A new approach to observe the radiative decay of the ${}^{229}\mathrm{Th}$ nuclear isomer, and to determine its energy and radiative lifetime, is presented. Situated at a uniquely low excitation energy, this nuclear state might be a key ingredient for the development of a nuclear clock or a nuclear laser and, the search for time variations of fundamental constants like the fine structure constant. The isomer's $\gamma$ decay towards the ground state will be studied with a high-resolution vacuum ultraviolet (VUV) spectrometer after its production by the $\beta$ decay of ${}^{229}\mathrm{Ac}$. The novel production method presents a number of advantages asserting its competitive nature with respect to the commonly used ${}^{233}\mathrm{U}\alpha$-decay recoil source. In this paper, a feasibility analysis of this new concept, and an experimental investigation of its key ingredients, using a pure ${}^{229}\mathrm{Ac}$ ion beam produced at the ISOLDE radioactive beam facility, is reported.

Background: $\beta$-decay rates play a decisive role in understanding the nucleosynthesis of heavy elements and are governed by microscopic nuclear-structure information. A sudden shortening of the half-lives of Ni isotopes beyond $N=50$ was observed at the RIKEN-RIBF. This is considered due to the persistence of the neutron magic number $N=50$ in the very neutron-rich Ni isotopes.

Purpose: By systematically studying the $\beta$-decay rates and strength distributions in the neutron-rich Ni isotopes around $N=50$, I try to understand the microscopic mechanism for the observed sudden shortening of the half-lives.

Methods: The $\beta$-strength distributions in the neutron-rich nuclei are described in the framework of nuclear density-functional theory. I employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the proton-neutron quasiparticle random-phase approximation (pnQRPA) for the transitions. Not only the allowed but the first-forbidden (FF) transitions are considered.

Results: The experimentally observed sudden shortening of the half-lives beyond $N=50$ is reproduced well by the calculations employing the Skyrme SkM* and SLy4 functionals in contrast to the monotonic shortening predicted in the preceding calculation using the SkO' functional.

Conclusions: The sudden shortening of the half-lives beyond $N=50$ in the neutron-rich Ni isotopes is due to the shell gap at $N=50$ and cooperatively with the high-energy transitions to the low-lying $0^{-}$ and $1^{-}$ states in the daughter nuclei. The onset of FF transitions pointed out around $N=82$ and 126 is preserved in the lower-mass nuclei around $N=50$. This study suggests that a microscopic calculation is needed, where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are self-consistenly taken into account for predicting $\beta$-decay rates of exotic nuclei in unknown region.

Background: Many nuclear structure calculations, even for full configuration interaction approaches, are performed with input Hamiltonians represented in a truncated model space. These often require consistent transformations of the operators to account for the missing physics beyond the active space, so that several recent efforts have been devoted to find compatible derivations of the effective operators. The effective charges employed in the shell-model calculations, and fitted to reproduce experimental data, can be seen as the phenomenological counterpart of such renormalization for electromagnetic operators.

Purpose: A coherent mapping of ab initio approaches into shell-model valence spaces requires a consistent derivation of effective electromagnetic operators. Here, we make a first step to lay the bases for their microscopic derivation in the context of the self-consistent Green's function approach.

Methods: We compute electric quadrupole ($E2$) effective charges from microscopic theory by coupling the single-nucleon propagators to core-polarization phonons, derived consistently from a realistic nuclear interaction. Nuclear correlations are included nonperturbatively according to the third-order algebraic diagrammatic construction and the Faddeev random-phase approximation. The polarization effects are included by evaluating the Feynman diagrams that couple the intermediate multinucleon configurations to the single-particle transitions induced by the electromagnetic operator.

