Vol49_2_2006 643 ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006 Key words high energy neutrino telescopes – deep- sea cabled laboratories 1. Introduction Ocean depths represent today a new frontier for the exploration of the Earth. The study of these vast regions is a scientific and technological challenge that has been undertaken by scientists of various disciplines. The main problem in the exploration of deep- sea regions is represented by the very harsh envi- ronment (high pressure, corrosion, etc.). Up to now studies have been limited by the possibility to gather data only from surface vessels or by moor- ing instruments for limited time periods on the sea bottom. Recent developments in the field of com- munication technology, robotics and sensors may now allow a completely different strategy: realis- ing permanent underwater infrastructures that can allow in situ continuous monitoring of deep-sea environments in real time. The possibilities that this new approach can open are of utmost impor- tance in many fields of research: oceanography, geophysics, seismology and deep-sea biology. Ocean depths have recently also attracted the interest of astrophysicists and high-energy physi- cists. Some years ago it was proposed to use deep waters as a detector of high energy cosmic parti- cles. The proposed detector could be realized by setting up a lattice of optical sensors able to de- tect the faint light produced by the passage of these particles through the water. These appara- tuses could detect the most elusive known parti- cle, the neutrino, and open the new field of neu- trino astronomy. 2. Astronomy with neutrinos Almost everything we know about the Uni- verse comes from its observation by means of Underwater laboratories for astroparticle physics and deep-sea science Emilio Migneco (1) (2), Antonio Capone (3) and Paolo Piattelli (1) (1) Laboratori Nazionali del Sud, Catania, Italy (2) Dipartimento di Fisica, Università degli Studi di Catania, Italy (3) Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Roma c/o Dipartimento di Fisica, Università degli Studi di Roma «La Sapienza», Roma, Italy Abstract The exploration of deep-sea environments is currently at the dawn of a new era: underwater laboratories, perma- nently installed on the seafloor and offering power and on-line data transmission links to the shore, will allow continuous monitoring of oceanographical properties. An important boost in this direction has been provided by the high energy physics scientific community, that aims at the realization of an underwater detector for cosmic high energy neutrinos. Neutrinos are considered a very promising probe for high energy astrophysics and many indications suggest that some of the most energetic sources known in the universe could also be high energy neu- trino sources. The expected neutrino fluxes indicate that a km3-scale detector must be realised to achieve this am- bitious aim. The quest for the realization of such a detector in the Mediterranean Sea has already started. Mailing address: Dr. Paolo Piattelli, Laboratori Nazio- nali del Sud, Via Sofia 62, 95123 Catania, Italy; e-mail: paolo.piattelli@lns.infn.it 644 Emilio Migneco, Antonio Capone and Paolo Piattelli electromagnetic waves. From the observational point of view, electromagnetic waves offer many advantages: they are copiously produced, they are stable, electrically neutral and therefore in- sensitive to magnetic fields, and they are easy to detect over a wide energy range that spans from low energy radio waves to infrared, visible, ultra- violet, X-rays up to high energy gamma rays. The disadvantage is that the internal regions of the astrophysical objects, where energy produc- tion takes place, are completely opaque to pho- tons and therefore inaccessible to direct observa- tion. For example the light that comes from the Sun comes from its «photosphere». Properties of its internal core, where nuclear reactions take place, can only be inferred indirectly. Nevertheless, the observation of the sky by means of electromagnetic radiation has allowed the discovery of extremely powerful sources, which are probably powered by massive black holes, located in the most remote regions of the observable Universe. Unfortunately, the obser- vation of these sources through their gamma ray emission is strongly unfavoured since high- energy gamma rays are absorbed by the cosmic Infrared and MicroWave background radiation. This limits the observable horizon for high en- ergy gamma rays. In fact, this horizon is re- stricted to less than one million light years for gamma rays of more than 1015 eV (1 PeV). Additional information comes from the ob- servation of cosmic rays: protons or heavier nu- clei. Unfortunately, due to their charge, these particles are deflected by the galactic and inter- galactic magnetic fields, thus preventing the identification of their sources. Moreover, the extremely high-energy charged particles inter- act with photons of the infrared radiation back- ground and with the cosmic microwave back- ground. This is known as the Greisen-Zatsepin- Kuz’min (GZK) effect (Greisen, 1966; Zatsepin and Kuz’min, 1966). In order to observe the inner workings of the astrophysical objects and to obtain a description of the Universe over a larger range of energies, we need a probe which is electrically neutral, such that its trajectory will not be affected by magnetic fields, stable, such that it will reach us from distant sources, and weakly interacting so that it will penetrate regions which are opaque to photons. The only candidate particle currently known to have these properties is the neutrino. Several low energy neutrino detectors are currently active around the world (Boger et al., 2000; Fukuda et al., 2003). These detectors have allowed us to study the neutrino fluxes from the Sun, resulting from the nuclear reac- tions that take place in its inside, which is not accessible to direct observation. These «tele- scopes» are also sensitive to neutrinos produced in Supernova explosions. Two detectors, Ka- miokande (Hirata et al., 1987) and IMB (Bion- ta et al., 1987), have in fact recorded a short burst of neutrinos in coincidence with the ex- plosion of Supernova 1987A, which was the lat- est Supernova explosion taking place at a rela- tively close distance from the Earth. Neutrinos interact so weakly with matter (Gandhi et al., 1996) that only huge detectors can collect a sizeable sample of their interac- tions. Thousands of tons are needed to detect the low energy neutrinos coming from the Sun core, but these volumes are still not sufficient to detect low energy neutrino sources at cosmo- logical distances. High energy neutrinos offer a better detection opportunity since their interac- tion cross section increases with energy and since the products of their interactions can be more easily detected as their energy increases. An instrument able to detect high-energy cos- mic neutrinos and identify their direction of ar- rival, a «Neutrino Telescope», would enlarge the observation horizon allowing the study of objects at the limit of the Universe. 