Time Calibration with Optical Beacons Pylos, Apr 2007 C.Bigongiari IFIC Pylos, Apr 2007 C.Bigongiari IFIC (CSIC – Universitat de València) KM3NeT WP3 – Calibration Session

Pylos, 16-19/04/2007C. Bigongiari2 Outline Time Calibration Optical Beacons –LED Beacons –LASER Beacons What we have learnt in ANTARES up to now –Time resolution –Optical Beacon illuminating same line OMs –Optical Beacon illuminating other line OMs Laser OB development Conclusions

Pylos, 16-19/04/2007C. Bigongiari3 Time Calibration The reconstruction of muons trajectories in a neutrino telescope heavily relies on the measurements of Cherenkov photons arrival times. A precise relative time calibration of the detector is of utmost importance to achieve a good angular resolution and a high reconstruction efficiency ν μ

Pylos, 16-19/04/2007C. Bigongiari4 Time Calibration Different systems are needed to measure time delays and time jitters in different parts of the electronic chain, from PMTs to DAQ. A redundant time calibration system is very useful to disentangle different effects In-situ calibration systems are mandatory to monitor the time calibration after the deployment

Pylos, 16-19/04/2007C. Bigongiari5 ANTARES Optical Beacons Optical Beacon  Well controlled pulsed light source, (LEDs or LASERs) LED Beacon LASER Beacon LASER LEDs

Pylos, 16-19/04/2007C. Bigongiari6 ANTARES LED Beacon Features LED Beacon without container  6 faces with 6 LED each = 36 LED.  3 groups (top, centre, four)  Internal PMT Hamamatsu H (RT=0.7 ns) to know the actual time emission of the light.  Wave length emission = 472 nm (blue)  Flux per max INT = 4 x 10 8 photons per LED.  Intensity: variable  Light emission: isotropic in 50º < θ < 120º range  Location along the line (storeys: 2, 9, 15, 21). TOP CENTRE FOUR

Pylos, 16-19/04/2007C. Bigongiari7 ANTARES LASER Beacon Features  Internal fast photodiode (jitter ~50 ps) to measure the actual time emission of the light.  Wave length emission = 532 nm (green).  Flux per shot = photons.  Intensity: fixed. Adjustable in new design  Light emission: Lambertian  Location at string bottom Glass rod to avoid biofouling

Pylos, 16-19/04/2007C. Bigongiari8 LED Optical Beacons Advantages: –There are Blue LEDs (472 ± 15) nm Absorption length ~ 60 m Effective scattering length > 200 m –LED light yield is tunable –LEDs are cheap Disadvantages: –Long rise time 2 ns –Expensive containers –Low yield => Many LEDs Cumbersome mounting Source spatially spread Hazy proper T0 Dull and tricky synchronization procedure needed

Pylos, 16-19/04/2007C. Bigongiari9 LASER Optical Beacons Advantages: –Extremely coherent light source –High light yield No synchronization needed Well defined proper T0 –Very good rise time (<0.5 ns) –Light yield is tunable (see following slides) Disadvantages: –Green light 532 nm Absorption length ~ 28 m –The emitted light is not isotropic –Expensive (~ 12K€ )

Pylos, 16-19/04/2007C. Bigongiari10 Photons emitted by Optical Beacons propagate through sea water and, if they reach OMs, can produce electronic signals in pretty the same way as Cherenkov photons do. Time calibration with Optical Beacons is therefore a very comprehensive system: –Many different aspects are involved Sea-water properties Detector positioning (Rotation included) Other timing systems OM response Readout system Time Calibration with Optical Beacons Shore Station Junction box

Pylos, 16-19/04/2007C. Bigongiari11 What we have learnt from ANTARES We studied the distribution of T1- T0 –T0 = Light emission time by OB –T1 = Arrival time of light on OM In the following results about: Electronic chain time resolution OB flashing same line OMs OB flashing other line OMs LASER beacon flashing Optical Beacon OB Optical Module OM

