|Flashback to the Future:
IceCube Captures its First Two Neutrino Events
|Contact: Lynn Yarris, [email protected]|
The more than 100 researchers representing 26 scientific institutions from the United States, Europe, Japan and New Zealand who are building IceCube, an observatory for studying high-energy neutrinos, have cause for celebration. Data from last year's deployment of a single string of 60 detectors more than a mile beneath the South Polar icecap revealed the capture of two neutrino events.
"This shows that the hardware and electronics are working as designed, and, in some areas, considerably better than called for in the design specifications," says Spencer Klein, an astrophysicist with Lawrence Berkeley National Laboratory (Berkeley Lab), one of the institutions participating in the IceCube collaboration. Klein heads the physics analysis team for Berkeley Lab's IceCube effort.
When completed in 2011, IceCube will feature at least 75 strings of detectors, spread out over a cubic kilometer of Antarctic ice. The observatory is designed to study high-energy neutrinos, subatomic particles with very small mass and no electrical charge that originate from supernovae (exploding stars), black holes, and other high energy sites in the Milky Way and beyond. Because these neutrinos travel to Earth virtually unobstructed, they can be used by scientists to "see" into regions of space otherwise obscured by dust or other matter.
Such observations should provide new insight into questions about the nature of dark matter, the origin of cosmic rays, the source of gamma-ray bursts, and other cosmic issues. IceCube can detect neutrinos with energies ranging from 100 billion to a quadrillion or more electron volts.
Berkeley Lab was responsible for the unique electronics package inside the digital optical modules (DOMs) that will enable IceCube to pick out the rare signal of a high-energy neutrino colliding with a molecule of water. A DOM consists of a pressurized glass sphere the size of a basketball, which houses an optical sensor called a photomultiplier tube that can detect photons and convert them into electronic signals scientists can analyze.
Equipped with onboard control, processing, and communications hardware and software, each DOM acts like a minisatellite. There are 60 DOMs attached to each IceCube string, an electrical cable connecting the DOMs to the surface. The DOMs operate at depths from 1,450 to 2,450 meters below the surface.
IceCube detects particles that come from all directions. However, since neutrinos are the only particles able to pass untouched through an entire planet, Earth can serve as a filter to screen out every type of particle but neutrinos. Trillions of neutrinos pass through every square centimeter of Earth's surface each second like ghosts, interacting with nothing. Once in a rare while, though, a neutrino will collide with an atom. This generates a muon, a subatomic particle resembling a heavy electron, which emits flashes of blue light called Cherenkov radiation as it passes through ice or water.
IceCube's DOMs can detect the Cherenkov light emitted as a muon moves upward through the ice. By measuring the intensity and arrival time of the light flashes as they travel through IceCube, scientists can reconstruct the muon's direction and energy. Knowing the direction is critical for separating a muon generated by a cosmic neutrino from the million-times more numerous downward-traveling muons generated by cosmic rays in the atmosphere.
While there are extensive ongoing studies of neutrinos emitted out of thermonuclear reactions in the core of the sun, as well as antineutrinos from nuclear reactors, IceCube is designed to study the high-energy neutrinos spawned in the most violent of astrophysical events.
That IceCube could detect neutrino events with only a single string of DOMs in place was a pleasant surprise. Francis Halzen, a University of Wisconsin-Madison professor of physics who is the principal investigator for IceCube, says the first DOMs performed "like a Swiss watch" in their first full year.
This past austral season at the South Pole eight more strings, bearing a total of 480 DOMs, were added to the single string deployed the previous season. This brings the current count of IceCube DOMS up to 540, and effectively doubles the size of AMANDA (the Antarctic muon and neutrino detector array), an existing South Pole neutrino telescope, which is still operational. In addition to the buried DOMs, there is another array of DOMs on the surface of the ice, instrumented in ice-filled tanks located where the deep ice strings emerge from the frozen depths. This surface array is called IceTop and will be used to study cosmic rays. When completed, IceCube will feature nearly 5,000 DOMs in all.
Successful drilling this past season was accomplished despite a 10-day delay at the start of the drilling season, which runs from November to mid-February. The strings are lowered into holes 2,500 meters deep in the ice made by a five-megawatt hot water drill. This past season, two drill towers were used, which enabled the drilling teams to set up and begin work on a subsequent hole within three days of completing the previous hole. As the season progressed, the drill team improved the rate of production from one hole every five days to one every four day. With an earlier start next year and a consistent cycle time of four days or less, 14 or more strings could be deployed next season.
"Drilling eight new holes this past season was a significant achievement because it shows that we can drill and instrument holes at production speeds," says Robert Stokstad of Berkeley Lab's Nuclear Science Division, who heads the Institute for Nuclear and Particle Astrophysics and is the leader of Berkeley Lab's IceCube effort. "This was a key point in demonstrating the success of IceCube as a construction project."
Construction of IceCube is projected to cost $272 million. The National Science Foundation will provide $242 million for the project, with an additional $30 million from Germany, Sweden, Belgium, Japan, New Zealand, the Netherlands, and the Wisconsin Alumni Research Foundation.
In addition to Berkeley Lab and the University of Wisconsin-Madison, other IceCube collaborating institutions from the United States are the University of California at Berkeley, the University of Maryland, Pennsylvania State University, the University of Wisconsin-River Falls, the University of Delaware, the University of Kansas, Clark Atlanta University, Southern University, the University of Alaska, and the Princeton Institute for Advanced Study.
More than 30 Berkeley Lab scientists and engineers have been involved in the IceCube project. Project leaders include Klein; Stokstad; astrophysicist and IceCube Operations manager Azriel Goldschmidt of the Nuclear Sciences Division; William Edwards of the Engineering Division, who is the Berkeley Lab project manager; and David Nygren of the Physics Division, an expert in particle detection. Nine Berkeley Lab researchers traveled to the South Pole this season to participate in IceCube's deployment and testing; in addition to Klein and Edwards they were Keith Beattie, David Hayes, Arthur Jones, Chuck McParland, Jerry Przybylski, Mike Solarz, and John Jacobsen, representing the Nuclear Science, Physics, Engineering, and Computational Research divisions.