Search for alpha decay of naturally occurring osmium nuclides accompanied by gamma quanta

Abstract

A search for $\hat{I}\pm$ 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 ${\hat{I}}^3$-detector with a volume of 112 cm${}^3$ and an ultra-pure 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 Ionisation Mass Spectrometry. After 15851 h of data taking with the ${\hat{I}}^3$-detector no effect has been detected, and lower limits on the $\hat{I}\pm$ decays were set at level of $\lim T_{1/2}\sim 10^{15}-10^{19}$ yr. The limits for the $\hat{I}\pm$ decays of ${}^{184}$Os and ${}^{186}$Os to the first excited levels of daughter nuclei, $T_{1/2}{(}^{184}$Os$)\ge 6.8\times 10^{15}$ yr and $T_{1/2}{(}^{186}$Os$)\ge 3.3\times 10^{17}$ yr (at 90% C.L.), exceed the present theoretical estimates of the decays half-lives. For ${}^{189}$Os and ${}^{192}$Os also decays to the ground states of the daughter nuclei were searched for due to the instability of the daughter nuclides relative to ${\hat{I}}^2$ decay.

19 pages, 6 figures, 5 tables