Fission product yields (FPYs) are used for a wide range of applications including nuclear fission theory, nuclear reactor design, reactor antineutrino anomaly, stellar nucleosynthesis, and nuclear forensics. These applications rely on FPYs that were last evaluated in 1993, which included measurements from all over the world up until 1993. In this work, the most recent measurement of 239Pu FPY and 237Np FPY using Godiva IV critical assembly is presented. Both measurements used the same experimental setup and same analysis codes. Discrepancies in γ-ray branching ratios were observed for both measurements and the data was limited to 45 minutes after irradiation. This led to the construction of the Lāpaki array in order to perform γ-γ coincidence measurements. An experiment to study 238U short-lived FPY was conducted using the Lāpaki array with the Oregon State University (OSU) Training, Research, Isotopes General Atomics (TRIGA) reactor rabbit facility. This allows for 238U fission product γ rays to be observed within seconds after irradiation. A new rabbit was designed and fabricated with high-purity polyethylene along with a new procedure was established to reduce as much background γ rays as possible when studying short-lived isotopes. A new digital data acquisition (DAQ) system from Mesytec was used for this experiment.

Investigating the properties of atomic nuclei through measuring their influence upon bound electrons is a powerful and well-established approach in modern nuclear physics [Yan23]. By measuring the hyperfine structure and isotope shift in the atomic structure of radioactive nuclei, nuclear spins, magnetic dipole and electric quadrupole moments and changes in mean-square charge radii can be determined in a nuclear model-independent manner. These observables offer critical and complementary insights into the single-particle structure and collective behavior of the ground- and isomeric states of atomic nuclei, enabling state-of-the-art models of nuclear theory to be tested.

Advances in experimental techniques have allowed laser spectroscopy techniques to push further away from stability, studying isotopes towards the extremes of existence. The unprecedented combination of experimental precision and sensitivity available to researchers has also enabled the first study of molecules containing radioactive nuclei [Gar20, Udr21], despite their significantly more complex structures.

In this seminar, I will present recent highlights from studying radioactive molecules of particular interest for fundamental symmetries studies at ISOLDE-CERN [Ath23b, Udr24, Wil23]. I will also outline the diverse range of research opportunities [Arr23, Ath23a] available for laser spectroscopy studies of radioactive atoms and molecules at the Facility for Rare Isotope Beams.

[Arr23] Arrowsmith-Kron, G. et al., arXiv 2302 02165 (2023)

[Ath23a] Athanasakis-Kaklamanakis, M. Phys. Rev. X 13 011015 (2023)

[Ath23b] Athanasakis-Kaklamanakis, M. arXiv 2308 14862 (2023)[Gar20] Garcia Ruiz, R. et al., Nature 581, 396 (2020)

[Udr21] Udrescu, S. et al., Phys. Rev. Lett 127, 033001 (2021)

[Udr23] Udrescu, S. et al., Nat. Phys. Accepted (2024)

[Wil23] Wilkins, S. et al., arXiv 2311 04121 (2023)

[Yan23] Yang, X. et al., PPNP 129, 104005 (2023)

One method of pursuit in our search for a more complete description of the spectroscopic properties of nuclei is through the isolation and impact of specific or well-developed mechanisms.

In the present work, the known consequences of a nuclear single-particle orbital wave function as it approaches its confining threshold have been shown to be at the root of various observed phenomena.

Specifically, the role of this weak-binding or so-called geometric behavior was explored in terms of descriptions of evolving single-particle orbitals, the presence and impact of a 'bubble' nucleus, the locations of the drip lines, and the origins of nuclear halo states.

Future and ongoing directions, which both build upon the insight gained through the aforementioned program as well as complement it, will be discussed.

