The ETscope Ground Array for the ULTRA Experiment

The ETscope detector designed for the ULTRA experiment is a small array made of 7 particle detection stations. Operating simultaneously with an optical telescope it detects the Extensive Air Showers in coincidence with the ˇ Cerenkov light, diffused by the impact on the ground. The main goal of the detector is the characterization of the impinging shower by the measurement of size and arrival direction. These informations, together with the UV light measurement and an accurate MC simulation, will allow the determination of the diffusing features of the ground. Since it must be placed on different surfaces including sea, it has been optimized as portable, floating and waterproof detector. First test has been performed during October 2002 at Mont-Cenis in the Alps region, at the France-Italy border. Detector performances and preliminary results will be discussed here. 1. The ETscope Detector The ETscope array of the ULTRA project [1] was located at Mont-Cenis site (1970 meters a.s.l, 45.3 North, 6.9 East) at an equivalent atmospheric depth of 805 g/cm 2 . In the used configuration 3 stations were placed at vertexes of a triangle, each one 23 m from the central fourth station (see fig. 2.). Each station includes a plastic NE102A scintillator 80 × 80 cm 2 wide and 4 cm thick seen by two photomultipliers XP3462. They are contained in a pyramidal shape box made with thin stainless steel, coated with white diffusing paint and placed in a 1 m 3 PVC container. A standard NIM and CAMAC electronics, controlled by a PC under LabView software, is used for data read-out. PMT calibration and gain stability are obtained through the measurement of single muon spectra from which the Vertical Equivalent Muon charge (VEM) is defined. Therefore the measured analog signal of each station converted in VEM units gives the particle density yield. A 3-fold, 150 ns coincidence between 3 external stations is required to trigger the DAQ. The hardware trigger threshold is set to 0.3 VEM/module and the dynamic range allows to measure particle densities up to 40 VEM.