Time Calibration of AMANDA

1. Time Calibration of AMANDA Three Variations on a Theme of T0 Kael D. Hanson Department of Physics & Astronomy University of Pennsylvania For the AMANDA Collaboration kaeld@hep.upenn.edu

2. The AMANDA Collaboration 7 US and 9 European institutions, about 110 current members: • Bartol Research Institute, University of Delaware, Newark, USA • BUGH Wuppertal, Germany • Universite Libre de Bruxelles, Brussels, Belgium • DESY-Zeuthen, Zeuthen, Germany • Dept. of Technology, Kalmar University, Kalmar, Sweden • Lawrence Berkeley National Laboratory, Berkeley, USA • Dept. of Physics, UC Berkeley, USA • Institute of Physics, University of Mainz, Mainz, Germany • University of Mons-Hainaut, Mons, Belgium • University of California, Irvine, CA • Dept. of Physics and Astronomy, University of Pennsylvania, Philadelphia, USA • Physics Department, University of Wisconsin, River Falls, USA • Physics Department, University of Wisconsin, Madison, USA • Division of High Energy Physics, Uppsala University, Uppsala, Sweden • Fysikum, Stockholm University, Stockholm, Sweden • Vrije Universiteit Brussel, Brussel, Belgium K. Hanson - Calor2002 – Pasadena, CA

3. The AMANDA Detector • Currently operating AMANDA-II, 677 OMs, 20 Megaton geometric volume. • Several artificial light sources deployed for calibrating AMANDA: • Nd:YAG surface laser • Timing • Geometry • N2 (UV) in situ lasers • UV LED ‘flashers.’ K. Hanson - Calor2002 – Pasadena, CA

4. Timing is Critical to AMANDA Event Reconstruction! • Muon/cascade particle id and reconstruction depend crucially on relative timing in the OMs. • Studies of reconstruction per-formance indicate 10 - 20 ns resolution sufficient for muon track reco (Biron, AIR-20001101). K. Hanson - Calor2002 – Pasadena, CA

5. Dealing with Discriminator Walk • Electrical signals suffer large amount of dispersion in cables  pulse risetime corrections necessary. • Timewalk effect is deter-ministic: pulse risetimes at surface are up to 100’s of ns but jitter is still at ns level. • Pulse risetimes only signi-ficant for electrical channels; optical channels K. Hanson - Calor2002 – Pasadena, CA

6. Variation 1: Laser T0 • T0 measures the signal propagation time from OM to TDC • Cable prop. time • Front-end electronics • Amplifiers (SWAMPs) • Discriminators • High power pulsed Nd:YAG laser at surface delivers 532 nm light via optical fibers to OM. K. Hanson - Calor2002 – Pasadena, CA

7. Laser T0 (continued) • Acrylic “diffuser ball” at OM isotropizes laser light. • Each OM on strings 1-4 and strings 11-19 equipped with diffuser ball. • Only even OMs on strings 5-10 have diffuser ball: neighboring OM used for those lacking diffuser. • Fiber lengths determined separately using OTDR equipment. K. Hanson - Calor2002 – Pasadena, CA

8. Laser Calibration Analysis • YAG intensity controlled via ND filters and optical atten-uators. Sample T0 in range of 1 – 5 photo-electrons. • TDC leading edge plotted vs. 1/SQRT(ADC) • Y intercept is T0 • Slope is timewalk coefficient • Fiber lengths must be sub-tracted to obtain signal propagation time. • Precision of laser cal esti-mated at ~ 5 ns. K. Hanson - Calor2002 – Pasadena, CA

9. Stability of AMANDA T0’s • Q: Is annual calibration sufficient? • Station closed for winter. • No HW changes unless catastrophic failure of equipment • Electronics in ice static • TDCs use crystal oscillator: very stable. • A: YES! K. Hanson - Calor2002 – Pasadena, CA

10. Laser T0 Summary • Laser T0 remains the default AMANDA time calibration method. • Very labor-intensive: full detector calibration done annually requires 1000 man-hours! • Useful for debugging detector (channel mapping errors) after hardware work. • Very easy to piggyback crosstalk mapping. • Only method that currently obtains timewalk coefficients. K. Hanson - Calor2002 – Pasadena, CA