Results: The effective charges for E2 static moments and transitions are computed for selected isotopes in the oxygen (${}^{14}\mathrm{O}, {}^{16}\mathrm{O}, {}^{22}\mathrm{O}$, and ${}^{24}\mathrm{O}$) and nickel (${}^{48}\mathrm{Ni}, {}^{56}\mathrm{Ni}, {}^{68}\mathrm{Ni}$, and ${}^{78}\mathrm{Ni}$) chains. The values found are orbital dependent, especially for the neutron effective charges, which also show a characteristic decreasing trend along each isotopic chain. In general, the values are compatible with the phenomenological ones commonly used for shell-model studies in the $0p1s0d$ and $1p0f0g_{\frac{9}{2}}$ valence spaces.

Conclusion: The phenomenological shell-model effective charges can be explained through ab initio approaches, where the sole experimental input comes from the fitting of the realistic nuclear interaction. Effective electromagnetic operators can be derived, which are tailored for different valence spaces and for specific numbers of active nucleons.

We study ground-state energies and charge radii of closed-shell medium-mass nuclei based on novel chiral nucleon-nucleon (NN) and three-nucleon ($3N$) interactions, with a focus on exploring the connections between finite nuclei and nuclear matter. To this end, we perform in-medium similarity renormalization group (IM-SRG) calculations based on chiral interactions at next-to-leading order (NLO), ${\mathrm{N}}^2\mathrm{LO}$, and ${\mathrm{N}}^3\mathrm{LO}$, where the $3N$ interactions at ${\mathrm{N}}^2\mathrm{LO}$ and ${\mathrm{N}}^3\mathrm{LO}$ are fit to the empirical saturation point of nuclear matter and to the triton binding energy. Our results for energies and radii at ${\mathrm{N}}^2\mathrm{LO}$ and ${\mathrm{N}}^3\mathrm{LO}$ overlap within uncertainties, and the cutoff variation of the interactions is within the EFT uncertainty band. We find underbound ground-state energies, as expected from the comparison to the empirical saturation point. The radii are systematically too large, but the agreement with experiment is better. We further explore variations of the $3N$ couplings to test their sensitivity in nuclei. While nuclear matter at saturation density is quite sensitive to the $3N$ couplings, we find a considerably weaker dependence in medium-mass nuclei. In addition, we explore a consistent momentum-space SRG evolution of these NN and $3N$ interactions, exhibiting improved many-body convergence. For the SRG-evolved interactions, the sensitivity to the $3N$ couplings is found to be stronger in medium-mass nuclei.

We have experimentally determined the production rate of ${}^{39}\mathrm{Ar}$ and ${}^{37}\mathrm{Ar}$ from cosmic ray neutron interactions in argon at sea level. Understanding these production rates is important for argon-based dark matter experiments that plan to utilize argon extracted from deep underground because it is imperative to know what the ingrowth of ${}^{39}\mathrm{Ar}$ will be during the production, transport, and storage of the underground argon. These measurements also allow for the prediction of ${}^{39}\mathrm{Ar}$ and ${}^{37}\mathrm{Ar}$ concentrations in the atmosphere which can be used to determine the presence of other sources of these isotopes. Through controlled irradiation with a neutron beam that mimics the cosmic ray neutron spectrum, followed by direct counting of ${}^{39}\mathrm{Ar}$ and ${}^{37}\mathrm{Ar}$ decays with sensitive ultralow background proportional counters, we determined that the production rate from cosmic ray neutrons at sea level is expected to be $(759\pm 128)\mathrm{atoms}/\left({\mathrm{kg}}_{\mathrm{Ar}}\mathrm{day}\right)$ for ${}^{39}\mathrm{Ar}$, and $(51.0\pm 7.4)\mathrm{atoms}/\left({\mathrm{kg}}_{\mathrm{Ar}}\mathrm{day}\right)$ for ${}^{37}\mathrm{Ar}$. We also performed a survey of the alternate production mechanisms based on the state of knowledge of the associated cross sections to obtain a total sea-level cosmic ray production rate of $(1048\pm 133)\mathrm{atoms}/\left({\mathrm{kg}}_{\mathrm{Ar}}\mathrm{day}\right)$ for ${}^{39}\mathrm{Ar}, (56.7\pm 7.5)\mathrm{atoms}/\left({\mathrm{kg}}_{\mathrm{Ar}}\mathrm{day}\right)$ for ${}^{37}\mathrm{Ar}$ in underground argon, and $(92\pm 13)\mathrm{atoms}/\left({\mathrm{kg}}_{\mathrm{Ar}}\mathrm{day}\right)$ for ${}^{37}\mathrm{Ar}$ in atmospheric argon.

