Two neutrino double-beta decay of {sup 100}Mo to the first excited 0{sup +} state in {sup 100}Ru

Double-beta decay from the ground state of {sup 100}Mo to the O{sup +} excited state at 1,130.29 keV in {sup 100}Ru has been observed. A sample of 956q of Mo metal powder isotopically enriched to 98.468% of {sup 100}Mo was counted in a Marinelli geometry with a well shielded, ultralow-background germanium detector. The cascade gamma-rays at 539.53 and 590.76 keV were observed. The resulting decay half-life is 1.1{sub {minus}0.2}{sup +0.3} {times} 10{sup 21} y at 68% CL.