Identification of a potential ultra-low Q value electron capture decay branch in ${}^{75}$Se via a precise Penning trap measurement of the mass of ${}^{75}$As

Abstract

Background: Low energy ${\hat{I}}^2$ and electron capture (EC) decays are important systems in neutrino mass determination experiments. An isotope with an ultra-low Q value ${\hat{I}}^2$-decay to an excited state in the daughter with Qes < 1 keV could provide a promising alternative candidate for future experiments. ${}^{75}$Se EC and ${}^{75}$Ge ${\hat{I}}^2$-decay represent such candidates, but a more precise determination of the mass of the common daughter, ${}^{75}$As, is required to evaluate whether their potential decay branches are energetically allowed and ultra-low. Purpose: Perform a precise atomic mass measurement of ${}^{75}$As and combine the result with the precisely known atomic masses of ${}^{75}$Se and ${}^{75}$Ge, along with nuclear energy level data for ${}^{75}$As to evaluate potential ultra-low Q value decay branches in the EC decay of ${}^{75}$Se and the ${\hat{I}}^2$-decay of ${}^{75}$Ge. 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 ${}^{75}$As via cyclotron frequency ratio measurements of ${}^{75}$As${}^{+}$ to a ${}^{12}$C${}_6^{+}$ reference ion. Results: The ${}^{75}$As mass excess was determined to be ME(${}^{75}$As)= -73 035.98(43) keV, from which the ground-state to ground-state Q values for ${}^{75}$Se EC and ${}^{75}$Ge ${\hat{I}}^2$-decay were determined to be 866.50(44) keV and 1179.01(44) keV, respectively. These results were compared to energies of excited states in ${}^{75}$As at 865.4(5) keV and 1172.0(6) keV to determine Q values of 1.1(7) keV and 7.0(7) keV for the potential ultra-low EC and ${\hat{I}}^2$-decay branches of ${}^{75}$Se and ${}^{75}$Ge, respectively. Conclusion: The ${}^{75}$Se EC decay to the 865.4 keV excited state in ${}^{75}$As is potentially ultra-low with Qes $\approx$ 1 keV. However, a more precise determination of the 865.4(5) keV level in ${}^{75}$As is required.

7 pages,3 figures