Mass measurements of As, Se and Br nuclei and their implication on the proton-neutron interaction strength towards the N=Z line

Abstract

Mass measurements of the ${}^{69}$As, ${}^{70,71}$Se and ${}^{71}$Br isotopes, produced via fragmentation of a ${}^{124}$Xe primary beam at the 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 1,000,000. For the ${}^{69}$As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the ${}^{70}$Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $\hat{I}´$m/m = 4.0$\times 10^{-8}$, with less than 500 events. The masses of the ${}^{71}$Se and ${}^{71}$Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the ${}^{70,71}$Se and ${}^{71}$Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the ${}^{69}$As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of ${}^{14}$N${}^{15}$N${}^{40}$Ar (A=69) with a relative accuracy of $\hat{I}´$m/m = 1.7$\times 10^{-8}$, the highest yet achieved with a MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction ($\hat{I}´$V${}_{pn}$) for odd-odd nuclei at the N=Z line above Z=29 (recently extended to Z=37) is hardly evident at N-Z=2, and not evident at N-Z=4. Nevertheless, detailed structure of $\hat{I}´$V${}_{pn}$ along the N-Z=2 and N-Z=4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing $\approx$500 keV discrepancy in the mass value of the ${}^{70}$Br isotope, which prevents including it in the world average of $Ft$-value for superallowed 0${}^{+}\to$ 0${}^{+}$ ${\hat{I}}^2$ 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 ${}^{70}$Br isotope.

11 pages, 4 figures, accepted for publication at Physical Review C