High-precision electron-capture Q value measurement of 111In for electron-neutrino mass determination

A precise determination of the ground state 111In (9/2+) electron capture to ground state of 111Cd (1/2+) Q value has been performed utilizing the double Penning trap mass spectrometer, JYFLTRAP. A value of 857.63(17) keV was obtained, which is nearly a factor of 20 more precise than the value extracted from the Atomic Mass Evaluation 2020 (AME2020). The high-precision electron-capture Q value measurement along with the nuclear energy level data of 866.60(6) keV, 864.8(3) keV, 855.6(10) keV, and 853.94(7) keV for 111Cd was used to determine whether the four states are energetically allowed for a potential ultra-low Q-value β decay or electron-capture decay. Our results confirm that the excited states of 866.60(6) keV with spin-parity (Jπ) of 3/2+ and 864.8(3) keV with Jπ=3/2+ are ruled out due to their deduced electron-capture Q value being smaller than 0 keV at the level of around 20σ and 50σ, respectively. Electron-capture decays to the excited states at 853.94(7) keV (Jπ=7/2+) and 855.6(10) keV (Jπ=3/2+), are energetically allowed with Q values of 3.69(19) keV and 2.0(10) keV, respectively. The allowed decay transition 111In (9/2+) → 111Cd (7/2+), with a Q value of 3.69(19) keV, is a potential new candidate for neutrino-mass measurements by future EC experiments featuring new powerful detection technologies. The results show that the indium level 2p1/2 for this decay branch leads to a significant increase in the number of EC events in the energy region sensitive to the electron neutrino mass.