Mass measurements of As, Se, and Br nuclei, and their implication on the proton-neutron interaction strength toward the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mi>Z</mml:mi></mml:mrow></mml:math> line

Mass measurements of the nuclides $^{69}\mathrm{As},\phantom{\rule{0.16em}{0ex}}^{70,71}\mathrm{Se}$, and $^{71}\mathrm{Br}$, produced via fragmentation of a $^{124}\mathrm{Xe}$ primary beam at the Fragment Separator (FRS) at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost $1\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}000$. Such high resolving power is the only way to achieve accurate results and resolve overlapping peaks of short-lived exotic nuclei, whose total number of accumulated events is always limited. For the nuclide $^{69}\mathrm{As}$, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only ten events. For the nuclide $^{70}\mathrm{Se}$, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $\ensuremath{\delta}m/m=4.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$, with less than 500 events. The masses of the nuclides $^{71}\mathrm{Se}$ and $^{71}\mathrm{Br}$ have been measured with an uncertainty of 23 and 16 keV, respectively. Our results for the nuclides $^{70,71}\mathrm{Se}$ and $^{71}\mathrm{Br}$ are in good agreement with the 2016 Atomic Mass Evaluation, and our result for the nuclide $^{69}\mathrm{As}$ resolves the discrepancy between the previous indirect measurements. We measured also the mass of the molecule $^{14}\mathrm{N}^{15}\mathrm{N}^{40}\mathrm{Ar}$ ($A=69$) with a relative accuracy of $\ensuremath{\delta}m/m=1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$, the highest yet achieved with an MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction ($\ensuremath{\delta}{V}_{pn}$) for odd-odd nuclei along the $N=Z$ line above $Z=29$ (recently extended to $Z=37$) is hardly evident at the $N\ensuremath{-}Z=2$ line, and not evident at the $N\ensuremath{-}Z=4$ line. Nevertheless, detailed structure of $\ensuremath{\delta}{V}_{pn}$ along the $N\ensuremath{-}Z=2$ and $N\ensuremath{-}Z=4$ lines, confirmed by our mass measurements, may provide a hint regarding the ongoing $\ensuremath{\approx}500$ keV discrepancy in the mass value of the nuclide $^{70}\mathrm{Br}$, which prevents including it in the world average of $Ft$ value for superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+} \ensuremath{\beta}$ decays. The reported work sets the stage for mass measurements with the FRS Ion Catcher of nuclei at and beyond the $N=Z$ line in the same region of the nuclear chart, including the nuclide $^{70}\mathrm{Br}$.

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