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 δm/m = 4.0×10 −8 , with less than 500 events. The masses of the 71 Se and 71 Br isotopes have been measured with an uncertainty of 23 and 16 keV, respectively. Our results for the 70,71 Se and 71 Br isotopes are in good agreement with the 2016 Atomic Mass Evaluation, and our result for the 69 As isotope resolves the discrepancy between the previous indirect measurements. We measured also the mass of the molecule 14 N 15 N 40 Ar (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 (δV pn ) for odd-odd nuclei along the N=Z line above Z=29 (recently extended to Z=37) is hardly evident at the N-Z=2 line, and not evident at the N-Z=4 line. Nevertheless, detailed structure of δV pn 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 70 Br isotope, which prevents including it in the world average of F t-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 70 Br isotope.