2.1. High energy neutrino sources Astrophysical sources of high-energy neu- trinos have not yet been observed directly, but their existence can be inferred from the proper- ties of cosmic rays. Primary cosmic rays are protons, with some admixture of heavier nuclei. The energy spec- trum follows a power law that extends up to ex- tremely high energies: values exceeding 1020 eV have been observed in recent years. Protons themselves have limited use as astrophysical messengers, because they are charged and there- fore subject to deflection by cosmic magnetic 645 Underwater laboratories for astroparticle physics and deep-sea science fields: only the very highest-energy protons are likely to retain any memory of their source direc- tion but, as already mentioned, they suffer from interaction with the background radiation. The production mechanism of the highest- energy cosmic rays is currently unknown, al- though Supernova Remnants and Active Galac- tic Nuclei have been proposed as their sources. Whatever the source, it is clear that accelerating protons to such high energies implies also the production of a large flux of pions, originated in the interaction with the photon background. Pi- ons subsequently decay to yield gamma rays and neutrinos. For these reasons it is generally assumed that the existence of very high-energy protons in the cosmic rays implies the existence of a flux of high-energy neutrinos. Extra-galactic objects, such as the sources of Gamma-Ray Bursts (GRBs) and Active Galac- tic Nuclei (AGN), plausibly generate cosmic rays up to the maximum observed energies, and are therefore likely sources of neutrinos in the TeV (1012 eV) to PeV (1015 eV) energy range. GRBs are transient flashes of gamma-rays, last- ing typically for 1÷100 s, that are observed from sources at cosmological distances. Although we do not yet understand in detail the internal mech- anisms that generate these powerful explosion, some evidences suggest that they are cataclysmic processes associated with the collapse of mas- sive stars to a black hole. AGN consist of both persistent and flaring sources with apparent lu- minosities reaching about 1048 erg/s. They are tought to be powered by mass accretion onto su- permassive (106÷ 109 solar-mass) black holes re- siding at the centre of galaxies. In both GRBs and AGN, the mechanism of mass accretion is believed to drive a relativistic plasma outflow that results in the acceleration of high-energy particles. Both AGN and GRBs have been proposed as high energy neutrino sources, and neutrino observations will provide unique information on the physics of the underlying engine, which is not well understood despite many years of re- search. Other theorized neutrino sources are as- sociated with compact astrophysical objects (SuperNova Remnants, X-ray binaries and mi- croQuasars) or with the annihilation of the yet unobserved particles which may constitute the dark matter. A comprehensive review of candi- date neutrino sources and flux model predic- tions can be found in Learned and Mannheim (2000) and in Halzen and Hooper (2002). Neutrino observations are specifically inter- esting because the detection of high-energy (>TeV) neutrinos will provide unambiguous ev- idence for cosmic acceleration of protons and nuclei, and their arrival direction will point to the location of the accelerators. Neutrino telescopes will permit us not only to look into the engines driving powerful sources such as distant AGN and GRBs that cannot be explored directly with photon observations, but also to look far into the universe. Cosmological sources cannot be observed at photon energies exceeding 100 GeV because of attenuation by γ- γ pair production on the diffuse intergalactic infrared background radiation. By contrast, high- energy neutrinos will propagate unhindered di- rectly to us from their sources. Thus, neutrinos can provide a new window to explore the high- energy phenomena in the distant universe. A question still remains on how large a high energy neutrino telescope must be to allow for an unambiguous detection of some sources. As we have seen the strongest case is based on the existence of extreme high energy cosmic rays. From this observed high energy charged parti- cle flux we can derive a neutrino flux assuming that a cosmic accelerator produces equal ener- gies in cosmic rays, gamma rays and neutrinos. These flux estimates set the needed scale for a high energy neutrino telescope: a 1 km2 effec- tive area telescope will be able to detect a few tens of events per year. 2.2. High energy neutrino detection The most promising method to achieve the ambitious goal of realizing a km2 scale Neutri- no Telescope is the tracking of secondary muons produced in the interaction of neutrinos in the volume close to the telescope. High ener- gy muons are extremely penetrating charged particles since they lose energy, interacting with matter, mainly through Coulomb scattering. Processes like the emission of high energy pho- tons (bremsstrahlung), that is typical of high 646 Emilio Migneco, Antonio Capone and Paolo Piattelli energy electrons, is strongly reduced since muons are heavier (≈ 200 times) than electrons. When a high-energy muon, having a veloc- ity υ close to the speed of light in vacuum (c), propagates in a transparent medium with index of refraction n > 1, its velocity will be greater than the speed of light in the medium that is equal to c/n. In these conditions visible light is radiated along the track with an angle θC, with respect to the muon direction, defined by the relation cos(θC)=1/(βn), where β= υ/c. This process, called Cherenkov effect, is similar to the shock wave, the sonic boom, produced when an airplane exceeds the speed of sound in air. In the water, that has a refraction index n ≈ 1.35 in the blue light region, θC is approxi- mately 42°. The light radiated by Cherenkov ef- fect forms a conical wavefront that propagates inside the detector, leading to a relation be- tween the muon direction and the arrival time of photons in different points of the space (fig. 1). The energy loss of a muon via Cherenkov ra- diation is only a negligible fraction of the total en- ergy loss and the number of Cherenkov photons emitted in water is roughly 300 per centimeter of track. Nevertheless, numerical simulations show that equipping a large volume of a natural trans- parent medium (like the oceans or the Antarctic ice) with optical sensors capable to detect even single photon signals one can identify a muon track and reconstruct its direction. Water optical properties will determine