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 α, double-α and double-β decays with γ 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 α decays of 184Os to the first 2+ 103.6 keV excited level of 180W (T1/2 ≥ 9.3 × 1015 yr) and of 186Os to the first 2+ 100.1 keV of 182W (T1/2 ≥ 4.8 × 1017 yr) exceed substantially the present theoretical predictions that are at level of T1/2 ~ (0.6–3) × 1015 yr for 184Os and T1/2 ~ (0.3–2) × 1017 yr for 186Os. New half-life limits on the 2EC and ECβ+ decay of 184Os to the ground and excited levels of 184W were set at level of T1/2 > 1016–1017 yr; a lower limit on the 2β– decay of 192Os to the 2+ 316.5 keV excited level of 192Pt was estimated as T1/2 ≥ 6.1 × 1020 yr. The half-life limits for 2α decay of 189Os and 192Os were set for the first time at level of T1/2 > 1020 yr.