Catching Supernovas in a Grain of Salt

21 August 1997 8:30 pm

Starry messenger. Supernova neutrino strikes a sodium nucleus in the wall of a salt deposit, dislodging a neutron that triggers a flash of cintillation.

An international team of physicists hopes to go almost a kilometer deep into the earth to see into the hearts of exploding stars. These researchers are laying plans to convert deep salt deposits into a pair of underground observatories that would capture thousands of the elusive particles called neutrinos, which spray from the core of a supernova.

If the project, called Observatory for Multiflavor Neutrinos from Supernovae (OMNIS), gets up and running, its low-maintenance detectors would wait years or decades until a burst of neutrinos announces a type II supernova in our galaxy. These supernovas are thought to explode when a massive star's core runs out of fusion fuel, cools, and collapses. It then rebounds, generating a ferocious shock wave. The shock slows as it plows into the outer layers of the star, but theorists believe that a blast of neutrinos released from the core revives the shock, which blows the outer part of the star into space.

The observatories, one in a salt deposit already excavated for a nuclear waste dump in Carlsbad, New Mexico, and the other in the Boulby Salt Mine in the United Kingdom, would detect the neutrinos by watching for neutrons thrown off when a tiny fraction of the neutrinos crash into nuclei in the salt walls of the mine or in metal slabs near the detectors. These neutrons would strike plastic or liquid scintillation detectors, producing countable flashes of light.

"The whole flood of 2000 events lasts only 10 seconds," says Peter F. Smith, a collaborator at the Rutherford Appleton Laboratory in the United Kingdom. The shape of this brief pulse could tell astrophysicists whether they really understand how such stars explode and clear up mysteries such as whether a black hole, from whose gravity nothing can escape, sometimes forms when the star's core collapses. OMNIS could also help settle the vexing question of whether the neutrino has mass. Unlike existing neutrino observatories, OMNIS could readily detect all three "flavors" of neutrinos, which would differ subtly in behavior if the particles do have mass--a finding that would open up new theories in physics and cosmology.

OMNIS collaborators are now putting together funding proposals for submission to the U.S. Department of Energy and the National Science Foundation. Estimates of construction costs range between $20 million and $40 million.

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