KM3NeT Optical Calibration Umberto Emanuele IFIC (CSIC – University of Valencia, Spain) WPF/L KM3NeT Meeting Nikhef 5 th - 7 th July

Outline Time Calibration requirements Optical Beacons: ANTARES experience Optical Beacons for KM3NeT In situ tests of KM3NeT LEDs LASER: ANTARES results and KM3NeT prospects Time Calibration Simulation Conclusions 2

Time Calibration requirements Absolute time resolution -> Can be determined with a precision better than 110 ns (enough to correlate with astrophysics processes). Relative time resolution -> Chromatic dispersion and scattering amounts to σ ∼ 2 ns (at 50 m) -> TTS and PMT electronics contributions are less than 2 ns -> Time precision required for calibration offsets is σ ≤ 1 ns 3 Before deployment, the time calibration constants are determined in the laboratory. Time resolution cross-checks -> For OMs in the same storey, K 40 can be used for charge and intra-storey time calibration. -> Reconstructed muons can be used to further refine and cross- check the determination of the time constants. OM 0 OM 1 OM 2 40 K 40 Ca   Gaussian peak on coincidence plot Taking difference s by pairs Cherenkov e - (  decay )

1. Time Offsets determination Time residuals should be characterized by mean values well centered at zero. 4 storey 2 storey 9 storey 15 storey 21 Line 2 However, there is a linear delay due to the combination of Early photon + Walk effect. Deviations from the straight line are used for calibration. OM 0OM 1OM 2 storey LOB-ARS (ns) Optical Beacons (ANTARES experience) 4

Water current ≠ 0  60 m 300 m  60 m Water current = 0 L1L1 L2L2 L1L1 L2L2 L 1 = L 2 L 1 ≠ L 2 Example with ANTARES Lines 3, 4 and 5 which are quite aligned 2. Illuminating adjacent lines Storey rotationAnticorrelation Time Difference LOB L4F2 flashing to OMs L3 LOB L5F2 flashing to OMs L3 5 Optical Beacons (ANTARES experience) Relative intra DU calibration using adjacent OB will be complex. Too many corrections to apply

The early photon effect appears at high light regimes. It produces a delay in the arrival time similar to the walk effect. The PMT is unable to resolve multiple photons arriving at the same time. Only the arrival time of the earliest photons is recorded. Toy MC RUN Early photon effect Early Photon Region Single Photoelectron Region 6 Optical Beacons (ANTARES experience) 4. Time Resolution σ ~ 0.4 ns Time difference between a LED OB and an OM Electronics contribution less than 0.5 ns 0 0 0

HV tuning Hint of some ‘bad’ OMs Back to standard ~0.5 ns RMS ~ 2 ns RMS ~ 0.6 ns Retuning of feeding HV can be corrected with the OBs 5. High Voltage retuning 7 An Optical Beacon system can correct the T 0 s in case of unexpected changes as HV retuning Optical Beacons (ANTARES experience)

6. Laser Beacons 8 Optical Beacons (ANTARES experience) L1 L2 L3 L4 L6 L9 L7 Laser Beacon L5 L8 L10 Inter DU calibration by means of the Laser Beacon provides a common reference with a fixed position. Range from 160 m (~p.e. level) to 240 m (lack of statistics). Mean value of the time difference Laser - ARS for each floor. Data points fitted to a flat distribution. Intra DU calibration possible. Data 2007 Data 2008

9 RMS ~0.5 ns NO positioning corrected positioning corrected L7 F1 6. Laser Beacons Optical Beacons (ANTARES experience) The laser is fixed on the anchor. It can be used to check the line movements. If the line is considered rigid and straight, the time residuals has a RMS ~ 2.3 ns. Taking into account the shape of the line the values are distributed within RMS ~ 0.5 ns. CROSS-CHECK POSITIONING RMS ~2.3ns

INTER D.U. Calibration : Laser 532 nm 485 nm - Higher intensity ( > μJ ) and shorter pulses (< 1 ns) - No synchronization needed - More expensive but less redundancy required - Tunable by Liquid Crystal Optical attenuator - Collimated beam -> Diffusion device needed Decoupling inter-intra D.U. Calibration systems INTRA D.U. Calibration : Nano-Beacon Upward single LED housed inside OMs - Less expensive and high redundancy - Not triggered by the clock to avoid electronic noise - Frequency of several kHz depending on the DAQ system (300 ANTARES is too low) - Avoid cumbersome synchronization process, only one (or two) LED per nanobeacon Optical Calibration for KM3NeT

Pulser: Auto-triggered – External clock not needed Controlled by one (or two) DC Voltage: Two designs: 1.- LED intensity and frequency inter-correlated 2.- LED intensity and frequency non-correlated Control Electronic board: Select the intensity of flashing Select the trigger (external or auto-trigger) Provides an external trigger signal Select the frequency of the external trigger Optical Beacon for KM3NeT 1. NanoBeacon – First Prototypes I1 – Positive Power Suply (3.3 or 5 Volts) I2 – GND - Negative Power Supply (0 Volts) I3 – SDA of the I2C bus I4 – SDL of the I2C bus O1 – Trigger output O2 – Voltage to set flash intensity O3 – Common GND

Two PVC made pieces: One piece glued to the glass sphere (fixed) The second one screwed (in) to the first one: Holds the LED + Electronics Protect the PMT from the Nano-Beacon Light Possibility of replacement The final design depends on the definitive OM solution First prototypes are going to be tested on ANTARES line 9 Optical Beacon for KM3NeT 2. Preliminary Mechanics

