XXth International Conference on Neutrino Physics and Astrophysics
R.Stokstad for the IceCube and AMANDA Collaborations
The Digital Optical Module - How IceCube will Acquire Data
Large-scale neutrino detectors pose challenging problems for signal
processing because of the long distance between the physical sensors
and the site at which data from many sensors can be combined to
recognize events of interest. It has been possible for AMANDA to use
analog transmission (via copper and fiber optic cables) to bring
photomultiplier tube signals a distance of ~2.5 km to the surface
where timing and digitization can be performed in one central
location. IceCube, the km-scale neutrino detector to be deployed near
AMANDA in the ice sheet at the South Pole, will have a distributed
system with a high-speed digitizer and very stable oscillator located
in each PMT module. Power and digital communication signals will be
transmitted over copper twisted-pair cable. This system offers
improved signal quality, increased dynamic range , and higher
reliability. The concept has been developed and tested by the AMANDA
collaboration with a string of 40 modules equipped with both digital
and analog transmission technologies. Digitized waveforms recorded by
a custom integrated circuit (the Analog Transient Waveform Digitizer)
at a rate of ~500 megasamples/s are sent to the surface for modules
satisfying a nearest-neighbor local coincidence requirement. The 16.8
MHz oscillators used to time-stamp the waveforms are calibrated to an
absolute accuracy of 5 ns by sending identical timing pulses up and
down at intervals of ~10 seconds. This technique will enable a fully
automatic time calibration procedure for IceCube's 4800 digital
optical modules. Results obtained with the AMANDA string will be
described along with plans for the IceCube DAQ.
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