The detection unit deployed in the KM3NeT-It site at a depth of 3449 m is a tower-like structure composed of eight horizontal elements (named floors). Each floor is a 8 m long marine grade aluminum structure connected to its next neighbours by means of eight tensioning ropes . The arrangement of these ropes is such that each floor is forced to a position perpendicular to its vertical neighbours, as shown in Fig. 4. An iron anchor fixes the structure to the seabed while an appropriate buoyancy at the top provides the pull to keep the structure taut. The floors are vertically spaced by 40 m, with the lowermost one positioned 100 m above the sea bottom. Each floor holds four Optical Modules (OM), two at each end, one looking vertically downwards and the other horizontally outwards. The structure is designed to be assembled and deployed in a compact configuration (Fig. 5) and to be unfurled once on the sea bottom under the pull provided by the buoy.
A schematics of the tower is shown in Fig. 6. In addition to the 32 OMs the instrumentation installed includes several sensors for calibration and environmental monitoring. In particular for positioning calibration purposes two hydrophones are installed close to the ends of each floor and two others on the tower base. Being the deep seawater a dynamical medium, monitoring of its oceanographic and optical properties during the detector operation is important since they can have an impact on the detector performance. For that reason environmental probes are installed on the tower: two Conductivity–Temperature–Depth (CTD) probes,Footnote 3 installed on the 1st and 7th floor; a light transmissometre used for the measurement of blue light attenuation in seawater (C*); a Doppler Current Sensor (DCS) used to monitor deep sea currents installed on the 5th floor.
Power distribution and data transmission along the tower is fulfilled by means of a “backbone” electro-optical cable. This is a lightweight umbilical subsea cable,Footnote 4 carrying 10 electrical conductors and 12 optical fibres.
The optical modules
The OM is the basic element of the NEMO Phase-2 detection unit [20, 21]. Each OM consists of a 13-inch pressure resistant (up to 700 bar) borosilicate glass sphere,Footnote 5 housing a 10-inch Hamamatsu photomultiplier (PMT R7081-SEL)  together with its high-voltage power supply, read-out electronics and calibration system (Fig. 7). Mechanical and optical contact between the PMT and the internal glass surface is ensured by an optical silicone gel. A \(\mu \)-metal cage shields the PMT from the Earth’s magnetic field. A detailed description is given in .
PMT signal digitization is provided by a Front-End Module board that is also housed inside the OM . The board samples the analog PMT signal using two 8-bit Fast Analog to Digital Converters (Fast-ADCs) running at 100 MHz and staggered by 5 ns. This technique gives the desired sampling rate yet allowing for a power dissipation lower than a single 200 MHz ADC. To match the large dynamic range of the PMT signal to the input voltage range of the ADCs, the signal is compressed by a non-linear circuit, which applies a quasi-logarithmic signal compression.
Four OMs were also equipped with a prototype RFID based system that allowed to acquire oceanographic data with readout through the glass sphere without the use of feedthroughs .
Electronics and cabling
A simplified scheme of the electronics and cabling of the tower is shown in Fig. 8. At the level of each floor the backbone is split by means of breakout boxes (BoB). Each breakout is a high-density polyethylene vessel filled with silicone oil and pressure compensated. The BoB is equipped with two hybrid penetrators, which are used to split the backbone, and two connectors (one electrical and one optical) from the backbone to the floor cabling system.
On each floor the four OM digital data are sent to a Floor Control Module board (FCM) housed in a pressure resistant vessel, the Floor Module Protective Oceanic Device (FMPOD). The FMPOD is fitted in the middle of the floor mechanical structure and houses the data acquisition, control and the power distribution systems of the floor. A similar setup is adopted for the tower base.
The 375 V supplied by the MVC is monitored at the level of the tower base and distributed to the eight floors. Inside the floor FMPOD a Power Control System board (PCS) provides conversion of the DC supply from 375 V to the low voltages needed by the electronics as well as monitoring and control of the main electrical parameters of the floor.
A Slow Control Interface board (SCI) provides the interface to the oceanographic instruments installed on the floor via RS-232 serial standard. In addition, each SCI has two analogue sensors to monitor humidity and temperature inside the FMPOD.
The acoustic board (AcuBoard) is a part of the positioning system described in Sect. 3.5.1.
Data transmission system
The link to the shore-station uses an optical fibre as physical layer and implements a high speed serial link using a proprietary data format . All data are encoded into a serial 800 Mb/s stream by a serializer, converted into optical signal by an electro-optical transceiver and transmitted to the shore station. In the communication protocol the data stream is divided into 125 \(\upmu \)s long frames of 10,000 bytes each.
A transmission system through optical links based on Dense Wavelength Division Multiplexing (DWDM) technology was chosen . It is implemented by means of add-and-drop passive devices that multiplex/demultiplex many optical channels at different wavelengths into/from the same fibre. Two wavelengths are associated to each floor of the tower, one for the sea-to-shore and one for the shore-to-sea communication. The optical wavelengths are chosen according to the ITU standard grid with 100 GHz frequency spacing in the C-Band thus allowing up to 45 channels per fibre. The DWDM network provides, indeed, a point-to-point communication between the shore-station equipment and the deep-sea apparatus. Each FCM contains an add-and-drop filter that allows to add or subtract the specific optical wavelength allocated to the floor. Data from all floors are thus transmitted through the backbone in the same fibre.