In a previous Letter [Phys. Rev. Lett. 96, 072502 (2006)], the multichannel algebraic scattering (MCAS) technique was used to calculate spectral properties for proton-unstable ${}^{15}\mathrm{F}$ and its mirror, ${}^{15}\mathrm{C}$. This achieved a close match to the then-new data for $p+{}^{14}\mathrm{O}$ elastic scattering and predicted several unusually narrow resonances at higher energies. Subsequently, such narrow resonance states were discovered. New cross section data has been published characterizing the shape of the $J^{\pi }={\frac{1}{2}}^{-}$ resonance. Herein we update that first MCAS analysis and its predictions. We also study the spectra of the set of mass-15 isobars ${}^{15}\mathrm{C}, {}^{15}\mathrm{N}, {}^{15}\mathrm{O}$, and ${}^{15}\mathrm{F}$, using the MCAS method seeking a consistent Hamiltonian for clusterization with a neutron and a proton, separately, coupled to core nuclei ${}^{14}\mathrm{C}$ and ${}^{14}\mathrm{O}$.

Background: Quasifission is the main reaction channel hindering the formation of superheavy nuclei (SHN). Its understanding will help to optimize entrance channels for SHN studies. Quasifission also provides a probe to understand the influence of shell effects in the formation of the fragments.

Purpose: Investigate the role of shell effects in quasifission and their interplay with the orientation of the deformed target in the entrance channel.

Methods: ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ collisions are studied with the time-dependent Hartree-Fock approach for a range of angular momenta and orientations.

Results: Unlike similar reactions with a ${}^{238}\mathrm{U}$ target, no significant shell effects which could be attributed to the ${}^{208}\mathrm{Pb}$ “doubly magic” nucleus are found. However, the octupole deformed shell gap at $N=56$ seems to strongly influence quasifission in the most-central collisions.

Conclusions: Shell effects similar to those observed in fission affect the formation of quasifission fragments. Mass-angle correlations could be used to experimentally isolate the fragments influenced by $N=56$ octupole shell gaps.

The static properties of the heaviest residue and unbound particles produced in central ${}^{64,58}\mathrm{Ni}$ on ${}^{64,58}\mathrm{Ni}$ collisions at 50 MeV/nucleon are predicted within the Boltzmann-Uehling-Uhlenbeck transport model, in order to explore the sensitivity of those observables to the density dependence of the symmetry energy. We include fluctuations in the collision integral and use a metamodelling for the mean-field which allows an independent variation of the different empirical parameters of the equation of state. We find that the isospin ratio of pre-equilibrium particles is a good estimator of the stiffness of the symmetry energy, in agreement with previous works. In addition to that, whatever be the functional form of the equation of state, we show that a higher symmetry energy at subsaturation densities leads to an increased size and isotopic ratio for the heaviest residue. This is understood in terms of energy sharing between the pre-equilibrium particles and the (quasi)fused system. The combination of the two observables might be an interesting tool to constrain the different density dependence below and above saturation, which is linked to the relatively poorly known parameter $K_{\mathrm{sym}}$.

Background: In current simulations of fission, the number of protons and neutrons in a given fission fragment is almost always obtained by integrating the total density of particles in the sector of space that contains the fragment. The semiclassical nature of this procedure and the antisymmetry of the many-body wave function of the whole nucleus systematically leads to noninteger numbers of particles in the fragment.

Purpose: We seek to estimate rigorously the probability of finding $Z$ protons and $N$ neutrons in a fission fragment, i.e., the dispersion in particle number (both charge and mass). Knowing the dispersion for any possible fragmentation of the fissioning nucleus will improve the accuracy of predictions of fission fragment distributions and the simulation of the fission spectrum with reaction models.

