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Undersea Telescope Detects Neutrinos

Photonics Spectra
Feb 1999
Hank Hogan

Collaborators will soon deploy an underwater telescope to detect and study high-energy cosmic neutrinos. Placed on the ocean floor 2.35 km below the surface, the telescope may eventually unlock fundamental secrets of the universe.


An artist's drawing portrays the proposed setup of arrays of photomultiplier tubes placed deep below the ocean's surface to detect high-energy neutrinos. Courtesy of Antares.

As part of the Antares project -- involving scientists from France, Spain, The Netherlands and the UK -- researchers plan to begin deploying the prototype this year. Several trial runs have already taken place off the coast of France, with 350-m-long strings of photomultiplier tubes unfurled on the ocean bottom and then recovered.

A single strand of photomultiplier tubes is just the beginning, according to François Montanet, a physicist at France's Center for Particle Physics who has been involved in the project since it began. "A next step will consist of seven other lines, or eight in total with a high photomultiplier tube density, or 16 lines with a larger photomultiplier tube spacing," he said. "These first eight to 16 lines will cover an effective surface of about a 10th of a square kilometer. This should be installed and running by the end of 2001."

The goal is to assemble photomultiplier tubes in an array that scans a cubic kilometer of water at a depth of more than 2 km. Montanet expects that to happen around 2005.

The motivation for Antares is that deep water is the best place to detect high-energy neutrinos. Ocean bottoms are relatively dark, undisturbed by high-energy cosmic rays and other phenomena. Under the protective blanket of the ocean, the arrays can look for the distinctive upward flash of light that marks the passage of a high-energy neutrino, avoiding random bioluminescence and radioactive decay that could trigger downward light flashes. Far below waves and swells, the tubes will sway gently, while acoustic sensors report their position to 10-cm accuracy. Trial runs have indicated such an underwater neutrino telescope is technically feasible.

Neutrinos interact very weakly with matter, so not many of them can be seen without a large volume of water. Previous experiments have been confined to mine shafts and caves filled with water.

One of the requirements of the Antares project is large photomultiplier tubes with fast response times, Montanet said. Originally, the project used commercially available 8-in. tubes, but he noted that the project's demands have led to the development of 11-in. tubes. Another photonic innovation arising from the research is the development of an intense, pulsed light source at 470 nm.

Neutrinos provide an extremely long-range glimpse into the universe. Because of their high energy, these particles are also messengers of little-understood events and objects.


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