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

A long-term measurement was conducted to search for \(\alpha\), double-\(\alpha\) and double-\(\beta\) decays with \(\gamma\) 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\(\beta^+\) 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 \(2\beta^-\) 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 \(2\alpha\) decay of \(^{189}\)Os and \(^{192}\)Os were set for the first time at level of \(T_{1/2} > 10^{20}\) yr.