- Email: [email protected]
The KM3NeT neutrino telescope: Status and prospects
Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The KM3NeT neutrino telescope: Status and prospects Juan José Hernández–Rey n IFIC - Instituto de Física Corpuscular, Universitat de València–CSIC, E-46100 Valencia, Spain
The KM3NeT Collaboration art ic l e i nf o
a b s t r a c t
Available online 6 December 2013
The KM3NeT Collaboration intends to build and operate a deep-sea research infrastructure that will host as its main facility a neutrino telescope of several cubic kilometres. The infrastructure will be distributed in three sites located near Toulon, France (40 km offshore and seabed at 2500 m), Capo Passero (80 km offshore and seabed at 3500 m) and Pylos, Greece (20 km offshore and seabed between 2500 and 5000 m). In this contribution we describe the technological choices made for the basic components of the three-dimensional array of photo-sensitive devices in which the telescope will consist. The Digital Optical Modules (DOMs) will use a 17 in. pressure-resistant sphere to accommodate 31 photomultipliers (PMTs) with a 3 in. photocathode each. These DOMs will be installed in detector units (DUs) consisting of vertical strings anchored to the seabed and kept taut by the appropriate buoyancy. These DUs will be deployed by means of launching vehicles that will allow several of them to be deployed in the same campaign. The main features of the telescope and its technical components, the science it can do and the recent progress made towards its construction will be described here. & 2013 Elsevier B.V. All rights reserved.
Keywords: Neutrino Astroparticles Astronomy
1. Introduction The detection of high-energy cosmic neutrinos can help solve the problem of the origin of high-energy cosmic rays and be a new tool to study the mechanisms of hadronic acceleration in astrophysical objects. In the low energy domain (few MeV to several GeV) the observation of extraterrestrial and atmospheric neutrinos gave rise to the discovery of neutrino oscillations and to one of the most direct experimental tests of the models of supernova explosions. In the high energy regime (several GeV to EeV), neutrinos can provide information on the particle acceleration mechanisms in the Universe. Several astrophysical objects both Galactic and extra-galactic have been proposed as sites of acceleration of protons and nuclei. Even though experimental evidence is accumulating for some of them, much more information is required to gain an in-depth knowledge of the mechanisms called into play. Neutrinos have several advantages as cosmic messengers. They are neutral, therefore they are not deflected by magnetic fields and point back to their sources. They are weakly interacting and thus can escape from very dense astrophysical objects and travel long distances without being absorbed by matter or background radiation. In cosmic sites where hadrons are accelerated, it is likely that neutrinos are generated in the decay of charged pions produced in
n
Tel.: þ 34 963543536. E-mail address: juan.j.hernandez@ific.uv.es
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.11.055
the interaction of hadrons with the surrounding matter or radiation, indicating that hadronic acceleration is taking place. Experimental methods to detect neutrinos exist and have been technologically proven. The major challenge in the field of Neutrino Astronomy is at present to reach a sensitivity high enough to detect cosmic neutrino sources with sufficient statistics. The recent results reported from the IceCube telescope indicate that the first detection of high-energy cosmic neutrinos may be taking place. 2. Detection principle and general facility characteristics A method to detect high-energy cosmic neutrinos is by means of the Cherenkov light induced by the superluminal interaction products when they traverse natural media such as the ice of the South Pole or the water of deep oceans or lakes [1]. These media act both as a shield for downgoing muons coming from cosmic-ray showers produced in the atmosphere and as a dark, optically transparent material suitable to observe the Cherenkov radiation of the neutrino products. An array of photo-sensors at sufficient depth can record the Cherenkov light induced by upgoing muon tracks that can only come from neutrinos that have crossed the Earth and interacted relatively close to the array. Thanks to the long muon path – that can reach several kilometres at high energy – the effective volume for this channel is especially large. Moreover, other neutrino flavours can also be detected through the hadronic and electromagnetic showers that they produce when they interact with matter. The muon channel
78
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
Fig. 1. Artistic view of the KM3NeT neutrino telescope. More than 12 000 optical modules housing 3 in. PMTs will be deployed in several 100-m long strings anchored to the sea bed.