Detector data are received onshore by a dedicated electronics board, named Ethernet FCM (EFCM), which is based on a Virtex-5Footnote 6 development board. This board collects the data received by the underwater electronics and transfers them to the DAQ and storage systems through a Gigabit Ethernet connection.
Data acquisition system
Thanks to the extremely large communication bandwidth available (2 Gpbs), no hardware triggers are implemented underwater: all the digitized signals are sent to shore. The data stream originated by each FCM is addressed on shore to the twin EFCM board, which transfers the data to the onshore Trigger and Data Acquisition System (TriDAS) .
Each detected photon pulse is sampled by the FEM and arranged by the FCM in a hit record with a mean size of 28 bytes. Consequently the tower averaged optical throughput is about 250 Mbps. The total amount of data from offshore includes also a stream of about 80 Mbps from the acoustic sensors and a negligible contribution of slow control data. The electronics were designed to deal with an optical signal up to 150 kHz continuous single rate on each PMT without dead time.
From the EFCMs, the PMT optical data-stream is routed through a 1 Gb Ethernet network to the first layer of the TriDAS, composed of two Hit Managers (HM) processes running on two CPUs. Each HM gathers data from half a tower and coherently time-slices the continuous stream of data into time intervals of 200 ms. All the data corresponding to a given interval of time are sent by the HMs to a single TriggerCPU process (TCPU). Subsequent time slices are addressed to the others TCPU processes, implementing the trigger algorithms for background rejection. A reduced selected stream is then addressed, through a 1 Gb Ethernet switch, from all the TCPUs to the Event Manager (EM) server which is deputed to write the trigger selected data on the local temporary storage device. Finally data are copied to the long lasting storage facility at the Laboratori Nazionali del Sud in Catania (Italy) by means of a dedicated 1 Gb Ethernet point to point connection.
Positioning and calibration
Acoustic positioning system
Since the tower structure is not rigid, floors are subject to the effects of deep-sea currents than can distort the vertical line shape of the tower, making the floors rotate and tilt. For a proper reconstruction of the muon tracks, which is based on space-time correlation of Cherenkov photons hitting the OMs, the knowledge of the position of each OM with a precision of the order of 10 cm is needed. This can be obtained by using a system based on acoustic triangulation as demonstrated by previous experience gained with the NEMO Phase-1 prototype .
The acoustic positioning system installed on the NEMO Phase-2 tower consists of couples of hydrophones mounted in fixed positions close to the end of each floor (H0 and H1 in Fig. 6) [28, 29]. For testing and validation purposes different types of hydrophones were used. The six lowermost floors are equipped with large broadband hydrophones (10 Hz–70 kHz).Footnote 7 A couple of free flooded rings (FFR) hydrophonesFootnote 8 is installed on the 7th floor. Finally, two of the OMs on the 8th floor are equipped with custom designed piezoelectric acoustic sensors glued to the internal surface of the glass sphere. Data from all acoustic receivers are sampled offshore at a frequency of 192 kHz with a 24 bit resolution and continuously sent to shore. Offshore data are time stamped with the absolute acquisition time by a dedicated synchronous system which is phased with a GPS time station.Footnote 9 The acoustic position system is completed with a set of autonomous acoustic beacons installed on the seafloor at a distance of approximately 400 m from the tower and one beacon at the base of the tower.
An independent real time monitoring of the floor orientation is provided by an Attitude Heading Reference System (AHRS), that includes triaxial gyroscopes, accelerometers and magnetometers, placed inside each FMPOD.
Time calibration system
The reconstruction of physical events (e.g., muon tracks) requires sub-nanosecond synchronisation between the OMs.
The synchronous communication protocol implemented on the tower provides synchronization with a unique clock source (Master Clock) for all electronics boards and OMs. The clock distribution system is based on a GPS station, which provides the absolute time, encoded in IRIG-B 100-1344 standard, and a high-stability 10 MHz clock which works as Master Clock for the detector .
Due to the clock propagation delay from shore to the tower, each OM has its own time offset to be measured and compensated. This is done using a fast light pulser installed in each OM. The light pulsers are controlled by dedicated FPGAFootnote 10 based control boards located in each FMPOD, which also allow setting the intensity and the frequency of the flashes of each light pulser. The system is operated from shore. The control boards can also perform measurements of the propagation delays of the commands to reach each of the light pulsers. The propagation delays have been proven to remain stable down to the level of the resolution of the measurements, i.e. within 100 ps.
Prior to the deployment, a calibration of the OMs of the full tower has been performed by using an external system exploiting a fast laser source (Hamamatsu PLP-10) coupled to a network of optical fibres of calibrated lengths. Multi-mode, graded index optical fibres have been used for this application.
In order to have redundancy in time calibration, for exploring new solutions proposed for a km\(^3\)-size detector as well as for measurements of the optical properties of water, the NEMO Phase-2 tower is also equipped with additional calibration devices comprising small LED pulsers, installed inside selected OMs in such a way to illuminate the OMs of the upper floors, and a laser beacon installed on the base of the tower.