Isomeric Excitation Energy for ${{}^{99}\mathrm{In}}^m$ from Mass Spectrometry Reveals Constant Trend Next to Doubly Magic ${}^{100}\mathrm{Sn}$

The excitation energy of the $1/2^{-}$ isomer in ${}^{99}\mathrm{In}$ at $N=50$ is measured to be 671(37) keV and the mass uncertainty of the $9/2^{+}$ ground state is significantly reduced using the ISOLTRAP mass spectrometer at ISOLDE/CERN. The measurements exploit a major improvement in the resolution of the multireflection time-of-flight mass spectrometer. The results reveal an intriguing constancy of the $1/2^{-}$ isomer excitation energies in neutron-deficient indium that persists down to the $N=50$ shell closure, even when all neutrons are removed from the valence shell. This trend is used to test large-scale shell model, ab initio, and density functional theory calculations. The models have difficulties describing both the isomer excitation energies and ground-state electromagnetic moments along the indium chain.

Figure 1

Time-of-flight spectrum for the $m/q=99$ beam in the MR-TOF MS with a hyperEMG fit [45] (red) to the data (see text for explanation). The top panel shows the 2018 dataset before the device improvements, and the lower panel shows the dataset from this Letter. The black (dash-dotted) line highlights the TOF for the strontium fluoride molecule. The vertical red lines show the ground state TOF (solid) and isomeric state TOF (dashed) of ${}^{99}\mathrm{In}$. The bin size is $\sim 73\mathrm{\mu }\mathrm{u}$.

Figure 2

Proton binding energies for nuclear states of the indium ($Z=49$, green and purple) and antimony ($Z=51$, blue) isotopic chains. Data taken from Ref. [48] (solid symbols) and this work (open symbols, red). The vertical dashed lines indicate the spherical shell closure at $N=50$ and the subshell closure at $N=64$.

Figure 3

Excitation energies for the $1/2^{-}$ states in odd-even neutron-deficient indium isotopes compared to large-scale shell model and ab initio calculations. The CCSM results are taken from [25].

Figure 4

Isomer excitation energies (a) and ground-state electromagnetic moments (b) and (c) of odd-even indium isotopes, compared to theoretical calculations. The experimental data are from [59] (black) and this work (red) for the excitation energies and from [10] for the moments. Both DFT-HF and VS-ISMRG calculations are from [10] and are extended in this work to reach $N=50$.