South Pole turned into giant neutrino detector

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ASA will help shed light on cosmogenic neutrinos, which are produced by energetic objects, such as this active galaxy NGC 1672(Credit: NASA)

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Mentioning Antarctica brings many things to mind, such as ice, penguins, and possibly even dead cities built by Lovecraftian horrors, but a gigantic radiation detector for unravelling the secrets of the Universe probably isn't high on the list. But that's just what an international team of scientists and engineers are building at the South Pole. Called the Askaryan Radio Array (ARA), will use the continent's vast expanses of dense transparent ice as an instrument for seeking radio waves thrown off by high-energy neutrinos from the depths of outer space.

The Universe has many secrets that we are only beginning to gain a dim understanding of. "Dimly" is an apt word because in order to understand a phenomenon, it's necessary to see it clearly. The trouble is, space is so cluttered by stars, galaxies, gas and dust, combined with magnetic and gravitational fields, that the traditional approach of looking through a telescope has only limited applications.

In the last century, things improved as astronomers started looking at UV light, microwaves, radio waves, and even X-rays and gamma rays, but even these make it to Earth in relatively limited amounts or not at all depending on distance and point of origin. Not only do all of these forms of light have trouble penetrating all the galactic clutter, but the information they carry is often distorted as they pass through it.

One promising candidate for making things more transparent is the neutrino. This subatomic particle is incredibly tiny, has almost no mass, is electrically neutral, and is unaffected by electromagnetic or strong nuclear forces. In fact, nothing much affects it. Despite the fact that the Sun, nuclear reactors, radioactive elements, and all sorts of other energetic phenomena produce so many neutrons that, according to the University of Kansas, 65 billion of them shoot through every square centimeter of the Earth every second, we are totally unaware of them.

"Partly because it's so tiny, a neutrino has this unique property — it's able to penetrate through matter very easily," sas David Besson, professor of physics and astronomy at the University of Kansas and spokesman for the ARA project. "A neutrino produced in the center of the Sun can typically require a light year's worth of solid lead to stop that neutrino, whereas light, in the form of photons from the Sun, you can block out easily with dark sunglasses."

One of the things that interests scientists about neutrinos is that what are called "cosmogenic neutrinos" are created by the highest-energy cosmic rays. Physicists are very keen on these rays because no one knows where they come from. Lower energy cosmic rays come from energetic objects, such as supernovae, which, when they explode, can outshine an entire galaxy, but the cosmogenic neutrinos have energies of 1020 eV, which means that something even more powerful is involved.

Because neutrinos can travel for billions of light years without interruption or distortion, Besson says they are the perfect messengers for understanding such cosmic goings on. The tricky bit is being able to detect the undetectable.

This is where ARA and all that ice comes in. If you've ever seen a photo of an open pool nuclear reactor, you may have noticed a blue glow surrounding the reactor proper in its pool of water. This is called Cherenkov radiation and is produced when charged particles pass through a particular medium, such as water, faster than the speed of light in that medium. Neutrinos produce a similar radiation thanks to the Askarya effect – only in this case, the radiation is in the form of radio waves rather than light. Unfortunately, neutrinos create this very rarely and in order to detect it one needs a colossal detector made of a radio-transparent material in an area with very little man made radio noise about.

For ARA, this medium is the vast, deep ice cap of Antarctica in the vicinity of the Amundsen–Scott South Pole Station. Here the ARA team is using steam drills to cut holes in the ice some 200 m (669 ft) deep in which to place radio detectors. Though there are only a handful of detectors in place at the moment, the plan is to have 37 antennae stations covering 200 km2 (77 mi2) that will turn hundreds of cubic kilometers of ice into one massive detector. So far, the system is working as expected.

"The first two stations are showing a lot of promise of detecting neutrinos," says Besson. "Radio transparency within the ice is very high. We measured this at the South Pole and were able to send radio waves through about four miles' worth of ice. So that's pretty transparent, and it means you can bury an antenna in the Antarctic ice and scan for several kilometers around for potential neutrino interactions."

A more down-to-Earth reason for ARA is that it's potentially very cost effective. It may seem pricey to go to Antarctica and cover a huge area with an array of buried antennae, but it's far cheaper than the alternative. Traditionally, high-energy particle physics has been the reserve of supercolliders like CERN, but these are becoming rarer and more expensive as the envelope is pushed farther and farther. With cosmic particles being punched out at 10 million times the energy of Earthbound colliders, taking advantage of a natural resource might be the less expensive option.

The progress of the ARA detector was published in the Physical Review.

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