In that work we study the scattering parameters of the water on the KM3Net site in Capo Passero. To this purpose we compare the real data from the time calibration runs of the detector to the results of a simulations of the light emission, propagation and detection according to the expeimental apparatus

Anghinolfi, M.

Briukhanova, N.

Hugon, C. M. F.

Nicolau, C.

Sanguineti, M.

English

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                   Sezione di Genova   ISTITUTO NAZIONALE DI FISICA NUCLEARE                INFN-17-05/GE        27th March 2017     The Water Properties of the Site in Capo Passero using  the LED Beacon of the Prototype Tower N. Briukhanova1, C.M.F. Hugon2, C. Nicolau3, M. Sanguineti4, M. Anghinolfi4  1) Moscow State University  2) Céntre de Physique des Particules-Marseille 3) INFN-Roma1  4) INFN, Sezione di Genova, Via Dodecaneso 33, I-16146 Genova, Italy   Abstract    In that work we study the scattering parameters of the water on the KM3Net site in Capo Passero. To this purpose we compare the real data from the time calibration runs of the detector to the results of a simulations of the light emission, propagation and detection according to the expeimental apparatus.     Pubblicato da SIDS–Pubblicazioni      Laboratori Nazionali di Frascati       PACS:11.30.Er,13.20.Eb;13.20Jf;29.40.Gx;29.40.Vj  1 IntroductionOne of the two detection infrastructures of KM3NeT [1] is located near Sicily, Italy. Ituses the Cherenkov light method to detect high energy neutrinos and requires a preciseknowledge of the light propagation in the water. So, in this note we will present somesteps that are leading to that knowledge. For this purpose we used a comparison betweenthe GEANT4 simulation [2] with different parameters and the real experimental data fromthe prototype tower [3]. These data are the arrival time distributions of the photons emittedby one LED in one beam of the tower and detected by an upper optical module. The peakof the distribution is used for timing calibration purposes while the tail mostly dependson the scattering properties of the water as discussed in this note. We expect that thisapproach can be extended to the analysis of the data from the new prototype strings [4].2 Experimental facilitiesThe test line used in this work is shown in fig.1. It had 8 floors, each level is an 8-meter-long beam, which are separated by the distance of 40 meters, the bottom one is 100 metersabove from the sea bottom. Each beam is perpendicular to its nearest neibourghs.Figure 1: The Tower Prototype2On each end of each beam there are two optical modules [5], one of which looksdownward, another is turned sidewards. The structure of the optical module is shown infig.2: a glass sphere contais one 8” Hamamatsu photomultiplier, electronic boards for thePMT’s operation, read out and for time calibration. The optical gel provides mechanicaland optical contact between the sphere and the PMT, and a µ-metal net protects the systemfrom the magnetic field.Figure 2: The Optical ModuleAs it is shown in fig.2, the upper side of the OM contains a support with an LED.The LED has a wavelength equal to 470 nm (that is blue, closer to the Cherenkov radi-ation’s higher intensity region) pointing upward. Three of them on the lower floors areused in the calibration runs. During the calibration runs, these beacons are blinking withthe frequency of 2 kHz.The analised data includes empty runs with background noise as well as the runswith active LEDs on different floors. Some of them have lower intensity, and suites betterfor our purpose. The reason for that will be explained later in the Scattering section.As an example fig.3 represents the charge detected in one OM as a function of thetime. In the same OM an LED was activated with different intensities corresponding to thedifferent charge levels. Noise level is represented by the few counts above t=700 whilea more detailed view of the background behavior is reported in fig.4. Here the spikescorrespond to bioluminescence activity close to the OM.We determined the charge calibration procedure using the noise runs. These runscontains mostly one-photon events, produced by biological activity or 40K decay, so if we3Figure 3: The Run with LEDFigure 4: The Run with Noiseplot the hystogram of the number of the events with different charge, the plot’s maximumwill correspond to the most probable charge produced by a single photon. So, the plot’s4peak was approximated by a gaussian curve and the charge was taken from the position ofthe maximum of the curve, as shown in fig.5. The charges were quite similar, as expected,but not exactly the same, so we had to consider that.Figure 5: Calibration3 ScatteringThe scattering is studied here using the Kopelevich model [6] that uses two sizes of par-ticles, large and small, instead of a variety of sizes. The resulting scattering probabilityfrom the particle is the following:bp = 1.34νs(550nmλ)1.7+0.312νl(550nmλ)0.3(1)where the suffix s stands for small, l for large and λ is the wave length. There is alsoa part that is related to the molecule