Metal powders are the fundamental starting point for many nuclear target fabrications. Elemental powder properties can differ drastically from batch-to-batch, even when using the same method. Therefore, the variation in morphology and size of metal powders can cause variability in quality and yield inconsistent results with otherwise proven techniques for manufacture of targets. Additive manufacturing has additional requirements for higher quality and more uniform feedstock. The production of spheroidized powders with uniform, reproducible properties and a narrow size distribution represents unexplored opportunities for experiments. Such opportunities include experimenting with solid metals that can now flow like liquids, new options for powder handling and dispensing, and new methods for target fabrications using additive manufacturing.

The Stable Isotope Materials and Chemistry Group at Oak Ridge National Laboratory obtained an AMAZEMET rePowder ultrasonic metal atomization tool for creating limited batches of fully dense, free flowing, spherical powders of extremely rare materials, such as isotopic materials that have a narrow size distribution. This presentation and manuscript present early results from several materials that were produced, explore the anticipated limits of this instrument with extremely rare materials (e.g., enriched stable isotopes), and highlight research into new fabrication techniques that provide additional options beneficial to the international nuclear target community.

Radioactive ion beam facilities offer unique access to unexplored regions of the nuclear chart. Due to short half-lives and low production yields of the most promising regions of the nuclear chart, next-generation high-precision techniques are crucial for characterizing fundamental nuclear properties.

This talk presents recent developments in ion trapping and laser spectroscopy techniques of radioactive isotopes that have enabled pioneering precision measurements of neutron-rich indium isotopes in the direct vicinity of the doubly-magic 100Sn(N=Z=50) at CERN/ISOLDE. Using precision mass measurements, we resolved a discrepancy in the β-decay energy of 100Sn, thereby providing an updated atomic mass value for 100Sn via its direct β-decay into 100In [Nature Phys. 17, 1099 (2021)]. Furthermore, using precision laser spectroscopy of the same indium isotopes, we shed light on 100Sn's doubly magic character through the evolution of nuclear deformation across the indium isotopic chain. [arXiv:2310.15093; under review with Nature Phys.] The impact of these measurements is further demonstrated through an assessment of state-of-the-art density-functional and ab initio nuclear theory approaches.

Lastly, I will introduce a new experiment in which both techniques are combined for precision measurements of fundamental symmetries and unknown effects of the nuclear electroweak structure [arXiv:2310.11192; under review with Phys. Rev. Lett.]. In particular, single trapped molecular ions can vastly amplify unknown nuclear-spin-dependent parity-violating effects such as the nuclear anapole moment. The tremendous leverage offered by radioactive molecules produced at LBNL is discussed.

This talk focuses on experimental work employing techniques including in-beam gamma-ray spectroscopy and ion trapping to probe short-lived atomic nuclei in exotic regions of the nuclear chart.

First, the ongoing analysis of 25Ne produced from a 18O on 9Be fusion evaporation experiment using the state-of-the-art Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) and the Fragment Mass Analyzer (FMA) at Argonne National Laboratory (ANL) will be presented. Here, gamma-ray angular distribution and polarization measurements will help clarify spin-parity assignments to help elucidate the evolution of nuclear structure along the Ne isotopic chain from stability at N=12 toward the N=20 Island of Inversion.

Furthermore, results from the first GRETINA experiment at the Facility for Rare Isotope Beams (FRIB), which studied shape coexistence near the N=40 Island of Inversion, will be relayed. The varying widths and shapes of the parallel momentum distributions following two-proton knockout into different excited states were utilized to probe levels in N=38 62Cr.

Finally, a precision measurement of 8B beta decay performed at ANL using the Beta-decay Paul Trap (BPT) will be discussed. From this work, the 8B neutrino energy spectrum, an important input for solar neutrino astrophysics, was reconstructed and stringent limits on the beyond Standard Model Tensor current contribution to the electroweak interaction were set.

