Search for α decay of naturally occurring osmium nuclides accompanied by γ quanta

A search for α decay of naturally occurring osmium isotopes to the lowest excited levels of daughter nuclei has been performed by using an ultra-low-background broad-energy germanium γ detector with a volume of 112 cm3 and an ultrapure osmium sample with a mass of 118 g at the Gran Sasso National Laboratory of the INFN (Italy). The isotopic composition of the osmium sample has been measured with high precision using negative thermal ionization mass spectrometry. After 15851 h of data taking with the γ detector, no effect has been detected, and lower limits on the α decays were set at level of limT1/2≈1015–1019 yr. The limits for the α decays of 184Os and 186Os to the first excited levels of daughter nuclei, T1/2(184Os)≥6.8×1015 yr and T1/2(186Os)≥3.3×1017 yr (at 90% C.L.), exceed the present theoretical estimates of the decays' half-lives. For 189Os and 192Os also decays to the ground states of the daughter nuclei were searched for due to the instability of the daughter nuclides relative to β decay.