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 69As, 70,71Se and 71Br isotopes, produced via fragmentation of a 124Xe 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 69As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the 70Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of δm/m = 4.0× 10-8, with less than 500 events. The masses of the 71Se and 71Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the 70,71Se and 71Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the 69As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of 14N15N40Ar (A=69) with a relative accuracy of δm/m = 1.7× 10-8, the highest yet achieved with a MR-TOF-MS. Our results show that the measured restrengthening of the proton-neutron interaction (δVpn) 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 δVpn along the N-Z=2 and N-Z=4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing ≈500 keV discrepancy in the mass value of the 70Br isotope, which prevents including it in the world average of Ft-value for superallowed 0+→ 0+ β 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 70Br isotope.