High-precision mass measurements of neutron deficient silver isotopes probe the robustness of the $N$ = 50 shell closure

Abstract

High-precision mass measurements of exotic ${}^{95-97}$Ag isotopes close to the $N=Z$ line have been conducted with the JYFLTRAP double Penning trap mass spectrometer, with the silver ions produced using the recently commissioned inductively-heated hot cavity catcher laser ion source at the Ion Guide Isotope Separator On-Line facility. The atomic mass of ${}^{95}$Ag was directly determined for the first time. In addition, the atomic masses of ${\hat{I}}^2$-decaying 2${}^{+}$ and 8${}^{+}$ states in ${}^{96}$Ag have been identified and measured for the first time, and the precision of the ${}^{97}$Ag mass has been improved. The newly measured masses, with a precision of $\approx$ 1 keV/c${}^2$, have been used to investigate the $N=$ 50 neutron shell closure confirming it to be robust. Empirical shell-gap and pairing energies determined with the new ground-state mass data are compared with the state-of-the-art \textit{ab initio} calculations with various chiral effective field theory Hamiltonians. The precise determination of the excitation energy of the ${}^{96m}$Ag isomer in particular serves as a benchmark for \textit{ab initio} predictions of nuclear properties beyond the ground state, specifically for odd-odd nuclei situated in proximity to the proton dripline below ${}^{100}$Sn. In addition, density functional theory (DFT) calculations and configuration-interaction shell-model (CISM) calculations are compared with the experimental results. All theoretical approaches face challenges to reproduce the trend of nuclear ground-state properties in the silver isotopic chain across the $N=$50 neutron shell and toward the proton drip-line.

9 pages, 3 figures