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Moonshadow Over Minnesota

10 June 1999 7:00 pm
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oudan-2 detected the moon's shadow in cosmic rays slamming into Earth's atmosphere.

Physicists working in an iron mine almost a kilometer underground have detected the "shadow" the moon casts when it blocks cosmic rays streaming toward Earth. Scientists, who announced the feat last week, say it's a step toward identifying the mysterious source of the rays.

When cosmic rays smash into Earth's atmosphere, the collisions spawn muons, hardy particles that can pass through meters of lead like a cannonball through tissue paper. This makes it impossible to tell where they come from. Although they will leave imprints on detectors like film, they will pass right through the mirrors of conventional telescopes, making it impossible to bring them into focus and thus identify the direction of the source Physicist John Cobb of Oxford University and his team are now attempting to track down the elusive sources of cosmic rays with a novel muon detector made from steel tubes packed with a mixture of argon and carbon dioxide gas at the bottom of a defunct mine in Soudan, Minnesota. Built a decade ago to measure a proton's lifetime, the Soudan-2 detector is now being pressed into service as, in effect, a muon telescope. As a muon runs through the gas, it knocks off electrons, leaving a detectable trail. "The rock acts as a filter to block all but the highest energy muons," says Cobb. That means the apparatus doesn't register muons generated when the solar wind hits Earth's atmosphere.

Because the moon blocks cosmic rays, it casts a shadow over the detector as long as the moon is visible from Minnesota. Using a computer to track the directions of incoming muons absorbed in the gas, Cobb's team created an image of the moon's "muon shadow." The reconstruction confirms that Soudan-2 can trace an incoming cosmic ray to the region of the sky it came from to "within a small fraction of 1 degree," says Cobb--precise enough to identify potential sources of the rays.

"It is a technical tour de force," says physicist John Learned of the University of Hawaii, Manoa. With the detector now up and running, adds Cobb, "we are in an excellent position to make a survey of the entire sky." That may allow researchers to figure out whether quasars--or some other kind of object--are the cosmic ray fountains.

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