How to make a telescope out of the Moon
The interaction of a neutrino with atoms in matter, when it does occur, creates a shock wave of sorts that releases energy called Cherenkov radiation. Other research teams look for this radiation under Antarctic ice, at the bottom of the sea and in abandoned mines. It has been used to confirm the existence of neutrinos on the lower end of the energy scale.
One type of this Cherenkov radiation, in the form of microwaves, was hypothesized in 1962. Gorham and Saltzberg, along with other colleagues, confirmed it last fall by firing an intense beam of photons -- equivalent to the energy of a neutrino -- into a giant indoor sand pit. The results of this experiment were published in the March 26, 2001 issue of Physical Review Letters.
The researchers are now using the Moon to hunt for cosmic versions of these intense but brief microwave pulses. They look for the signs of neutrinos that penetrate at an angle through the top 32 feet (10 meters) of the Moon's surface.
"If you're lucky about the way the whole thing was oriented, this [microwave] Cherenkov radiation will hit the Earth," Saltzberg said.
Cheap time
Here on Earth, NASA's Deep Space Network, an array of highly sensitive radio telescopes usually used to monitor distant spacecraft, can be used to try and capture that luck.
"We point these antennas at the Moon, and look for a little blip," Saltzberg explained. To make sure the reading is not an artifact, or something from a nearby source, the scientists monitor two telescopes 12.5 miles (20 kilometers) apart and, by measuring the difference in time, they can deduce that the signal came from the Moon.
"The pulse we're looking for is about a nanosecond long," Saltzberg said. Depending on where the Moon is in the sky, the delay between the two telescopes is between 70 and 200 microseconds, or typically more than 100,000 times the length of the pulse.
If the project makes a detection, a position of the original source could be estimated "with some crude accuracy," Saltzberg said, but further research would be needed to pin down an exact location. Perhaps more importantly, with enough detections the researchers could estimate a rate at which neutrinos arrive, giving scientists much to chew on regarding the quantity of high-energy sources in the universe.
Why not the Moon?
Other projects have detected lower energy neutrinos. One, an array of detectors buried in Antarctic ice, is called
AMANDA (Antarctic Muon and Neutrino Detector Array) and uses a giant ice field as a collection device. A planned expansion of the AMANDA project, called IceCube, would distribute 4,800 optical sensors through a cubic kilometer of Antarctic Ice.
"Because we cannot afford to build large particle detectors for astrophysics by using the conventional techniques invented for accelerator experiments, we have to find 'detectors' provided by nature, such as deep-sea water and Antarctic ice," said Frances Halzen, who runs the AMANDA project. "Why not the Moon?"
Other giant detectors are on the drawing boards.
A group of European researchers plans an underwater neutrino detector called
ANTARES (Astronomy Neutrino Telescope Abyss Research) for 2003. ANTARES would sit on the floor of the Mediterranean Sea. A Russian project called Baikal and the Greek NESTOR Project also are moving forward.
But none of these would be capable of detecting the highest energy neutrinos, which pack a hundred million times the energy of the most powerful human-made particle accelerators.
"At the moment, it seems that the Moon experiment has the best chance to get a hint at the highest energy neutrinos," said Learned, who works on another neutrino detection project called K2K. But, he added, the Moon is "sort of [a] clumsy exploratory tool" and therefore, "if something is found, more detailed instruments will be needed to elucidate the results."
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