ββ(2ν) decay of 100Mo to the first excited 0+ state of 100Ru

The rate of the double-beta ββ(2ν) decay of 100Mo to the first 0+ excited state in 100Ru is measured by a γγ-coincidence technique in which two HPGe detectors are used to detect the two γ rays (Eγ1 = 590.76 keV and Eγ2 539.53 keV) from the daughter nucleus 100Ru as it deexcites to the ground state via the transition sequence 0+ → 2+ → 0+. In contrast to all previous ββ-decay experiments, this technique provides data that are essentially background-free. By using a 1.05-kg isotopically enriched (98.4%) disk of 100Mo, 22 coincidence events (with the background estimated at 2.5 events) are detected in 440 d of measuring time, which translates into a half-life time of [5.9-1.1+1.7(stat.) ± 0.6(syst.)] x 1020 yr