Detector in polar ice to hunt for neutrinos

Frances Halzen, a UW–Madison scientist who helped develop AMANDA, shows the inside of one of the 422 basketball-sized glass orbs, called photomultiplier tubes, used to catch the fleeting signals left by high-energy neutrinos.

AMANDA: Facts at a glance What is AMANDA? AMANDA stands for Antarctic Muon and Neutrino Detector Array. A sort of telescope, it consists of strings of photomultiplier tubes sunk deep into the Antarctic ice using a hot-water drill. AMANDA is designed to detect the fleeting signatures of light left by cosmic neutrinos as they pass through the polar ice cap. What are neutrinos? Neutrinos are particles without electric charge and, as far as scientists know, they have no mass. Neutrinos are hard to find because their interaction with matter is extremely feeble, producing nothing more than a fleeting burst of light. Large, very sensitive detectors like AMANDA have the best chance to find the distant and exotic sources of cosmic neutrinos. Where do neutrinos come from? Scientists believe cosmic neutrinos are constantly bombarding the Earth from many distant sources, including: Black holes: Massive black holes in the centers of galaxies swallow up most everything with huge gravitational pull. Other galaxies: Supernovae, which are exploding stars, distant galaxies are believed to emit neutrinos. Quasars: Quasars (quasi-stellar objects) have been enigmatic because they emit prodigious amounts of energy from a very compact source. Who cares? Scientists are searching for cosmic neutrinos because they promise insight into some of the most violent and enigmatic objects in the universe: black holes, supernovae and quasars. Finding the elusive particle would open a new realm of astronomy. Who pays? AMANDA is funded primarily by the National Science Foundation. UW–Madison and University of California-Irvine and UC-Berkeley have also made substantial contributions. Other partners in the AMANDA collaboration include Lawrence Berkeley National Laboratory, Stockholm University, University of Uppsala and DESY, the German national high-energy physics laboratory.

The hunt for the cosmic neutrino is on.

This winter, after an extensive shakedown period, the Antarctic Muon and Neutrino Detector Array or AMANDA, a novel telescope set kilometers deep in the ice at the South Pole, began its search for the ghost-like cosmic neutrino. The nearly massless particle is rocketed through space, scientists think, by supernovas, black holes, quasars, gamma ray bursts and whirling neutron stars.

Unlike any other astronomical telescope ever built, AMANDA is not a telescope in the conventional sense. It is composed of 422 basketball-sized glass orbs, photomultiplier tubes arranged on cables and sunk deep into the Antarctic ice in concentric rings.

The device looks down through the Earth and is designed to catch the fleeting signals left by cosmic neutrinos, high-energy particles that are believed to emanate from objects deep in space and whose bizarre properties permit them to pass through entire planets without skipping a beat.

If AMANDA successfully detects cosmic neutrinos and traces their paths back to the objects from which they come, it will open a new window to the universe, permitting scientists to study some of the most intriguing phenomena in the cosmos, according to Francis Halzen, a UW–Madison scientist who helped develop the telescope.

“We’ve spent over a year understanding the idiosyncratic nature of this instrument,” says Halzen. “Nobody’s ever built anything like this before.”

AMANDA was built with extensive support from the National Science Foundation and in collaboration with other institutions in Europe and the United States.

The AMANDA telescope works by detecting the fleeting flashes of blue light created by muons, particles created when neutrinos occasionally collide with other subatomic particles called nucleons. The muon’s flash of light creates a bow wave much like that made by a boat in water. In theory, the bow wave will point back to the source from which the neutrino comes.

The deep Antarctic ice is crystal clear and, at great depths, is free of air bubbles and nearly free of other imperfections. It serves as an ideal medium in which to look for the rare signals left by the billions of neutrinos that continuously pass through the Earth.

To detect these signals, AMANDA looks down through the Earth to suspected neutrino sources in the sky of the Northern Hemisphere.

“If something emits a lot of gamma rays, it’s a good bet there are a lot of neutrinos there,” says Robert Morse, a UW–Madison physicist who has spent years helping oversee the construction of the AMANDA telescope.

Suspected sources include black holes, the remains of supernovas, and neutron stars, planet-sized, burned out husks of stars that spin at amazing speeds. Other potential sources are what scientists call active galactic nuclei, things like quasars and blazers, extremely bright and energetic objects at the centers of distant galaxies.

What all of these objects have in common, says Morse, is that they act like enormous versions of the accelerators scientists build on Earth to study high-energy, subatomic particles. They also are at great distances from Earth.

“The sources are far away. Gamma ray bursts, for instance, could be three to five billion light years away, or maybe even half way to the suspected edge of the universe. So you need a big detector,” Morse says.

In conventional forms of astronomy, the photon, the particle that makes up visible light and other parts of the electromagnetic spectrum, is what is sampled by telescopes on remote mountaintops, satellites and radio telescopes. But photons can be deflected and absorbed as they traverse space and encounter interstellar dust and pockets of gas and radiation. The cosmic neutrino, on the other hand, is unhindered by such obstacles.

The tradeoff, says Morse, is that neutrinos are very hard to detect. Moreover, the sun and cosmic rays crashing into the Earth’s atmosphere also make neutrinos, creating a soup of high-energy particles. But neutrinos from different sources, whether the sun or from a distant black hole, have defining characteristics that would permit scientists to identify the particles of interest.

“It’s like a police line-up,” says Morse. “They have to pass the test.”

Over the past year, the AMANDA telescope has been tuned and tested and has succeeded in sampling neutrinos, but not the cosmic neutrinos of interest.

“We’ve gotten the apparatus tuned up to the point that what we’re seeing really are neutrinos,” Halzen says. “But the majority of the neutrinos we’ve seen are atmospheric neutrinos. What we have to do now is pick out that one event out of 10 million.”

Yet the neutrinos now being sampled by AMANDA are the highest energy neutrinos ever detected, according to Albrecht Karle, a UW–Madison physicist. And the muons they spawn are tracked in the AMANDA detector for distances of up to 400 meters through the crystal clear Antarctic ice.

Constructed at a cost of $7 million over seven years, the AMANDA detector will nearly double in size next year with the addition of seven more strings, each with 48 photomultiplier tubes. The ultimate configuration, says Morse, is a proposed cubic kilometer detector of 80 to 100 strings with as many as 5,000 to 6,000 photomultiplier tubes.

The larger telescope will not only make a bigger target for the elusive cosmic neutrino, but also will make a key diagnostic test, measuring the energy of neutrino particles more precisely. That enhancement would permit a search for neutrino oscillations on a cosmological scale, says Morse.

“Neutrinos can bring us a message of the most violent and cataclysmic processes occurring at the very edge of the universe — colliding black holes, neutron stars and maybe even colliding galaxies,” Morse says. “But it’s very difficult to make the measurements. AMANDA, we think, is our best bet to do that.”