provides a good angular resolution since at high energies the angle between the muon and its parent neutrino is low and the resolution is mainly limited by instrumental effects (the optical properties of the medium and the time and spatial resolution of the telescope). The energy of the neutrino, E, can be estimated within a factor 2–3 through the energy deposited in the detector by the corresponding track. Showers can offer yet better energy resolutions of the order of 0.1–0.2 in log E. Sea or lake water and ice are therefore natural media that are used to deploy huge neutrino detectors using the Earth as a shield. The main backgrounds for this sort of telescope are downgoing atmospheric muons that could be wrongly reconstructed as upgoing and the unavoidable flux of upgoing muons coming from atmospheric neutrinos produced by cosmic ray showers at the other side of the Earth. The goal is therefore to detect an excess of neutrinos either diffuse (in all directions) or point-like (localized in a given point of the sky) above this irreducible background. Other methods have been developed to detect also downgoing neutrinos either taking advantage of the rapidly falling spectrum of atmospheric neutrinos with energy or using the containment in the detector of the interaction point to reject atmospheric muons and neutrinos. KM3NeT is a new generation neutrino telescope that will be deployed in the Mediterranean Sea at a depth between 2 and 5 km (see Fig. 1). In its final form, the facility is expected to contain more than 12 000 optical modules that will be installed in long strings, called detector units. The telescope will be deployed distributed in three different sites in several groups of detector units. The telescope will be optimised to detect Galactic sources via muon neutrinos, but will also be able to detect showers produced by neutrinos of other flavours. The facility will also provide a platform for studies in a variety of Earth and Sea sciences, yielding long-term data useful for oceanography, geophysics and marine biology [2].
larger than that of one single large PMT; smaller PMTs are less sensitive to the Earth's magnetic field and no additional system has to be installed to reduce its effect; the optical background can be better rejected and directional information of the detected photons can be obtained and used to improve the reconstruction efficiency and angular resolution of the particle track or shower. The Digital Optical Module of KM3NeT consists of 31 3-in. PMTs inside a 17-in. diameter pressure-resistant sphere (Fig. 2). All the required electronics to control the PMTs, digitise the signal and send the data to shore, are located inside the sphere. In order to maximize the detector sensitivity, each PMT will be surrounded by an expansion cone which collects photons that would otherwise miss the photocathode. Results for various angles of incidence with respect to the PMT surface indicate an increase in collection efficiency by 30% on average for angles up to 451 with respect to the perpendicular [3]. Each PMT is connected to an active base that allows the onshore control of its high voltage and the setting of the relevant thresholds. The bases are equipped with ASICs that digitise the signals, so that the threshold crossing time and the time-overthreshold information of each digitised hit can be sent to shore. This multi-PMT scheme provides a factor 10 better identification purity for hits with multiplicity 2 than the 10-in. PMTs and improves by 30% the detection efficiency for neutrinos in the 1–50 TeV energy range. The synchronization of the DOMs will be performed using a clock signal distributed from shore. The time offsets of the individual PMTs will be calibrated onshore before deployment. The light pulses emitted from several beacons will be used to monitor and correct if needed these offsets in situ. Laser beacons will be located at the sea bottom, while the so-called nanobeacons containing LEDs will be installed in the DOMs themselves. Additionally, each DOM contains a piezo-sensor for acoustic positioning, a tiltmeter, a compass and temperature and humidity sensors inside. The internal structure of the DOM has been carefully designed so as to efficiently remove heat from the electronics and transfer it to the sea through the glass sphere. A main central logic board manages the data acquisition and communication with the shore. Within this logic board an optical interface allows the transmission of data to shore with a DWDM technique with 50 GHz spacing. The DOM has a 1 Gb/s readout
3. The optical module and the detection unit Traditionally, neutrino telescopes have used large-area photocathode photomultipliers as light sensors. In KM3NeT an alternative approach has been selected, namely the use of many small PMTs inside the same pressure-resistant glass sphere. This has several advantages compared to the traditional solution: the photocathode surface inside the same size sphere can be made
Fig. 2. The Digital Optical Module of the KM3NeT neutrino telescope. 31 3 in. PMTs are housed in a 17 in. pressure-resistant sphere. The module also contains the electronics for readout and control and some other devices for calibration and monitoring.