Methods: Given a partition of the full space ${\mathbb{R}}^3$ in two sectors corresponding to the two prefragments, we discuss two different methods. The first one is based on standard projection techniques extended to arbitrary partitions of space. We also introduce a novel sampling method that depends only on a relevant single-particle basis for the whole nucleus and the occupation probability of each basis function in each of the two sectors. We estimate the number of particles $A$ in the left (right) fragment by statistical sampling of the joint probability of having $A$ single-particle states in the left (right) sector of space.

Results: We use both methods to estimate the charge and mass number dispersion of several scission configurations in ${}^{240}\mathrm{Pu}$ using either a macroscopic-microscopic approach or full Hartree-Fock-Bogoliubov calculations. We show that restoring particle-number symmetry naturally produces odd-even effects in the charge probability, which could explain the well-known odd-even staggering effects of charge distributions.

Conclusions: We discuss two methods to estimate particle-number dispersion in fission fragments. In the limit of well-separated fragments, the two methods give identical results. It can then be advantageous to use the sampling method since it provides a $N$-body basis for each prefragment, which can be used to estimate fragment properties at scission. When the two fragments are still substantially entangled, the sampling method breaks down, and one has to use projector techniques, which gives the exact particle-number dispersion even in that limit. Note that, in this paper, we have assumed that scission configurations could be described well by a static Bogoliubov vacuum: the strong odd-even staggering in the charge distributions could be somewhat attenuated when going beyond this hypothesis.

The production of neutron-rich Rf isotopes in multinucleon transfer reactions is investigated using the dinuclear system model with gemini$++$ code. The reaction ${}^{238}\mathrm{U}+{}^{252}\mathrm{Cf}$ is more advantageous to generate neutron-rich Rf isotopes than ${}^{238}\mathrm{U}+{}^{249}\mathrm{Bk}$ and ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cf}$, because ${}^{252}\mathrm{Cf}$ has a larger $N/Z$ ratio and neutron number. The influence of incident angular momentum on the production cross sections in the ${}^{238}\mathrm{U}+{}^{252}\mathrm{Cf}$ reaction is investigated. The cross sections of primary products become larger with increasing incident energy, while the final yields of the unknown isotopes at $1.1V_{\text{C}}$ are slightly larger than at $1.05V_{\text{C}}$ and $1.2V_{\text{C}}$. Hence, $1.10V_{\text{C}}$ is a suitable incident energy in the ${}^{238}\mathrm{U}+{}^{252}\mathrm{Cf}$ reaction to produce Rf isotopes. Four unknown Rf isotopes, ${}^{264}\mathrm{Rf}$, ${}^{266}\mathrm{Rf}$, ${}^{268}\mathrm{Rf}$, and ${}^{269}\mathrm{Rf}$, are synthesized in the ${}^{238}\mathrm{U}+{}^{252}\mathrm{Cf}$ reaction with cross sections 97.2, 31.1, 0.61, and 0.04 nb, respectively. Considering the beam intensity and thickness of the target, the counts per day of those four unknown isotopes are 2724, 872, 17, and 1, respectively.

We construct nuclear matter based on an extended parity doublet model including four light nucleons, $N\left(939\right), N\left(1440\right), N\left(1535\right)$, and $N\left(1650\right)$. We exclude some values of the chiral invariant masses by requiring the saturation properties of normal nuclear matter: saturation density, binding energy, incompressibility, and symmetry energy. We find a further constraint on the chiral invariant masses from the tidal deformability determined by the observation of the gravitational waves from neutron star merger GW170817. Our result shows that the chiral invariant masses are larger than about 600 MeV. We also give some predictions on the symmetry energy and the slope parameters in the high density region, which will be measured in future experiments.

