The ground state to ground state electron-capture $Q$ value of ${}^{159}\mathrm{Dy}$ ($3/2^{-}$) has been measured directly using the double Penning trap mass spectrometer JYFLTRAP. A value of 364.73(19) keV was obtained from a measurement of the cyclotron frequency ratio of the decay parent ${}^{159}\mathrm{Dy}$ and the decay daughter ${}^{159}\mathrm{Tb}$ ions using the novel phase-imaging ion-cyclotron resonance technique. The $Q$ values for allowed Gamow-Teller transition to $5/2^{-}$ and the third-forbidden unique transition to $11/2^{+}$ state with excitation energies of 363.5449(14) keV and 362.050(40) keV in ${}^{159}\mathrm{Tb}$ were determined to be 1.18(19) keV and 2.68(19) keV, respectively. The high-precision $Q$ value of transition $3/2^{-}\to 5/2^{-}$ from this work, revealing itself as the lowest electron-capture $Q$ value, is used to unambiguously characterize all the possible lines that are present in its electron-capture spectrum. We performed atomic many-body calculations for both transitions to determine electron-capture probabilities from various atomic orbitals and found an order of magnitude enhancement in the event rates near the end point of energy spectrum in the transition to the $5/2^{-}$ nuclear excited state, which can become very interesting once the experimental challenges of identifying decays into excited states are overcome. The transition to the $11/2^{+}$ state is strongly suppressed and found unsuitable for measuring the neutrino mass. These results show that the electron-capture in the ${}^{159}\mathrm{Dy}$ atom, going to the $5/2^{-}$ state of the ${}^{159}\mathrm{Tb}$ nucleus, is a new candidate that may open the way to determine the electron-neutrino mass in the sub-eV region by studying electron-capture. Further experimental feasibility studies, including coincidence measurements with realistic detectors, will be of great interest.

• Received 14 July 2021
• Revised 22 September 2021
• Accepted 30 November 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.272301