Using the Moon as a Cosmic Ray Detector

Some of the ultrahigh-energy cosmic ray neutrinos striking the Moon's soil are expected to set up shock waves of Cerenkov radiation, the light given off by particles (in this case charged particles spawned by neutrinos) traveling faster than light itself in that medium (see schematic drawing at Physics News Graphics).

For the cosmic rays of greatest interest, those with an energy above 10²⁰ eV, the Cerenkov radiation peaks in the microwave region of the electromagnetic spectrum. To test the validity of this "Askaryan effect" (named for the Armenian-Russian scientist Gurgen Askaryan), a consortium of scientists (David Saltzberg, 310-206-4542, saltzberg@physics.ucla.edu; Peter Gorham, peter.w.gorham@jpl.nasa.gov) have shot gamma rays into a bed of sand at the SLAC Final Focus Test Beam. Sure enough, the expected coherent microwaves appeared. The scientists are pointing two JPL radio telescopes (part of the Deep Space Network) toward the Moon to look for such radiation from cosmic ray neutrinos. Right now their calibration involves pointing at distant quasars, but it would be nice to have some source of microwave pulses on the Moon itself, a luxury not possible at present.

Some of the more optimistic estimates place the number of possible 10²⁰ eV cosmic ray neutrino events as high as one every 10 to 20 hours or so (Saltzberg et al., Physical Review Letters, 26 March; text at Physics News Select; web site here). By the way, in this week's issue of Nature, members of the AMANDA collaboration report the observation of cosmic-ray neutrinos, also via the emission of Cerenkov radiation, but in this case the detectors are buried in Antarctic ice (Andres et al., Nature, 22 March 2001.)