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The IceCube Neutrino Detector is a neutrino telescope currently under construction at the South Pole. Like its predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA), IceCube is being constructed in deep Antarctic ice by deploying an array of optical sensors (or Digital Optical Modules, DOMs) at depths between 1,450 and 2,450 meters.
The sensors are deployed on "strings" of sixty modules each, into holes in the ice melted using a hot water drill. The completed detector will consist of 80 strings and a total of 4800 DOMs.
Figure 1 shows an artistic impression of what the detector would look like, with the Eiffel tower (324 m height) to scale.
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Fig 1: Artist rendering of the IceCube detector
The AMANDA prototype size is shown as the shaded cylinder |
Each DOM consists of a photomultiplier tube (PMT) and associated electronics enclosed in a
glass sphere. The electrical cables ("strings") that hold the optical sensors serves also to provide the high voltage to the PMT and to transmit the signals to the surface electronics. IceCube uses
25 cm diameter Hamamatsu PMTs, with a spherical photocathode and 10 dynodes. The PMTs are operated at a gain of 107 at voltages in the range 1200-1400 V. Each DOM is a highly complex and autonomous detector unit. The PMT waveforms are recorded and
digitized locally, which allows transmission to the surface without the inherent signal smearing that analog signals would suffer while travelling through more than two kilometers of cable. The waveforms are also time-stamped by a local oscillator, which is
calibrated periodically (every few seconds) against a master clock at the surface. This allows to achieve a time resolution of less than
2 ns.
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Fig 2: Attaching a DOM
to the main cable |
IceCube includes an air shower detector array on the surface of the ice, IceTop. The concept of IceTop is similar to other
air shower arrays under construction. The detector consists of an array of 160 ice tanks with two DOMs in each tank. Two tanks
are placed at the surface position of each of the IceCube strings, as indicated by the surface dots in Fig 1. The IceTop DOMs are the same as the IceCube DOMs.
The two DOMs in a tank are run at different gains in order to increase the dynamic range of the tank. The IceTop array detects showers of
electrons and muons from cosmic ray interactions in the atmosphere, and can serve both as a veto and a coincident detector
to IceCube. The combination of a surface and a deep detector at the same site provides a unique instrument for the study
of ultra high cosmic ray physics.
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| Fig 3: Inside view of an IceTop tank |
Neutrinos are detected indirectly, by the Cherenkov radiation of the secondary particles produced in the rare interaction of
an incoming neutrino with an atomic nucleus of the ice surrounding the detector. In the interaction, a "shower" of particles is produced, along with a lepton of the same flavour as the incoming neutrino. An artist view of this process is shown in figure 4, in this case representing a muon being produced at the interaction point, which then traverses the detector emitting Cherenkov radiation in the typical cone shape.
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Fig 4: Artist view of a neutrino interaction, producing
a muon traversing IceCube |
The individual DOMs detect the Cherenkov photons with nanosecond resolution and this allows to reconstruct the direction of the muon, and therefore of the incoming neutrino. The total number of registered photons is
related to the original neutrino energy.
Fig 5 shows a real event taken by the AMANDA detector, illustrating how the neutrino direction and energy can be reconstructed
by registering the Cherenkov light pattern of the individual photons. Each coloured dot represents an optical module that registered at least one photon. The color coding is related to the time of arrival of the photon;
red is earlier, blue later. The size of the dots is related to the number of photons registered by
each photomultiplier. With this information, both the muon direction and an estimate of its energy can be
derived.
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| Fig 5: A real neutrino event in AMANDA |
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