Rooted in Quantum Chromodynamics, the strong force is the underlying interaction that binds atomic nuclei and is giving rise to nuclear structure. Despite understanding stable nuclei, predictive power of nuclear models diminishes for nuclei and nuclear matter under extreme conditions, like in neutron stars. To address those questions and provide a comprehensive insight into the nuclear interaction, spanning both long- and short-range characteristics, studies of nuclear systems with extreme neutron excess and phenomena related to close nucleons in nuclei are essential. As a key region, I will discuss the impact of long-range forces on the structure evolution of the most neutron-rich fluorine isotopes 29,30 F as studied through the spectroscopy of bound and neutron-unbound states using nucleon-knockout reaction measurements at RIBF. In the second part of my talk, I will discuss efforts to leverage similar experimental techniques in terms of hadronic and electron scattering to probe short-range correlations, manifested in close-proximity nucleon-nucleon pairs, at JINR, GSI-FAIR, and JLab. Looking ahead, I will present a program of measurements using the new FRIB facility towards an understanding of nuclei as open quantum systems and its interactions.

As current and next-generation rare isotope beam facilities race to produce the most exotic nuclei far from stability, experiments must evolve in stride. In this talk I will give an overview of two different experiments which have undergone recent upgrades to permit more efficient studies of never-before-studied nuclei. First, I will discuss highlights from the CARIBU era at Argonne National Laboratory of the Canadian Penning Trap (CPT) mass spectrometer where the implementation of a phase-imaging mass measurement technique led to a revamped sensitivity, providing access to the most exotic beams ever produced by the facility. I will show results from a CPT mass survey of the neutron-rich rare-earth region near N = 100 where nuclear structure effects observed in these nuclei were used to explore the dynamic formation of the rare-earth elemental abundance peak during the astrophysical rapid neutron-capture process. Several instances of isomer discoveries in this region will also be addressed. In the second half, I will briefly summarize the heavy element experimental program at the 88-inch cyclotron facility of Berkeley Lab where the properties of heavy and superheavy elements are studied using the Berkeley Gas-filled Separator (BGS) and the FIONA apparatus. I will highlight a series of recent upgrades, including the commissioning of the superheavy recoil (SHREC) detector at the BGS focal plane in preparation for an upcoming campaign to search for element 120 using the 50Ti + 249Cf reaction. Finally, I will consider the prospects of studying new nuclei produced through multi-nucleon transfer reactions, both here at the 88-inch cyclotron facility and at Argonne's upcoming N = 126 factory.

Core-collapse supernovae (CCSNe) nucleosynthesis contributes significantly to the production of thechemical elements in the galaxy but the scarcity of high-quality spectroscopic observations (through nofault of astronomers) hinders our understanding of these processes. What nuclear process producesthese nuclei and how much is expelled into the interstellar medium by CCSNe? I will discuss theconcerted, multi- disciplinary approach being pursued by the nuclear astrophysics team at LLNL toprobe explosive nucleosynthesis. We combine radioactive beam measurements of important reactionrates with state-of-the-art nucleosynthesis network models to make predictions for isotopic abundancesthat we then compare to data from in-house nano-analytical measurements of presolar CCSNe grainsobtained from meteorites. I will present our first results and on-going work. I will also briefly discuss recent results from the second FRIB Decay Station initiator (FDSi) experiment.

Precise measurements of nuclear beta decays provide a unique insight into the Standard Model due to their connection to electroweak interactions. These decays can provide constraints on the unitarity or non-unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) quark mixing matrix, where non-unitarity would signal potential physics Beyond the Standard Model. The most precise of these tests involves the matrix element V­ud as determined from superallowed pure Fermi beta decays, and indicates a deviation from unitarity on the order of ~2.4σ. As such, cross-checks from additional methods, including superallowed mixed mirror beta decays, are necessary. V­ud precision from mirror decays is currently limited by the absence of precise Fermi-to-Gamow Teller mixing ratios, which are most sensitively determined via the angular correlation of the neutrino and beta particle emitted during the decay. At the Nuclear Science Laboratory (NSL) at the University of Notre Dame, the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is being constructed to determine the beta-neutrino angular correlation parameter of various mirror decays. We plan on measuring this correlation parameter for the beta decays of nuclei ranging from ­­11C to 41Sc using radioactive ion beams from the NSL’s TwinSol separator, which will result in significantly improved precision of the V­ud element of the CKM matrix from superallowed mirror transitions. The St. Benedict facility, its developmental status and first commissioning experiments will be presented.