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
79
bandwidth. Its consumption is around 10 W and the electronics are arranged in supports that allow the efficient transfer of heat to the surrounding water. The DOMs will be located along the detection units (DUs). The final adopted design for the DU consists of a flexible linear structure formed by two dyneema ropes with a backbone that provides connections for each DOM through two conductors for electrical power and two optical fibres for data flow and communication (see Fig. 3) The DU will be anchored to the sea floor and kept taut by the buoyancy of the DOMs and by additional buoyancy at the upper section of the DU. To prevent excessive tension on the backbone, sufficient slack is foreseen during DU construction so as to accommodate the stretch of the ropes once the structure is in place. Each DU will contain 18 DOMs vertically separated by 36 m. The lowest DOM will be located 100 m above the sea bottom. The total height of a DU will be slightly over 700 m.
4. Building blocks, layout, sites and deployment The density and the total number of DOMs deployed determine the response of the detector as a function of the energy of the parent neutrinos. Simulation studies have been carried out in order to estimate the performance of different configurations as a function of the vertical spacing between DOMs, the number of DOMs per DU, the horizontal separation between DUs and their total number. The results of these studies indicate that the detection efficiency increases with the number of DUs and of DOMs per DU, but that it levels out after 120 DUs and 18 DOMs per DU. These values are a consequence of the selected energy spectrum of the source used in the simulation, which is rather hard and has a cut-off, corresponding to what is foreseen for the expected Galactic sources, such as supernova remnants. This defines the size of a “building block” – the smallest segmentation of the whole detector that is fully efficient. The total size of the final telescope is expected to be of the order of six of these building blocks. Technically, this segmentation is important for the design of the distribution of the secondary junction boxes that will supply the detector with power and will allow the communication with the shore station. The building blocks do not necessarily have to be installed side by side in the same location. As a matter of fact, an agreement has been reached to deploy the telescope in the three different sites willing to host it. The fact that the technology will be the same in the different sites and that the building block concept ensures an optimal use of the deployed DUs makes the gain associated to the multi-site concept – in terms of additional funding and human resources – well worth the price of the possible overheads incurred in. The onshore facilities are under construction at La Seyne sur Mer, near Toulon in France and the 40-km electro-optical cable is expected to be deployed soon. The depth in this site is round 2500 m below sea level. The onshore station is ready at Capo Passero, Italy, and connected to a 100 km electro-optical cable. The depth of this site is 3500 m below sea level. In order to deploy such a large number of DUs a special technique has been devised. Each DU (a string) is wound on a launcher vehicle (see Fig. 4). This vehicle is then lowered from a vessel to the seabed, the buoy is released and the DU unfurls until it deploys completely. Several launcher vehicles will be used to deploy several strings during a single campaign. Moreover, the launcher vehicles can be recovered after each deployment.
Fig. 3. Schematic view of a KM3NeT Detector Unit. The DOMs in the string-like structure will be held by two ropes of high performance polyethylene (dyneema). The DU is anchored to the sea bed and the buoyancy is provided by the DOMs and by several buoys at the top of the structure.
The total cost of the construction of the infrastructure is estimated to be between 220 and 250 million euros. The operational costs of the infrastructure are estimated to be 3% of the investment. These figures are estimated assuming the distributed site scenario.
5. Scientific goals As previously mentioned, the main goal of the telescope is the observation of astrophysical sources of neutrinos which will likely shed light on the origin of the cosmic rays. Due to its location and the optimisation of its layout the primary target is Galactic sources and the primary channel charged current interactions of muon neutrinos. Nevertheless, other sources and channels will be targets as well. A study was made to estimate the sensitivity of KM3NeT to the young shell-type supernova remnant (SNR) RXJ1713.7-3946. This SNR has been observed by HESS in several campaigns [4] and its energy spectrum in gammas has been measured up to around 100 TeV. It will be visible around 75% of the time for KM3NeT. Using some approximations for its morphology and for the energy spectrum of the emitted neutrinos, simulations indicate that the signal will produce a 5s fluctuation over the background with a 50% probability in 5 years. Furthermore, with this same 50% probability a 3s fluctuation above the background will be observed in only 2 years of data taking. Vela X is one of the closest pulsar wind nebulae and is associated to the powerful Vela pulsar PSR B0833-45. TeV gamma-ray emission has been observed from Vela X which hints that hadronic acceleration may be taking place. A recent analysis
80
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
Fig. 5. Prototype KM3NeT DOM integrated in the instrumentation line of the ANTARES telescope and deployed in April 2013.
