector: a small EAS array (ETscope) formed by five scintillators (using conventional sampling technique) disposed in a centered parallelogram, and an UV optical device (300-400 nm interval) to detect the Cherenkov light, including: two narrow field detectors (UVscope), pointing toward the ETscope central station, and four wide field detectors (Belenos), two of them pointing to zenith (Belenos-up), the other two pointing to nadir (Belenos-down).
ETscope, UVscope and Belenos operated simultaneously to detect EAS in coincidence with the UV light reflected/diffused from its impact on sea surface. The ETscope array allowed measuring the core position, arrival direction and size of the incoming EAS; the UV devices were used to determine the amount of direct and diffused coincident Cherenkov light.
The atmospheric transmission properties were also studied using the UV light detectors and a laser emitter.
Conceived and designed in the framework of the EUSO mission, ULTRA was firstly implemented and tested (in a partial configuration) at the sites of Mont-Cenis and Grenoble, France, in various periods from October 2002 and June 2004. At the sametime, a set of measurements was performed on Madonie Mountains, near Palermo, Italy, to test the UVscope performance. Then, ULTRA was fully implemented and a calibration and optimization phase on the entire apparatus was carried out in Grenoble, France, with acquisitions during the winter 2004-2005. The first complete campaign of measurements had been performed at the Capo Granitola site, Italy, along more than seven months from Spring to Winter 2005. ULTRA was there mounted with the ETscope and Belenos setup very close to the Grenoble one, and with the UVscope observing the center of the array far from it, at a distance from land of about 50 m and from an altitude of about 10 m asl.
The Capo Granitola campaign was successfully carried out. Starting from 3 June 2005, the entire apparatus operated under optimal conditions, giving us useful acquisition data, suitable for a complete analysis, for more than 300 hours.
With different commitments and tasks, the following IASF-Palermo personnel was part of the ULTRA group: Osvaldo Catalano, Gaetano Agnetta, Benedetto Biondo, Giacomo D’Alì Staiti, Salvo Giarrusso, Giovanni La Rosa, Maria Concetta Maccarone, Angelo Mangano, Francesco Russo, Alberto Segreto.