The effect of pasta phases on the quark-hadron phase transition is investigated for a set of relativistic mean-field equations of state for both hadron and quark matter. The results of the full numerical solution with pasta phases are compared with those of an interpolating construction used in previous works, for which we demonstrate an adequate description of the numerical results. A one-to-one mapping of the free parameter of the construction to the physical surface tension of the quark-hadron interface is obtained for which a fit formula is given. For each pair of quark and hadron matter models the critical value of the surface tension is determined, above which the phase transition becomes close to the Maxwell construction. This result agrees well with earlier theoretical estimates. The study is extended to neutron star matter in beta equilibrium with electrons and muons and is applied to investigate the effect of pasta phases on the structure of hybrid compact stars and the robustness of a possible third family solution.

Differences in the equation of state (EOS) of dense matter translate into differences in astrophysical simulations and their multimessenger signatures. Thus, extending the number of EOSs for astrophysical simulations allows us to probe the effect of different aspects of the EOS in astrophysical phenomena. In this work, we construct the EOS of hot and dense matter based on the Akmal, Pandharipande, and Ravenhall (APR) model and thereby extend the open-source SROEOS code which computes EOSs of hot dense matter for Skyrme-type parametrizations of the nuclear forces. Unlike Skrme-type models, in which parameters of the interaction are fit to reproduce the energy density of nuclear matter and/or properties of heavy nuclei, the EOS of APR is obtained from potentials resulting from fits to nucleon-nucleon scattering and properties of light nuclei. In addition, this EOS features a phase transition to a spin-isospin ordered state of nucleons, termed a neutral pion condensate, at supranuclear densities. We show that differences in the effective masses between EOSs have consequences for the properties of nuclei in the subnuclear inhomogeneous phase of matter. We also test the new EOS of APR in spherically symmetric core-collapse of massive stars with $15M_{\odot }$ and $40M_{\odot }$, respectively. We find that the phase transition in the EOS of APR speeds up the collapse of the star. However, this phase transition does not generate a second shock wave or another neutrino burst as reported for the hadron-to-quark phase transition. The reason for this difference is that the width of the coexistence region and the latent heat in the EOS of APR are substantially smaller than in the quark-to-hadron transition employed earlier, which results in a significantly smaller softening of the high density EOS.

The $\beta$-intensity distributions of the decays of ${}^{100\text{gs},100\text{m}}\mathrm{Nb}$ and ${}^{102\text{gs},102\text{m}}\mathrm{Nb}$ have been determined using the total absorption $\gamma$-ray spectroscopy technique. The JYFLTRAP double Penning trap system was employed in a campaign of challenging measurements performed with the decay total absorption $\gamma$-ray spectrometer at the Ion Guide Isotope Separator On-Line facility in Jyväskylä. Different strategies were applied to disentangle the isomeric states involved, lying very close in energy. The low-spin component of each niobium case was populated through the decay of the zirconium parent, which was treated as a contaminant. We have applied a method to extract this contamination, and additionally we have obtained $\beta$-intensity distributions for these zirconium decays. The $\beta$-strength distributions evaluated with these results were compared with calculations in a quasiparticle random-phase approximation, suggesting a prolate configuration for the ground states of ${}^{100,102}\mathrm{Zr}$. The footprint of the Pandemonium effect was found when comparing our results for the analyses of the niobium isotopes with previous decay data. The $\beta$-intensities of the decay of ${}^{102\text{m}}\mathrm{Nb}$, for which there were no previous data, were obtained. A careful evaluation of the uncertainties was carried out, and the consistency of our results was validated taking advantage of the segmentation of our spectrometer. The final results were used as input in reactor summation calculations. A large impact on antineutrino spectrum calculations was already reported, and here we detail the significant impact on decay heat calculations.