Fig. 4. A launcher vehicle ready to go. The DU (a mockup in this case) is housed in a compact structure that is lowered to the seabed, the buoys are released and the detector unit unfurls until it reaches its full length.
indicates that under certain assumptions a neutrino signal can be detected with 50% probability at a 5s (3s) level in 3.3 (1.2) years of data taking of KM3NeT. A study of the possibility of detecting neutrinos coming from the Fermi Bubbles has also been carried out. Under certain assumptions, the forecast is rather positive [5]. Lately, the possibility of building a first phase of KM3NeT dedicated to disentangling the neutrino mass hierarchy has been raised. This measurement would require a very different layout than that for a high energy neutrino telescope, with a much denser detector designed to observe atmospheric neutrinos with energies down to a few GeV. Studies are being carried out to see if this is possible with the proviso that the same technology as the one just described be used. If these studies conclude that such a measurement is indeed feasible, KM3NeT will seriously consider a temporary “detour” of its main goal, neutrino astronomy. The KM3NeT facility will also provide a very good platform for studies in other fields of Earth and Sea sciences.
Fig. 6. Coincidence rate as a function of the number of coincident hits. The data is shown as black points. The estimated contributions from the combinatorial background – aleatorial coincidences – from 40K decays and from downgoing muon tracks are indicated.
6. Recent progress and future steps An exhaustive qualification programme is being carried out by the KM3NeT Collaboration. Within this programme, a prototype DOM has been integrated in the instrumentation line of the ANTARES telescope and has been deployed recently (see Fig. 5). This DOM is working according to specifications and providing very useful data which is being currently analysed. As an example of some preliminary results, the coincidence rate as a function of the number of coincidences in a 20 ns window is shown in Fig. 6. To these coincidences contribute uncorrelated random hits, those from 40 K decays and Cherenkov light coming from downgoing atmospheric muons. As can be seen, when the number of coincidences is high (4 5 hits) they come mainly from muon tracks. Thus, even one
Fig. 7. Rate of single hits in a 10 in. PMT of the Capo Passero tower as a function of time (2 h of data-taking). A low continuous background and a very moderate burst rate are observed, in agreement with previous measurements.
single DOM is able to start disentangling hits produced by tracks from those coming from the optical background. In Capo Passero, Siciliy, a prototype structure built with the so-called tower architecture has been deployed and connected in March 2013. This tower is working according to specifications and providing high quality data. In Fig. 7, an example of the single rate on a 10-in. PMT is shown. The rate shows a low background in
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
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these waters, both in terms of continuous rate and bursts, confirming previous measurements with standalone equipment. A campaign comprising five deployment tests of the unfurling technique mentioned in Section 4 (see Fig. 4) was carried out near Málaga, Spain. The know-how acquired will be used to make the first deployment of a mechanical model of a full-size DU in the Toulon coast. A pre-production model DU with a reduced size and containing three final DOMs will be built and installed at Capo Passero in the coming months. It is expected that one final, fully equipped DU will be deployed by the end of 2014.
Acknowledgements
7. Conclusions
References
With the successful deployment, operation and scientific exploitation of the ANTARES neutrino telescope the need to proceed soon to a larger detector has become a priority. The first step towards such a telescope, the so-called KM3NeT phase 1, has already secured the funding. This will demonstrate the feasibility of the concept and pave the way to a full detector by the end of this decade.
[1] M.A. Markov, On high energy neutrino physics, in: Proceedings of 10th Annual International Conference on High Energy Physics, 578, Rochester 1960. [2] 〈www.km3net.org〉. [3] Adrián-Martinez, et al., KM3NeT Collaboration, Journal of Instrumentation 8 (2013) T03006 〈http://iopscience.iop.org/1748-0221/8/03/T03006〉. [4] F. Aharonian, et al., Astronomy & Astrophysics 464 (2007) 235. [5] Adrián-Martinez, et al., KM3NeT Collaboration, Astroparticle Physics 42 (2013) 7.