Pylos, 16-19/04/2007C. Bigongiari12 ANTARES Optical Beacons Layout Floor 2 Floor 9 Floor 15 Floor 21Sector 5 Sector 3 Sector 2 Sector 1 Line 5 MILOM ~101 m ~87 m ~94 m ~114 m Line 1 Laser  7+1 Lines deployed so far  5+1 Lines connected  20+9 LED Optical Beacons.  1 Laser Optical Beacon. LED Line 2 Line 4Line 3 JUST RECOVERED

Pylos, 16-19/04/2007C. Bigongiari13 T OM – T OB (ANTARES Data) The distribution is clearly asymmetric due to the photon scattering. We consider only the rising part, which is less affected by scattering, early photon effect, and fit it with a Gaussian. Better results can be obtained with a two-steps fit: Second step: Gaussian from 10% of the mean bin content to the 90% over passed bin. 90% 10% 100% 20% First step: Gaussian fit from 20% of maximum to the 100 % T 100 = Gaussian meanSigma = Gaussian sigma T 50 = 50%

Pylos, 16-19/04/2007C. Bigongiari14 Time Resolution Flashing OMs in the storey above the OB at high intensity we can measure the electronic chain contribution to the overall time resolution. The σ of the distribution is well below the requested resolution, 0.5 ns, for all the OMs

Pylos, 16-19/04/2007C. Bigongiari15 OB illuminating same line OMs 1.A LED OB is able to illuminate its own storey OMs and even the ones in the storey below 2.The statistic is enough to perform the fit up to 8 storeys above (about 116m far away) OB

Pylos, 16-19/04/2007C. Bigongiari16 OB illuminating same line OMs 1.A LED OB is able to illuminate its own storey OMs and even the ones in the storey below 2.The statistic is enough to perform the fit up to 8 storeys above (about 116m far away) OB

Pylos, 16-19/04/2007C. Bigongiari17 OB illuminating same line OMs The T100 grows as function of the distance/storey (up to 10 ns) more or less linearly with slope ~1.7 ns/storey The observed slope is due to the ‘early photons’ effect The RISE TIME of the light source must be smaller that the requested time resolution Or Illuminate OMs at 1 phe level low intensity  t [ns] Number of events [a.u.] ~ 9 ns high intensity

Pylos, 16-19/04/2007C. Bigongiari18 Sigma as function of OB-OM distance ~1.7 ns~1.3 ns The sigma of the fit is a measure of the direct photons peak width. This is the result of the convolution of source time spread (~1.7 ns) and phototube TTS (~1.3 ns). N pe, and therefore on the OB-OM distance. At large distance we reach the phe level -> Sigma =√(1.7ˆ ˆ 2 ) = 2.1 ns TTS depend on the number of photoelectrons N pe, and therefore on the OB-OM distance. At large distance we reach the phe level -> Sigma =√(1.7ˆ ˆ 2 ) = 2.1 ns

Pylos, 16-19/04/2007C. Bigongiari19 OB illuminating another line OMs OMs in storeys 1 to 13 of line 3 illuminated by the beacon of storey 2 lines 5. There is the same linear dependency on the OB-OM distance of the previous case due to early photon effect. There is an anti-correlation between OMs in the same storey due to storey rotation We can notice some wrong T0s We need an independent measurement of storey position/rotation

Pylos, 16-19/04/2007C. Bigongiari20 Laser OB – Correcting for position σ = 2.3 ns σ = 0.6 ns Corrected Not Corrected  60 m 300 m

Pylos, 16-19/04/2007C. Bigongiari21 LASER Beacon Development The light emitted by the LASER can be varied using a Voltage Controlled Optical Attenuator, a linear polarizer followed by a liquid-crystal retarder and another linear polarizer. Varying the voltage applied to the retarder the polarization of outgoing light changes. In this way the transmission of the attenuator can be varied.

