Search for two-neutrino double-β decay of 96Zr to excited states of 96Mo

Background: Double-beta decay is a rare second-order nuclear decay. The importance of this decay stems from the possibility of neutrinoless double-beta decay and its applications to neutrino physics. Purpose: A search was conducted for the 2 nu beta beta decay of Zr-96 to excited final states of the daughter nucleus, Mo-96. Measurements of this decay are important to test nuclear matrix element calculations, which are necessary to extract the neutrino mass from a measurement of the neutrinoless double-beta decay half-life. Method: Two coaxial high-purity germanium detectors were used in coincidence to detect gamma rays produced by the daughter nucleus as it de-excited to the ground state. The experiment was carried out at the Kimballton Underground Research Facility and produced 685.7 d of data with a 17.91 g enriched sample. Results: No counts were seen above background. For the decay to the first excited 0(+) state, a limit of T-1/2 > 3.1 x 10(20) yr was produced. Limits to higher excited states are also reported. Conclusion: The new limits on double-beta decay are an improvement over previous experiments by a factor of 2 to 5 for the various excited states. The nuclear matrix element for the double-beta decay to the first excited 0(+) state is found to be <0.13.