We wish to acknowledge the following agencies and grants that have supported the KM3NeT initiative: The European Commission under Contract 011937 (Sixth Framework Programme) and Grant agreement 212525 (Seventh Framework Programme); The Spanish Ministry of Science and Innovation and the Generalitat Valenciana under grants FPA2009-13983-C02-01, FPA2012-37528C02-01, PROMETEO/2009/026 and the Consolider MultiDark, CSD 2009-00064.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The KM3NeT neutrino telescope: Status and prospects Juan José Hernández–Rey n IFIC - Instituto de Física Corpuscular, Universitat de València–CSIC, E-46100 Valencia, Spain
The KM3NeT Collaboration art ic l e i nf o
a b s t r a c t
Available online 6 December 2013
The KM3NeT Collaboration intends to build and operate a deep-sea research infrastructure that will host as its main facility a neutrino telescope of several cubic kilometres. The infrastructure will be distributed in three sites located near Toulon, France (40 km offshore and seabed at 2500 m), Capo Passero (80 km offshore and seabed at 3500 m) and Pylos, Greece (20 km offshore and seabed between 2500 and 5000 m). In this contribution we describe the technological choices made for the basic components of the three-dimensional array of photo-sensitive devices in which the telescope will consist. The Digital Optical Modules (DOMs) will use a 17 in. pressure-resistant sphere to accommodate 31 photomultipliers (PMTs) with a 3 in. photocathode each. These DOMs will be installed in detector units (DUs) consisting of vertical strings anchored to the seabed and kept taut by the appropriate buoyancy. These DUs will be deployed by means of launching vehicles that will allow several of them to be deployed in the same campaign. The main features of the telescope and its technical components, the science it can do and the recent progress made towards its construction will be described here. & 2013 Elsevier B.V. All rights reserved.
Keywords: Neutrino Astroparticles Astronomy
1. Introduction The detection of high-energy cosmic neutrinos can help solve the problem of the origin of high-energy cosmic rays and be a new tool to study the mechanisms of hadronic acceleration in astrophysical objects. In the low energy domain (few MeV to several GeV) the observation of extraterrestrial and atmospheric neutrinos gave rise to the discovery of neutrino oscillations and to one of the most direct experimental tests of the models of supernova explosions. In the high energy regime (several GeV to EeV), neutrinos can provide information on the particle acceleration mechanisms in the Universe. Several astrophysical objects both Galactic and extra-galactic have been proposed as sites of acceleration of protons and nuclei. Even though experimental evidence is accumulating for some of them, much more information is required to gain an in-depth knowledge of the mechanisms called into play. Neutrinos have several advantages as cosmic messengers. They are neutral, therefore they are not deflected by magnetic fields and point back to their sources. They are weakly interacting and thus can escape from very dense astrophysical objects and travel long distances without being absorbed by matter or background radiation. In cosmic sites where hadrons are accelerated, it is likely that neutrinos are generated in the decay of charged pions produced in
n
Tel.: þ 34 963543536. E-mail address: juan.j.hernandez@ific.uv.es
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.11.055
the interaction of hadrons with the surrounding matter or radiation, indicating that hadronic acceleration is taking place. Experimental methods to detect neutrinos exist and have been technologically proven. The major challenge in the field of Neutrino Astronomy is at present to reach a sensitivity high enough to detect cosmic neutrino sources with sufficient statistics. The recent results reported from the IceCube telescope indicate that the first detection of high-energy cosmic neutrinos may be taking place. 2. Detection principle and general facility characteristics A method to detect high-energy cosmic neutrinos is by means of the Cherenkov light induced by the superluminal interaction products when they traverse natural media such as the ice of the South Pole or the water of deep oceans or lakes [1]. These media act both as a shield for downgoing muons coming from cosmic-ray showers produced in the atmosphere and as a dark, optically transparent material suitable to observe the Cherenkov radiation of the neutrino products. An array of photo-sensors at sufficient depth can record the Cherenkov light induced by upgoing muon tracks that can only come from neutrinos that have crossed the Earth and interacted relatively close to the array. Thanks to the long muon path – that can reach several kilometres at high energy – the effective volume for this channel is especially large. Moreover, other neutrino flavours can also be detected through the hadronic and electromagnetic showers that they produce when they interact with matter. The muon channel
78
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
Fig. 1. Artistic view of the KM3NeT neutrino telescope. More than 12 000 optical modules housing 3 in. PMTs will be deployed in several 100-m long strings anchored to the sea bed.
