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Invited Review Article: IceCube: An instrument for neutrino astronomy
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Image of FIG. 1.
FIG. 1.

(Top) Conceptual design of a large neutrino detector. A neutrino, selected by the fact that it traveled through the Earth, interacts with a nucleus in the ice and produces a muon that is detected by the wake of Cherenkov photons it leaves inside the detector. A high-energy neutrino has a reduced mean free path , and the secondary muon an increased range , so the probability for observing a muon increases with energy; it is about for a 1 TeV neutrino (Ref. 13). (Bottom) Actual design of the IceCube neutrino detector with 5160 optical sensors viewing a kilometer cubed of natural ice. The signals detected by each sensor are transmitted to the surface over the 86 cables to which the sensors are attached. IceCube encloses its smaller predecessor, AMANDA.

Image of FIG. 2.
FIG. 2.

The cosmic-neutrino spectrum. Sources are the Big Bang , the Sun, supernovae (SN), atmospheric neutrinos, active galactic nuclei galaxies, and GZK neutrinos. The data points are from detectors at the Frejus underground laboratory (Ref. 21) to the right at the top of the figure, and the upper portion of the Atmospheric line at the bottom of the figure, and from AMANDA (Ref. 26) pp and B at the top and the lower part of the Atmospheric line.

Image of FIG. 3.
FIG. 3.

At the energies of interest here, the cosmic-ray spectrum follows a sequence of three power laws. The first two are separated by the knee, the second and third by the ankle. Cosmic rays beyond the ankle are a new population of particles produced in extragalactic sources (Ref. 27).

Image of FIG. 4.
FIG. 4.

Simulated sky map from IceCube in galactic coordinates after 5 yr of operation of the completed detector. Two Milagro sources are visible “by eye” with four events for MGRO and three for MGRO with energy in excess of 40 TeV. These, along with background events, have been randomly distributed according to the resolution of the detector and the size of the sources.

Image of FIG. 5.
FIG. 5.

Contrasting Cherenkov light patterns produced by muons (left) and by showers initiated by electron and tau neutrinos (right) and by neutral-current interactions. The patterns are often referred to as tracks and cascades (or showers). Cascades are produced by a (approximately) point source of light with respect to the dimensions of the detector. At PeV energies, leptons travel hundreds of meters before decaying, producing a third topology, with two cascades—one when the interacts and the second when the decays (Ref. 43). This is the double bang signature; a simulated event is shown in Fig. 22.

Image of FIG. 6.
FIG. 6.

The average number of photoelectrons observed by an 8 in. AMANDA PMT is shown as a function of the minimum distance to a minimum-ionizing muon track. The result for the average PMT direction (dashed line) and the direction toward the Cherenkov cone (solid line) are shown. On the average, a minimum-ionizing particle is visible up to 20 m from a PMT.

Image of FIG. 7.
FIG. 7.

IceCube drilling site at Amundsen–Scott South Pole Station. The hole into which the optical sensors will be lowered is drilled under the tower building to the center-right. Hot water is pumped from the heaters (not shown) producing 5 MW of hot water under 1000 psi pressure through the hose at the left. The cylindrical hose reel holds 2500 m of hose which unreels as drilling proceeds. The trench at the right holds two IceTop tanks filled with ice. Reprinted from F. Halzen and S. R. Klein, Phys. Today 61 (5), 29 (2008).

Image of FIG. 8.
FIG. 8.

Data from the IceCube dust logger, compared with AMANDA measurements based on light scattering from sources to optical modules, along with measurements from two other Antarctic sites (Ref. 54).

Image of FIG. 9.
FIG. 9.

Absorption (left) and scattering (right) lengths of light in South Polar ice as a function of depth and wavelength, from 300 to 600 nm (Ref. 53).

Image of FIG. 10.
FIG. 10.

Schematic drawing of a digital optical module.

Image of FIG. 11.
FIG. 11.

The single photoelectron charge spectrum observed in the PMT at a gain of . Reprinted from R. Abbasi et al., Nucl. Instrum. Methods Phys. Res. A 618, 139 (2010).

Image of FIG. 12.
FIG. 12.

The distribution of arrival times of single photoelectrons at the PMT. A tail of late-arriving photons follows the main pulse (Ref. 56). The dashed line shows the contribution to late light due to laser afterglow plus random background.

Image of FIG. 13.
FIG. 13.

PMT signals for different input amplitudes, showing the effect of saturation. Saturation compresses the instantaneous current, so that small “features” such as prepulsing and afterpulsing grow in relative size when saturation sets in Ref. 56.

Image of FIG. 14.
FIG. 14.

A simplified block diagram of the IceCube main board electronics (Ref. 58).

