Abstract
The ground-state-to-ground-state beta-decay Q value of Cs-135(7/ 2(+)) -> Ba-135(3/2(+)) has been directly measured for the first time. The measurement was done utilizing both the phase-imaging ion-cyclotron resonance technique and the time-of-flight ion-cyclotron resonance technique at the JYFLTRAP Penningtrap setup and yielded a mass difference of 268.66(30) keV between Cs-135(7/2(+)) and Ba-135(3/2(+)). With this very small uncertainty, this measurement is a factor of 3 more precise than the currently adopted Q value in the Atomic Mass Evaluation 2016. The measurement confirms that the first-forbidden unique beta(-)-decay transition Cs-135(7/2(+)) -> Ba-135(11/2(-)) is a candidate for antineutrino mass measurements with an ultralow Q value of 0.44(31) keV. This Q value is almost an order of magnitude smaller than those of nuclides presently used in running or planned direct (anti)neutrino mass experiment.