Direct mass measurement of Rh 91 and Ru 89 and mid-shell trend of the one-proton separation energies for N = Z + 1 nuclei

Neutron-deficient nuclei in the N ≈ Z , A ≈ 90 region were produced by fragmentation of a relativistic Ag 107 beam at the GSI Helmholtz Centre for Heavy Ion Research (GSI), separated in the fragment separator FRS, slowed down, and thermalized, and high-precision mass measurements were performed employing the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher. The masses of Rh 91 , 93 , 95 , Ru 89 , Tc 88 , Mo 87 , 88 , 89 , and Nb 87 , 88 , 89 were determined. The mass of Rh 91 was measured for the first time. For Ru 89 , the measured mass excess deviates by more than ten standard deviations (361 keV) from the previous measurement. A possible misidentification of the measured nuclei, including Ru 89 , with contaminants was excluded using a newly developed identification technique for MR-TOF-MS measurements. For the other nuclides, the measured masses agree well with previous measurements. The impact of the new masses on the one-proton separation energy ( S p ) of the nuclei with constant total isospin projection ( T z ) in the island of inversion around N = Z = 40 is investigated, and signatures of the deformed shell gap at Z = 38 are found. A comparison of the experimental S p values with the predictions of several mass models shows a general agreement of the trends.