11. Variation 2: Using CR Muons • AMANDA-II receives ample supply (~100 Hz) of downgoing muons. • If T0’s known well enough to give track reco then possible to iteratively refine T0 guesses. • Premise: shift in T0 will appear as offset in timing residual Timing residual = Measured hit time – Hit time expected from track parameters K. Hanson - Calor2002 – Pasadena, CA

12. Outline of Algorithm • Reconstruct muon tracks using best knowledge of T0 calibration constants. • Accumulate time residuals from tracks. • Determine time offset from residual distribution. • Apply offset as correction to T0 constant. • Go to step #1. Repeat until T0 converges to a fixed point. K. Hanson - Calor2002 – Pasadena, CA

13. Offset Determination • Timing residuals have complex structure: some suggestions for getting the ‘zero’ point: • Take maximum of distribution, • Fit gaussian around max, • Cross-correlation with template • We chose 3 since it was overall most robust. • Fast implementation using FFTs. K. Hanson - Calor2002 – Pasadena, CA

14. Convergence • Convergence is monitored by plotting width of distri-bution of offsets for each iteration. • Applying only a fraction of offset at each iteration seems to stabilize method against oscillations: • Terminal value of this width gives rough estimate of precision of calibration. • Still not clear how close initial guess must be in order to ensure convergence. K. Hanson - Calor2002 – Pasadena, CA

15. Testing Muon-T0 • Take standard AMANDA T0 constants and shift by known amount (black line in figure). • Run 25 iterations of muon-T0 procedure. • Corrections shown as red points. K. Hanson - Calor2002 – Pasadena, CA

16. Muon-T0 Summary • Systematic drift over string hampers adoption by AMANDA as primary calibration – however it has been used in conjunction with laser T0: • Incorrect laser T0’s (fiber lengths incorrectly measured), • Fibers leaking at OM optical penetrator, • Can quickly check validity of T0’s for any given run period throughout the year or even previous years’ data! • 2001 calibration done using muon-T0 to determine which offsets have changed: only run laser cal on those channels (~50 as opposed to 700). • No special runs necessary (eliminates 1000 man-hour task in favor of 25 man-hour task). • Does not (yet) calibrate timewalk coefficients. • Eventually hope to use improved muon calibration. K. Hanson - Calor2002 – Pasadena, CA

17. Variation 3: IceCube Clock Synchronization • Baseline technology for IceCube is DOM: • Waveforms digitized in situ, stamped with local DOM time, and sent to surface as digital packet. • Each DOM (~5000 total) has independent clock oscillator. • Surface clocks are synchronized to GPS clock using high precision rubidium clock • RAPCal (Reciprocal Active Pulsing Calibration) method: • Surface electronics sends pulse on communication line at time T1, • DOM digitizes pulse /w/ local timestamp T2, sends mirror pulse at time T3, • Surface electronics digitizes pulse /w/ timestamp T4. K. Hanson - Calor2002 – Pasadena, CA

18. RAPCal Waveform Analysis • Pulses sent at time clock is latched, • Received pulses arrive asynchronously, must fit WF to get higher precision than 33 MHz clock. • Linear fit to leading edge extrapolated to baseline. Tarrival K. Hanson - Calor2002 – Pasadena, CA

19. RAPCal (continued) • RAPCal calibrates: • Cable propagation delay (T0!) • Ratio of clock frequencies fSURF/fDOM • RAPCal/DOM technology being tested now in AMANDA-II (18th string DOMs) • String 18 RAPCal done approximately 0.1 Hz, achieves time resolution of approx. 5 ns RMS. K. Hanson - Calor2002 – Pasadena, CA

20. RAPCal Summary • Hardware built into DOM and DOMHub (front-end surface electronics). • Software runs as application in DOMHub. • RAPCal must run in realtime: IceCube trigger depends on globally time-ordered hits. Simple linear fits currently implemented seem adequate to achieve desired time resolution of ~ ns. • RAPCal in IceCube eliminates need for explicit T0 calibration. K. Hanson - Calor2002 – Pasadena, CA