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Time Calibration with Optical Beacons

Time Calibration with Optical Beacons

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Time Calibration with Optical Beacons

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  1. Time Calibration with Optical Beacons Pylos, 16-19 Apr 2007 C.Bigongiari IFIC (CSIC – Universitat de València) KM3NeT WP3 – Calibration Session

  2. 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 C. Bigongiari

  3. Time Calibration The reconstruction of muons trajectories in a neutrino telescope heavily relies on the measurements of Cherenkov photons arrival times. μ A precise relative time calibration of the detector is of utmost importance to achieve a good angular resolution and a high reconstruction efficiency ν C. Bigongiari

  4. 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 C. Bigongiari

  5. LEDs LASER ANTARES Optical Beacons Optical Beacon  Well controlled pulsed light source, (LEDs or LASERs) LASER Beacon LED Beacon C. Bigongiari

  6. TOP FOUR CENTRE ANTARES LED Beacon Features LED Beacon without container • 6 faces with 6 LED each = 36 LED. • 3 groups (top, centre, four) • Internal PMT Hamamatsu H6780-03 (RT=0.7 ns) to know the actual time emission of the light. • Wave length emission = 472 nm (blue) • Flux per shot @ max INT = 4 x 108 photons per LED. • Intensity: variable • Light emission: isotropic in 50º < θ < 120º range • Location along the line (storeys: 2, 9, 15, 21). C. Bigongiari

  7. ANTARES LASER Beacon Features Glass rod to avoid biofouling • Internal fast photodiode (jitter ~50 ps) to measure the actual time emission of the light. • Wave length emission = 532 nm (green). • Flux per shot = 1012 photons. • Intensity: fixed. Adjustable in new design • Light emission: Lambertian • Location at string bottom C. Bigongiari

  8. 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 LED Optical Beacons C. Bigongiari

  9. 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€ ) LASER Optical Beacons C. Bigongiari

  10. Shore Station Junction box Time Calibration with Optical Beacons 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 C. Bigongiari

  11. Optical Beacon OB Optical ModuleOM 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 C. Bigongiari

  12. ANTARES Optical Beacons Layout LED Sector 5 Floor 21 ~87 m Floor 15 Sector 3 • 7+1 Lines deployed so far • 5+1 Lines connected • 20+9 LED Optical Beacons. • 1 Laser Optical Beacon. ~87 m Floor 9 Sector 2 ~101 m Floor 2 Sector 1 ~94 m JUST RECOVERED Line 1 ~114 m MILOM Line 2 Line 5 Line 4 Line 3 Laser C. Bigongiari

  13. 100% 90% 20% 10% TOM – TOB (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. • First step: Gaussian fit from 20% of maximum to the 100 % T100 = Gaussian mean Sigma = Gaussian sigma T50 = Time @ 50% C. Bigongiari

  14. 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 allthe OMs C. Bigongiari

  15. OB OB OB OB OB OB OB illuminating same line OMs • A LED OB is able to illuminate its own storey OMs and even the ones in the storey below • The statistic is enough to perform the fit up to 8 storeys above (about 116m far away) C. Bigongiari

  16. high intensity low intensity ~ 9 ns Number of events [a.u.] Dt [ns] 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 C. Bigongiari

  17. Sigma as function of OB-OM distance 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). TTS depend on the number of photoelectrons Npe, and therefore on the OB-OM distance. At large distance we reach the phe level -> Sigma =√(1.7ˆ2 + 1.3ˆ2) = 2.1 ns C. Bigongiari

  18. 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 C. Bigongiari

  19. 60 m 300 m Laser OB – Correcting for position σ = 2.3 ns Not Corrected Corrected σ = 0.6 ns C. Bigongiari

  20. 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. C. Bigongiari

  21. Variable Voltage Liquid Crystal Head Laser Head Polarizing cube beam-splitter 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 C. Bigongiari

  22. 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 C. Bigongiari

  23. 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 C. Bigongiari

  24. 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 C. Bigongiari

  25. Optical Beacon Cost • Laser Beacon Cost • Laser 5000 € • Container 4500 € • Optical attenuator 2000 € • Electronics 500 € TOTAL 12000 € • 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 C. Bigongiari

  26. ~33Hz ~330Hz Overdrive Mode The LED OB flashing frequency has been recently increased by a factor ten Everything worked as expected C. Bigongiari

  27. Y X 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-2006-002 (G. Lelaizant) • Rotation matrix from ANTARES-SLOW/1999-001 (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. rOM_0=(0.437, 0, 0) rOM_2=(-0.218, 0.378, 0) rcorr = R * ri + r rOM_1=(-0.218, -0.378, 0) rLOB=(0, 0, 1.003) C. Bigongiari

  28. Effect of scattering From MonteCarlo we know that scattering has a minor effect in the rising edge of time difference distributions, although systematic shift of in T100 peaks of +0.5 ns is expected. However delays larger than ~3 ns are unlikely due to scattering. ~1 ns ANTARES-Cali/2000-005 The T100 delay depends on the increasing fraction of scattered photons as we move away from the light source. Therefore T100 depends on the water properties. However a delay greater than 3 ns is not expected. C. Bigongiari

  29. Early Photons Effect • Order statistic: • Theorem: • If X1,…,Xn 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 Npe All Monte-Carlo (Calibob) simulations were done at phe level, hence this effect was not considered C. Bigongiari