Credit: NASA
SYDNEY: Astronomers have found a new way to search for high-energy cosmic rays, the most energetic particles in the universe, by scanning the face of the Moon.
Cosmic rays stream through space and constantly bombard the Earth's atmosphere. They pose little threat to us on the planet's surface, but can cause electronic glitches in satellites in space and, in high doses, are dangerous for astronauts.
Ultra-high-energy cosmic rays, with energies exceeding 1020 electron volts, were first detected in 1962.
Black holes
Their origin has not been proven, although theories suggest they may be created by the supermassive black holes thought to be at the heart of galaxies or result from the decay of massive particles leftover from the Big Bang.
A variety of large, Earth-bound experiments have been constructed to detect these cosmic rays.
In 2007 one statistical study, based on data from the Pierre Auger observatory in Argentina, showed that some of the highest energy cosmic rays detected seem to come from the direction of known active galactic nuclei (see Black holes: source of high-energy cosmic rays, Cosmos Online).
Now, Australian and Spanish researchers say they have found a bigger and better target for detecting the rays - the regolith (dusty rock) of the Moon. The research was presented at the 31st International Cosmic Ray Conference, held in Poland last month, and is also is posted on the arXiv.org physics web site.
Useful target
The team, led by astrophysicists Ray Protheroe from the University of Adelaide and Ron Ekers of the CSIRO's Australia Telescope National Facility, looked for high-energy neutrinos – particles with little mass, which are similar to electrons, but neutrally charged.
"Whenever cosmic rays are accelerated to these tremendous energies in space you'll also get neutrinos of comparable energies produced," Protheroe told Cosmos Online. "Not being charged they travel in a straight line through space, and are not deflected by magnetic fields."
This makes neutrinos a useful target for pinpointing the origin of cosmic rays, he said. But because they interact only weakly with other matter and are very rare, you need a huge target and sensitive instruments to detect them.
"At these energies - 1020 electron volts - we needed something the size of the Moon, so we used part of the Moon as a target," said Protheroe.
The researchers used the Australia Telescope Compact Array, an array of six 22-metre radio astronomy antennas located near Narrabri in New South Wales.
They were seeking nanosecond-length radio pulses from Cherenkov radiation produced by cascades of electrons and positrons created when neutrinos pass through the Moon's regolith. Cherenkov radiation is generated by particles passing through material at near light speed.
Dark matter
Although they didn't yet find the characteristic signature of high-energy neutrinos, the result narrows the search criteria for these particles. "We've proved the technique works and it's a good way of doing ultra-high-energy neutrino astronomy," said Protheroe.
Charley Lineweaver is an astrophysicist from the Planetary Science Institute of the Australian National University, in Canberra, and was not involved in the research.
He said that the sensitivity of the instrument was better than any other instrument at these extremely high energies for observing our own galactic centre, and also Centaurus A, the nearest active galaxy to Earth and a presumed source of cosmic rays.
"The search for the highest energy cosmic rays and the highest energy neutrinos could be our most promising way to track down dark matter. So the development of improved technologies to detect these particles is important," he said.
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