Scientists plan on turning the Moon into a giant particle detector

The SKA array will detect UHE cosmic rays as they strike the lunar surface (Image: SKA Project)

What is the Moon good for? Aside from inspiring poets, helping you see at night, and giving Neil Armstrong some place for a stroll, what can you do with it? If you ask scientists at the University of Southampton, they’ll tell you that it makes a cracking particle detector. With the help of the Square Kilometre Array (SKA) radio telescope, the team hopes to use the mass of the satellite to detect the most energetic particles known; Ultra-High-Energy (UHE) cosmic rays.

For all the advances of science over the past century, the universe still throws some major mysteries our way. One of the biggest of these is UHE cosmic rays. The particles that make up UHE cosmic rays have a kinetic energy many orders of magnitude greater than that of other cosmic ray particles, measuring over 1018 eV. One particle, called the Oh-My-God particle, was detected in 1961 with an energy of 3×1020 eV, which is the equivalent of a baseball traveling at 62 mph (100 km/h) – an alarming amount of energy to stuff into a subatomic particle. What particularly intrigues physicists about these rays is that they must be relatively young, otherwise their energy would have dissipated due to photon scattering, so where they come from is a major question mark in modern cosmology.

According to the Southampton team, the problem is that these high-energy particles are extremely rare and hard to detect. Normally, they’re found when they hit the Earth’s upper atmosphere, where they collide with an atom, and spray a nanosecond burst of radio energy. But with UHE cosmic rays hitting at a rate of one particle per square kilometer per century, that’s a pretty small flow of data to work with.

One way that astronomers have tried to get around this has been to use much larger detectors, such as the current largest, the 3,000 sq km (1,200 sq mi) radio telescope array at the Pierre Auger Observatory in Argentina. Southampton plans to go one better, by using the Moon as a target for colliding cosmic rays.

As the rays hit the surface of the 19 million sq km (7.3 million sq mi) near side of the Moon, they will give off bursts of radio waves which will be detected by the 33,000 sq km (12,700 sq mi) giant SKA array spread across South Africa and Australia. In effect, this turns the Moon into the largest-ever reflecting telescope with the SKA array acting as the eyepiece.

When operational, the Southampton team expects to detect about 165 UHE cosmic ray events, which is a considerable improvement on the 15 per year currently seen.

"Cosmic rays at these energies are so rare that you need an enormous detector to collect a significant number of them – but the moon dwarfs any particle detector that has been built so far," said Justin Bray, a Research Fellow in Cosmic Magnetism at the University of Southampton. "If we can make this work, it should give us our best chance yet to figure out where they’re coming from."

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