provides a good angular resolution since at high energies the angle between the muon and its parent neutrino is low and the resolution is mainly limited by instrumental effects (the optical properties of the medium and the time and spatial resolution of the telescope). The energy of the neutrino, E, can be estimated within a factor 2–3 through the energy deposited in the detector by the corresponding track. Showers can offer yet better energy resolutions of the order of 0.1–0.2 in log E. Sea or lake water and ice are therefore natural media that are used to deploy huge neutrino detectors using the Earth as a shield. The main backgrounds for this sort of telescope are downgoing atmospheric muons that could be wrongly reconstructed as upgoing and the unavoidable flux of upgoing muons coming from atmospheric neutrinos produced by cosmic ray showers at the other side of the Earth. The goal is therefore to detect an excess of neutrinos either diffuse (in all directions) or point-like (localized in a given point of the sky) above this irreducible background. Other methods have been developed to detect also downgoing neutrinos either taking advantage of the rapidly falling spectrum of atmospheric neutrinos with energy or using the containment in the detector of the interaction point to reject atmospheric muons and neutrinos. KM3NeT is a new generation neutrino telescope that will be deployed in the Mediterranean Sea at a depth between 2 and 5 km (see Fig. 1). In its final form, the facility is expected to contain more than 12 000 optical modules that will be installed in long strings, called detector units. The telescope will be deployed distributed in three different sites in several groups of detector units. The telescope will be optimised to detect Galactic sources via muon neutrinos, but will also be able to detect showers produced by neutrinos of other flavours. The facility will also provide a platform for studies in a variety of Earth and Sea sciences, yielding long-term data useful for oceanography, geophysics and marine biology [2].
larger than that of one single large PMT; smaller PMTs are less sensitive to the Earth's magnetic field and no additional system has to be installed to reduce its effect; the optical background can be better rejected and directional information of the detected photons can be obtained and used to improve the reconstruction efficiency and angular resolution of the particle track or shower. The Digital Optical Module of KM3NeT consists of 31 3-in. PMTs inside a 17-in. diameter pressure-resistant sphere (Fig. 2). All the required electronics to control the PMTs, digitise the signal and send the data to shore, are located inside the sphere. In order to maximize the detector sensitivity, each PMT will be surrounded by an expansion cone which collects photons that would otherwise miss the photocathode. Results for various angles of incidence with respect to the PMT surface indicate an increase in collection efficiency by 30% on average for angles up to 451 with respect to the perpendicular [3]. Each PMT is connected to an active base that allows the onshore control of its high voltage and the setting of the relevant thresholds. The bases are equipped with ASICs that digitise the signals, so that the threshold crossing time and the time-overthreshold information of each digitised hit can be sent to shore. This multi-PMT scheme provides a factor 10 better identification purity for hits with multiplicity 2 than the 10-in. PMTs and improves by 30% the detection efficiency for neutrinos in the 1–50 TeV energy range. The synchronization of the DOMs will be performed using a clock signal distributed from shore. The time offsets of the individual PMTs will be calibrated onshore before deployment. The light pulses emitted from several beacons will be used to monitor and correct if needed these offsets in situ. Laser beacons will be located at the sea bottom, while the so-called nanobeacons containing LEDs will be installed in the DOMs themselves. Additionally, each DOM contains a piezo-sensor for acoustic positioning, a tiltmeter, a compass and temperature and humidity sensors inside. The internal structure of the DOM has been carefully designed so as to efficiently remove heat from the electronics and transfer it to the sea through the glass sphere. A main central logic board manages the data acquisition and communication with the shore. Within this logic board an optical interface allows the transmission of data to shore with a DWDM technique with 50 GHz spacing. The DOM has a 1 Gb/s readout
3. The optical module and the detection unit Traditionally, neutrino telescopes have used large-area photocathode photomultipliers as light sensors. In KM3NeT an alternative approach has been selected, namely the use of many small PMTs inside the same pressure-resistant glass sphere. This has several advantages compared to the traditional solution: the photocathode surface inside the same size sphere can be made
Fig. 2. The Digital Optical Module of the KM3NeT neutrino telescope. 31 3 in. PMTs are housed in a 17 in. pressure-resistant sphere. The module also contains the electronics for readout and control and some other devices for calibration and monitoring.
