Tuning into neutrinos
ARIANNA proposes to use radio waves to capture the elusive particles
Posted March 26, 2010
The Chinese philosopher Lao Tzu once wrote that a journey of a thousand miles begins with a single step.
For a team of physicists hoping to learn more about the high-energy universe, the journey toward building an array of 10,000 instruments for just that purpose began this past season with a single prototype deployed on a 600-meter-thick ice shelf.
“We’re trying to find the sources of ultra-high-energy cosmic rays in the universe,” explained Spencer Klein , leader of the three-person field team that set up the detector in an area called Moore’s Bay, more than 100 kilometers from McMurdo Station . Those galactic cosmic rays — rays being something of a misnomer for the highly charged particles — pack the energy of a well-hit tennis ball in just one particle as it hits the Earth’s upper atmosphere and bursts into a trillion smaller bits.
Nothing on Earth — not even the Large Hadron Collider — is capable of producing a particle of comparable energy, according to Klein. To put it in perspective: One would need to build an accelerator around the sun to produce these particles, Klein said.
“That tells us that somewhere there are these extremely high-energy accelerators in the universe,” said Klein, a physicist in the Nuclear Science Division at Lawrence Berkeley National Laboratory (LBNL) . “We would very much like to know where these cosmic accelerators are, and how they work.”
Unfortunately, cosmic rays themselves are lousy tracers for getting back to the source of these high-powered galactic accelerators. Interstellar magnetic fields bend the electrically charged particles as they hurl through space, meaning there’s not a straight line back to the spot that first spit them out.
Physicists need a different type of probe — a neutrino. Yes, the same subatomic particle with no charge and little mass that the IceCube Neutrino Observatory at the South Pole is being built to detect.
IceCube uses strings of sensors buried deep in the icecap to detect not the neutrino itself but the light created by neutrino collisions in the ice. The collision creates a negatively charged particle called a muon, which continues in the same direction as the neutrino. And as it travels through the ice, it produces a cone of blue light (called Cherenkov radiation) that IceCube detects.
But Klein and his colleagues, including the project’s principal investigator Steve Barwick with the University of California-Irvine , want to detect even higher-energy neutrino events than IceCube will with its strings of ice-encased photomultipliers. At ultra-high energies, IceCube is too small to detect the tiny flux, Klein said. They want to use radio waves.
The grand scheme is called ARIANNA, for Antarctic Ross Ice shelf Antenna Neutrino Array. It would consist of up to 10,000 stations, each consisting of four to eight TV-like antennae embedded just below the surface of the ice, spread over more than 100 square kilometers. The antennae are connected to an electronics box for processing the radio pulses. Solar panels would charge the detector in the summer.
For the winter, Klein said the team is testing a wind generator on the prototype instrument, which would place limits on power consumption during the dark Antarctic months.
The ARIANNA detectors search for radio waves produced by neutrino interactions in the ice. The radio pulses come from the particle showers produced when the neutrino converts its energy into matter. Radio waves in ice are particularly attractive, Klein said, because you get radio emissions from the entire particle shower. Also, radio waves can travel up to a kilometer in the ice.
“That means if you’re building stations you can put them on a roughly 1-kilometer grid, and you need fewer of them,” Klein said.
The physicists also believe the water below the ice shelf is an additional advantage. The ice-water interface reflects radio waves, bouncing back any emissions from downward-traveling neutrinos.
Popular theories for the source of high-energy particles include something called active galactic nuclei, which are galaxies with a super massive black hole at the center. Colliding black holes or collapsing giant stars are also possible sources.
“Those are just theories, and it could just as well be something else,” Klein said.
Klein, LBNL engineer Thorsten Stezelberger, and Martha Story from McMurdo’s Berg Field Center, spent 11 days in a field camp setting up the prototype radio detector, which will stay out for a year. In addition, work included characterizing properties of the ice, such as measuring the radio wave reflections from where the ice meets the water.
Klein, also involved with the IceCube experiment, has been to the South Pole, but the ARIANNA fieldwork was new for the physicist. “Going out with three people and a tent was a very different experience. It was a small group, so everybody was doing everything,” he said.
If all goes as planned — the prototype station survives the winter, the site demonstrates the necessary properties — the ARIANNA team hopes to move forward a proposal to deploy a five- to seven-station array with new hardware specifically designed for the project.
“That would be the next step. You don’t go from one station to thousands,” Klein said.
NSF-funded research in this story: Steven Barwick, University of California-Irvine, Award No. 0839133 .