The ground-state-to-ground-state <math altimg="si1.svg"><msup><mrow><mi>β</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math>-decay <sup loc="post">131</sup>I (7/2<sup loc="post">+</sup>) → <sup loc="post">131</sup>Xe (3/2<sup loc="post">+</sup>) Q value was determined with high precision utilizing the double Penning trap mass spectrometer JYFLTRAP at the IGISOL facility. The Q value of this <math altimg="si1.svg"><msup><mrow><mi>β</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math>-decay was found to be Q = 972.25(19) keV through a cyclotron frequency ratio measurement with a relative precision of 1.6 × 10<sup loc="post">−9</sup>. This was realized using the phase-imaging ion-cyclotron-resonance technique. The new Q value is more than 3 times more precise and 2.3σ higher (1.45 keV) than the value extracted from the Atomic Mass Evaluation 2020. Our measurement confirms that the <math altimg="si1.svg"><msup><mrow><mi>β</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math>-decay to the <math altimg="si2.svg"><mn>9</mn><mo stretchy="false">/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup></math> excited state at 971.22(13) keV in <sup loc="post">131</sup>Xe is energetically allowed with a Q value of 1.03(23) keV while the decay to the <math altimg="si3.svg"><mn>7</mn><mo stretchy="false">/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup></math> state at 973.11(14) keV was found to be energetically forbidden. Nuclear shell-model calculations with established two-body interactions, alongside an accurate phase-space factor and a statistical analysis of the logft values of known allowed β decays, were used to estimate the partial half-life for the low-Q-value transition to the <math altimg="si2.svg"><mn>9</mn><mo stretchy="false">/</mo><msup><mrow><mn>2</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup></math> state. The half-life was found to be (<math altimg="si4.svg"><msubsup><mrow><mn>1.97</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo><mn>0.89</mn></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo><mn>2.24</mn></mrow></msubsup></math>) <math altimg="si5.svg"><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>7</mn></mrow></msup></math> years, which makes this candidate feasible for neutrino mass searches.Read less <