Pylos, 16-19/04/2007C. Bigongiari22 Variable intensity LASER Beacon Schematic view of the Variable Intensity Laser Beacon. The amount of outgoing light can be changed by varying the voltage V applied to the liquid crystal retarder Liquid Crystal Retarder Polarizing Beam-Splitter Laser Head Polarizing cube beam-splitter Liquid Crystal Head Variable Voltage

Pylos, 16-19/04/2007C. Bigongiari23 Variable Intensity LASER Beacon We measured the energy per pulse emitted at different pulsing frequencies as function of the applied voltage. The maximum output is above 1 μJ for all frequencies considered. A variable intensity LASER Beacon has been already installed on line 7

Pylos, 16-19/04/2007C. Bigongiari24 Conclusions (1/2) OB allow an in-situ time calibration and monitoring of the detector From ANTARES data we have learnt: –The overall time resolution is below 0.5 ns –LED_OB-OM time difference depends on the distance due to early photons effect Very short rise time light sources are needed Otherwise operate at 1 phe level to avoid this effect The Optical Beacon system is very comprehensive –Sensitive to Optical Module position (Rotation included) –Sensitive to sea-water conditions

Pylos, 16-19/04/2007C. Bigongiari25 Conclusions (2/2) This is an advantage: –We can get information on other aspects of the detector Attenuation and scattering lengths of water Efficiency of bright point reconstruction Cross-check of position measurements But it has also drawbacks: –What you get is the convolution of many different phenomena –It’s hard to get precise results without additional information Optical Beacons are not cheap -> Reduce cost. –Containers make a non negligible fraction of the cost. Study alternative solutions. –Mass production should help Km3Net to reduce the cost A variable intensity LASER beacon already realized, tested and installed

Pylos, 16-19/04/2007C. Bigongiari26 Optical Beacon Cost LED Beacon Cost –Container 4000 € –Mounting 600 € –Faces (6 x 40) 240 € –Motherboard 300 € –PMT 600 € TOTAL 5740 € Pressure and Climatic internal tests not included Laser Beacon Cost –Laser 5000 € –Container 4500 € –Optical attenuator 2000 € –Electronics 500 € TOTAL €

Pylos, 16-19/04/2007C. Bigongiari27 Overdrive Mode Everything worked as expected ~33Hz ~330Hz The LED OB flashing frequency has been recently increased by a factor ten

Pylos, 16-19/04/2007C. Bigongiari28 Correcting for position Fixed geometry takes, for the position of the OMs in a storey, the point in the centre of the OMs plane. Correction for position takes: Geometric constants from ANTARES-CALI (G. Lelaizant) Rotation matrix from ANTARES-SLOW/ (F. Cassol) Euler angles (A1,A2,A3) from table “ALIGNMENT_VALUES2”. They are referred to the centre of the OMs plane (0,0, 0.576) We took the ALIG_VALUES which are closer in time w.r.t. the start of the OB run. Y X r OM_0 =(0.437, 0, 0) r OM_1 =(-0.218, , 0) r OM_2 =(-0.218, 0.378, 0) r LOB =(0, 0, 1.003) r corr = R * r i + r

Pylos, 16-19/04/2007C. Bigongiari29 Effect of scattering ~3 ns From MonteCarlo we know that scattering has a minor effect in the rising edge of time difference distributions, although systematic shift of in T 100 peaks of +0.5 ns is expected. However delays larger than ~3 ns are unlikely due to scattering. ~1 ns The T 100 delay depends on the increasing fraction of scattered photons as we move away from the light source. Therefore T 100 depends on the water properties. However a delay greater than 3 ns is not expected. ANTARES-Cali/

Pylos, 16-19/04/2007C. Bigongiari30 Early Photons Effect Order statistic: –Theorem: If X 1,…,X n are r.v. following with density f and distribution function F, then the minimum has density function: Naive Monte-Carlo simulation (No real data): The time measured in the OM is given by the early photons There is a backward shift in the arrival time distribution which is function of the N pe All Monte-Carlo (Calibob) simulations were done at phe level, hence this effect was not considered