Measurement of the beta beta-decay rate of Mo-100 to the first excited 0(+) state of Ru-100

The double-beta beta beta (2 nu)-decay rate of Mo-100 to the first excited 0(+) state of Ru-100 has been measured by a gamma-gamma coincidence techniq ue that uses two HPGe detectors to observe the two gamma rays (E-gamma1 = 5 90.76 keV; E-gamma2 = 539.53 keV) from the Ru-100 nucleus as it deexcites t o the ground state via the 0(+) --> 2(+) --> 0(+) sequence. Unlike all prev ious beta beta -decay experiments, this technique provides data which have a large signal-to-background ratio. After a 440-day measurement of a 1.05-k g isotopically enriched (98.4%) disk of Mo-100, 22 detected coincidence eve nts (with an estimated background of 2.5 events) yield a half-life of [5.9( -1.1)(+1.7)(stat) +/- 0.6(syst)] x 10(20) years.