Each detector of the KM3NeT neutrino research infrastructure is a three dimensional array of sensor modules distributed over large volumes of the transparent water in the deep Mediterranean sea. The sensor modules are pressure resistant glass spheres housing photomultiplier tubes for light detection and several instruments to determine the geometrical position of the sensor modules and to calibrate the measured signals. The sensor module is also referred to as Digital Optical Module or DOM for short. The modules are arranged in vertical string-like detection units, DUs or strings for short.
The sensor module – DOM for short
The sensor modules register the time of arrival of the Cherenkov light generated in the sea water by charged particles created in neutrino induced interaction with the water inside or close to the detector; the modules also measure the brightness of the light and the geometrical position of the sensor at the time of arrival of the light. The measurements are transmitted to computers in a shore station via a network of optical fibres. In the shore station, intelligent software filters the data and reconstructs from the measurements the traces of the particles traversing the detector. The results are sent over the public internet to the KM3NeT data centres for further study by the scientists. The sensor modules of KM3NeT are referred to as Digital Optical Modules – or DOMs for short.
A DOM consists of a glass spherical vessel with a wall thickness sufficient to withstand the enormous pressure of up to 350 times normal atmospheric pressure that exists at the bottom of the sea. In this glass vessel 31 photomultiplier tubes have been arranged to look in all directions for the faint light emitted by particles passing by.
The photomultiplier tubes need to be provided with a high voltage of around 1000V. This is provided by a custom made circuit on the back of the photomultiplier tube. This board has been miniaturized in order to fit in the limited space available in the innards of the glass sphere.
A small electrical pulse is created by the photomultiplier tube when a quantum of light hits it. This pulse is then amplified and transformed into a square wave pulse, by the time-over-threshold technique (the amount of light is transformed to an amount of charge which is in turn translated to the length of the square wave pulse), and sent to the central processing logic housed in a Field programmable gate Array (FPGA) where its arrival time and its pulse length is registered and stored for later transfer to shore. The DOM also contains other sensors used for calibration purposes. A compass makes it possible to know in what direction each of the photomultipliers is pointing. Accelerometers allow tilt, pitch and yaw of the module to be determined. A piezo-acoustic sensor allows for the determination of the position of the DOM in 3D using a sonar technique. All these measurements are important as the DOMs move under the influence of sea currents.
The detection unit – DU or string for short
A collection of 18 DOMs connected by an electro-optical cable and arranged along a vertical structure with two ropes is called a Detection Unit or DU (or string) for short. The glass vessel of the DOM is surrounded by a collar made of titanium which allows for the DOM to be connected to two 4 mm ropes made of Dyneema, a high strength manmade fibre, that connect the DOMs together. Apart from the DOMs, a detection unit has an anchor, the weight of which keeps the detection unit firmly connected to the seabed. The power conductors and optical fibre enter the glass sphere via a penetrator. Even though the string design of the detection unit minimises drag and by itself is buoyant, additional buoyancy is introduced at the top of the string to reduce the horizontal displacement of the top relative to the base for the case of large sea currents. Attached to the ropes is the vertical electro-optical cable, a pressure balanced, oil-filled, plastic tube that contains two copper wires for the power transmission (400 VDC) and 18 optical fibres for the data transmission. At each storey two power conductors and a single fibre are branched out via the breakout box. The breakout box also contains a DC/DC converter (400 V to 12 V).
The ARCA and ORCA type neutrino detectors
The first construction phase of KM3NeT consists of 30 such detection units. The later incarnations of the KM3NeT detectors will consist of blocks of 115 detection units. Detection blocks with two different granularity of light sensor modules will be installed: ARCA– and ORCA-types. For the KM3NeT 2.0 construction phase two ARCA-type blocks will be installed at the KM3NeT-It site off the coast of Sicily where the distance between DOMs along the string units is about 40 m and the distance between units is about 100 m. The height of these blocks is about 700 m. The instrumented volume of the two blocks about 1 cubic kilometre. An ORCA-type block of 115 units will be different in that the distance between DOMs along the unit will be 9 m and the distance between units will be 20 m. This arrangement instruments almost 4 cubic hectometer – almost 250 times smaller than the ARCA detector – optimised for the lower neutrino energies necessary for the measurement of the neutrino mass hierarchy.
A very important milestone of the project is the in situ prototype validation. In May 2013, a prototype sensor module was successfully installed on an ANTARES detector line and the next year, in May 2014, a prototype string comprising three DOMs was successfully deployed and connected to the KM3NeT-It site at 3500 m depth. Both prototypes operated for more than a year thus providing not only a validation of the technologies, but also a check of the detection capability via the comparison of the response of the prototypes to background light from the decay of radioactive potassium-40 (40K) in the sea water and atmospheric muons expected from Monte Carlo simulation with data.