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Hot on the Trail of Cosmic Rays

January 2, 2008

The
mysterious origins of cosmic rays that slam into the Earth’s atmosphere could
soon be revealed, thanks to a better ground-based sensor that costs less than
balloons or satellites.

Cosmic rays
are thought to come from either the center
of the galaxy
or a nearby supernova, and knowing which is true will help
astrophysicists paint a more accurate picture of the cosmos.

“Cosmic
rays are not a spectator phenomenon in the galaxy — they have a role in
galactic dynamics,” said Scott Wakely, a University of Chicago physicist. “To understand the
galaxy in a full sense, you need to understand cosmic rays.”

That
understanding depends on ground and space-based instruments. Satellites and
balloons first detect a blue flash — known as Cerenkov radiation — when cosmic
rays smash into the upper atmosphere and release energy.

The cosmic
ray particles then break into a shower of smaller pieces and produce a second
blue flash. Ground sensors usually only detect the second flash.

Tens of
thousands of particles may bombard an area the size of a small parking lot on
Earth daily, while rarer high-energy particles strike less than once a year in the
same area. Satellites and balloons do a better job of detection by rising above
the atmosphere, but they can only cover a small area.

“A
$400 million satellite is only a couple particles per year, and you want
hundreds of thousands,” Wakely told SPACE.com. “You always want to
look for new ways to do this.”

Wakely set
out with colleague Simon Swordy, a physicist at the University of Chicago, to create a ground-based instrument that could detect both
the first and second blue flashes. The instrument will have roughly 10 times
the resolution and power of current ground-based detectors.

Scientists
can use information from both blue flashes to identify a particle as a certain
element and maybe even its origin. For instance, some elements will more likely
come from the fiery
outburst of a supernova
.

“We
can say that was iron or that was uranium,” noted Wakely. “Those are
the kinds of data you need to make progress in this business.”

No one
thought ground-based instruments could detect the first blue flash, until
Wakely and Swordy proposed the idea with other colleagues in 2001. A team of
researchers in Namibia confirmed the concept using a
telescope array called HESS. Wakely later made his own observations using a
telescope array called VERITAS.

“That
was direct evidence that it [the technique] works,” said Wakely. “The
goal of this [new] instrument is to combine large area detection with the high
precision of space-based sensors.”

An improved
instrument could also help solve at least one mystery about the energy range of
cosmic ray particles. Higher energy particles — such as those from the nuclei
of heavy elements like iron — are rarer than common, lower-energy particles
such as protons. But physicists have puzzled over a sudden drop-off in
frequency of high-energy particles at a certain point in the energy spectrum,
labeling the strange kink “the Knee” because of its shape.

Some
researchers suggest that supernovas which they claim produced all the cosmic
rays suddenly run out of energy at “the Knee,” and a new source of
cosmic rays takes over on the other side. Others think that a new model of
physics takes over that is beyond current scientific understanding, but no one
knows for sure, without more measurements of high-energy particles from
“the Knee” region.

If all goes
well, Wakely and Swordy plan to submit a proposal in three years to build the
instrument they are designing. The National Science Foundation has already
given a five-year, $625,000 grant to start drawing up the concept.


Source: imaginova