Investigating the Double Beta Decay of 58 Ni

Double beta decay (β + EC , EC/EC ) of 58 Ni is investigated at France’s Modane Underground Laboratory (4800 m water equivalent) using the OBELIX ultralow-background HPGe detector with a sensitive volume of 600 cm 3 and a natural nickel sample of ~68% 58Ni with a mass of ~21.7 kg. After preliminary analysis of the experimental data accumulated over ~144 days, new experimental limits are obtained for the 2νβ + EC decay of 58 Ni to the 0 + ground state and the 2 1 + , 811 keV excited state of 58 Fe, and for the 2ν EC/EC decay of 58 Ni to the 2 1 + , 811 keV and 2 2 + , 1675 keV excited states of 58 Fe. The limits are T 1/2 (β + EC ,0→0 + ) > 1.7 × 10 22 yr, T 1/2 (β + EC ,0→2 1 + ) > 2.3 × 10 22 yr, T 1/2 ( EC/EC ,0→2 1 + ) > 3.3 × 10 22 yr, and T 1/2 ( EC/EC ,0→2 2 + ) > 3.4 × 10 22 yr. Experimental limit T 1/2 (0ν EC/EC –res, 1918 keV > 4.1 × 10 22 yr is obtained for resonant neutrinoless radiative EC/EC decay with an energy of 1918.3 keV. All limits are at 90% CL.