Shell model and multiparticle-multihole configuration-mixing calculations indicate a missing quadrupole collectivity needed to explain the transition strengths in the even-even Ne isotopes. Even with a significant scaling of the effective charges all details of the data cannot be reproduced. The effect is very pronounced in ${}^{20,22}\mathrm{Ne}$. Therefore new lifetime measurements in these nuclei were undertaken by us with the Doppler-shift attenuation method to recheck experimentally earlier findings. They confirmed the previous result in the case of ${}^{20}\mathrm{Ne}$ while in ${}^{22}\mathrm{Ne}$ the $2_1^{+}$ level lifetime was found to be shorter by 43%. It turned out that in the latter nucleus the lifetime of the $4_1^{+}$ level also has to be corrected. The $B(E2;2_1^{+}\to 0_1^{+})$ reduced transition strengths derived from the newly determined lifetimes as well as those for the heavier Ne isotopes are reasonably described by involving a mixing of $\alpha$-cluster states of, e.g., the type $\alpha \otimes 16\mathrm{O}$ with normal, nearly spherical states. An extended version of the coupled-cluster effective interaction method published earlier describes well the spin dependence of the $B\left(E2\right)$'s in ${}^{20}\mathrm{Ne}$.

An outstanding problem in the theory of nuclear fission is to understand the Hamiltonian dynamics at the scission point. In this work the fissioning nucleus is modeled in self-consistent mean-field theory as a set of generator coordinate method (GCM) configurations passing through the scission point. In contrast to previous methods, the configurations are constructed in the Hartree-Fock approximation with axially symmetric mean fields and using the $K$-partition numbers as additional constraints. The goal of this work is to find paths through the scission point where the overlaps between neighboring configurations are large. A measure of distance along the path is proposed that is insensitive to the division of the path into short segments. For most of the tested $K$ partitions two shape degrees of freedom are adequate to define smooth paths. However, some of the configurations and candidate paths have sticking points where there are substantial changes in the many-body wave function, especially if quasiparticle excitations are present. The excitation energy deposited in fission fragments arising from thermal excitations in the pre-scission configurations is determined by tracking orbital occupation numbers along the scission paths. This allows us to assess the validity of the well-known scission-point statistical model, in which the scission process is assumed to be fully equilibrated up to the separated fission fragments. The nucleus ${}^{236}\mathrm{U}$ is taken as a representative example in the calculations.

A microscopic thermodynamically consistent approach is applied to compute electron capture (EC) rates and cross sections on nuclei in hot stellar environments. The cross-section calculations are based on the Donnelly-Walecka multipole expansion method for treatment of semileptonic processes in nuclei. To take into account thermal effects, we express the electron capture cross section in terms of temperature- and momentum-dependent spectral functions for respective multipole charge-changing operators. The spectral functions are computed by employing the self-consistent thermal quasiparticle random-phase approximation (TQRPA) with the Skyrme effective interaction. Three different Skyrme parametrizations (${\mathrm{SkM}}^{*}$, SGII, and SLy4) are used to investigate thermal effects on EC for ${}^{56}\mathrm{Fe}$ and ${}^{78}\mathrm{Ni}$. For ${}^{56}\mathrm{Fe}$, the impact of thermally unblocked Gamow-Teller ${\mathrm{GT}}_{+}$ transitions on EC is discussed and the results are compared with those from shell-model calculations. In particular, it is shown that for some temperature and density regimes the TQRPA rates exceed the shell-model rates due to violation of the Brink-Axel hypothesis within the TQRPA. For neutron-rich ${}^{78}\mathrm{Ni}$, the full momentum dependence of multipole transition operators is considered and it is found that not only thermally unblocked allowed $1^{+}$ transitions but also thermally unblocked first-forbidden $1^{-}$ and $2^{-}$ transitions favor EC.

Since their discovery, fluctuations in the initial state of heavy-ion collisions have been understood as originating mostly from the random positions of nucleons within the colliding nuclei. We consider an alternative approach where all the focus is on fluctuations generated by QCD interactions that we evaluate at leading logarithmic accuracy in the color glass condensate effective theory. We validate our approach using BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) data on anisotropic flow. In particular, we show that, compared to standard Glauber-inspired calculations, our formalism provides a better description of the centrality dependence of the ratio of elliptic flow and triangular flow. It also naturally explains the evolution of elliptic flow fluctuations between RHIC and LHC energies.

