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HOW do you detect the most energetic particles in the Universe? Easy,
according to two physicists based in the US and Spain. You just point a radio
telescope at the Moon and listen for the ping as “cosmic ray” particles drill
their way into the lunar soil.

The highest-energy cosmic rays, primarily protons and atomic nuclei, are
millions of times more energetic than those from Earth-bound particle
accelerators with energies of more than 10²⁰ electronvolts. Such particles are
also exceptionally rare: each square kilometre of the Earth’s surface will be
struck by one only once a century on average.

The best way to improve the chances of detecting one of these particles is to
have a very large collecting area. The ICECUBE detector in Antarctica, for
instance, makes use of vast expanses of ice. “But the biggest volume of material
in our neighbourhood is the Moon,” says Jaime Alvarez-Muñiz of the
University of Delaware in Newark.

Alvarez-Muñiz and his colleague Enrique Zas of the University of
Santiago de Compostela in Spain have calculated in detail the electromagnetic
signal that should be generated when an ultra-high-energy particle hits the
nearside of the Moon.

The cosmic rays burrow a few centimetres into the lunar surface,
generating a shower of particles. These particles produce “Cerenkov radiation”,
an electromagnetic emission given off by particles travelling through a medium
faster than the speed of light in that medium. The radiation is emitted across
the whole electromagnetic spectrum, but radio waves escape the soil most easily.
“We predict a nanosecond burst of radio waves, which should be easiest to detect
at a wavelength of a few tens of centimetres,” says Alvarez-Muñiz. “The
best place to look is near the circumference of the Moon.”

The two physicists also simulated the impact of ultra-high-energy neutrinos
on the Moon. “These also generate showers of particles which produce radio
waves,” he says.

According to Alvarez-Muñiz and Zas’s calculations, radio telescopes
looking at the Moon should be able to detect as many as 250 ultra-high-energy
cosmic rays a year. “Currently, only 17 particles with energies greater than
10²⁰ electronvolts have ever been detected, so the technique could be very
important,” says Alvarez-Muñiz.

A radio search for ultra-high-energy particles is planned by a team led by
Peter Gorham of the NASA’s Jet Propulsion Laboratory in Pasadena, using two
dishes at Goldstone in California. “We are quite hopeful we will see something
within the next two years,” says Gorham.