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
79
bandwidth. Its consumption is around 10 W and the electronics are arranged in supports that allow the efficient transfer of heat to the surrounding water. The DOMs will be located along the detection units (DUs). The final adopted design for the DU consists of a flexible linear structure formed by two dyneema ropes with a backbone that provides connections for each DOM through two conductors for electrical power and two optical fibres for data flow and communication (see Fig. 3) The DU will be anchored to the sea floor and kept taut by the buoyancy of the DOMs and by additional buoyancy at the upper section of the DU. To prevent excessive tension on the backbone, sufficient slack is foreseen during DU construction so as to accommodate the stretch of the ropes once the structure is in place. Each DU will contain 18 DOMs vertically separated by 36 m. The lowest DOM will be located 100 m above the sea bottom. The total height of a DU will be slightly over 700 m.
4. Building blocks, layout, sites and deployment The density and the total number of DOMs deployed determine the response of the detector as a function of the energy of the parent neutrinos. Simulation studies have been carried out in order to estimate the performance of different configurations as a function of the vertical spacing between DOMs, the number of DOMs per DU, the horizontal separation between DUs and their total number. The results of these studies indicate that the detection efficiency increases with the number of DUs and of DOMs per DU, but that it levels out after 120 DUs and 18 DOMs per DU. These values are a consequence of the selected energy spectrum of the source used in the simulation, which is rather hard and has a cut-off, corresponding to what is foreseen for the expected Galactic sources, such as supernova remnants. This defines the size of a “building block” – the smallest segmentation of the whole detector that is fully efficient. The total size of the final telescope is expected to be of the order of six of these building blocks. Technically, this segmentation is important for the design of the distribution of the secondary junction boxes that will supply the detector with power and will allow the communication with the shore station. The building blocks do not necessarily have to be installed side by side in the same location. As a matter of fact, an agreement has been reached to deploy the telescope in the three different sites willing to host it. The fact that the technology will be the same in the different sites and that the building block concept ensures an optimal use of the deployed DUs makes the gain associated to the multi-site concept – in terms of additional funding and human resources – well worth the price of the possible overheads incurred in. The onshore facilities are under construction at La Seyne sur Mer, near Toulon in France and the 40-km electro-optical cable is expected to be deployed soon. The depth in this site is round 2500 m below sea level. The onshore station is ready at Capo Passero, Italy, and connected to a 100 km electro-optical cable. The depth of this site is 3500 m below sea level. In order to deploy such a large number of DUs a special technique has been devised. Each DU (a string) is wound on a launcher vehicle (see Fig. 4). This vehicle is then lowered from a vessel to the seabed, the buoy is released and the DU unfurls until it deploys completely. Several launcher vehicles will be used to deploy several strings during a single campaign. Moreover, the launcher vehicles can be recovered after each deployment.
Fig. 3. Schematic view of a KM3NeT Detector Unit. The DOMs in the string-like structure will be held by two ropes of high performance polyethylene (dyneema). The DU is anchored to the sea bed and the buoyancy is provided by the DOMs and by several buoys at the top of the structure.
The total cost of the construction of the infrastructure is estimated to be between 220 and 250 million euros. The operational costs of the infrastructure are estimated to be 3% of the investment. These figures are estimated assuming the distributed site scenario.
5. Scientific goals As previously mentioned, the main goal of the telescope is the observation of astrophysical sources of neutrinos which will likely shed light on the origin of the cosmic rays. Due to its location and the optimisation of its layout the primary target is Galactic sources and the primary channel charged current interactions of muon neutrinos. Nevertheless, other sources and channels will be targets as well. A study was made to estimate the sensitivity of KM3NeT to the young shell-type supernova remnant (SNR) RXJ1713.7-3946. This SNR has been observed by HESS in several campaigns [4] and its energy spectrum in gammas has been measured up to around 100 TeV. It will be visible around 75% of the time for KM3NeT. Using some approximations for its morphology and for the energy spectrum of the emitted neutrinos, simulations indicate that the signal will produce a 5s fluctuation over the background with a 50% probability in 5 years. Furthermore, with this same 50% probability a 3s fluctuation above the background will be observed in only 2 years of data taking. Vela X is one of the closest pulsar wind nebulae and is associated to the powerful Vela pulsar PSR B0833-45. TeV gamma-ray emission has been observed from Vela X which hints that hadronic acceleration may be taking place. A recent analysis
80
J.J. Hernández–Rey / Nuclear Instruments and Methods in Physics Research A 742 (2014) 77–81
Fig. 5. Prototype KM3NeT DOM integrated in the instrumentation line of the ANTARES telescope and deployed in April 2013.