the detector granularity (i.e. the optical sensor density). Water is transpar- ent only in a narrow range of wavelengths (350 ≤ ≤ µ ≤ 550 nm) in the blue light region of the spec- trum. In this region the absorption length in clear ocean waters is about 70 m. This number rough- ly sets the spacing distance between the optical sensors of the detector. Muons carry, on average, a significant frac- tion of the neutrino energy and propagate with nearly the same direction of the incident neutri- no. For neutrino energies larger than 10 TeV the angle between the originating neutrino and the emerging muon is on the average less than 0.2°. Therefore the reconstruction of the muon, if per- formed with an accuracy of the order of 0.2° or better, can infer the direction of the incoming neutrino. A significant excess of neutrino events coming from the same direction would yield the identification of an astrophysical source. A high energy muon can travel up to sever- al km in water. This implies that the Cherenkov Neutrino Telescope will be sensitive to neutrino interactions occurring even far outside the de- tector instrumented volume. Muons originated by astrophysical neutri- nos have to be selected out of the more intense flux of muons (the background for our signal) produced in atmospheric showers resulting from the interactions of primary cosmic rays (protons and heavier nuclei) in the atmosphere. The atmospheric muon flux at sea level is about times 1011 times more intense than the flux of muons produced by astrophysical neutrinos but is typically less energetic, therefore these at- mospheric muons cannot penetrate large thick- nesses of matter. This property can be used to reduce the background by installing the detec- tors in an underground laboratory (like the Gran Sasso Laboratory, near L’Aquila in Italy). Obvi- ously a km3 detector, like the one needed for high energy neutrino astronomy, cannot be hosted in a tunnel or in a natural cave, but one can obtain the same result by deploying it in the ocean depths where the overlaying water would reach the same effect. At 3000 m the flux of at- mospheric muons would be reduced by a a fac- tor 106 making it feasible the search for astro- physical neutrinos. Fig. 1. Detection principle of an underwater neutri- no telescope. Astrophysical neutrinos that interact close to the detector generate secondary muons that can be detected by means of their Cherenkov light emission. The measure of arrival times of the Che- renkov light on the optical modules allows to recon- struct the muon direction. 647 Underwater laboratories for astroparticle physics and deep-sea science The role played by seawater is therefore threefold: it constitutes the target where astro- physical neutrinos interact to produce the sec- ondary muons; it acts as a transparent radiator where relativistic muons induce the Cherenkov light; it acts as a shield against the cosmic muon background. Seawater above the detector can reduce the atmospheric muon background but not totally eliminate it. For each astrophysical neutrino about 105 down-going atmospheric muons will cross the detector. But none of these atmospher- ic muons can cross the Earth, unlike neutrinos that can traverse the whole thickness of the Earth and interact in the vicinity of the detector originating an up-going muon (fig. 2). This fact completely eliminates the atmospheric muon background: a track reconstructed as crossing the detector with up-going direction will be a clear signature of a neutrino-induced muon. The Neutrino Telescope will then be sensitive to neutrinos, originated in astrophysical sources or in atmospheric showers, coming from the Southern Hemisphere, from the «bottom» and not from the «sky» as all the other astronomical instruments we are used to. However, a remaining background exists, due to neutrinos that are produced in the interac- tions of charged particles in the atmosphere and are therefore called «atmospheric neutrinos». This background is isotropic and becomes neg- ligible at very high energy (above 10 TeV). Point like sources of cosmic neutrinos can be identified if they can emerge from this back- ground. 3. High-energy neutrino telescopes In recent years the water Cherenkov tech- nique has been successfully used to detect solar and atmospheric neutrinos in 103-104 m3 scale detectors, like SNO (Boger et al., 2000) and Su- perKamiokande (Fukuda et al., 2003). As we have seen, the expected number of neutrino events from astrophysical sources in the 1012÷ 1015 eV range is of the order of 10 ÷ 100 events per km2 per year, much less than the Fig. 2. Detection principle of an underwater neutrino telescope. This instrument will use the whole thickness of the Earth to shield the charged particles cosmic flux and therefore its field of view will be restricted to the hemisphere below the horizon. Nevertheless, a background of atmospheric neutrinos, originated by the interac- tion of charged particle in the atmosphere in the opposte hemisphere, will always be present. 648 Emilio Migneco, Antonio Capone and Paolo Piattelli number of events expected in the low energy range. Therefore, only 109 m3 scale detectors could allow the source identification. As we have seen, conceptually the idea at the base of a Cherenkov high energy Neutrino Tele- scope is simple. A high-energy muon neutrino in- teracts with a nucleus in the water and produces a muon travelling in nearly the same direction as the neutrino. The high-energy muon track can range from several hundreds metres up to few km, depending on its energy. The Cherenkov light emitted along the muon track, with a well defined angle with the track itself, can be detect- ed by a sparse array of optical sensors, deployed in deep-sea (3000-4000 m depth) and arranged in a geometry that allows the reconstruction of the track direction. The average distance in between the sensors will be a function of the light propa- gation properties in the water: the lower the light absorption, the higher can be the distance be- tween sensors. Also the detector cost is a function of the water transparency: the longer is the ab- sorption length of deep-sea water the less is the number of light sensors needed to equip the same instrumented volume. Light sensors, also called Optical Modules (OM), constitute the active part of the detector. They are realised with large area (8 to 13 inches) PhotoMultiplier Tubes (PMT) hosted inside pressure resistant glass spheres. They have to provide the information of the in- tensity of the light that hits their surface and of the photons arrival time. These quantities are transformed locally into digital information that can be transmitted, by means of optical fibres, to a shore laboratory. On shore all the data received can be stored and analysed. The idea to construct a high energy neutrino detector by instrumenting