Identification of a potential ultralow-Q-value electron-capture decay branch in 75Se via a precise Penning trap measurement of the mass of 75As

Background: Low-energy 0 and electron-capture (EC) decays are important systems in neutrino mass deter-mination experiments. An isotope with an ultralow-Q-value 0 decay to an excited state in the daughter with Qes < 1 keV could provide a promising alternative candidate for future experiments. 75Se EC decay and 75Ge 0 decay represent such candidates, but a more precise determination of the mass of the common daughter, 75As, is required to evaluate whether their potential decay branches are energetically allowed and ultralow.Purpose: Our purpose is to perform a precise atomic mass measurement of 75As and combine the result with the precisely known atomic masses of 75Se and 75Ge, along with nuclear energy level data for 75As to evaluate potential ultralow-Q-value decay branches in the EC decay of 75Se and the 0 decay of 75Ge.Method: The LEBIT Penning trap mass spectrometer at the Facility for Rare Isotope Beams was used to perform a high-precision measurement of the atomic mass of 75As via cyclotron frequency ratio measurements of 75As+ to a 12C6+ reference ion.Results: The 75As mass excess was determined to be ME(75As) = -73 035.98(43) keV, from which the ground -state to ground-state Q values for 75Se EC decay and 75Ge 0 decay were determined to be 866.50(44) and 1179.01(44) keV, respectively. These results were compared to energies of excited states in 75As at 865.4(5) and 1172.0(6) keV to determine Q values of 1.1(7) and 7.0(7) keV for the potential ultralow EC-decay and 0-decay branches of 75Se and 75Ge, respectively.Conclusion: The candidate ultralow-Q-value EC-decay and 0-decay branches of 75Se and 75Ge to excited states in 75As, identified by Gamage et al. [N. D. Gamage, R. Bhandari, M. H. Gamage, R. Sandler, and M. Redshaw, Hyperfine Interact. 240, 43 (2019)] are seen to be energetically allowed. The 75Se EC decay to the 865.4-keV excited state in 75As is potentially ultralow with Qes approximate to 1 keV. However, a more precise determination of the 865.4(5)-keV level in 75As is required to draw a more definite conclusion about the energy of this potential decay channel.