Clue to Shadowy Neutrinos

Illinois researchers have made the first measurement of the rate of a very rare form of radioactive decay of uranium called “double-beta decay.” In the process, uranium-238 turns into plutonium-238. Chemist Anthony Turkevich and his colleagues at the University of Chicago report in today’s Physical Review Letters that the half-life of the reaction--the time required for half of the uranium to be converted into plutonium--is 2 billion trillion years.

The discovery of the rate is important because it sheds light on a question currently of great interest to physicists--whether fundamental particles called neutrinos have mass. The “standard model” of physics that is now widely accepted says that neutrinos travel at the speed of light and have no mass.

Scientists are unable to observe these ghostlike particles directly, but neutrinos are known to be emitted during double-beta decay. The standard model predicts that the half-life for the double-beta decay should be 100 times as long as the Chicago physicists observed. The fact that the half-life is shorter, Turkevich said, is strong evidence that neutrinos have a small but finite mass.