large volumes of nat- ural media was proposed long time ago by Markov (1960) and Markov and Zheleznykh (1961), but only recently the technological de- velopments in mechanics, electronics and data transmission system seems to be sufficient to transform the project in reality. The payoff for this choice is that one has to face several technological problems for the construction, deployment and maintenance of the instrument. The construction of km3 scale neutrino telescopes requires, in fact, detailed preliminary studies and intense R&D efforts. The layout of the detector must be opti- mized to achieve an effective detection area close to 1 km2 together with a pointing accura- cy close to the intrinsic uncertainty due to the neutrino interaction and an energy resolution of the order of some tens percent. The choice of the underwater installation site must be carefully investigated, since water properties strongly influence the detector per- formance. The electronics design must limit power con- sumption and allow the transmission of high data flows from the detector to the shore. The mechan- ical design must allow easy deployment (and pos- sibly maintenance and recovery) operations and the deployed structures must be reliable over a period of more than 10 years. The positioning system must be realised to determine the position of optical sensors with 10 cm accuracy. The pioneering effort to develop a neutrino telescope was carried out by the DUMAND col- laboration (Babson et al., 1990), starting more than twentyfive years ago in the ocean waters off- shore Hawaii Island. After several years the proj- ect was abandoned, mainly because of the prob- lems encountered in the deployment of the equip- ment in the sea (Roberts, 1992). At present two small scale neutrino telescopes are in operation, one at Lake Baikal and the other, AMANDA, under the ice of the South Pole. Two other projects, ANTARES and NES- TOR, are aiming at the realization of deep-sea prototypal detectors in the Mediterranean Sea. Studies and R&D activities towards the realiza- tion of a deep-sea km3 scale neutrino detector in the Mediterranean have been up to now conduct- ed by the Italian NEMO collaboration. These ac- tivities are expected to converge in the future in one large international collaboration to realize the Mediterranean km3 detector. 3.1. The running neutrino telescopes 3.1.1. The Lake Baikal experiment Baikal was the first collaboration to install an underwater neutrino telescope, which, after more than ten years of operation, is still the on- ly one located in the Northern Hemisphere. The 649 Underwater laboratories for astroparticle physics and deep-sea science Lake Baikal Neutrino Telescope (Belolaptikov et al., 1997) exploits the deep fresh water of the great Siberian lake as a detection medium for high energy neutrinos. Its lifetime spans almost two decades from the small initial NT-36 detec- tor, that has proven the capability of the exper- iment to search for neutrinos by the detection of first neutrino candidates (Balkanov et al., 1999, 2000), to the present neutrino telescope NT- 200, which was put into operation in 1998. The experiment is located in the southern part of Lake Baikal, 3.6 km from the shore. The NT- 200 detector is an array of 200 optical modules moored between a depth of 1000 and 1100 m. The deployment and recovery operations are car- ried out at the end of the winter season, when a thick ice cap (about 1 m) is still present over the lake. The limited depth and the qualities of lake water (light transmission length of 15 ÷ 20 m, high sedimentation and bio-fouling rate, optical background due to bioluminescence) limit the de- tector performances as a neutrino telescope. 3.1.2. AMANDA The AMANDA (Antarctic Muon and Neutri- no Detector Array) detector (Andres et al., 2000) is currently the largest neutrino telescope in- stalled. It is located close to the Amundsen-Scott Research Station at the South Pole and uses the deep Antarctic ice as detection medium (more in- formation on the AMANDA detector can be found on the experiment’s web site at http:// amanda.uci.edu). In the present stage, named AMANDA-II, the detector consists of 677 Opti- cal Modules all downward oriented. Optical modules are arranged in 19 vertical strings de- ployed in holes drilled in the ice between 1.3 and 2.4 km depth, where the ice optical properties are suitable for Cherenkov detection. Thanks to the high absorption length in ice (about 100 m), AMANDA is a good calorimeter for astrophysical events. On the contrary, due to the small light scattering length in ice (tens of cm), the detector angular resolution is worse than the one expected for underwater neutrino tele- scopes. The main advantage of the AMANDA lo- cation is the absence of optical noise sources, like 40K and bioluminescent organisms in the bulk ice. The AMANDA data have permitted to meas- ure for the first time the upgoing atmospheric neu- trino spectrum in the energy range from few TeV to 300 TeV, proving the capabilities of the Che- renkov detection technique and allowing to set what is up to now the most restrictive experimen- tal bound on the diffuse high energy neutrino flux (Ahrens et al., 2003; Ackermann et al., 2005a). A search for cosmic point-like sources has also been attempted using a targeted search strategy focus- ing on known objects known to be candidate neu- trino sources. However, among the collected sam- ple of neutrino induced events, no significant ex- cess due to astrophysical point sources has been observed (Ackermann et al., 2005b). 3.2. Small scale deep-sea demonstrator detectors 3.2.1. NESTOR NESTOR (Neutrino Extended Submarine Telescope with Oceanographic Research) was the first collaboration to propose the installation of a neutrino telescope in the Mediterranean Sea (Tzamarias, 2003). The goal was to deploy a modular detector at about 4000 m in the Ion- ian Sea (more information on the NESTOR ex- periment can be found on the experiment’s web site at http://www.nestor.org.gr). The NESTOR site is located 20 km SW off- shore Methoni, in the Peloponnese (Greece). The proposed array should comprise a series of semi rigid structures called «towers». Each tower would be 360 m high and 32 m in diame- ter and should be equipped with about 170 PMTs looking both in upward and downward direc- tions. In march 2003, after a long R&D activity, NESTOR has successfully deployed 12 Optical Modules at a depth of 3800 m acquiring, on- shore, underwater optical noise data and cosmic muon signals (745 events reconstructed) for about one month (Tzamarias, 2005). 