Moon may radio cosmic rays' biggest hits.

Now and then, ultra-high-energy cosmic rays from space slam Earth's atmosphere with energies too huge to be explainer. Scientists suspect they may have exotic origins, such as yet-to-be-discovered relics of the Big Bang. They rank among the major puzzles of astrophysics (SN: 8/15/98, p. 101).

Only a handful of such rays has been detected, but researchers hope to soon observe greater numbers of them with a supergiant detector--the moon. Building on theories from the 1960s, scientists suspect that ultra-high-energy particles striking the moon may interact with lunar soil to produce detectable bursts of microwaves. A new experiment simulating that burst-generating effect in a giant sandbox suggests that a moon-based detector might just work.

"We've shown that the effect is real," says David Saltzberg of the University of California, Los Angeles. At the Stanford (Calif.) Linear Accelerator Center, Saltzberg and his colleagues fired gamma rays into 3.5 metric tons of sand and measured a strong microwave signal coming out. Their findings appear in the March 26 PHYSICAL REVIEW LETTERS.

In the sand, gamma rays initiate cascades of particle-ejecting reactions, Saltzberg explains. Similar chain reactions should take place in lunar soil or other materials such as Antarctic ice. Cosmic rays may be protons, neutrinos, or other particles. They strike Earth's atmosphere or the lunar surface at nearly the speed of light and with up to 100 million times the energy that can be produced in the most powerful accelerators.

The microwaves emitted by the sand are a form of so-called Cerenkov radiation. Although electromagnetic radiation slows down as it passes through many materials, including sand, highly accelerated charged particles may actually traverse the material faster than the electromagnetic radiation does. When this happens, Cerenkov radiation is emitted. Scientists already routinely use visible forms of this radiation to detect neutrinos in large underground tanks of water (SN: 1/30/99, p. 76).

The Stanford experiment confirmed that invisible, microwave Cerenkov radiation does exist. What's more, compared with ordinary cosmic rays, ultra-high-energy cosmic rays elicit particularly intense and detectable bursts of microwave Cerenkov radiation, according to the researchers. "That's why the lunar observations ought to be possible," Saltzberg says. So far, he notes, radio telescopes haven't spotted the cosmic ray-induced microwaves.

The new data should also help scientists refine models of microwave Cerenkov radiation, says George M. Frichter of Florida State University in Tallahassee.

Neutrino astronomy is a closely related area that may benefit from the newly measured effect. Recently, a decade-long effort to build a Cerenkov-light--detecting neutrino telescope has finally begun to pay off. In the March 22 NATURE, Francis Halzen of the University of Wisconsin-Madison and his colleagues report the first neutrino detections by their array of photomultiplier tubes embedded in Antarctic ice (SN: 3/27/99, p. 207). In the same spot, Frichter and his colleagues have placed microwave detectors that may glimpse higher-energy neutrinos that the photomultiplier tubes can't pick up.