Fig. 4. A launcher vehicle ready to go. The DU (a mockup in this case) is housed in a compact structure that is lowered to the seabed, the buoys are released and the detector unit unfurls until it reaches its full length.
indicates that under certain assumptions a neutrino signal can be detected with 50% probability at a 5s (3s) level in 3.3 (1.2) years of data taking of KM3NeT. A study of the possibility of detecting neutrinos coming from the Fermi Bubbles has also been carried out. Under certain assumptions, the forecast is rather positive [5]. Lately, the possibility of building a first phase of KM3NeT dedicated to disentangling the neutrino mass hierarchy has been raised. This measurement would require a very different layout than that for a high energy neutrino telescope, with a much denser detector designed to observe atmospheric neutrinos with energies down to a few GeV. Studies are being carried out to see if this is possible with the proviso that the same technology as the one just described be used. If these studies conclude that such a measurement is indeed feasible, KM3NeT will seriously consider a temporary “detour” of its main goal, neutrino astronomy. The KM3NeT facility will also provide a very good platform for studies in other fields of Earth and Sea sciences.
Fig. 6. Coincidence rate as a function of the number of coincident hits. The data is shown as black points. The estimated contributions from the combinatorial background – aleatorial coincidences – from 40K decays and from downgoing muon tracks are indicated.
6. Recent progress and future steps An exhaustive qualification programme is being carried out by the KM3NeT Collaboration. Within this programme, a prototype DOM has been integrated in the instrumentation line of the ANTARES telescope and has been deployed recently (see Fig. 5). This DOM is working according to specifications and providing very useful data which is being currently analysed. As an example of some preliminary results, the coincidence rate as a function of the number of coincidences in a 20 ns window is shown in Fig. 6. To these coincidences contribute uncorrelated random hits, those from 40 K decays and Cherenkov light coming from downgoing atmospheric muons. As can be seen, when the number of coincidences is high (4 5 hits) they come mainly from muon tracks. Thus, even one
Fig. 7. Rate of single hits in a 10 in. PMT of the Capo Passero tower as a function of time (2 h of data-taking). A low continuous background and a very moderate burst rate are observed, in agreement with previous measurements.
single DOM is able to start disentangling hits produced by tracks from those coming from the optical background. In Capo Passero, Siciliy, a prototype structure built with the so-called tower architecture has been deployed and connected in March 2013. This tower is working according to specifications and providing high quality data. In Fig. 7, an example of the single rate on a 10-in. PMT is shown. The rate shows a low background in
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these waters, both in terms of continuous rate and bursts, confirming previous measurements with standalone equipment. A campaign comprising five deployment tests of the unfurling technique mentioned in Section 4 (see Fig. 4) was carried out near Málaga, Spain. The know-how acquired will be used to make the first deployment of a mechanical model of a full-size DU in the Toulon coast. A pre-production model DU with a reduced size and containing three final DOMs will be built and installed at Capo Passero in the coming months. It is expected that one final, fully equipped DU will be deployed by the end of 2014.
Acknowledgements
7. Conclusions
References
With the successful deployment, operation and scientific exploitation of the ANTARES neutrino telescope the need to proceed soon to a larger detector has become a priority. The first step towards such a telescope, the so-called KM3NeT phase 1, has already secured the funding. This will demonstrate the feasibility of the concept and pave the way to a full detector by the end of this decade.
[1] M.A. Markov, On high energy neutrino physics, in: Proceedings of 10th Annual International Conference on High Energy Physics, 578, Rochester 1960. [2] 〈www.km3net.org〉. [3] Adrián-Martinez, et al., KM3NeT Collaboration, Journal of Instrumentation 8 (2013) T03006 〈http://iopscience.iop.org/1748-0221/8/03/T03006〉. [4] F. Aharonian, et al., Astronomy & Astrophysics 464 (2007) 235. [5] Adrián-Martinez, et al., KM3NeT Collaboration, Astroparticle Physics 42 (2013) 7.
We wish to acknowledge the following agencies and grants that have supported the KM3NeT initiative: The European Commission under Contract 011937 (Sixth Framework Programme) and Grant agreement 212525 (Seventh Framework Programme); The Spanish Ministry of Science and Innovation and the Generalitat Valenciana under grants FPA2009-13983-C02-01, FPA2012-37528C02-01, PROMETEO/2009/026 and the Consolider MultiDark, CSD 2009-00064.