Construction is now underway for a most unusual telescope, one whose light collecting “mirror” will be buried more than a mile beneath the South Pole ice cap. Dubbed IceCube, because its array of detectors covers a cubic kilometer of ice, this telescope is designed not to capture starlight, but to study the high-energy variety of the ghostlike subatomic particles known as neutrinos.
Originating from the Milky Way and beyond, and traveling to Earth virtually unobstructed, high-energy neutrinos serve as windows back through time, and should provide new insight into questions about the nature of dark matter, the origin of cosmic rays, and other cosmic issues.
IceCube is an international collaborative effort made up of more than 150 scientists, engineers and computer scientists, from 26 institutions in the United States, Europe, Japan and New Zealand. The principal investigator for the project is Francis Halzen, a University of Wisconsin-Madison professor of physics, and the collaborating institutes include the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
Berkeley Lab researchers were 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, that houses an optical sensor, called a photomultiplier tube, which can detect photons and convert them into electronic signals that scientists can analyze.
“Each of these DOMs is like a mini-computer server that you can log onto and download data from, or upload software to,” says Robert Stokstad, of Berkeley Lab’s Nuclear Science Division (NSD), who heads the Institute for Nuclear and Particle Astrophysics (INPA) and is the leader of Berkeley Lab’s IceCube effort.
Equipped with on-board control, processing and communications hardware and software, and connected in long strings of 60 each via an electrical cable, the DOMs can detect neutrinos with energies ranging from 200 billion to one quadrillion (1015) or more electron volts. In the past few weeks, the first IceCube cable, with its 60 DOMs, was lowered down into a hole drilled through the Antarctic ice using jets of hot water. Plans call for a total of 4,200 DOMS to be put in place over the next five years. The Antarctic summer season, during which the weather is “mild” enough for work on IceCube to proceed, lasts only from mid-October to mid-February. After that, winter sets in and the climate is much too harsh for any outdoor work to be done.
When completed, IceCube’s total volume of detectors will be about 20 times greater than that of its predecessor, another South Pole high energy neutrino telescope called AMANDA, for Antarctic Muon And Neutrino Detector Array, which has 680 optical sensors.
“The South Pole might seem like an unusual place to build neutrino telescopes, but the Antarctic ice is very clear and very stable, and has relatively low background radiation levels,” says NSD astrophysicist Spencer Klein, who heads the physics analysis team for Berkeley Lab’s IceCube effort.
Neutrinos are one of the most common and mysterious particles in the universe. Produced by the decay of radioactive elements and certain elementary particles, they carry no electrical charge and scarcely a hint of mass, which means they are unaffected by magnetic fields and rarely interact with other forms of matter. Able to escape from anything other than a black hole, their pathway to earth is essentially a straight line from their point of origin. Because these neutrinos are the only known particles able to pass through Earth untouched, scientists can point telescopes like IceCube and AMANDA to the northern skies and use the planet to filter out every type of particle but neutrinos.
While there are extensive on-going studies of the 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 neutrinos spawned in the most violent of astrophysical events, i.e., supernovas, gamma-ray bursts and cataclysmic phenomena involving black holes and neutron stars. In studying these high-energy neutrinos, scientists hope to be able to produce a map of the neutrino sky.
In addition, IceCube can be used to search for neutrinos produced by the annihilation of weakly interacting massive particles (WIMPs) that have been captured in the gravitational fields of the earth or the sun. This research should shed new light on the nature of dark matter. Scientific measurements of the mass of the galaxies we observe suggest that 90-percent of our universe is made up of a “dark” form of matter that we cannot see. WIMPS have been proposed as constituents of dark matter.
“IceCube will also search for oddball hypothesized particles like magnetic monopoles and Q-balls,” says Klein. “Because of IceCube’s huge sensitive area, the search limits for such particles will be extended more than an order of magnitude below existing experiments.”
Trillions of neutrinos pass through each square centimeter of Earth’s surface every second without a trace of impact. However, every so often, a neutrino does collide with an atom. This rare collision generates a muon, a heavy electron-like subatomic particle that, as it passes through ice or water, emits flashes of blueish light called Cherenkov radiation. IceCube’s DOMs can detect this light and scientists, by measuring the intensity and arrival time of the light at multiple DOMs, can reconstruct the directional path of the muon and determine the type, direction, and energy of the neutrino that helped create it. This is critical for separating a muon generated by a cosmic neutrino from the millions more muons generated by cosmic rays in the atmosphere.
Says Stokstad, “Each DOM begins collecting data when it detects a single photon. Data are collected with a custom waveform-digitizer chip and the photomultiplier tube waveform is sampled 128 times at 300 megasamples per second. Adjacent DOMs can communicate via local-coincidence cables, allowing for the possibility of coincidence triggers.”
On the surface of the ice, located where the IceCube detectors emerge from the frozen depths, there is another array of detectors called IceTop. This past season, eight of the 160 tanks that will make up the completed IceTop were installed. Each tank is about two meters in diameter and will hold two IceCube DOMs frozen in water. A pair of tanks will be connected to each IceCube cable. IceTop will be used to calibrate IceCube and to study high-energy cosmic rays.
“For practical reasons, it was important that the DOMs be built around an integrated circuit that could give us fast sampling times (pulses on a nanosecond timescale) with low power demands,” says Azriel Goldschmidt, another NSD astrophysicist who has been working on DOM data analysis. “Each DOM runs on only about five watts of power.”
The DOM integrated circuit was custom made at Berkeley Lab based on architecture developed by Stuart Kleinfelder, formerly of the Engineering Division, now at UC Irvine. More than 30 Berkeley Lab scientists and engineers are involved in the IceCube project. Project leaders, in addition to Stokstad, Klein and Goldschmidt, include William Edwards, of the Engineering Division, who is the project manager, and David Nygren, of the Physics Division, one of the world’s foremost experts in particle detection.
Says Klein, “One of the most exciting things about IceCube is that we just don’t know what we will find. When you open up a new window into the universe, you open up the possibility of entirely new discoveries.”
Construction of IceCube is projected to cost $272 million. The National Science Foundation will provide $242 million for the project, and an additional $30 million will come from foreign partners.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov.
From Berkeley Lab