PDD - Executive Summary
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2 Executive Summary
The IceCube Project at the South Pole is a logical extension of the research and development work performed over the past several years by the AMANDA Collaboration. The optical properties of ice deep below the Pole have been established, and the detection of high-energy neutrinos has been demonstrated with the existing detector. This accomplishment represents a proof of concept for commissioning a new instrument, IceCube, with superior detector performance and an effective telescope size at or above the kilometer-scale.
IceCube scientific goals require that the detector have an effective area for muons generated by cosmic neutrinos of one square kilometer. The detector will utilize South Pole ice instrumented at depth with optical sensors that detect the Cherenkov light from secondary particles produced in interactions of high-energy neutrinos inside or near the instrumented volume.
The design for the IceCube neutrino telescope is an array of 4800 photomultiplier tubes (PMTs) each enclosed in a transparent pressure sphere to comprise an optical module (OM) similar to those in AMANDA. In the IceCube design, 80 strings are regularly spaced by 125 m over an area of approximately one square kilometer, with OMs at depths of 1.4 to 2.4 km below the surface. Each string consists of OMs connected electrically and mechanically to a long cable, which brings OM signals to the surface. The array is deployed one string at a time. For each string, a hot-water drill creates a hole in the ice to a depth of about 2.4 km. The drill is then removed from the hole and a string with 60 OMs spaced by 17 m is deployed before the water freezes. The signal cables from all the strings are brought to a central location, which houses the data acquisition electronics, other electronics, and computing equipment.
Each OM contains a PMT that detects individual photons of Cherenkov light generated in the optically clear ice by muons and electrons moving with velocities near the speed of light. Signal events consist primarily of upgoing muons produced in neutrino interactions in the bedrock or the ice. In addition, the detector can discriminate electromagnetic and hadronic showers ("cascades") from interactions of νe and ντ inside the detector volume provided they are sufficiently energetic (a few 100 TeV or higher). Background events are mainly downward-going muons from cosmic ray interactions in the atmosphere above the detector. The background is monitored for calibration purposes by the IceTop air shower array covering the detector.
Signals from the optical modules are digitized and transmitted to the surface such that a photon's time of arrival at an OM can be determined to within a few nanoseconds. The electronics at the surface determines when an event has occurred (e.g., that a muon traversed or passed near the array) and records the information for subsequent event reconstruction and analysis.
At the South Pole site (see fig. 1), a computer system accepts the data from the event trigger through the data acquisition system. The event rate, which is dominated by down-going cosmic ray muons, is estimated to be 1–2 kHz. This will produce a large amount of data and requires filtering and compression of this data stream at the South Pole. There are two ways for the data to be transported to the Northern Hemisphere. The first and preferred method is via satellite transmission. The second method is by hand-carrying the data tapes north once the station reopens in the austral summer. Even in this case, a reduced set of data must be transferred daily by satellite to monitor the detector and to access important data. Once at the data archive, the data are catalogued, unpacked, checked, filtered, and calibrated. Interesting events are reconstructed and distributed to the collaboration for scientific analysis.
The technology that will be employed in IceCube has been developed, tested, and demonstrated in AMANDA deployments, in laboratory testing, and in simulations. This includes the instrument architecture, technology, deployment, calibration, and scientific utilization of the proposed detector. There have been yearly improvements in the AMANDA system, especially in the OMs, and in the overall quality of the information obtained from the detector. In the 1999/2000 season, a string was deployed with optical modules containing readout electronics inside the module. The information is sent digitally to the surface over twisted-pair electrical cable. This option eliminates the need for optical fiber cables and simplifies calibration of the detector elements. This digital technology is the baseline technology of IceCube.
For more details and references, see http://www.icecube.wisc.edu.
- Figure 1: The South Pole site, showing the residential dome and associated buildings, the skiway where planes land, the dark sector with the Martin A. Pomerantz Observatory in which the AMANDA electronics are housed, and a rough outline of where IceCube strings are to be placed.