Image of FIG. 15.
FIG. 15.

A photograph of the DOM Main board. The circular board fits in the pressure vessel, while the cutout provides room for the neck of the PMT (Ref. 59).

Image of FIG. 16.
FIG. 16.

Diagram of the digital electronics including the calibration and monitoring circuitry.

Image of FIG. 17.
FIG. 17.

RapCal timing waveforms, as received by the DOM, and on the surface (“DOR side”). Initially narrow pulses are now wide (Ref. 62).

Image of FIG. 18.
FIG. 18.

(Left) The time distribution of the first photons arriving at DOM 46, string 21, when DOM 47 on the same string is flashing; the time difference is consistent with the 17 m separation, and the 1.26 ns sigma shows that the relative timing is accurately calibrated. (Right) The distribution of rms time differences from the 59 DOM pairs on string 21 (Ref. 62).

Image of FIG. 19.
FIG. 19.

A block diagram of the surface electronics. The string hub computers contain DOR cards that receive the signals from the DOMs. They pass these signals on to the trigger; hits within (typically) of a trigger are sent to the event builder to be saved.

Image of FIG. 20.
FIG. 20.

A block diagram of a string hub, showing the interfaces to the DOR cards.

Image of FIG. 21.
FIG. 21.

The ATWD digitizer output from a typical event; multiple photoelectrons are clearly visible. Each time sample is 3.3 ns. The waveform is decomposed into a list of photon arrival times, which is used for event reconstruction (Ref. 65).

Image of FIG. 22.
FIG. 22.

Simulated events of the three types of neutrino interactions in IceCube: (a) (top), (b) (middle), and (c) a double bang, from (bottom). Each circle represents one active optical module; the size of the circles shows the number of detected photons, while the color represents the time, from red (earliest) to blue (latest). In the top panel, the white shows the stochastic muon energy deposition along its track (Ref. 14).

Image of FIG. 23.
FIG. 23.

The azimuthal angle for downward-going or near downward-going muons in IceCube 22-string data, after tight cuts, compared with the results of cosmic-ray muon (blue) and neutrino (green) simulations. The coincident muon background is largely eliminated (four downward-going events expected) and not shown here. Reprinted from S. R. Klein, IEEE Trans. Nucl. Sci. 56, 1141 (2009).

Image of FIG. 24.
FIG. 24.

The neutrino effective area (averaged over the Northern Hemisphere) from IceCube simulation (histogram) is compared to the convolution of the approximate muon effective area (Ref. 36) (solid line) that is used in the estimates of event rates throughout this paper.

Image of FIG. 25.
FIG. 25.

Deficit of cosmic-ray muons in the direction of the Moon. Cosmic rays are blocked by the Moon, creating a shadow of 0.5° in the IceCube sky map. The shadow is visible as a deficit of secondary muons from cosmic-ray interactions in the atmosphere. The more-than- deficit of events in the 40-string data confirms the pointing accuracy of the telescope (Ref. 70).

Image of FIG. 26.
FIG. 26.

Using declination and right ascension as coordinates, the map shows the probability for a point source of high-energy neutrinos with energies not readily accommodated by the steeply falling atmospheric-neutrino flux. Their energies range from 100 GeV to several 100 TeV. This map was obtained by operating IceCube with 40 strings for half a year (Ref. 71). The “hottest spot” in the map has an excess of 7 events, an excursion from the atmospheric background with a probability of . After taking into account trial factors, the probability to get a spot this hot somewhere in the sky is not significant. The map contains 6796 neutrino candidates in the Northern Hemisphere and 10 981 down-going muons rejected to the level in the Southern Hemisphere, shown as black dots.

Image of FIG. 27.
FIG. 27.

The solid and dashed lines (bottom right) show the 90% confidence-level upper limits on the spin-dependent interactions of dark matter particles with ordinary matter (Ref. 86). The two lines represent extreme cases where neutrinos originate mostly from heavy quarks (“soft,” top line) and weak bosons (“hard,” bottom line) produced in the annihilation of dark-matter particles. Also shown is the reach of the complete IceCube and DeepCore with 5 yr of data. The shaded area represents supersymmetric models not disfavored by direct searches for dark matter. Also shown are previous limits from direct experiments and from the SuperK experiment. The results improve by two orders of magnitude on the sensitivity previously obtained by direct experiments.

Image of FIG. 28.
FIG. 28.

The arrival direction of cosmic-ray muons detected with 22 IceCube strings. The color scale represents the relative intensity (Ref. 79). The star indicates the direction of Vela, the brightest gamma-ray source in the sky.


Generic image for table
Table I.

Some IceCube acronyms and their meanings.


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Scitation: Invited Review Article: IceCube: An instrument for neutrino astronomy