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
I. Mardor, S. Ayet San Andrés, T. Dickel, Daler Amanbayev, Sönke Beck, J. Bergmann, H. Geißel, Lizzy Gröf, E. Haettner, C. Hornung, N. Kalantar‐Nayestanaki, Gabriella Kripkó-Koncz, Ivan Miskun, Ali Mollaebrahimi, W. R. Plaß, C. Scheidenberger, H. Weick, S. Bagchi, D. L. Balabanski, A. A. Bezbakh, Z. Brencic, Olga Charviakova, V. Chudoba, P. Constantin, M. Dehghan, А. С. Фомичев, Л. В. Григоренко, O. Hall, M.N. Harakeh, Jean-Paul Hucka, A. Kankainen, O. Kiselev, R. Knöbel, D. Kostyleva, S. A. Krupko, N. Yu. Kurkova, N. Kuzminchuk, I. Mukha, I. A. Muzalevskii, D. Nichita, C. Nociforo, Z. Patyk, M. Pfützner, S. Piétri, S. Purushothaman, M. P. Reiter, Heidi Roesch, F. Schirru, P. G. Sharov, A. Spătaru, Goran Stanić, A. State, Y. Tanaka, M. Vencelj, Michael I. Yavor, J. Zhao
Abstract
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 $^{70}\mathrm{Br}$.