Design and operation of novel underwater acoustic detectors: applications to particle physics and multidisciplinary science for the NEMO-SMO and KM3NeT projects

Abstract

Neutrino-astronomy is one of the most interesting frontiers of the astro-particle physics. Since neutrinos interact only via weak interaction, they are an optimal probe to explore the Universe and regions close to black-holes, where the radiation and matter densities hinder the photon emission. The detection of high energy astrophysical neutrinos, not yet claimed by present experiments, will provide powerful information to improve astrophysical models describing Supernova Remnants, Active Galactic Nuclei and Gamma-Ray Burst or to study the so called cosmological neutrinos at Ultra High Energy. The most promising method to detect neutrinos with energy higher than 10^18 eV is based on the detection of acoustic waves produced by deposition of a large amount of energy following a UHE interaction of neutrinos in sea/lake-water. Thanks to the long attenuation length of the acoustic waves in water, the acoustic detection technique permits instrumentation of very large detection volumes with a sparse array of acoustic sensors. The activity reported in this thesis, was mainly addressed to the construction and test of a novel underwater acoustic detector for particle physics and multidisciplinary science. The detector, developed in the framework of the project SMO (Submarine Multidisciplinary Observatory), funded by Futuro in Ricerca 2008 funds of MIUR, is installed onboard NEMO Phase-II tower, a prototype of a detection unit for an underwater km3-scale neutrino telescope, that will be installed at a depth of about 3500 m in the first months of 2013, 100 km off-shore South-East Sicily coast. SMO consists of a 3D array of 14 broad-band (10 Hz - 70 kHz) hydrophones. The data acquisition system of the SMO acoustic array is fully integrated with the NEMO Phase-II detector one. The analysis of the acoustic signals is entirely performed on-shore, i.e. the acquisition system is based on an all data to shore philosophy. All acoustic signals acquired by the acoustic sensors are sampled underwater and continuously sent to shore through the NEMO Phase-II data transmission system. A novel technology was implemented to time-stamp audio data underwater with absolute GPS time off-shore. In this work tests and results on the performances of the SMO acoustic array are presented. In particular, the electronic noise of the data acquisition electronics has been measured, using dedicated test benches, set up at LNS-INFN. These measurements define the smallest amplitude of the acoustic signals that the SMO array can detect as function of the frequency. The very low electronics noise makes the SMO detector suitable for searching for impulsive signals in the frequency range of interests of the acoustic neutrino detection and makes this detector perfectly complaint with the goal of detecting biological signals (namely mammals sounds).Dedicated tests carried out at the waterpool facility of CNR-IDASC and NATO-URC laboratories allowed full characterization of the detector. These measurements, also, show that acoustic data are time-stamped underwater with a known and measurable latency (time delay) with respect to the GPS absolute time. This feature will permit the alignment in time of data recorded on shore from all sensors. This feature will be used, for the first time, to search for acoustic impulsive signals in coincidence with events reconstructed by the NEMO Phase-II Cherenkov array, allowing preliminary studies on acoustic neutrino detection.