Background: Magnetic dipole (M1) excitation is the leading mode of nuclear excitation by the magnetic field, which couples unnatural-parity states. Since the M1 excitation occurs mainly in open-shell nuclei, the nuclear pairing effect is expected to play a role. Considering the M1 operator form, it can provide the information on the spin-related properties, including the spin component of dinucleon correlations.

Purpose: We investigate the M1 excitation of the systems with two valence nucleons above the closed-shell core. The corresponding version of the M1 sum rule, which can be used to validate theoretical and experimental approaches, is also discussed.

Methods: Three-body model, which consists of a rigid spherical core and two valence nucleons, is employed. Interactions for its two-body subsystems are phenomenologically determined so as to reproduce the two-body and three-body energies. The M1 operator up to the one-body-operator level is utilized to compute the M1 transitions between the $0^{+}$ ground state and $1^{+}$ excited states.

Results: The three-body-model calculations for ${}^{18}\mathrm{O}, {}^{18}\mathrm{Ne}$, and ${}^{42}\mathrm{Ca}$ nuclei demonstrate a significant effect of the pairing correlation on the low-lying M1 transitions. The introduced M1 sum rule can be utilized as a benchmark for model calculations of these systems. The total sum of the M1 transition strength is shown to be relative with the coupled spin of valence nucleons in the open shell.

Conclusions: The M1 excitation can be a suitable tool to investigate the pairing correlation in medium. Further studies of M1 transitions are on demand to validate and optimize the pairing models.

Background: Ultralow $Q$-value $\beta$ decays are interesting processes to study with potential applications to nuclear $\beta$-decay theory and neutrino physics. While a number of potential ultralow $Q$-value $\beta$-decay candidates exist, improved mass measurements are necessary to determine which of these are energetically allowed.

Purpose: To perform precise atomic mass measurements of ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$. Use these new measurements along with the precisely known atomic masses of ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ and nuclear energy level data for ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$ to determine if there could be an ultralow $Q$-value decay branch in the $\beta$ decay of ${}^{89}\mathrm{Sr}\to {}^{89}\mathrm{Y}$ or ${}^{139}\mathrm{Ba}\to {}^{139}\mathrm{La}$.

Method: High-precision Penning trap mass spectrometry was used to determine the atomic mass of ${}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{La}$, from which $\beta$-decay $Q$ values for ${}^{89}\mathrm{Sr}$ and ${}^{139}\mathrm{Ba}$ were obtained.

Results: The ${}^{89}\mathrm{Sr}\to {}^{89}\mathrm{Y}$ and ${}^{139}\mathrm{Ba}\to {}^{139}\mathrm{La}\beta$-decay $Q$ values were measured to be $Q_{\mathrm{Sr}}=1502.20\left(0.35\right)$ keV and $Q_{\mathrm{Ba}}=2308.37\left(0.68\right)$ keV. These results were compared to energies of excited states in ${}^{89}\mathrm{Y}$ at 1507.4(0.1) keV, and in ${}^{139}\mathrm{La}$ at 2310(19) keV and 2313(1) keV to determine $Q$ values of $-5.20\left(0.37\right)$ keV for the potential ultralow $\beta$-decay branch of ${}^{89}\mathrm{Sr}$ and $-1.6\left(19.0\right)$ keV and $-4.6\left(1.2\right)$ keV for those of ${}^{139}\mathrm{Ba}$.

Conclusion: The potential ultralow $Q$-value decay branch of ${}^{89}\mathrm{Sr}$ to the ${}^{89}\mathrm{Y}$ ($3/2^{-}$, 1507.4 keV) state is energetically forbidden and has been ruled out. The potential ultralow $Q$-value decay branch of ${}^{139}\mathrm{Ba}$ to the 2313 keV state in ${}^{139}\mathrm{La}$ with unknown $J^{\pi }$ has also been ruled out at the $4\sigma$ level, while more precise energy level data is needed for the ${}^{139}\mathrm{La}$ ($1/2^{+}$, 2310 keV) state to determine if an ultralow $Q$-value $\beta$-decay branch to this state is energetically allowed.