The ULTRA experiment : a supporting activity for the Euso project

The ULTRA experiment [4] has been designed in the framework of the EUSO project to measure the reflected/diffused signal produced by the EAS impacting on the Earth surface. EUSO will detect the EECRs measuring the fluorescent light produced by the interaction with the Earth atmosphere. With this method, the particle track can be measured together with its relative depth, but not its absolute position. ˇ Cerenkov light associated to the EAS is emitted in a narrow cone and hits the Earth surface, being partially absorbed and partially diffused. This signal will give an absolute reference for the track, allowing the measurement of the shower maximum and making also easier the separation between neutrino and hadronic showers. Moreover, ˇ Cerenkov light can give an independent estimation of the shower energy if the reflectivity of the shower impinging surface is known. The ULTRA detector includes a UV optical device to collect the ˇ Cerenkov light diffused by various surfaces and an array of scintillators to detect, in coincidence, cosmic ray showers. First tests have been done last October in Mont-Cenis (France, 1970 m a.s.l.); next measurement campaign is