SYMMETRY Magazine: 2021-04-27
Dark matter’s signature could be written in stone
挺有意思的一个创新方法。
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A proposed dark-matter detection method would look for tracks of dark matter etched into billion-year-old mineral samples.
Now researchers have proposed a new way to look for dark matter, one
that leverages time over size. Instead of waiting for dark matter to
come to them, these scientists hope to discover ancient signs of it in
minerals buried deep in the Earth. A small, billion-year-old mineral
sample might work better than a new, thousand-pound detector, the
scientists behind the work say.
“I think building bigger and bigger is not necessarily better,” says
Andrzej Drukier, a physicist at Stockholm University who—along with
Sebastian Baum, Katherine Freese, Maciej Górski and Patrick Stengel—is
one of the original researchers behind the so-called paleodetectors. “We
need some new class of detectors.”
...
Goliath or the grandparent
Dark-matter detectors look for the fallout caused by an invisible
dark-matter particle as it hits a nucleus. Whether in a liquid detector
or ancient rock, this collision—and the resulting transfer of energy—is
expected to leave a mark. For liquid detectors, scientists look for the
brief flashes as atoms are ionized. In the case of the rock, such a
collision would cause the nucleus to shoot off, leaving a tiny trail
only tens of nanometers long but permanently etched in stone.
The paleodetector researchers have estimated that it would take only a
100-gram sample of billion-year-old rock to find such signatures. Using
new technology, such as helium-ion beam microscopy or small-angle X-ray
spectroscopy, scientists could measure tracks in a rock to learn more
about the dark-matter particles that passed there. Since the track
length would correspond to particle mass, scientists could use the
tracks to differentiate dark-matter particles from other particles such
as neutrons and neutrinos, and to determine their mass.
High-energy particles like cosmic rays could also leave footprints.
But these could be avoided by digging up rocks from deep underground,
where such particles could not readily penetrate.
A bigger issue for contamination would be from within the Earth
itself. Radioactive decay from uranium, which does exist deep
underground, could similarly leave marks in rock.
To find a way to avoid this, the scientists spent months studying
thousands of minerals in search of the perfect canvas for dark matter.
They eventually closed in on marine evaporates, such as halite, which
form out of sea water and are less likely to contain uranium.
“We worked very hard to define materials that would be optimal for
not having too much radioactive decay from uranium,” says Freese, a
theoretical astrophysicist at the University of Texas, Austin. “We're
very careful with dealing with that problem.”
...
Experiments with paleodetectors could also be somewhere between 10 to 50
times less expensive than conventional experiments. Members of the
paleodetector group say samples would need to originate at least 5
kilometers underground, and existing scientific boreholes that could
supply such samples already surpass 12 kilometers.
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Reference: Dark matter’s signature could be written in stone
以上。
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