Double-electron capture on ¹¹²Sn to the excited 1871 keV state in ¹¹²Cd: A possible alternative to double-β decay

We report the first use of a coincidence technique to study neutrinoless double-electron capture (0νECEC) to an excited state in the daughter nucleus. We investigated 0νECEC by ¹¹²Sn leading to the possibly degenerate 1871 keV excited state in ¹¹²Cd by searching for its deexcitation γ rays of 1253 keV and 618 keV in coincidence. The experiment was performed at ground level. A sample of 3.91 g of tin enriched to 99.5% in ¹¹²Sn was placed between two high-purity germanium γ-ray detectors. In order to enhance the sample material, rods of natural tin totaling 1.2 kg (natural abundance 0.97% ¹¹²Sn) surrounded the cylindrical surface of our two γ-ray detectors. After an exposure of 1.59 kg × days of ¹¹²Sn, no decays were observed. From this null result we determine a lower limit for the half-life time of T_1/2>2.7(1.3)×10¹⁹ yr (68%(90%) CL). We hope to achieve a sensitivity in the 10²³ to 10²⁴ yr range with a sample of a few kg of ¹¹²Sn and improved γ-ray detectors in an underground facility.