Tremendous success has been achieved in the low-excitation region of the nuclear structure study. There, a wealth of knowledge has been gained, thanks to precise measurements of discrete levels and detailed calculations by advanced nuclear theories. However, less is known about highly excited states, except the detailed study along the Yrast line in high-spin physics. The current question is: how much do we understand the highly excited states extending to the neutron separation energy and to the high spin region beyond the termination of rotational bands?

There is a growing need to better understand the highly excited states. For nuclear astrophysics and practical applications, such as reactor technology, radiative neutron capture is one of the important types of reactions, which is commonly described by the Hauser-Feshbach theory that requires the nuclear level density (NLD), and -ray strength function (SF) as model inputs. In the fission study, each primary fission fragment is treated as a compound nucleus, for which knowledge of the initial spin-distribution (SD) is crucial. All these (NLD, SF, SD) are purely nuclear structure quantities, but exhibit statistical properties. However, statistical models cannot provide quantitative information on NLD, SF, and SD. 

With new breakthroughs in many-body calculations, shell-model calculations for highly excited states are now possible in the Projected Shell Model (PSM). This novel shell model, designed for arbitrarily heavy systems, starts from a deformed mean-field solution, transforms the basis states from the intrinsic to the laboratory frame through angular–momentum-projection, builds the configurations in the projected space, and then performs shell-model diagonalization in the laboratory frame. The obtained states are eigenstates of spin and parity, and the well-defined wavefunctions can be used to calculate any observables. 

This talk introduces PSM through recent examples in applications and discusses potential future applications. Taking deformed heavy nuclei as examples, we demonstrate how to solve the eigenvalue problem, H |Ψ> = E |Ψ>, to obtain microscopic NLD and SF. Excited scissors states are included in the calculation of SF. We discuss how the scissors M1 resonances and the low-energy enhancements (LEE) in magnetic dipole transitions evolve as deformation changes. To prove that other observables can also be given in the same model, electron-capture rates, Gamow-Teller and first-forbidden transitions of  decay for highly excited states are mentioned with calculated examples. Finally, interpretive questions are raised and new emerging physics at excitation extremes is speculated.

In this talk, I will discuss recent advancements in the study of exotic nuclei and rare decay modes using particle and γ-ray coincidence spectroscopy developed during my PhD research work. 

First, a bismuth-germanate (BGO) anti-Compton shield (ACS) for five Compex germanium detector modules was designed, built, and tested. These Compex detectors, equipped with ACS, are planned for diverse applications, including local environmental sample measurements, future nuclear structure studies at the GSI Helmholtz Centre for Heavy Ion Research, Germany, and the superheavy element research programme at Lawrence Berkeley National Laboratory, USA.

Second, an experimental campaign at ANL in 2020 focused on odd-Z, neutron-deficient nuclei near 56Ni. Two highly pixelated double-sided Si-strip detectors were added to the Gammasphere-Microball setup, improving in-beam proton-γ coincidence spectroscopy and enabling in-beam particle tracking for beam-spot position determination. Excited states in 61Ga were populated via the fusion-evaporation reaction 24Mg(40Ca,p2n)61Ga, leading to the identification of a proton-emitting stateat Ex = 2150(34) keV, interpreted as the πg9/2 single-particle state. This discovery refined shell-model calculations through studying isospin-breaking parameters for the A = 61, Tz= =+/- 1/2 mirror nuclei 61Ga and 61Zn.

In the same experiment, deuteron evaporation from 52Fe*, 56Ni*, and 64Ge* was observed, based on enhanced light-charged particle sensitivity. A study was conducted for a few evaporation channels comparing relative production rates for deuteron vs. proton-neutron evaporation as a function of angular momentum and excitation energy.