Eyes in the Ocean

November 09, 2008

Welcome to the fourth and final installment of Neutrino Week at Twisted Physics. In prior posts, I explored the first detection of solar neutrinos; the subsequent realization that neutrinos can change their "flavor" and thus have a tiny bit of mass; and a couple of South Pole experiments (AMANDA and IceCube) to further investigate the intriguing properties of this complicated little "ghost particle." But there are also two competing neutrino telescopes under construction that will take the hunt down to the depths of the sea.

It's admittedly an unusual approach to astronomy, which generally involves looking up to scan the heavens for clues to our universe, gleaned from light, radio waves, x-rays, infrared radiation, or gamma rays. But any instrument designed to do so must grapple with how best to filter out all the extraneous "noise" to focus on the most useful data. In recent years, astronomers realized that because the Earth is largely opaque to most of the aforementioned forms of radiation, if you sink neutrino detectors deep under water (or bury it under a lot of Antarctic ice) you'll mostly just see neutrinos -- which are otherwise extremely elusive.

That's where ANTARES comes in, short for Astronomy with a Neutrino Telescope and Abyss environmental RESearch project. (Clearly, they took some liberties with the acronym, in part because it's also the name of a prominent star.) It's comprised of an array of 12 vertical stings of photomultiplier terms serving as its "eyes", anchored to the bottom of the Mediterranean Sea just off the coast of Toulon, France.

ANTARES goes about detecting neutrinos in much the same way as AMANDA, looking for telltale flashes of Cherenkov radiation on the rare occasion when a neutrino collides head-on with an atomic nuclei in the salty water. In fact, ANTARES is a companion telescope of sorts to AMANDA's successor, IceCube: the two projects employ similar operating principles, merely point toward opposite hemispheres. ANTARES was completed in May and over the next few years, its scientists hope to produce a detailed map of cosmic sources for neutrinos, as well as helping with the ongoing search for dark matter.

The main rival to ANTARES in the search for neutrinos under the sea is the project known as NESTOR (Neutrino Extended Submarine Telescope with Oceanographic Research), slated for deployment on the sea floor just off Pylos, Greece. Its "eyes" consist of lots of glass balls holding the usual photomultiplier tubes, connected via star-shaped titanium frames that are stacked on top of each other into a tower that will ultimately boast 12 "floors." The various detectors are connected via a deep sea optic fiber to the communications system.

What do astronomers gain from deploying their neutrino telescopes in the ocean? Well, light scatters less in water than it does in ice, so one advantage of the deep-sea environment is slightly better resolving power. The downside is that ocean water contains the radioactive isotope potassium-40, giving rise to more sources of background light than ice. Nor is the deep sea environment especially gentle: underwater currents can be quite powerful, and in fact, NESTOR has struggled to deploy its various components in bad weather.

And then there was the surprising discovery by astronomers -- marine biologists have known for centuries -- that the ocean depths are teeming with multiple forms of bioluminescent marine life -- not something easily confused with Cherenkov radiation, but still, it's that much more background light that must be filtered out of the collected data before the weak neutrino signatures can be detected.

ANTARES, at least, incorporates a special camera system to automatically track bioluminescent organisms, and whatever it finds will be shared with the various ocean science institutes participating in the collaboration. This is interdisciplinary cooperation in action -- even if it happened a bit after the fact. Perhaps we'll end up learning as much about deep-sea creatures as we will about the cosmos.

Photo: Artist's conception of the ANTARES array. Source: ANTARES collaboration.