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- A shadow cast by the Moon has been detected
700 m underground.
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- The Soudan Mine in northern Minnesota
contains a detector sensitive to sub-atomic particles called muons. Having
the detector underground means that it is isolated from interference that
it would be plagued with on the surface.
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- The shadow occurs in the "rain"
of cosmic rays - particles from outer space which create charged particles
known as muons when they collide with the Earth's atmosphere.
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- It appears on a map of the sky as "seen"
by muons through the detector. The observed shadow results from approximately
120 muons missing from a total of 33 million detected in the Soudan mines
detectors over its 10 years of operation.
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- The cross at the centre of the map indicates
the true position of the Moon. The shadow is slightly offset, due to the
small bending of the electrically charged cosmic rays in the Earth's magnetic
field.
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- Although the Moon can sometimes seem
to dominate the sky, it is relatively small - 0.5 degrees or about the
same width as a thumb held at arm's length.
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- So the Moon's effect in shielding the
Earth from cosmic rays, which bombard the planet from all directions, is
rather like using your thumb as an umbrella!
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- The Soudan 2 detector is run by a team
from the Argonne National Laboratory, and Minnesota, Tufts and Western
Washington Universities US, and Oxford University and the Rutherford Appleton
Laboratory in the UK.
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- The detector, key features of which were
developed at Oxford University, was built originally to search for the
decay of protons, although no evidence for this phenomenon has been found.
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- However, the detector is ideal for detecting
muons that penetrate hundreds of metres below ground.
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