DISCOVER Vol. 22 No. 5 (May 2001)
Table of Contents

Ice Fishing for Neutrinos
By Tim Stoddard

Deep inside a glacier at the south pole, the world's most unconventional telescope is facilitating a new kind of astronomy based not on light but on neutrinos, ghostly particles that emerge from the hearts of supernovas and quasars. The Antarctic Muon and Neutrino Detector Array--AMANDA for short--has no mirror, no eyepiece, and no dome. Instead, it consists of about 700 bowling-ball-sized glass sensors that pick up the faint blue flashes given off when neutrinos collide with atoms more than a mile down in the Antarctic ice.

A sensor about the size of a basketball is one of many buried in Antarctic ice to detect elusive high-energy neutrinos. Photo by Jeff Miller/University of Wisconsin-Madison.

With no electrical charge and little or no mass, neutrinos zip through the universe largely unimpeded by gravity or magnetic fields, passing blithely through stars, planets, and your body. But one time in a billion a passing neutrino will strike a proton. The collision ejects a heavy electron, or muon, that travels in the same direction as the neutrino and leaves a trail of blue light as it sheds energy, much like a meteorite burning up in the atmosphere. AMANDA's photoreceptors absorb that telltale blue flash, turning the light into a measurable and meaningful electrical signal. A computer then compares the signals from several photoreceptors to calculate the path of the light streak in three dimensions. From that, scientists have a good idea of the neutrino's point of origin. "The muon tells us the direction the neutrino came from, and then we have a telescope, because you can point the neutrino back up into the sky," says Francis Halzen, a physicist at the University of Wisconsin at Madison.

High-energy neutrinos emerge from some of the most violent phenomena in the universe, including supernovas, quasars, and other types of active galaxies. Because neutrinos barely interact with matter, they reach Earth still carrying unadulterated information about the cosmic events that produced them. Photons of visible light, in contrast, can get absorbed, obscured, and altered by intervening matter on its way to Earth. "Photons are very gregarious. They interact with everything. Only neutrinos can bring us unvarnished information," says Robert Morse, Halzen's colleague at Wisconsin and AMANDA's project leader.

Just finding neutrinos is not good enough--it's the rare, very energetic ones that Morse and Halzen are after. But Earth is constantly showered with a far greater abundance of low-energy neutrinos generated in the sun or created when cosmic rays strike atoms in the upper atmosphere. Scientists have built giant underground water tanks to detect these solar neutrinos. Even the largest of these neutrino observatories--the 12.5-million-gallon Super Kamiokande in Japan--is too small to catch the few high-energy neutrinos, however.

A hole drilled about a mile down into Antarctic ice becomes the new home for a string of 10 to 16 photoreceptors, which will detect the light produced when a neutrino collides with a proton. Courtesy of Robert Morse.

Since it is prohibitively expensive to build a sufficiently large tank of water, physicists Halzen and Morse pursued a suggestion from a glaciologist: Look for neutrinos in the vast expanses of ultra-clear ice in Antarctica. At depths greater than about three quarters of a mile, the pressure inside the glaciers squeezes out air bubbles, creating an extremely transparent medium in which a photon of light travels an average of 700 feet before being absorbed. Down there, the ice is bathed in a continuous blue glow from millions of sparking muons.

The primary task of the AMANDA sensors is to study this glow and track how the muons travel through the ice. All the downward-moving muons come from low-energy neutrinos created in the atmosphere above the south pole. The interesting ones will be those moving upward, which are mostly more energetic particles originating from the sun or from somewhere far beyond the solar system. The intensity of each blue flash reveals the energy of the neutrino that produced it.

So far, AMANDA successfully detected and tracked the background of low-energy neutrinos from the sun. To pick up the long-sought high-energy particles from intergalactic space, Morse and his Wisconsin colleagues are adding 5,000 detectors to transform AMANDA into IceCube. At 3,000 feet in each dimension, IceCube will be the largest single scientific instrument ever built. Finding even a handful of neutrinos from quasars and their ilk could allow the first direct measurement of the massive, galaxy-shaping, star-swallowing black holes believed to lie at the center of these celestial bodies. "The hope is that a particle that is almost nothing may tell us everything about the universe," says Halzen.

— Posted 04/24/01

See a graphic of how AMANDA works at http://www.news.wisc.edu/misc/amanda.html

The AMANDA project page is http://amanda.berkeley.edu/amanda/amanda.html

Halzen's Web site is http://phenom.physics.wisc.edu/~halzen/