Only intra D.U. calibration 1 LED integrated in the upper part of the OMs -> Higher Intensity and range (Not Cleaved) -> Simple electronics and mechanics (inexpensive) a.u. Optical Beacon for KM3NeT 3. Opening Angle A 15°opening angle is sufficient to illuminate OMs above the beacon even in perpendicular arrangement including potential misalignment 3m 40m

A LED Beacon (L12F2) with 4 LED candidates for KM3NeT is actually working in ANTARES. LED modelRise time (ns) (nm) FWHM (º)Intensity (pJ) CB CB NSPB500S AB Four Calibration Runs have been analyzed 2 UV LEDs and 4 different Blue LEDs In situ tests of KM3NeT LEDs RUNLEDN. Flashes 45161CB CB AB NSPB500S20061 Standard ANTARES setup Frequency = 300 Hz Duration = 1-2 min Standard ANTARES setup Frequency = 300 Hz Duration = 1-2 min 14 Special LED Beacon More calibration runs with more statistics under study

Run LED CB26 In situ tests of KM3NeT LEDs Intra DU calibration with KM3NeT LEDs 15 Early Photon Region Photoelectron Region

In situ tests of KM3NeT LEDs In KM3NeT Separation between floors bigger (30-40 m), less points in the fits. Longer D.U. distances, ~600 m. Optical Calibration will be performed in both early and photoelectron regions. But calibration in the Photoelectron region requires. 10 TIMES BIGGER FREQUENCY !!! OR 10 TIMES LONGER CALIBRATION RUNS !!! IMPORTANT POINT What will be the maximum KM3NeT DAQ acquisition frequency? Optical calibration will depend strongly on that ! Calibration regions and maximum distance 16

In situ tests of KM3NeT LEDs 17 Calibration regions and maximum distance 0.1 Hits per flash 1 Hits per flash Photoelectron region ANTARES max distance 150 m, Early-photon region limit LED MODEL 1 p.e./ flash 0.1 p.e./ flash ANTARES STD 70m150m CB26145m230m CB30120m195m AB87145m235m NSPB500150m250m

LASER: ANTARES results and KM3NeT prospects ENERGY per PULSE 0.1 p.e./ flash 0.01 p.e./ flash 0.2 mJ170.0 m210.0 m 0.4 mJ205.0 m250.0 m 1.2 mJ240.0 m285.0 m 18 KM3NeT LASERs will have higher intensity (≥ 12 mJ) and longer range * Run High Intensity (1.2 mJ) * Run Low Intensity (0.4 mJ) * Run Low Intensity (0.2 mJ) # Hits/# Flash LASER range in ANTARES Laser intensity can be changed by liquid crystal optical attenuator 0.1 Hits per flash 0.01 Hits per flash

Time Calibration simulation (LED & Laser) Water optical properties simulation Time Calibration simulation (LED & Laser) Water optical properties simulation 19 Time Calibration Simulation CALIBOB Special simulation code for timing calibration with Optical Beacons (includes OM angular acceptance, emission light distribution and ARS response) Special simulation code for timing calibration with Optical Beacons (includes OM angular acceptance, emission light distribution and ARS response) Used currently in ANTARES for: DATA MC, CALIBOB Laser Beacon time calibration simulation

Time Calibration software has been optimized for KM3NeT Already tuned to KM3NeT layouts Time Calibration Simulations are well working Scattering, early photon and photoelectron regions are well reproduced Time Calibration software has been optimized for KM3NeT Already tuned to KM3NeT layouts Time Calibration Simulations are well working Scattering, early photon and photoelectron regions are well reproduced Time Calibration Simulation Kalibob for KM3NeT 20 λ OF SCATTERING = ∞ λ OF SCATTERING = 60 m λ OF SCATTERING = 40 m Gaus+Exponential convolution fit will improve time offsets values

21 Conclusions Decoupling intra/inter DU calibration  Global movements of the DU don’t allow a “trouble-free” calibration Intra DU calibration  Time calibration less subject to positioning and DU rotation  Less intensity required (may be just 1-2 LED per floor)  Scattering + attenuation corrections not so critical  Cross-check of first storeys by means Laser Beacons Inter DU calibration  Possible with Laser Beacons  Directly correlated with detector angular resolution  Cross-check with the positioning system HV  By means of the Optical Beacons system is possible to correct the T 0 s in case of unexpected changes as HV retuning DAQ  High frequency rate of DAQ is fundamental to calibrate in the photo-electron region (enough statistics to fit the time residual distributions) Time Calibration Software  MC simulation tool (KALIBOB) ready to optimize the time calibration

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θ OM SPHERE NANOBEACON θ ~ 20º or 30º In principle and following our experience with the LED Beacons in ANTARES the bio- fouling is negligible. According to the studies carried out in ANTARES an angle of 20 º would reduce the effects of the bio-fouling. Light Optical Beacon for KM3NeT 4. Zenith Angle

Nano-Beacon Beacon LED 5mmØ, ap. 30° Alumium screen system for cooling e/o & e/e conversion PCB Screen to shield conversion board Space for sphere logic (incl. LED driver) provisional

2. Illuminating adjacent lines ANTARES experience advises against illuminating adjacent lines with Optical Beacons Natural movements of the DU do not allow a “trouble-free” calibration. Small rotations of a DU determine a big variation in angular acceptance of PMTs. Displacements of the DU change distances and T 0 s. Inter & Intra DU calibration by lateral LEDs will be complex. Too many corrections to apply. Not advisable 26 Optical Beacons (ANTARES experience)