By Hamish Johnston
When it comes to designing detectors for neutrinos, the bigger the better.
At the South Pole, for example, physicists have begun work on the IceCube experiment, which will pepper a cubic kilometre of ice with over 4000 photomultiplier tubes with the aim of detecting tiny bursts of light created when neutrinos interact with the ice.
However, this experiment is tiny by comparison to the NuMoon experiment, which is trying to use the Moon to detect ultra-high energy neutrinos from the far reaches of the universe.
The NuMoon collaboration has just published an analysis of the first 10 hours of observation on the arXiv preprint server.
NuMoon uses the Westerbork Synthesis Radio Telescope in the Netherlands to look for short radio pulses that are believed to occur when an ultra-high energy neutrino creates a cascade of charged particles within the layer of rocks and sand that covers the Moon.
These particles move through this rubble at speeds faster than the local speed of light, creating pulses of Cherenkov radiation that can be detected on Earth using a radio telescope.
The NuMoon team reckon that 100 hours of data will allow them to set the best limit yet on the “GZK neutrino flux” — the number of neutrinos with energies in excess of 1020 eV that pass through the Moon.
Such neutrinos are believed to be produced when ultra-high energy cosmic rays from distant sources scatter from the cosmic microwave background. While this scattering prevents the cosmic rays themselves from reaching Earth, much could be learned about their origins (such as massive black holes) by studying GZK neutrinos.
This is not the first time that astronomers have tried to use the moon as a giant neutrino detector. The idea was first proposed about 20 years ago and there have since been two other experimental attempts — including the GLUE experiment, which failed to spot any ultra-high energy neutrinos.
And if NuMoon fails to detect any neutrinos, the team plan to use successively more powerful radio telescopes such as the Lofar array currently under construction in the Netherlands and ultimately the Square Kilometer Array that should be built by 2020 in either South Africa or Australia.