Speaker
Description
Double electron-capture (EC/EC) of $^{58}$Ni on excited states of $^{58}$Fe is investigated at Baksan neutrino observatory INR RAS in DULB-4900 laboratory (4900 m w. e.) using the ultralow-background HPGe detector with a sensitive volume of 200 cm$^3$ ($\sim1$ kg mass) and a natural nickel sample of $\sim68$% $^{58}$Ni with a mass of $\sim 6$ kg. The detector is surrounded by low-background shield consist of 180 mm oxygen-free copper, 150 mm lead, 1 mm cadmium and 80 mm polyethylene. After preliminarily analysis of the experimental data accumulated over 3200 hours, the experimental limits are obtained for the 2νEC/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$_{1/2}$ (EC/EC, 0 → 2$^{+}_{1}$) > 4$\cdot$10$^{21}$ yr, and T$_{1/2}$ (EC/EC, 0 → 2$^{+}_{2}$) > 7$\cdot$10$^{21}$ yr. At the same time, the sensitivity of the experimental setup for one year of measurements to the processes mentioned above is: S (EC/EC, 0 → 2$^{+}_{1}$) = 2$\cdot$10$^{22}$ yr, and S (EC/EC, 0 → 2$^{+}_{2}$) = 1.3$\cdot$10$^{22}$ yr. All limits are at 90% CL.
