University of Wisconsin-Madison

IceCube in Depth

IceCube is an international project sponsored and conducted by the United States and several non-U.S. countries and funding agencies. The project is defined by the IceCube science requirements and is carried out by the IceCube Collaboration. The science requirements demand an extensive set of engineering requirements on the entire IceCube system. With several different institutions involved in all phases of the project, clear allocation of responsibilities and coordination of effort is key to project efficiency and success.

Each collaboration participant has its own established practices, traditions, and local requirements, which must be accommodated and tailored to meet collaboration needs. Communication and maintenance of the engineering requirements, and when necessary, managing their modification, are critical tasks. Much of the construction and operation of the project will take place in an extremely harsh and difficult environment, making the issues of reliability and safety of paramount importance.

IceCube is a neutrino observatory for astrophysics that was installed at the South Pole during Austral summers beginning in 2004 and ending in December 2010. The IceCube in-ice detector consists of 5,160 optical modules deployed on 86 vertical strings buried 1450 to 2450 meters under the surface of the ice. A surface component called IceTop includes an additional 324 DOMs. Although IceCube was only recently completed, it has been providing data since the completion of the nine string array in 2006. The completed detector will be operated for at least 20 years.

The detector was designed, built, installed and is now used for research by an international IceCube Collaboration. Funding institutions provided funds for the construction of the detector and continue to support data analysis. The United States National Science Foundation (NSF) supplied funds for the design, development, fabrication, procurement, testing, drilling and operations of the project at the South Pole. Funding for construction is provided through the Major Research Equipment and Facilities Construction (MREFC) budget account. Operations funding will be provided through the Research and Related Activities (R&RA) budget account. As the host institution, the University of Wisconsin-Madison, with support from the National Science Foundation, provides oversight and staffing for the execution of the project and development of key components.

Scientific Goals

Many parts of the universe are difficult to study using conventional or historical methods. Protons do not carry directional information because of their deflection by magnetic fields, neutrons decay before reaching the earth and high-energy photons are absorbed. Neutrinos, being nearly massless and without charge, are ideal messengers.

IceCube opens unexplored wavelength bands for astronomy including the PeV (1015 eV) energy region. IceCube looks at fundamental questions about the physical conditions in gamma ray bursts and whether the multi-TeV photons, originating in the Crab supernova remnant and near the super massive black holes of active galaxies, are of hadronic or electromagnetic origin.

Diffuse Neutrino Flux vs. Energy for various models of sources contributing to the diffuse background. The horizontal line in red indicates IceCube sensitivity after three years.
Diffuse Neutrino Flux vs. Energy for various models of sources contributing to the diffuse background. The horizontal line in red indicates IceCube sensitivity after three years.

Data from the Chandra X-ray satellite demonstrate that the "diffuse" extragalactic X-ray background (as observed with instruments of low angular resolution like IceCube) consists mostly of accumulated radiation from all active galaxy sources. Models of the radiation producing processes in active galaxies can provide a prediction as to the "diffuse" flux of high-energy neutrinos. Figure 1-1 shows the results of a variety of such models and illustrates the discovery reach of IceCube. The broad, dark, steep spectrum is the atmospheric background. IceCube also occupies a unique place in the multi-prong attack on the particle nature of dark matter.

With unmatched sensitivity to cold dark matter particles approaching TeV masses, IceCube will be sensitive to supernova within our galaxy. As a particle physics detector capable of detecting neutrinos with energies far above those produced at accelerators, IceCube searches for super-symmetric particles and the topological defects created during grand unified phase transitions in the early universe. The detection of cosmic neutrino beams will also make it possible to study neutrino oscillations over megaparsec baselines.

As history has demonstrated, the opening of each new astronomical window has led to unexpected discoveries. The true possibility of IceCube is discovery.


Technical Description

The target geometry of the IceCube Neutrino Observatory consists of a regularly spaced array of 5,160 photomultiplier tubes buried between 1450 to 2450 meters below the surface of the South Pole ice and a surface air shower detector, IceTop. The array covers an area of about 1 square km as shown in Figure 1-2. The pilot project AMANDA is contained within the IceCube detector, but was decommissioned in 2009.


The configuration of an IceCube string is depicted schematically in Figure 1-3.

The photomultiplier tubes record Cerenkov radiation from charged particle progenitors of neutrino interactions in the ice. Each photomultiplier is enclosed in a transparent pressure sphere, a Digital Optical Module (DOM). The DOM also contains a digitally controlled high voltage supply to power the photomultiplier, an analog transient waveform digitizer and LED flashers. The signals digitized in the Digital Optical Modules (DOM) are communicated to the surface, via twisted pairs, to the data acquisition and data analysis system at the surface. Data is transmitted via satellite from the South Pole to data storage facilities accessible to the collaboration for its data analysis effort.

Authorizing Agencies

The U.S. National Science Foundation (NSF)

The NSF is the Executive Agent with responsibility for seeing that the Project meets its baseline requirements of cost, schedule, scope, and technical performance. The NSF has a special role in IceCube because of its host laboratory responsibilities in managing operation of the Amundson-Scott South Pole Station. These responsibilities include: safety; physical qualification of project staff; environmental protection; transport of personnel, fuel and equipment; and the provision of housing, food service, support personnel, logistical support, IT support, and general infrastructure support.

International Oversight and Finance Group (IOFG)

An IOFG provides oversight and financial support for the IceCube project. The group organized annual reviews of the construction project and meets annually to discuss project performance. The group also sets policies for receiving periodic progress reports on all aspects of the project and by all the performers in the project, and for conducting external reviews when appropriate.

Membership

A representative of the National Science Foundation chairs the IOFG. Membership is comprised of representatives of the funding agencies in the partner countries supporting the construction and operation of IceCube Neutrino Observatory. The group is informed by the spokesperson of the collaboration, the project director, the principal investigator and others as appropriate.