ANTARES time calibration International Workshop on a Very Large Volume Neutrino Telescope for the Mediterranean Sea (VLVnT08) Toulon, Var, France, 22-24 April 2008 ANTARES time calibration Francisco Salesa Greus IFIC (CSIC–Universitat de València, Spain) On behalf of the ANTARES collaboration

Outline The ANTARES detector Why time calibration? Calibration Systems of ANTARES: Laser by optical fibre Echo-based clock system Internal LED Optical Beacons K40 Summary F. Salesa - 23/04/2008

The ANTARES detector (see M.Circella’s talk in the plenary session) Horizontal layout Storey Buoy Electronics container OM 40 km electro-optical cable to shore OM 350 m 12 lines 25 storeys/line 3 PMTs/storey 900 PMTs in total OM Junction box Submersible 100 m 40 km off Toulon coast (France) 2500 m depth 42º50’N 6º10’E 10 lines (+1 instrumentation) taking data ! 12 lines 3x25 PMT/line 60-75 m Interlink cables Anchor F. Salesa - 23/04/2008

Importance of time calibration The ability to point back to the detected sources is critical in a neutrino telescope. In order to get the best possible angular resolution a good positioning and time calibration are required. In ANTARES the attainable angular resolution is better than 0.3º for En greater than 10 TeV. The absolute time resolution (fix a specific time for each neutrino event w.r.t. UT) depends on: Time offsets. Electronic paths (Electro-optical cable between the junction box and the shore station). In order to obtain correlations with the physics phenomena (e.g. GRB) an accuracy of ~1 ms is required in the absolute timing. Relative timing resolution (among OMs) depends on: Transit time spread (TTS) of the PMTs (s~1.3 ns). Optical properties of the sea water: light scattering, chromatic dispersion (s~1.5 ns). Electronics (s<0.5 ns). In order to get the best angular accuracy (i.e. only limited by reconstruction) the relative time uncertainty coming from the electronics must be < 0.5 ns. F. Salesa - 23/04/2008

Offset = TREF - TOM/LOB - clock_ph – fibre_path Laser by optical fibre The computation of the relative time-offsets between the OMs of the detector are performed in the integration sites in dedicated black boxes or a dark room. A laser provides a common light signal sent through optical fibre to every OM and LED Optical Beacon. The time difference between both signals corrected by the fibre path gives the relative time offset. All the offsets are referred to a particular OM. In addition, calibrations of the read-out electronic cards are also done using the data obtained with the laser system. LED beacon Example of LED OB time offset calculation Flower pot Offset = TREF - TOM/LOB - clock_ph – fibre_path Optical fibres F. Salesa - 23/04/2008 Laser Signal splitter

Echo-based clock system On-shore Station In-situ EMC Cable 1550 nm1310 nm Time Digital Converter  Dt from anchor to a particular OM Inter Link Cables 200-500 m fibre START STOP GPS E/O/E TX RX TDC Main Electro-optical cables (42 km) (1549 nm1532 nm ) s = 12 ps Dt MEOC Junction Box optical splitter The system has shown a stability of 2ns/208μs during one year of operation The 20 MHz clock signal (synchronized with the GPS) is generated on shore and distributed throughout the detector. Start and stop signals are generated and sent to the TDC which measures the round-trip delay Local Control Modules (LCM) clock boards Very good absolute calibration! F. Salesa - 23/04/2008

The transit time of each OM stable within 0.5 ns Internal LED Every OM has an internal blue LED glued to the back of the PMT which can illuminate the photocathode from inside. The aim of this system is to monitor the transit time (TT) of the PMTs measuring the difference between the OM signal arrival time and the time of the LED flash. Blue LED Agilent HLMP-CB15 The transit time of each OM stable within 0.5 ns F. Salesa - 23/04/2008

Optical Beacon system 60 m The Optical Beacons are controlled sources of light with a well-known time emission. There are two kinds of OB: Laser and LED. Four LED Beacons are placed along each line. Two Laser Beacons are placed at the bottom of two central lines. 60 m 300 m F. Salesa - 23/04/2008

Optical Beacon system LED Laser The time of the Optical Beacon is known very precisely by means of a small internal PMT (LED OB) and by a built-in photodiode (Laser OB). Both signals are very stable in rise-time. Flashing a nearby OM the time resolution is checked, because the only uncertainty is the one coming from the electronics. With the LED OB we can estimate the change of the offsets once in situ w.r.t. the ones computed in the laboratory. DAQ waveform signal from the small PMT LED DR - OB offset difference σ = 0.4 ns Lines 1-10 RMS 0.7 ns DAQ waveform signal from the internal photodiode Laser Time difference between the LED OB and an OM Electronics contribution less than 0.5 ns Only 15% are larger than 1 ns F. Salesa - 23/04/2008

The attenuation length @ 472nm has been computed with the LED OB Optical Beacon system The effects of the orientation and the scattering are seen in the time distribution given by the LED OB runs. storey 2 storey 9 storey 15 storey 21 Instrumentation Line Line 1 storey 3 LOB light direction The attenuation length @ 472nm has been computed with the LED OB PRELIMINARY F. Salesa - 23/04/2008

Gaussian peak on coincidence plot K40 calibration The K40 present in the salt water can be used for charge and time calibration of the detector. Taking differences by pairs OM 0 OM 1 OM 2 Gaussian peak on coincidence plot g RMS 0.7 ns g Cherenkov e- (b decay) 40K 40Ca F. Salesa - 23/04/2008

Cross-check between systems The K40 test is done by pairs because we know neither the position of the source nor the time emission of the light. With the LED Beacon we can reproduce this test with more precision and with a more intense light source. Lines 1-5 LED OB K40 40K 40Ca e- (b decay) Cherenkov light g K40 The offsets calculated with the LED OBs are validated by the K40 (independent calibration procedure) LED OB – K40 LED OB F. Salesa - 23/04/2008

Cross-check with positioning Without alignment With alignment The laser is fixed on the anchor. Therefore it can be used to check the line movements. If the line is considered rigid and straight, the time difference distribution has a RMS ~ 2.3 ns. Taking into account the shape of the line the values are distributed within RMS ~ 0.6 ns. σ=0.6 ns F. Salesa - 23/04/2008

Summary The ANTARES calibration system has been successfully tested in the laboratory and once deployed in the sea. The absolute time resolution required (~1ms) is ensured by the high precision of the clock system. The relative time resolution required (~0.5 ns) has been confirmed with the OB system. The OB system provides the time offset corrections. The K40 analysis validates these results in an independent way. The time calibration system has also shown a good agreement with the positioning. With the offsets corrected and the relative time resolution checked, the angular accuracy of the detector of < 0.3º for high-energy neutrinos can be reach. F. Salesa - 23/04/2008