3.2.2. ANTARES The construction of the proposed ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) detector (AN- 650 Emilio Migneco, Antonio Capone and Paolo Piattelli TARES collaboration, 1999) is currently in a well advanced stage (more information on the AN- TARES experiment can be found on the experi- ment’s web site at http://antares.in2p3.fr). ANTA- RES will be a demonstrator with a detection area of 0.1 km2 for high energy muons generated by astrophysical neutrinos, and will be a fundamen- tal step towards the km3 telescope in deep-sea. The ANTARES site is located at 2400 m depth, 40 km SE off-shore the city of Toulon (France). In the present design ANTARES will be a high granularity detector consisting of 12 strings, each one equipped with 75 Optical Modules, placed at an average distance of 60 m. This con- figuration is optimized to reduce the muon detec- tion threshold down to about 10 GeV allowing the investigation of lower energy phenomena such as atmospheric neutrino oscillations and search for dark matter. With respect to AMAN- DA and BAIKAL, strongly affected by light scat- tering in the medium, the dense ANTARES de- tector is expected to reach a pointing accuracy which will be close to 0.1°. In December 2002 the detector installation started with the deployment of the so called junc- tion box, which will interconnect electro-optical cables from the strings to the shore. After a first operation of two prototypal lines in spring 2003 a new and improved version of a short line equip- ped with oceanographic instruments and optical modules has been put in operation in april 2005 and is taking data since then. Data recorded by the optical modules show an unexpectedly high optical background, strongly fluctuating, as a function of time, from a level of 50 kHz to a lev- el of more than 250 kHz, with a strong contribu- tion of bioluminescence bursts (Anton, 2005). 3.3. Future km3 neutrino telescopes The simultaneous observation of the full sky is an extremely important issue, since the distri- bution of some neutrino sources is expected not to be isotropic. To achieve this goal two km3 scale neutrino telescopes, one in each Earth hemisphere, are needed. In the Southern Hemisphere, and therefore looking at the northern sky, the ICECUBE tele- scope (Botner, 2005) will be the natural extension of AMANDA to km3 size. Its construction started in January 2005 with the deployment of the first string. When completed in 2010 it will be an ar- ray of 4800 PMT arranged in 80 strings. All the Optical Modules will be downward looking as in AMANDA and simulations show that an average spacing of 125 m between the strings is an opti- mal compromise between the two requirements of angular resolution of better than 1° and a high energy muon detection area of approximately 1 km2. It is worth mentioning that under-ice detec- tors are not affected by radioactive and biological optical noise. This makes them suitable for the search of low energy neutrino fluxes from galac- tic Supernova explosions (more information on the IceCube detector can be found on the experi- ment’s web site at http://icecube.wisc.edu). In the Northern Hemisphere the Mediter- ranean Sea offers the most favourable conditions for the construction and maintenance of an under- water km3 Cherenkov neutrino detector: optimal water characteristics, deep-sea sites at close dis- tance from shore, proximity to scientific and in- dustrial infrastructures, good weather and sea conditions for a large fraction of the year. An underwater detector offers, compared to ICECUBE, the possibility to be recovered, maintained and reconfigured. The long light scattering length of the Mediterranean abyssal seawater preserves the Cherenkov photon direc- tionality and will permit excellent pointing ac- curacy (of the order of 0.1° for 10 TeV muons). Unlike deep polar ice, the sea is a biologically active environment where organisms produce background light. The selection of a marine site with optimal oceanographic and optical charac- teristics is therefore a major task for the collab- orations involved in the km3 project. At present, the effort towards the construction of a large area underwater detector is focused on the development of submarine technologies. Deep-sea is an extremely hostile environment where pressure (100 bars per 1000 m depth), to- gether with salinity, reduces the lifetime of most of metals and alloys used in surface and shallow water applications. The efforts presently conduct- ed by the three Mediterranean collaborations (ANTARES, NEMO and NESTOR) will repre- sent a valuable experience in the construction of 651 Underwater laboratories for astroparticle physics and deep-sea science the underwater km3 detector, which should take place in the coming years. 4. Research and development for the km3 detector The construction of an underwater km3 scale neutrino telescopes requires detailed pre- liminary studies: the choice of the underwater installation site must be carefully investigated to optimise detector performance; the readout electronics must have a very low power con- sumption; the data transmission system must allow data flow transmission, as high as 100 Gbps, to shore; the mechanical design must al- low easy detector deployment and recovery op- erations, moreover the deployed structures must be reliable over more than 10 years; the posi- tion monitoring system has to determine the po- sition of OM within ≈10 cm accuracy. In order to propose feasible and reliable so- lutions for the km3 installation the NEMO (NEutrino Mediterranean Observatory) collab- oration has been conducting an intense R&D activity on all the above subjects since 1998 (Migneco et al., 2004a), which will be briefly described in this section. 4.1. Site selection and characterization The needs of the underwater neutrino tele- scope impose a series of requirements that the site must fulfill. Depth – Thickness of the overlaying water has to be enough to filter out the down-going at- mospheric muon background to allow the selec- tion capability of the up-going tracks originated from neutrino interactions in the Earth and/or the water near the detector. Distance from shore –The data transmission to the on-shore laboratory, as well as the trans- mission of power from the laboratory to the off- shore detector, will be performed via an electro- optical multi-fibre cable. At distances closer than 100 km from the coast, commercial sys- tems allow data and power transmission with- out special hardware requirements (e.g., ampli- fiers) that would increase the cost and reduce the reliability of the project. Moreover, the proximity to the coast and to shore infrastruc- tures simplifies the access to the site for deploy- ment and maintenance operations. Site geology – The seabed has to be flat and stable to allow the mooring of the telescope structures. For the detector safety, it should al- so be located at some tens of km far from the shelf break and submarine canyons. Water transparency – The detector perform- ance is not only directly determined by the ex- tension of the instrumented volume but