Observation of an ultralow-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Q</mml:mi></mml:math>-value electron-capture channel decaying to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>As</mml:mi><mml:mprescripts/><mml:none/><mml:mn>75</mml:mn></mml:mmultiscripts></mml:math> via a high-precision mass measurement
M. Ramalho, Z. W. Ge, T. Eronen, D. A. Nesterenko, Jukka Jaatinen, A. Jokinen, A. Kankainen, Joel Kostensalo, J. Kotila, M. I. Krivoruchenko, J. Suhonen, K. S. Tyrin, V. Virtanen
Abstract
A precise determination of the atomic mass of $^{75}\mathrm{As}$ has been performed utilizing the double Penning trap mass spectrometer, JYFLTRAP. The mass excess is measured to be $\ensuremath{-}73035.519(42)\phantom{\rule{0.16em}{0ex}}\mathrm{keV}/{c}^{2}$, which is a factor of 21 more precise and $1.3(9)\phantom{\rule{0.16em}{0ex}}\mathrm{keV}/{c}^{2}$ lower than the adopted value in the newest Atomic Mass Evaluation (AME2020). This value has been used to determine the ground-state--to--ground-state electron-capture decay $Q$ value of $^{75}\mathrm{Se}$ and ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay $Q$ value of $^{75}\mathrm{Ge}$, which are derived to be 866.041(81) keV and 1178.561(65) keV, respectively. Using the nuclear energy-level data of 860.00(40) keV, 865.40(50) keV (final states of electron capture), and 1172.00(60) keV (final state of ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay) for the excited states of $^{75}\mathrm{As}^{*}$, we have determined the ground-state--to--excited-state $Q$ values for two transitions of $^{75}\mathrm{Se}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{75}\mathrm{As}^{*}$ and one transition of $^{75}\mathrm{Ge}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{75}\mathrm{As}^{*}$. The ground-state--to--excited-state $Q$ values are determined to be 6.04(41) keV, 0.64(51) keV, and 6.56(60) keV, respectively, thus confirming that the three low $Q$-value transitions are all energetically valid and one of them is a possible candidate channel for antineutrino mass determination. Furthermore, the ground-state--to--excited-state $Q$ value of transition $^{75}\mathrm{Se}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{75}\mathrm{As}^{*}$ [865.40(50) keV] is revealed to be ultralow ($<1$ keV) and the first-ever confirmed electron capture transition possessing an ultralow $Q$ value from direct measurements.