Speaker
Description
Summary
Construction of the IceCube Neutrino Observatory reached a milestone in January 2005
with the deployment of the first deep-ice string of IceCube sensors and the first
elements of the surface air-shower array, IceTop, located directly above IceCube.
The completed Observatory will instrument a cubic kilometer of deep ice with at
least 70 new strings plus the existing 19 AMANDA strings.
The major scientific goal of the Observatory is to detect both diffuse and point
sources of high-energy neutrinos coming from galactic and extra galactic regions -
in essence, to map the high-energy neutrino sky and contribute to the understanding
of the origin of the ultra high energy cosmic rays.
IceCube incorporates novel electronic features that are suited for the requirement
that it detect high-energy muons (E>100 GeV) and cascades at a remote location using
sensors that are inaccessible once deployed. Electrical power is at a premium at
the South Pole, making low power consumption in the sensor modules a priority. A
robust system is required for long-term reliability at temperatures down to -55oC
and to survive the stress of deployment, the subsequent refreezing of the ice
around the modules, and the presence of electromagnetic interference from other
installations.
At the same time, high quality of information from each event has to be combined
with autonomous operation and calibration.
PMT signals are digitized using a custom integrated circuit called the Analog
Transient Waveform Digitizer, or ATWD. The ATWD is a switched-capacitor array (128
samples deep) having four channels. Launched by a discriminator on the PMT signal,
it digitizes (10 bits) all 128 samples simultaneously after waveform capture. The
sampling rate is adjustable and set at 300 megasamples/s. With three ATWD channels
set at different gains, dynamic range exceeds 150 photoelectrons for a time interval
of 15 ns.
A combined FPGA and CPU (Altera Excalibur) and on-board memory are central to the
system’s operation, providing logic, timing, fast signal processing, data handling,
and communication with the surface via ADCs and DACs. PMT signals receive a coarse
time stamp from a stable (df/f < 10-10/s) local oscillator running at 20 MHz. This
clock is calibrated within a few nanoseconds against a master clock on surface. The
calibration procedure is automatic and repeats at typically ten-second intervals,
with negligible consumption of bandwidth. A single copper twisted pair with a total
bandwidth of one Mbit/s serves two modules. Adjacent modules are connected to each
other by a short twisted pair, which enables a local time coincidence. This
effectively eliminates the consumption of bandwidth that would arise from sending
the ~800 Hz of PMT noise to the surface.
System performance is evaluated by detecting the cosmic-ray muons that penetrate
from the surface to the detector below. Analysis of these events, as well as events
induced by LED flashers in each module, indicate that the intrinsic time resolution
is ~4 ns. These results will be presented along with data indicating that the
first IceCube string and IceTop surface modules are performing according to design
and expectation.