is also strongly affected by the light transmission properties of the water. Mainly two microscop- ic processes affect the propagation of light in the water: absorption and scattering. Light ab- sorption directly reduces the effective area of the detector, the scattering spreads the photon arrival times and therefore worsens the detector angular resolution. Optical background – Optical background in seawater comes from two natural causes: the decay of 40K, which is present in seawater, and the so called bioluminescence that is the light produced by biological organisms. The first one shows up as a constant rate background noise on the optical modules (of the order of 30 kHz for a single 10m PMT). The second one, when present, may induce large fluctuations (both in the baseline and as presence of high rate spikes) in the noise rate. The background directly af- fects the detector performances, in particular the quality of muon track reconstruction. In a high background environment severe cuts to the photon detector data must be applied, hence re- ducing the detector effective area. Downward sediment flux – The presence of sediments in the water can seriously affect the performances of the detector. Sediments in- crease the light scattering, therefore worsening the track reconstruction angular resolution. Moreover, a deposit on the sensitive part of photon detectors, i.e. large surface photomulti- pliers, reduces the global detector efficiency. A direct consequence is that in a high sedimenta- tion rate environment the operation of upward looking optical modules will not be possible. Deep-sea currents – These must have low intensity and a stable direction. This is impor- tant for several reasons: 652 Emilio Migneco, Antonio Capone and Paolo Piattelli – it does not imply special requirements on the mechanical structures; – the detector deployment and positioning is easier if the water current is limited; – the optical noise due to bioluminescence, mainly excited by variation of the water cur- rents, is reduced. The Mediterranean Sea offers optimal condi- tions, on a worldwide scale, to locate an underwa- ter neutrino telescope. The NEMO Collaboration has performed a long-term research program to characterize deep-sea sites that could be appropri- ate for the installation of a deep-sea high-energy neutrino detector. Studies of deep-sea water opti- cal properties (absorption and diffusion) and the sites environmental properties (water temperature, salinity, biological activity, optical background, water currents and sedimentation) that will be briefly described in the following have been car- ried out. This activity has demonstrated that the abyssal plateau in the Ionian Sea (fig. 3) close to the southernmost cape of the coast of Sicily (Capo Passero) shows excellent characteristics to host the km3 underwater neutrino detector. The site was selected after a series of measure- ments in the region at varying distances from the coast. Its location at about 50 km from the shelf break about 80 km from the coast was chosen to ensure the best condition of stability in time of the water parameters and avoid any perturbation com- ing from the presence of the shelf break. A geological survey of the area verified the flatness and the absence of any evidence of recent turbidity events. Deep-sea currents were meas- ured to be, in the average, as low as 3 cm/s, nev- er exceeding 15 cm/s. The study of optical properties in the selected site is extremely important and must be carried out with a long-term program of characterisation carried out in all different seasons. Seawater, in- deed, absorbs and scatters photons as a function of water temperature, salinity and concentration, dimension and refraction index of dissolved and suspended, organic/inorganic particulate. These parameters are different in different marine sites and change as a function of time. Seawater oce- anographic parameters (temperature and salinity) and inherent optical properties (light absorption and attenuation) were measured as a function of depth, showing that, while at shallow depths wa- ter properties change as a function of season, at large depth (>1500) the water column has stable characteristics. The average value of blue (λ = = 440 nm) light absorption length is of the order of 70 m, comparable with that of pure salt water. The optical background noise was measured at 3000 m depth in Capo Passero. Data collect- ed in Spring 2002 and 2003, for several months, show that optical background induces on the optical modules a constant rate of 20-30 kHz (compatible with the one expected from 40K de- cay), with negligible contribution of biolumi- nescence bursts. These results were confirmed by biological analysis that show, at depth larger than 2500, extremely small concentration of dissolved bioluminescent organisms. 4.2. The underwater km3 detector concept The underwater neutrino detector design must be optimised in order to reach the needed Fig. 3. The south Ionian Sea, showing the location of the site selected and characterized by the NEMO collaboration for the installation of the km3 neutrino detector. 653 Underwater laboratories for astroparticle physics and deep-sea science effective area of ≈1 km2, a pointing accuracy better than 0.1° for 10 TeV muons, an energy resolution of the order of some tens percent in log(E) and an energy threshold close to 100 GeV. 4.2.1. Detector architecture It is clear that the architecture of the km3 de- tector should stem from a compromise between performances and technical feasibility of the detector. As a first approach one can consider filling up a volume of about one km3 with a lattice of equally spaced sensors, with a spacing of the same order of the absorption length of light in water. Taking the horizontal and vertical distance between down looking sensors equal to 60 m turns out in a total number of 5600 sensors arranged in 400 strings. Although the solution of arranging the sen- sors along a string presents some advantages in terms of simplicity, this architecture can be dif- ficult to realise due to the close distance be- tween strings (60 m) compared with their total height (780 m). Moreover, the large number of structures to be moored and connected raises the cost of the detector. Therefore, alternative solutions should be studied for the architecture, in which the number of structures is reduced by gathering a relatively large number of sensors on each of them. In par- ticular, constraints on the distance between struc- tures (larger than about 120 m) and on their height (smaller than 1 km) were suggested by a preliminary feasibility study in terms of con- struction, deployment and maintenance opera- tions of the detector within reasonable costs. Fol- lowing these suggestions a structure composed by a square array of «NEMO towers», shown in fig. 4, was proposed. The proposed architecture is «modular», in the sense that it is expandable with the addition of extra towers, and config- urable with different seafloor layouts. The performances of this architecture in terms of effective area and angular resolution were evaluated by means of computer simula- tions, which have shown that a detector built with this architecture can meet the required goal of a detection area larger than 1 km2 for muon energies larger than 10 TeV with the re- quired angular resolution. 