scattering. The angular distributions for these twovariants of scattering are different, the particle one has forward going angular distribution,while the molecule scattering has isotropic distribution. The scattering distribution fromthe molecules is, however, relatively well known, while the parameters of the particlesvary from one location to the other.In the next stage of the work we developed a method to select the LED pulses andwe accumulated statistics to well determine the arrival time distribution as detected in one5PMT. The LED was blinking in a regular manner, so it was possible to sum the signalfrom all the flashes. As these flashes had the same duration and the same intensity, it waspossible to do that. The intervals of 500 µs that includes a shorter flash of light togetherwith all the scattering effects, were summed together to collect a significant data array.Figure 6: An example of the time arrival distribution of the photons from the LED locatedin floor 1 and detected in floor 8 compared to the GEANT4 calculationIn this analysis we considered only the runs at low intensity to guarantee that onlyone photon at time reach the PMTThe simulation uses GEANT4 to model of the propagation of the photons throughthe water. That model works only with the assumption that only one photon is detected atthe same moment. That assumption is quite rough, of course, but without it the diffilcutiesof creating the simulation will increase significantly. So, instead of doing that, we selecteda low-intensity run.An example of the time arrival distribution of the photons from the LED locatedon floor 1 and detected in floor 8 is reported in fig. 6. The measurement (blue line) iscompared to the GEANT4 simulation (red line).4 AnalysisThe simulation was repeated several times, with two parameters varied. One was a scaligfactor which was applied to the total scattering legth that is the primary aim of that study,6another was the aperture angle of the LED which was varied within its specifications±5%. The results of the simulation of the arrival time distribution was at first approxi-mated by an exponential decay, and after this smoothing it was compared with real data.That was done to simplify the analysis.The analysis was made at first using the data only from the seventh floor of thesystem, but afterwards the study was extended to the three highest floors.From each value of the scattering factor and of aperture angle we performed thesimulation and compared to the real data. Some parameters, like height of the peak or thestart time of the signal, were changed to fit the real values better (so there was a time shiftand vertical scaling). After that we took chi-square for each pair of parameters. Finaly, aplot representing the dependency of χ-square from the scattering lenght and the appertureangle was made. The result of the two-dimensional χ-square distribution is reported infig.7. The darker color represents lower χ-square values , that is better fit to the data.The chisquare distribution does not show a clear behavior but there is an evidence thatand LED angular aperture close to the nominal values together with a scaling factor of0.9 better fit the experimental distributions. The scaling factor 0.9 is applied to the totalscattering length used in the simulation indicated in the figure as ’0.9 ANTARES sca.’This result indicates that the method we have described is potentially usable to determinethe scattering properties of the environment. We are confident that the application of thisprocedure to the new data from the strings will give more results.Figure 7: The Result of the χ-square definition for different parameters75 ConclusionTo sum up, the main result of this work is the development of a method to determine thescattering length of enviromental water in the Capo Passero site. Unluckily, the data fromthe tower prototype does not seem to be sufficiently accurate for the task, so the waterparameters should be re-evaluated using the data from the string, that seems to providemore accurate information.References[1] KM3NeT collaboration, Journal of physics G: Nuclear and Particle Physics, 43 (8),084001, 2016[2] C.M.F. Hugon on behalf of the ANTARES and KM3NeT coll: ”GEANT4 simu-lation of optical modules in neutrino telescopes” Proceedings of Science (2015),ICRC[3] S. Adrian-Martinez et al., Eur. Phys. J. C (2016) 76:68[4] KM3NeT Collaboration, The Prototype detection unit of the KM3NeT detector, Eur.Phys. J. C (2016) 76:54[5] S. Aiello et al., The optical modules of the phase-w of the NEMO project, Journalof instrumentation, Volume 8, July 2013[6] O.V. Kopelevich, Small-parameter model of optical properties of sea water (in Rus-sian), Chapter 8 in A.S. Monin (ed.), Ocean Optics, vol 1: Physical Ocean Optics,Nauka Pub., Moscow (1983).8

The Water Properties of the Site in Capo Passero using the LED Beacon of the Prototype Tower

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The Water Properties of the Site in Capo Passero using the LED Beacon of the Prototype Tower

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