APS Nuclear Science Division Meeting

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Time and Position Calibration of IceCube Optical
Modules W.J. Robbins for the Collaboration

• APS Nuclear Science Division Meeting
• Kapalua, HI
• September 2005

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Abstract

• In order to reconstruct the path of
  extraterrestrial neutrino-induced muons, the
  IceCube high-energy neutrino telescope uses
  optical modules (OMs- each containing a
  photomultiplier tube) embedded in glacial ice to
  detect muon-generated Cherenkov photons. To
  facilitate the accurate reconstruction of muon
  tracks, thereby allowing the performance of
  neutrino astronomy, uncertainties in the optical
  module (OM) positions and the arrival times of
  photons at each OM are minimized through a series
  redundant detector calibrations. The IceCube
  Collaboration -- in the process of constructing a
  kilometer-scale detector near the South Pole--
  recently deployed a string of sixty OMs deep in
  the Antarctic Ice and sixteen OMs at the surface
  above the string. This work will describe a few
  of the detector calibration techniques.

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Why is this Important?

• The Project
• Use PeV energy range multi-messenger astronomy
• Black holes
• Maybe observe dark matter (TeV mass range),
  nuclearities (strange quark matter) or magnetic
  monopoles
• Study neutrino oscillations on mega parsec scales

• Detector Verification
• Maximum resolution requires precise knowledge of
  OM positions and detector timing
• We want to confirm detector performs as it was
  designed to
• Scattering near OM can cause 5ns effects
  require better resolution

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Neutrinos Good Information Carriers

• Dont experience the strong force travel huge
  distances unimpeded
• Experience little gravitational force no
  gravitational lensing
• Unaffected by magnetic fields excellent for
  finding neutrino sources
• Small mean angular deviation of muon from
  inducing neutrino (typically
• ?µ? 0.65((1012eV )/ E? )0.48

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The Office Amundsen-Scott South Pole Station
Projection of detector volume onto surface at
south pole station
South Pole
Actual detector 1km away from SPS
Hopefully, my future cubicle
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High Voltage Board
PMT Collar
Penetrator Assembly
Delay Board
Flasher Board
IceCube with Up-Going Muon
DOM Main Board
Pressure Sphere
PMT
Mu Metal Shield Assembly
µ
Optical Module
OM Deployment
60 OMs/string 80 strings 4800 OMs
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Simulated Examples
?e ? e ? Shower (Multiple Cherenkov Cones)
In General ? N lepton hadrons
?m? m via Charged Current Interaction
En 375 TeV
Optimum resolution for Eµ between 500GeV 1PeV
300m _at_ E?t 1 PeV
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Multiplicity (Nch) and Energy
Nch number of OMs which detect photon intensity
above a set threshold
As muon energy increases, the event multiplicity
grows increasingly larger
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How Signals are Processed Inside the OM Main Board
Input signal
ATWD bin size set to 3.3ns ssingle photoelectron
8ns
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Rapcal Timing Calibration

• Pulse sent from surface hardware to OM and back
  every 3-10 seconds
• Pulse trip-time (?Tdt)/2
• dt is well-known
• Successive pulses can determine OM oscillator
  frequency deviations
• Allen Variance of OM clock drift every 10 s!

Surface Hardware
µs rise time
?T/2
ns rise time
dt
With OM time stamp
Optical Module
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Rapcal Results from String-21

• Rapcal is repeatable with small standard
  deviation
• Effective for all OMs

OMs on Surface known as IceTop
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OM Flasher Timing Calibration

• 405 nm LEDs mounted in OM produce 6 billion
  photons/LED pulse directed upward and outward
  large statistics
• Variations in the difference between arrival time
  of first photon between adjacent OMs leads to
  resolution measurement
• Ice properties can be probed through this method
  also

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Flasher Timing Results for String-21
Distribution in Time Difference Between OMs 58
59 with OM 60 Flashing
Resolution for All Adjacent OM Pairs on String-21
entries
RMS 1.2 ns
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OM Flasher Position Calibration

• Pulsed mode LED flashes
• Random walk photons fit to modified (for PMT
  jitter) Pandel function
• Assuming timing and knowledge of ice properties,
  time of 1st arrival gives information about
  relative position

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Gamma Distribution of Photons

• Properties
• Normalized
• Can be integrated over tres
• Increased probability of unscattered event at
  small distance
• Large distance leads to larger delay time
  distribution
• For distances?, power of tres changes sign-
  probability of undelayed photon goes to 0

Dependencies Ci Speed of light in medium ?a
Absorption length ? Tau functions of geometry
and distance determined by Monte Carlo
methods tres is residual time thit- tgeom
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Down-Going Muon Calibration
Cherenkov Cone

• Reconstruct muon track with data from OMk omitted
• Assume correct track fix non-OMk parameters
• Minimize -log(L) to determine position of OM

Z
?c
k-th Optical Module
r
µ
?c Cos-1(1/ (index of refraction ß)) 41 in
ice1
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Log-Likelihood (L ) Optimization

• Properties of this method
• For OMi,
• ?(photons) f(r, Eµ, other non-geometric
  parameters5)
• Does not depend on ice properties, but can
  provide information about them

• L (X a) p P(Xi a)
• P probability density
• Xi measured quantities for i-th OM (r, t0,
  pµ, E0)
• a parameters (x,y,z-positions OMk) to optimize

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Single String Results
Time Residual Modes Coincide with Scattering
Coefficients
Distribution of difference between arrival time
and retarded time for photons from muon track
reconstructed without OM 15 data
Note anomalous OM 60 omitted from discussion
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Conclusion

• Facts
• km sized detector
• Some OMs separated by more than 6 km of cable
• Verification methods agree 2ns relative
  detector timing resolution!
• (now we need to finish building the detector)

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Acknowledgments

• This material is based upon work supported by
  the National Science Foundation under Grant Nos.
  OPP-9980474 (AMANDA) and OPP-0236449 (IceCube).
  Any opinions, findings, and conclusions or
  recommendations expressed in this material are
  those of the author and do not necessarily
  reflect the views of the National Science
  Foundation. Thanks to John Pretz, Erik Blaufuss,
  Sourav Mandal, John Aytag, Thorsten Stezlberger,
  Dave Hardtke, Kirill Filimonov, Warren Rogers and
  the CEU staff, the Nuclear Science Division, the
  NSF and the DoE. Lastly, Id like to thank LBNL,
  Karen Edwards, Jeanne Miller and IceCubers
  especially, the following individuals

Tom McCauley
Dima Chirkin
Arthur Jones
Spencer Klein
Dave Nygren
Azriel Goldschmidt
Bill Edwards
Bob Stokstad
Howard Matis
Jerry Przybylski
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Ill have copies of these _at_ the session

• 1 J. Ahrens. Muon Track Reconstruction in Amanda.
  
• 2 S. Barwick. Physics and Operation of AMANDA-II
  High Energy Neutrino Telescope. SPIE, 2002
• 3 C. Speiring. Science Potential for the IceCube
  Detector
• 4 A. Goldschmidt. The IceCube Detector. ICRC
  2001 1237
• 5 D. Chirkin. Locating OM Position with Muons

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