Final results of the measurement to search for rare decays of naturally occurring osmium isotopes with ultra-low background gamma-ray spectrometry

Abstract

A long-term measurement was conducted to search for $\hat{I}\pm$, double-$\hat{I}\pm$ and double-${\hat{I}}^2$ decays with ${\hat{I}}^3$ quanta emission in naturally occurring osmium isotopes. This study took advantage of two ultra-low background HPGe detectors and one ultra-low background BEGe detector at the Gran Sasso National Laboratory (LNGS) of the INFN. Over almost 5 years of data were taken using high-purity osmium samples of approximately 173 g. The half-life limits set for a decays of ${}^{184}$Os to the first $2^{+}$ 103.6 keV excited level of ${}^{180}$W ($T_{1/2}>9.3\times 10^{15}$ yr) and of ${}^{186}$Os to the first $2^{+}$ 100.1 keV of ${}^{182}$W ($T_{1/2}>4.8\times 10^{17}$ yr) exceed substantially the present theoretical predictions that are at level of $T_{1/2}\sim (0.6-3)\times 10^{15}$ yr for ${}^{184}$Os and $T_{1/2}\sim (0.3-2)\times 10^{17}$ yr for ${}^{186}$Os. New half-life limits on the 2EC and EC$β^+$ decay of ${}^{184}$Os to the ground and excited levels of ${}^{184}$W were set at level of $T_{1/2}>10^{16}-10^{17}$ yr; a lower limit on the $2β^-$ decay of ${}^{192}$Os to the $2^{+}$ 316.5 keV excited level of ${}^{192}$Pt was estimated as $T_{1/2}>6.1\times 10^{20}$ yr. The half-life limits for $2\hat{I}\pm$ decay of ${}^{189}$Os and ${}^{192}$Os were set for the first time at level of $T_{1/2}>10^{20}$ yr.

38 pages, 17 figures, 5 tables