We report high-precision mass measurements of ${}^{50–55}\mathrm{Sc}$ isotopes performed at the LEBIT facility at NSCL and at the TITAN facility at TRIUMF. Our results provide a substantial reduction of their uncertainties and indicate significant deviations, up to 0.7 MeV, from the previously recommended mass values for ${}^{53–55}\mathrm{Sc}$. The results of this work provide an important update to the description of emerging closed-shell phenomena at neutron numbers $N=32$ and $N=34$ above proton-magic $Z=20$. In particular, they finally enable a complete and precise characterization of the trends in ground state binding energies along the $N=32$ isotone, confirming that the empirical neutron shell gap energies peak at the doubly magic ${}^{52}\mathrm{Ca}$. Moreover, our data, combined with other recent measurements, do not support the existence of a closed neutron shell in ${}^{55}\mathrm{Sc}$ at $N=34$. The results were compared to predictions from both ab initio and phenomenological nuclear theories, which all had success describing $N=32$ neutron shell gap energies but were highly disparate in the description of the $N=34$ isotone.

A nuclear mass survey of rare-earth isotopes has been conducted with the Canadian Penning Trap mass spectrometer using the most neutron-rich nuclei thus far extracted from the CARIBU facility. We present a collection of 12 nuclear masses determined with a precision of $\le 10$ keV/$c{}^2$ for $Z=58–63$ nuclei near $N=100$. Independently, a detailed study exploring the role of nuclear masses in the formation of the $r$-process rare-earth abundance peak has been performed. Employing a Markov chain Monte Carlo (MCMC) technique, mass predictions of lanthanide isotopes have been made which uniquely reproduce the observed solar abundances near $A=164$ under three distinct astrophysical outflow conditions. We demonstrate that the mass surface trends thus far mapped out by our measurements are most consistent with MCMC mass predictions given an $r$ process that forms the rare-earth peak during an extended $(n,\gamma )\rightleftarrows (\gamma ,n)$ equilibrium.

Nuclear properties across the chart of nuclides are key to improving and validating our understanding of the strong interaction in nuclear physics. We present high-precision mass measurements of neutron-rich Fe isotopes performed at the TITAN facility. The multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS), achieving a resolving power greater than $600000$ for the first time, enabled the measurement of ${}^{63–70}\mathrm{Fe}$, including first-time high-precision direct measurements ($\delta m/m\approx {10}^{-7}$) of ${}^{68–70}\mathrm{Fe}$, as well as the discovery of a long-lived isomeric state in ${}^{69}\mathrm{Fe}$. These measurements are accompanied by both mean-field and ab initio calculations using the most recent realizations which enable theoretical assignment of the spin-parities of the ${}^{69}\mathrm{Fe}$ ground and isomeric states. Together with mean-field calculations of quadrupole deformation parameters for the Fe isotope chain, these results benchmark a maximum of deformation in the $N=40$ island of inversion in Fe and shed light on trends in level densities indicated in the newly refined mass surface.

Masses of neutron-rich, odd-odd Pm, Eu, and Tb nuclei near $N=98$ were measured using the Canadian Penning Trap mass spectrometer at the Californium Rare Isotope Breeder Upgrade (CARIBU) facility. High-resolution mass measurements yielded the discovery of spin-trap isomers at $N=97$ in ${}^{162}\mathrm{Tb}$, and in the $N=99$ isotones of ${}^{160}\mathrm{Pm}$ and ${}^{164}\mathrm{Tb}$. Furthermore, no evidence of long-lived isomers were observed at $N=95$ in ${}^{158}\mathrm{Eu}$, at $N=97$ in ${}^{158}\mathrm{Pm}$, nor at $N=101$ in ${}^{164}\mathrm{Eu}$ and ${}^{166}\mathrm{Tb}$. These experimental observations are compared to results from multiquasiparticle blocking calculations.

Nuclear mass measurements of isotopes are key to improving our understanding of nuclear structure across the chart of nuclides, in particular, for the determination of the appearance or disappearance of nuclear shell closures. We present high-precision mass measurements of neutron-rich Ca, Ti, and V isotopes performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN) and the Low Energy Beam and Ion Trap (LEBIT) facilities. These measurements were made using the TITAN multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS) and the LEBIT 9.4T Penning trap mass spectrometer. In total, 13 masses were measured, 8 of which represent increases in precision over previous measurements. These measurements refine trends in the mass surface around $N=32$ and $N=34$, and support the disappearance of the $N=32$ shell closure with increasing proton number. Additionally, our data do not support the presence of a shell closure at $N=34$.

Background: An ultralow $Q$-value $\beta$ decay can occur from a parent nuclide to an excited nuclear state in the daughter such that $Q_{\text{UL}}\lesssim$ 1 keV. These decay processes are of interest for nuclear $\beta$-decay theory and as potential candidates in neutrino mass determination experiments. To date, only one ultralow $Q$-value $\beta$ decay has been observed—that of ${}^{115}\mathrm{In}$ with $Q_{\beta }=147\left(10\right)$ eV. A number of other potential candidates exist, but improved mass measurements are necessary to determine if these decay channels are energetically allowed and, in fact, ultralow.