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 (T_1/2 ≥ 9.3 × 10¹⁵ yr) and of ¹⁸⁶Os to the first 2⁺ 100.1 keV of ¹⁸²W (T_1/2 ≥ 4.8 × 10¹⁷ yr) exceed substantially the present theoretical predictions that are at level of T_1/2 ~ (0.6–3) × 10¹⁵ yr for ¹⁸⁴Os and T_1/2 ~ (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 T_1/2 > 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 T_1/2 ≥ 6.1 × 10²⁰ yr. The half-life limits for 2α decay of ¹⁸⁹Os and ¹⁹²Os were set for the first time at level of T_1/2 > 10²⁰ yr.