Double-beta decay caught in the act.

Double-Beta Decay Caught in the Act

The first direct, laboratory evidence of the rarest radioactive decay process ever observed in nature was announced this week by researchers at the University of California at Irvine. Michael K. Moe and his colleagues reported at the national meeting of the American Chemical Society in New Orleans that the half-life of this process--called "two-neutrino double-beta decay'--is around 10 billion times the age of the universe, or about 10(20) years.

According to Peter Rosen at Los Alamos (N.M.) National Laboratory, the finding is an experimental triumph for scientists, who have been searching for this particular decay scheme--the emission of two electrons (which are known historically as beta rays) and two antineutrinos --for 40 years. And while the value of the measured half-life does not jibe exactly with some nuclear theories, the observation of the decay confirms a part of conventional particle physics theory, he says.

The first indirect observations of two-neutrino double-beta decay came from recent geochemical studies of billion-year-old ores of selenium and other minerals. By measuring the abundance of krypton-82, which is created when two neutrons in selenium-82 simultaneously become protons by casting off two electrons, researchers came up with a value for the half-life that was about 10 times longer than that predicted by nuclear theories, according to Moe. This disagreement and the possibility that krypton could have escaped from the rocks during their long history--tarnishing the half-life measurement--caused some to question the geochemical results. The matter could only be resolved in the laboratory.

However, detecting two-neutrino double-beta decay in the laboratory is not an easy feat, since the energies of the emitted particles are comparable to those found in much more common radioactive decay modes from elements such as uranium, which is present in most materials. With Alan A. Hahn and graduate student Steve R. Elliott, Moe found a way to pinpoint the double-beta decay of other reactions by using a relatively new device called a projection time chamber.

The chamber, when filled with helium gas that becomes ionized by particles escaping from a thin selenium-82 foil in its center, enabled the researchers to reconstruct the three-dimensional paths of emitted particles. Two electrons emerging from a common point and moving with the appropriate energies at the appropriate angles signal two-neutrino double-beta decay. Moe says they have seen one such event every three or four days.

"Mike Moe . . . has been doing this experiment for quite a few years now, and little by little he's come to understand all the tricky background [decay events] that you normally don't think about,' says Rosen.

From the recorded events, Moe's group measured a half-life that supports the earlier geochemical results. "By having the geochemical measurements and the direct measurements in agreement,' he says, "the theorists [who calculate the probability of this decay mode occurring in the nucleus] have to face the fact that the half-life value we obtained is the correct one.'

If nuclear physicists can fine-tune their theories in accordance to the new results, they will be in a much better position to interpret another class of ongoing experiments, which are hunting for postulated double-beta-decay reactions that do not emit neutrinos. These decay schemes, if found, would herald new physics beyond the standard model, which links the strong, weak and electromagnetic interactions between particles. These experiments are part of a larger slew of studies trying to determine whether the neutrino has mass (SN: 4/11/87, p.231; 5/30/87, p.342). The half-life measured in two-neutrino double-beta decay may indirectly help physicists get a better handle on the upper limit of neutrino mass in the neutrino-less double-beta-decay work.

As for confirming part of the conventional particle theory with the two-neutrino double-beta decay, says Moe, "It's an interesting result. It certainly [won't bring] a Nobel prize, but it's fun.'