Eur. Phys. J. A (2026) 62: 123
https://doi.org/10.1140/epja/s10050-026-01883-8

Regular Article - Experimental Physics

High-precision mass and ${\beta }^{-}$ decay Q-value measurement of $_{}^{129}$Sb and its isomer

¹ Department of Physics, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
² Université de Bordeaux, CNRS/IN2P3, LP2I Bordeaux, UMR 5797, 33170, Gradignan, France
³ Department of Physics, University of Liverpool, L69 7ZE, Liverpool, UK
⁴ Université de Caen Normandie, CNRS/IN2P3, LPC Caen UMR6534, 14000, Caen, France
⁵ Max-Planck-Institut Für Kernphysik, 69117, Heidelberg, Germany
⁶ KU Leuven, Instituut voor Kern- en Stralingsfysica, 3001, Leuven, Belgium
⁷ GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291, Darmstadt, Germany
⁸ Institut Laue-Langevin, 38042, Grenoble, France

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Received: 23 February 2026
Accepted: 18 May 2026
Published online: 23 June 2026

Abstract

The ${\beta }^{-}$ decay Q value and mass excess of $_{}^{129}$Sb were measured using the JYFLTRAP double Penning trap. The $_{}^{129}$Sb isotopes were produced and separated on-line using the IGISOL facility to precisely measure these quantities. The ground-state-to-ground-state $Q_{\beta }$ value was determined to be 2411.3(12) keV, which is a factor of $\sim$ 18 improvement in precision and 36(21) keV ($\approx$ 1.7$\sigma$) higher than the adopted value in the most recent Atomic Mass Evaluation AME2020. With this new $Q_{\beta }$, the five branches previously considered as potential ultra-low-$Q^{\ast }$${\beta }^{-}$ from the ground state to excited levels in $_{}^{129}$Te, ranging from 2360 to 2380 keV, are refined to have $Q_{\beta }^{\ast }>31$ keV. This places them well outside the ultra-low regime required for enhanced sensitivity in future electron antineutrino mass experiments, definitively ruling them out as viable candidates. With the new ground-state mass, we also revised the mass-excess value for the isomeric state $_{}^{129m}$Sb ($19/2^{-}$), which is a well-known candidate for Nuclear Excitation by Electron Capture (NEEC). The new value reveals a potential ultra-low $\beta$ transition from $_{}^{129m}$Sb with $Q_{\beta }^{\ast }=3.3(13)$ keV. However, the spin-parity is most likely unfavorable for the beta transition. Furthermore, the newly determined mass excess of $_{}^{129}$Sb, $-84,593.6(14)$ keV, is used to deduce the $Q_{\beta }$ values of ground and isomeric states of $_{}^{129}$Sn, which are relevant for the r-process nucleosynthesis models and kilonova.