4.2.2. Mechanical structures The NEMO collaboration has proposed an innovative structure, alternative to the string concept, to host optical modules: the NEMO tower. It is a three dimensional flexible struc- ture composed by a sequence of stories (that host the instrumentation) interlinked by a sys- tem of cables and anchored on the seabed. The structure is kept vertical by appropriate buoyan- cy on the top. The final features of the tower (number and length of stories, number of optical modules per storey, distance between the stories) can be op- timized following the results of numerical sim- ulations. However, the modular structure of the Fig. 4. The proposed layout of the km3 detector. A hierarchical arrangement of Junction Boxes (JBs), a main one connected to the main electro-optical cable and a number of secondary JBs, will allow to concen- trate the data flow coming from the detector structures and, at the same time, distribute the power. 654 Emilio Migneco, Antonio Capone and Paolo Piattelli tower will permit us to adjust these parameters to the experimental needs. In the preliminary design a 16-storey tower, where each storey is made with a 20 m long structure hosting two optical modules at each end (4 OM per storey), has been considered. The vertical separation between storeys is fixed at 40 m, giving a total active height of 680 m. An additional spacing of 150 m is added at the base of the tower, between the anchor and the lowermost storey to allow for a sufficient water volume below the detector. In its working posi- tion each storey will be rotated by 90°, with re- spect to the up and down adjacent ones, around the vertical axis of the tower. The tower will also be equipped with an acoustic triangulation system for precise posi- tion reconstruction purposes and with environ- mental sensors. One of the advantages of this structure is the fact that it can be compacted, by piling each storey upon the other, to simplify transport and deployment. The structure is unfurled, reaching its operating configuration, only after its de- ployment on the seabed. 4.3. Technological challenges for the km3 detector In recent years many innovations have been applied to underwater technology: DWDM (Dense Wavelength Division Multiplexing) is permitting a large increase in speed and band- width of the optical fibres data transmission; newly developed materials (synthetic fibres, al- loys, plastics, ...) can improve long term under- water reliability of complex deployed struc- tures; deep-sea (≈3000 m depth) operations with Remotely Operated Vehicles (ROV) or Au- tonomous Underwater Vehicles (AUV) have been developed and standardised. The design of the Mediterranean km3 will directly profit of these ultimate technologies. Concerning the km3 data transmission system, it is considered a wise approach to transmit all PMT signals, acquired at low threshold level, to shore. In such a way all filtering and event recon- struction procedures can be applied by on-shore data acquisition systems. This approach results in a very high data transmission rate (order of 100 Gbps) that has to be transmitted about 100 km off- shore. The above considerations impose the use of a fibre optics transmission system based on DWDM telecommunication technique and com- posed of mainly passive optical components to re- duce power consumption and increase reliability. The proposed NEMO architecture is designed to collect data hierarchically: each storey (hosting 4 OMs) transmits data to a tower main module us- ing an assigned wavelength (or colour). Each tow- er module collects data from the stories and sends all data, multiplexed in one single fibre, to a col- lector. This collector can receive data from a num- ber of towers, multiplexes them and sends them to the shore. A mirror system on shore de-multiplex- es the signals, recovering data from each storey, then from each very single OM. To cope with the problem of pressure and corrosion resistance of the underwater contain- ers that have to host the electronics, a new con- cept has been proposed by the NEMO collabo- ration. This innovative vessel decouples the two problems by placing an inner pressure resistant steel vessel inside an outer glass epoxy vessel filled with non-corrosive oil. Another crucial issue for deep-sea detector installation is the feasibility and reliability of underwater connections. However, the tech- nologies of underwater connections have con- siderably advanced in recent years and reliable connection systems exist that can be operated underwater by means of ROVs. These connec- tors and their operability have been tested both by the ANTARES and NEMO collaborations. 4.4. The NEMO Phase 1 project: a multidisciplinary underwater laboratory at 2000 m The technological solutions proposed for the km3 detector require an adequate process of vali- dation. For this reason the realization of a techno- logical demonstrator is underway. It will consist of a subset of the proposed km3 detector, includ- ing some critical elements like a junction box, a tower complete with data transmission, power and calibration systems. This project is called NEMO Phase 1 (fig. 5) and will be installed at the 655 Underwater laboratories for astroparticle physics and deep-sea science Underwater Test Site of the Laboratori Nazionali del Sud in Catania at a depth of 2000 m. The test site of the Laboratori Nazionali del Sud consists of an electro-optical submarine ca- ble, that connects the underwater installation to shore, and a shore station. The cable system is composed of a 25 km main electro-optical cable, split in two branches, each one 5 km long. One branch dedicated to the NEMO Phase 1 experiment (Migneco et al., 2004b; Piattelli, 2005), and the other one to the SN-1 underwater seismic monitoring station re- alized by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) (Favali et al., 2003). A shore station, located inside the port of Catania, will host the energy power system of the laboratory, the instrumentation control sys- tem, the landing station of the data transmission system and the data acquisition, as well as a mechanics and electronics laboratory for the as- sembly of the components. In January 2005 the project achieved a ma- jor milestone with the installation of the two ca- ble termination frames (that host the wet mate- able underwater plugs) and the deployment and connection of a small acoustic detection station on the first branch and the SN-1 station on the second branch. Both these stations have been operational ever since, continuously transmit- ting data to shore. The project will be completed in 2006 with the installation of the junction box and a proto- type four storey tower. In its final configuration this project will not only represent a fundamen- tal test of the technologies for the km3 detector but it will also be a fully functioning multidis- ciplinary laboratory at 2000 m depth, offering the possibility of on line connection to a variety of experiments. 