Investigation of Double Beta Decay of 58Ni with the Obelix Spectrometer.

Investigation of double beta decay processes (β+EC, EC/EC) of 58Ni was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.) using ultra low-background HPGe detector Obelix with sensitive volume of 600 cm3 and a sample of natural nickel, containing ∼68% of 58Ni with a mass of ∼21.7 kg. New experimental limits on 2νβ+EC decay of 58Ni to the ground 0+ and 2+1 , 811 keV excited state of 58Fe, and 2νEC/EC decay of 58Ni to 2+1 , 811 keV and 2+2 , 1 675 keV excited states of 58Fe were obtained from the preliminary calculation of experimental data accumulated in ∼88d. They are - T1/2(β+EC, 0+ → 0+) > 1.4 ×1021 y; T1/2 (β+EC, 0+ → 2+1 ) > 1.7 ×1022 y, T1/2 (EC/EC, 0+ → 2+1 ) > 2.4 ×1022 y, T1/2 (EC/EC,0+ → 2+2 ) > 2.1 × 1022 y. For resonant neutrino-less radiative EC/EC decay with energy of 1 918.3 keV a new experimental limit of T1/2 (0νEC/EC − res, 1918keV) > 4.7 ×1022 y was also obtained. All limits are at 90 % CL. [ABSTRACT FROM AUTHOR]