Calibrations in ANTARES Heide Costantini INFN Genova Italia

Calibrations in ANTARES Heide Costantini INFN, Genova, Italia Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Motivation for calibrations in ANTARES • To achieve good angular resolution (0. 3 o for E >10 Te. V), we need precise: • Position calibration: Lines are displaced due to the sea current OM positions monitoring is needed • Time calibration: ns time resolution is required to allow high detector performances • Charge calibration: enables to translate signal amplitudes into number of photo-electrons • To determine detector efficiency we need: • the absorption length at ANTARES site • OM efficiency and angular acceptance Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Position calibration: acoustic system Acoustic system (5 hydrophones in each line) every 2 minutes the relative positions of the hydrophones is recorded Tiltmeter-Compass sensor board (1 in each storey) The local tilt and heading angles of each storey with optical modules is obtained Additional devices provide independent sound velocity measurements The positions of all OM is obtained every two minutes performing a fit of the line using the above inputs Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Position calibration: line fit accuracy L 3 storey 25 x = x(Hydrophone) - x(Linefit) < x>=10 cm Sea current velocity comparison Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Position calibration: absolute positioning • position of the boat: DGPS ( x, y ~ 1 m) • postion of BSS in respect to boat: LF acoustic ( x, y ~ 1 m) • positions between BSS: HF acoustic ( d ~ 1 cm) Absolute orientation of the detector known with 0. 14° precision Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Time calibration • Absolute time determination with precision ~1 ms is required to correlate detected events with variations of an astrophysical phenomenon • Relative timing resolution (among OMs) : – TTS transit time spread (TTS) of the PMTs ( ~1. 3 ns). – water coming from optical properties of the sea water: light scattering, chromatic dispersion ( ~1. 5 ns for 40 m distance). – elec coming from electronics ( <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. Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Time calibration: clock • A 20 MHz clock signal is generated on shore and distributed throughout the detector to each storey. • The time offsets between all storeys is determined through an echo-based system time resolution is ~ 100 ps. • The clock signal is synchronized with GPS and a timestamp is assigned to the data with precision of ~ 10 s • Timing calibration with clock system allows a relative timing resolution between all storey boards of 100 ps. • Absolute timing resolution is ~ 10 s Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Time calibration: optical beacons Four LED Beacons are placed along each line. 300 m Relative time calibration of different OMs using independent and well-controlled pulsed light sources 60 m Two Laser Beacons are placed at the bottom of two central lines. Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Time calibration: elec Time difference between the LED OB and an OM T 0 distribution example = 0. 4 ns • during OB runs Npe and N is large electronic timing resolution is obtained Electronics contribution less than 0. 5 ns Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

OB system: attenuation length measurements Rmin Rmax All points at R < RMin are skipped in the fit to avoid ARS token ring dead time pe level All points at R > RMax are skipped in the fit because signal is compatible with noise fluctuations The biggest challenge is to determine the separate contribution of absorption and scattering Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Charge calibration: the ARS chip • 2 ARS (Analogue Ring Sampler) chips (in a “token ring”) measure the charge and time information of a single PMT - The ARS can acquire the signals in two different modes: - integrating the pulse and transmit the digitized signal to shore (AVC) –default mode - sampling the full waveform of the OM signal - Settings of each individual chip can be remotely configured from the shore Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Charge calibration: offshore -Regular pedestal and single photo-electron (SPE) runs are taken to convert charge into photoelectron units. -Pedestal runs are special runs reading the PMT current at random times -SPE runs can be obtained by background events: 40 K and bioluminescent bacteria produce single photons at the photocathode level. Photo-electron peak Effective threshold SPE run Joint Meeting Ice Cube-ANTARES Charge in pe Berlin 2009 H. Costantini

OM angular acceptance and efficiency Has to be known to compute reconstruction efficiencies and effective areas Measurements were performed in a water tank Photon scattering affects the measurements PMT angular acceptance determination is not reliable at large PMT Detailed GEANT 4 simulation of the OM LED Dedicated measurements of the photocathode surface of OMs Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini

Conclusions • Calibrations are performed regularly in situ • position calibration is performed every 2 minutes x = 10 cm the automation of the full positioning calibration is almost complete • time calibration is performed every week telec=0. 5 ns time resolution is stable after several months of deployment • charge calibration is done monthly • Investigations to determine the absorption length at the ANTARES site and the OM angular acceptance are ongoing Joint Meeting Ice Cube-ANTARES Berlin 2009 H. Costantini