Complete Majorana Instrument, shielding removed.

A very rare form of nuclear disintegration called double beta decay was observed for the first time in 1986. This process, in which a nucleus emits two electrons and two antineutrinos, was observed in an isotope of selenium by Michael Moe and colleagues at the University of California, Irvine, with Office of Science support. Later, double beta decay was seen in other nuclei, including forms of calcium, germanium, and molybdenum. The methods developed for the experiment are being used to search for another rare decay mode, neutrino-less double beta decay, in which only the two electrons are emitted. This process could occur only if an electron neutrino is its own antiparticle and if neutrinos have mass, in violation of the Standard Model (physicists' current theory of matter and the forces of nature). Given the prevalence of neutrinos in the universe, even a tiny mass could account for much of the mysterious "dark matter" that emits no visible radiation but constitutes 90 percent of the mass of the universe.

Scientific Impact: The measurement of the double beta decay rate provides key information that will be needed to determine neutrino mass once neutrino-less double beta decay is observed. In addition, the ultrasensitive techniques developed for these experiments are being used to search for other forms of dark matter.

Social Impact: Although research on double beta decay is new, single beta decay has many applications, including diagnostic and therapeutic medicine, and archeological dating. For example, doctors can examine functional images of patients' organs using positron emission tomography or treat thyroid disorders using a radioisotope of iodine, and carbon-14 dating is used to establish the age of ancient objects.

Reference: "Double Beta Decay," S. P. Rosen and Michael K. Moe, Scientific American November, 1989, p. 30.

URL: http://majorana.pnl.gov

Technical Contact: Dr. Peter Rosen, peter.rosen@science.doe.gov

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics