R. Bhandari

We report the first direct measurement of the nuclear electron capture (EC) decay $Q$-value of $^{7}$Be $\rightarrow$ $^{7}$Li via high-precision Penning trap mass spectrometry (PTMS). This was performed using the LEBIT Penning trap located at the National Superconducting Cyclotron Laboratory/Facility for Rare Isotope Beams (NSCL/FRIB) using the newly commissioned Batch-Mode Ion-Source (BMIS) to deliver the unstable $^{7}$Be$^{+}$ samples. With a measured value of $Q_{EC}$ = 861.963(23) keV this result is also three times more precise than any previous determination of this quantity. This improved precision, and accuracy of the $^7$Be EC decay $Q$-value is critical for ongoing experiments that measure the recoiling nucleus in this system as a signature to search for beyond Standard Model (BSM) neutrino physics using $^7$Be-doped superconducting sensors.

Background: Low energy $β$ and electron capture (EC) decays are important systems in neutrino mass determination experiments. An isotope with an ultra-low Q value $β$-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 $β$-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 $β$-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 $β$-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 $β$-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.