An array of HELYCON scintillation counters can be used as a SeaTop calibration infrastructure, floating on top of an underwater neutrino telescope. Such an array can detect the copiously produced, high energy and small zenith angle, Extensive Air Showers (EAS) that contain energetic muons able to penetrate the sea water and reach the neutrino detector. These muons are detectable by the deep-sea telescope and the muon track parameters are estimated with high accuracy. The synchronous detection of EAS, by the HELYCON array and the neutrino telescope, can be used to investigate systematic bias in determining the direction of cosmic messengers by the neutrino telescope, as well as to provide the absolute position of the undersea infrastructure.

A similar procedure has been established in the IceCube neutrino telescope using the IceTop array. The IceTop detection units are frozen-water tanks (similar to the water tanks of Pier Auger cosmic ray observatory) fixed on the surface, just above IceCube. However, scintillation counters are more suitable for a sea level floating calibration system. A floating platform will carry an array of 16 HELYCON charged particle detectors, each of 1m2 effective area, arranged on a two dimensional grid (5m cell size) covering a total area of about 360 m2. The detectors on each platform will form an autonomous station equipped with a GPS receiver, for absolute synchronization and positioning, digitization and control electronics, as well as a data acquisition system controlled by a personal computer.

Three independent floating stations, during a 10-day operation, will accumulate enough data in order to estimate a possible systematic offset (in the zenith and/or azimuth angles determination by the neutrino telescope) with a much better accuracy than the foreseen angular resolution of the telescope.

The performance of this calibration system has been studied using the HOURS software package. The HOU Reconstruction & Simulation (HOURS) software package comprises a realistic simulation package of the neutrino telescope and HELYCON detector’ s response, including an accurate description of all the relevant physical processes, the production of signal and background, the response of sensors and digitization electronics. Furthermore HOURS provides several tools for analysis strategies, for triggering and pattern recognition, event reconstruction, tracking and energy estimation.  HOURS has been used to simulate the development of a very large number of EAS, the HELYCON array response to these showers as well as the response of the neutrino telescope to muons arriving at the detector depth. The HOURS reconstruction tools have been used to analyze the simulated detector signals in order to reconstruct the muon track and the EAS parameters.  The calibration potential of the HELYCON floating arrays was evaluated by comparing the directional parameters of the EAS, detected by the array, with the direction of the reconstructed muon (or muon bundle) by the neutrino telescope. Two different comparison strategies (calibration techniques) have been evaluated.


The first calibration method (“using the shower axis reconstruction”) is based on the comparison of the shower axis direction, reconstructed using the arrival times and amplitudes of the HELYCON detector waveforms, with the muon track directionality found by the neutrino telescope. It has been shown that the direction of energetic muons, produced during the shower development and reaching the neutrino detector, are in avery good approximation parallel to the shower axis. Consequently, in absence of systematic bias, the difference between the reconstructed zenith angles of the shower axis and the muon track should follow a symmetric distribution around zero. Any statistically significant deviation from zero, indicates a systematic angular offset.

Although the shower axis direction is determined, by a single HELYCON station, with an accuracy of 6.7 degrees and the expected resolution of a KM3 neutrino telescope is about 0.1 degrees, the proposed calibration technique can estimate much more accurately any possible systematic angular offset. This is due to the fact that the technique uses as an estimator of the angular bias the mean value of the difference, which scales inversely proportional to the square root of number of reconstructed showers. It has been shown that by accumulating data using 3 independent floating stations in a 10-day operation, the accuracy in measuring a systematic bias of the neutrino telescope, in   reconstructing the direction of cosmic messengers, is about 0.04 degrees (km3 ν-Telescope at 3500m depth)

The performance of the above technique, when applied to smaller neutrino telescopes, such as ANTARES, has been also studied. In these simulation studies the calibration potential of a reduced set-up of HELYCON detectors, located on a platform dragged by a boat or on the boat itself, was evaluated. Monte Carlo simulations were performed in order to study the calibration resolution of four different detector layouts when positioned at two different locations (just above the ANTARES detector and 1 km away from the centre of the detector). The simulation study concluded that a 5 day sea campaign with a surface array made of 10 HELYCON counters, distributed on an area of , would reveal systematic angular errors with an  accuracy better than 0.5° in the zenith angle and 1.5° in the azimuth angle determination, which is much better than the reconstruction resolution of the ANTARES pilot telescope.


Each HELYCON autonomous station-array can be used to select EASs passing close to the center of the floating platform (e.g. by requiring at least 5 active counters on the array, with a  collective response corresponding to more than 20 minimum ionizing particles [mips]) . In these cases a “simple direction estimation” of the energetic muons  (or muon bundles), which reach and activate several optical modules (OM) of the neutrino telescope, can be achieved following simple geometrical arguments.  That is the straight line connecting the position of the center of the platform with the weighted mean position (weighted by the number of Cherenkov photons, detected by the OM) of the active optical modules. Detailed simulation studies have shown that this “simple direction estimation” is a very good approximation (within 0.8 degrees) of the real muon direction.

This simple direction estimation of the muon is compared, on an shower by shower basis, to the reconstructed muon track direction by the neutrino telescope. The distribution of the difference between the simple direction estimation and the corresponding parameters of the reconstructed muon track, exhibits a central Gaussian structure which is used for evaluating  possible systematic angular offsets.


Both the above calibration techniques were studied, in order to quantify their performance, when used to calibrate  two different telescope configurations (“Slender String” and “Flexible Towers” configurations), both evaluated in the framework of the KM3NeT Design Study, in two different deployment depths. It was found that the so called, “simple estimation” calibration approach performs better (than the  “using shower axis reconstruction” technique) due to the fact that employs loose event selection criteria, which results in a much larger selected event sample for a certain operation time. On the other hand, a telescope deployed in deeper sea is better protected from atmospheric muons with a consequence that fewer showers are simultaneously detected by the surface array and the neutrino telescope. Finally, the so called “Slender String” configuration employs multi-PMTs as optical units, offering an almost 4π field of view improving thus the reconstruction efficiency of the muon  track by the ν-telescope especially for down coming muons at small zenith angles.