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©
Per Olof Hulth
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At
240 meters, a single lamp from the module illuminates
the hole.
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A
literary essay about AMANDA by Francis Halzen
page 8
At
10:30 P.M. on December 24, 1993, when the first of four strings
of photomultipliers was deployed and ready for testing, I
was at my family’s house in Tienen, Belgium, sitting
down to a late Christmas Eve dinner. As usual in a Belgian
home, the spread was magnificent, but I hardly paid attention.
When the news finally arrived, dessert was being served and
my laptop was propped on my knees. "First string deployed,"
the E-mail message read. The sender was too tired to write
anything more.
IT
WOULD HAVE BEEN EASY to build a more conventional neutrino
telescope than AMANDA. We would have covered a square kilometer
with spark chambers, shielding them from cosmic radiation
with lead plates a few inches thick. The end result would
have detected neutrinos beautifully and cost about $10 billion—a
thousand times as much as we could afford. Instead, we resigned
ourselves to using the simplest instruments possible to detect
neutrinos across the greatest possible volume of water at
the least possible cost. We would build a telescope that barely
works.
Unfortunately, such a design depends, to some extent, on nature’s
cooperation. So it was that our initial euphoria, on Christmas
Eve, turned quickly to perplexity. We knew that some downward-traveling
muons, created by cosmic-ray events at the surface, would
reach our photomultipliers—even 800 to 1,000 meters beneath
the surface. But we detected a hundred times as many as we
expected. And though we had expected that bubbles, at that
depth, would scatter the Cherenkov light to some degree, what
we saw instead was a nearly meaningless blur.
Everything glaciologists had told us about Antarctic ice,
it seemed, was wrong. To begin with, the ice down there was
far more transparent than anyone had expected. Condensed from
snow that fell 10,000 years ago, at the end of the last ice
age, it could transmit a streak of blue light as far as one
hundred meters, not just the eight meters that had been predicted.
(The discrepancy seemed to arise from the fact that glaciologists
carried out their tests with distilled water, which was much
less pure than Antarctic ice.) In fact, because our photomultipliers
operate at wavelengths where neither atomic nor molecular
excitations absorb photons, the ice was almost infinitely
transparent. That implied we could detect a few more upward-traveling
muons than we had hoped, but it also explained the excess
downward-traveling muons.
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