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

A search for $\ensuremath{\alpha}$ 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 $\ensuremath{\gamma}$ detector with a volume of 112 ${\mathrm{cm}}^{3}$ 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 $\ensuremath{\gamma}$ detector, no effect has been detected, and lower limits on the $\ensuremath{\alpha}$ decays were set at level of $lim{T}_{1/2}\ensuremath{\approx}{10}^{15}--{10}^{19}$ yr. The limits for the $\ensuremath{\alpha}$ decays of $^{184}\mathrm{Os}$ and $^{186}\mathrm{Os}$ to the first excited levels of daughter nuclei, ${T}_{1/2}(^{184}\mathrm{Os})\ensuremath{\ge}6.8\ifmmode\times\else\texttimes\fi{}{10}^{15}$ yr and ${T}_{1/2}(^{186}\mathrm{Os})\ensuremath{\ge}3.3\ifmmode\times\else\texttimes\fi{}{10}^{17}$ yr (at 90% C.L.), exceed the present theoretical estimates of the decays' half-lives. For $^{189}\mathrm{Os}$ and $^{192}\mathrm{Os}$ also decays to the ground states of the daughter nuclei were searched for due to the instability of the daughter nuclides relative to $\ensuremath{\beta}$ decay.