4.5. The NEMO Phase 2 project: a deep-sea infrastructure at 3500 m Although the Phase 1 project will provide a fundamental test of the technologies proposed for the realization and installation of the detec- tor, these must be finally validated at the depths needed for the km3 detector. For these motiva- tions the realization of an infrastructure on the site of Capo Passero has been started. This in- frastructure will consist of a 100 km deep-sea cable, linking the 3500 m deep-sea site to the shore, a shore station, located inside the har- bour area of Portopalo di Capo Passero, and the underwater infrastructures needed to connect prototypes of the km3 detector. The construction details of the cable have still to be defined, but it will have characteristics of power load (greater than 50 kW) and number of Fig. 5. Schematic layout of the NEMO Phase 1 project. 656 Emilio Migneco, Antonio Capone and Paolo Piattelli optical fibres (20 or more) sufficient to serve a de- tector such as the proposed NEMO neutrino tele- scope. Construction work to restore an already ex- isting building that will become the shore sta- tion is going to start soon. The station will host the power system, data acquisition and control facilities, together with a large assembling area. The completion of this project is foreseen by the end of 2007. At this time it will be pos- sible to connect one or more prototypes of de- tector structures, allowing a full test at 3500 m of the deployment and connection procedures. This project will also allow a continuous long term on-line monitoring of the site properties (light transparency, optical background, water currents, …) whose knowledge is essential for the installation of the full detector. The possibil- ity of installing multidisciplinary observatories on the site is also foreseen. 5. Multidisciplinary researches in a km3 scale underwater laboratory The core discipline served by an underwater Neutrino Telescope is astroparticle physics, but it also has potential to attract significant interest from other scientific communities. In fact, the possibility to monitor continuously over long periods and in real time the sea bottom at depths of the order of 3000 m is of utmost in- terest to a large number of sea-related disci- plines like geophysics, biology, chemistry and environmental sciences. The underwater world has not yet been ex- tensively explored. Current technology allows autonomous vehicles or Remotely Operated Ve- hicles (ROVs) to carry out scientific experi- ments at great depths only for relatively short periods and with accompanying support ships. A deep-sea neutrino observatory will provide the community of deep-sea scientists with a continuous supply of power and a high band- width data channel, enabling them to make lo- cal real-time studies. Moreover, the information gathered by the optical sensors and other oceanographic monitoring equipment needed for running the neutrino telescope can be of use to oceanographers and marine biologists. More generally, other types of instrumenta- tion can be added to the observatory array. For example, ocean bottom seismometers can trans- mit their recordings in real time, thus permit- ting the localization of the epicenters of seismic events with greater accuracy. The effectiveness of this approach is shown by the realization of the Test Site Laboratory of the NEMO project, realized with the primary goal of testing technologies for the km3 project at 2000 m, but also hosting the SN-1 deep-sea seismic and environmental observatory of the Is- tituto Nazionale di Geofisica e Vulcanologia (Fa- vali et al., 2003), which constitutes the first ac- tive node of the European seafloor Observatory Network – ESONET (more information about the ESONET network can be found on the web site at http://www.oceanlab.abdn.ac.uk/research/ ESONET.shtml) (Priede et al., 2004). 6. Conclusions The realization of a km3 telescope for high- energy astrophysical neutrinos is a challenging task at the frontiers of astroparticle science. Several collaborations in Europe are already working on the realization of first generation demonstrators. More efforts are needed to de- velop a project for the km3 detector. In its five years of activity the NEMO col- laboration has contributed in this direction by performing an intense R&D activity. An extensive study on a site close to the coast of Sicily has demonstrated that it has op- timal characteristics for telescope installation, in particular as concerns the water optical prop- erties and its proximity to already existing in- frastructures like the LNS in Catania. A complete study has been performed to analyse all the detector components both in term of their technical feasibility and their in- stallation. This study, which has produced as a result a preliminary design of the detector, has shown that a detector with effective area over 1 km2 is realizable at an affordable cost. The realization of a demonstrator of some of the key technological solutions proposed for the km3 detector has been started at the under- water Test Site in Catania. This project foresees 657 Underwater laboratories for astroparticle physics and deep-sea science the realization of an underwater laboratory in- cluding prototypes of the proposed structures. The completion of this project is foreseen by the end of 2006. The design, construction and operation of a km3 neutrino telescope is a great challenge that must be pursued by a large international collab- oration. Recently, the scientific and technical ex- periences gathered by the ANTARES, NEMO and NESTOR collaborations have come togeth- er in the KM3NeT consortium (more informa- tion can be found on the KM3NeT web site at http://www.km3net.org), formed around the Eu- ropean institutes currently involved in the neutri- no astronomy projects (Thompson, 2005), with the aim of carrying out a Design Study for the km3 neutrino telescope. Based on the leading ex- pertise of these research groups, the develop- ment of the km3 telescope is envisaged to be achieved within a period of three years for preparatory R&D work plus five years for con- struction and deployment. 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