Dam fine physics.

Dam fine physics

Two-neutrino double-beta decay is the rarest radioactive decay process ever observed in nature. Now, physicist Michael K. Moe of the University of California, Irvine, who was the first to obtain direct, laboratory evidence for this process (SN: 9/5/87, p.148), is setting a trap for an even more elusive

quarry: neutrinoless double-beta decay. Recently, he received permission from the U.S. Department of the Interior to install his particle-detecting apparatus within one of the tunnels serving the Hoover Dam near Boulder City, Nev. There, about 400 feet of rock will shield the apparatus, which contains a sample of selenium metal, from the effects of cosmic rays, which would otherwise confuse the data collected.

In the two-neutrino decay process, one neutron in the nucleus of the isotope selenium-82 decays into a proton, neutrino and electron (beta particle). The selenium atom fleetingly turns into an atom of bromine-82, which decays almost instanteneously into krypton-82 by converting a second neutron into a proton and releasing a neutrino and beta particle. Selenium-82 has a half-life of 10.sup.20 years, or 10 billion times the age of the universe, so that even with a sample containing trillions of atoms, researchers would expect to record no more than two or three decay events per week. Moe's apparatus, known as a "projection time chamber," allows him to trace the paths of the two beta particles released during a decay event and to compute their energies. Any missing energy is attributed to the emitted neutrinos, which can't be detected directly.

After more than a year of observing such decays, Moe noticed that a few events seemed to show little or no leftover energy, hinting that no neutrinos were released. The attempt to confirm whether such a neutrinoless decay process actually occurs forced him to seek a more shielded location for his 10-ton apparatus. The Hoover Dam was the closes convenient site he could find. Moe hopes to install his equipment this fall and run the experiment for at least a year.

If he manages to confirm the occurrence of neutrinoless decays, the finding would force scientists to reevaluate current theories about the nature and behavior of subatomic particles. It would mean, for example, that a neutrino and an antineutrino are really the same particle, and that neutrinos have a definite mass. "The search for the two-neutrino mode was a long, hard quest, but it wasn't quite such a long shot," Moe says. "The zero-neutrino mode is a very long shot. The rewards of finding it are very great. The chances of finding it are very slim. The hint is such that I think we'd be foolish not to pursue it and find out whether it's real."