Precision mass measurements of neutron-rich Co isotopes beyond $N=40$

The region near $Z=28$ and $N=40$ is a subject of great interest for nuclear structure studies due to spectroscopic signatures in ${}^{68}\mathrm{Ni}$ suggesting a subshell closure at $N=40$. Trends in nuclear masses and their derivatives provide a complementary approach to shell structure investigations via separation energies. Penning trap mass spectrometry has provided precise measurements for a number of nuclei in this region; however, a complete picture of the mass surfaces has so far been limited by the large uncertainty remaining for nuclei with $N>40$ along the iron ($Z=26$) and cobalt ($Z=27$) chains because these species are not available from traditional isotope separator online rare isotope facilities. The Low-Energy Beam and Ion Trap Facility at the National Superconducting Cyclotron Laboratory is the first and only Penning trap mass spectrometer coupled to a fragmentation facility and therefore presents the unique opportunity to perform precise mass measurements of these elusive isotopes. Here we present the first Penning trap measurements of ${}^{68,69}\mathrm{Co}$, carried out at this facility. Some ambiguity remains as to whether the measured values are ground-state or isomeric-state masses. A detailed discussion is presented to evaluate this question and to motivate future work. In addition, we perform ab initio calculations of ground-state and two-neutron separation energies of cobalt isotopes with the valence-space in-medium similarity renormalization group approach based on a particular set of two- and three-nucleon forces that predict saturation in infinite matter. We discuss the importance of these measurements and calculations for understanding the evolution of nuclear structure near ${}^{68}\mathrm{Ni}$.

Figure 1

Schematic overview of the major experimental components used for this work.

Figure 2

Example of the ${}^{68}\mathrm{Co}^{2+}$ TOF-ICR resonances used for the determination of ${\nu }_c$ (${}^{68}\mathrm{Co}^{2+}$). This resonance contains 6674 ions. The solid line is a theoretical line shape [19] fit to the data.

Figure 3

The $\beta$-delayed $\gamma$-ray spectrum detected following the decay of ${}^{68}\mathrm{Co}$. Dashed vertical lines mark the energies of $\gamma$ rays known to follow primarily decay from only one of the two $\beta$-decaying states of ${}^{68}\mathrm{Co}$. $\gamma$ rays at 478 keV follow decay from the low-spin state and $\gamma$ rays at 324 and 595 keV follow decay from the high-spin state. The presence of clear peaks at 324 and 595 keV and the lack of a peak at 478 keV suggest that the high-spin $\beta$-decaying state of ${}^{68}\mathrm{Co}$ was measured in this experiment. Other peaks in the spectrum have all been identified from either natural background radiation or ${}^{68}\mathrm{Co}$ decays common to both $\beta$-decaying states.

Figure 4

Two-neutron separation energies $S_{2n}$ plotted as a function of neutron number $N$ for isotopes of Fe, Co, Ni, and Cu. Lighter (green) square points correspond to new Co values from this work and darker (black) circles correspond to data from AME2016 [14]. Points plotted with an x marker indicate values derived not from purely experimental data in the AME2016.

Figure 5

Neutron shell gap ${\mathrm{\Delta }}_N$ plotted as a function of neutron number $N$ for isotopes of Fe, Co, Ni, and Cu. The lighter (green) curve corresponds to new Co values from this work and the darker (black) curves correspond to data from AME2016 [14]. Points plotted with an x marker indicate values derived not from purely experimental data in the AME2016. Inset: Neutron shell gap plotted as a function of proton number $Z$ for isotopes with $N=40$ neutrons.

Figure 6

Two-neutron separation energies $S_{2n}$ plotted as a function of neutron number $N$ for the Ni isotopic chain. The solid black data correspond to data from AME2016, and the dashed lines correspond to IM-SRG theoretical calculations. Blue triangles use a ${}^{40}\mathrm{Ca}$ core for $N\le 40(\mathrm{pf}$ neutron valence space) and a ${}^{60}\mathrm{Ca}$ core for $N>40$ (sdg neutron valence space). Red squares use a $p{f}_{5/2}{g}_{9/2}$ valence space for protons and neutrons for the entire chain starting from a ${}^{56}\mathrm{Ni}$ core.