High-precision electron-capture $Q$ value measurement of $^{111}$In for electron-neutrino mass determination

A precise determination of the ground state $^{111}$In ($9/2^+$) electron capture to ground state of $^{111}$Cd ($1/2^+$) $Q$ 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 $^{111}$Cd 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 ($J^π$) of 3/2$^{+}$ and 864.8(3) keV with $J^π$ = 3/2$^{+}$ are ruled out due to their deduced electron-capture $Q$ value being smaller than 0 keV at the level of around 20$σ$ and 50$σ$, respectively. Electron-capture decays to the excited states at 853.94(7) keV ($J^π$ = 7/2$^+$) and 855.6(10) keV ($J^π$ = 3/2$^+$), are energetically allowed with $Q$ values of 3.69(19) keV and 2.0(10) keV, respectively. The allowed decay transition $^{111}$In (9/2$^{+}$) $\rightarrow$ $^{111}$Cd (7/2$^{+}$), with a $Q$ value of 3.69(19) keV, is a potential a new candidate for neutrino-mass measurements by future EC experiments featuring new powerful detection technologies. The results show that the indium level $2p_{1/2}$ 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.

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