Nuclear Structure 2024

Jul 21, 2024, 5:30 PM → Jul 26, 2024, 3:00 PM America/Chicago

Auditorium (APS Conference Center, Argonne)

Benjamin Kay, Melina Avila Coronado

This conference serves to highlight recent experimental and theoretical research on atomic nuclear structure at the limits of mass, isospin, excitation, and angular momentum from around the world. This will be the 19th of series of nuclear structure conferences organized biennially by North American national laboratories. We will use this occasion to celebrate the 75th anniversary of Hans Jensen's and (Argonne's) Maria Goeppert Mayer's seminal 1949 work.

The conference was held in the Auditorium of the APS Conference Center at Argonne National Laboratory on July 22-26, 2024. 

Local Organizing Committee:

Melina Avila (co-chair)
Michael Carpenter
Calem Hoffman
Ben Kay (co-chair)
Filip Kondev
Anna McCoy
Walter Reviol
Guy Savard
Darek Seweryniak
Marco Siciliano
Ivan Tolstukhin

Local Arrangements:

Colleen Tobolic (email)

group_photo_high_res.jpg

Sunday, July 21

6:00 PM → 8:00 PM Nuclear Structure 2024 reception

Monday, July 22

8:00 AM → 9:00 AM Registration, Coffee & Snacks

9:00 AM → 10:20 AM Reflections on the Shell Model

Convener: Robert Janssens

9:00 AM Welcoming remarks

Speakers: Ben Kay, Melina Avila

9:05 AM Magic numbers, shell model and the inspiring experimental data

The magic numbers that inspired Maria Goeppert-Mayer and Hans Jensen to propose the shell model were deduced from early experimental observations. The success of the model, at the beginning only able to describe the structure of nuclei near the closed
shells, has been extended in the last decades to the description of well deformed nuclei
with several valence particles in large model spaces. The possibility to extend gammaspectroscopy studies to nuclei very far from stability has shown an evolution in the shell structure with the disappearing of the historical magic numbers with the development of new regions of deformation and the appearance of new magic numbers.
After a brief historical introduction, the experimental data that has inspired the
continuous development and improvement of the shell model calculations will be shown
and discussed

Speaker: Silvia Lenzi (Dipartimento di Fisica e Astronomia, Università di Padova, Italy)

9:30 AM The Nuclear Shell Model; a Long and Winding Road

In my presentation commemorating the 75 years gone since the founding papers of Maria Goeppert-Mayer and Hans Jensen, I will give a personal view of the evolution of the shell model approach to the structure of the atomic nucleus. The virtues and the limitations of the original independent particle model (IPM) and its microscopic justification will be discussed, as well as its fundamental role providing the natural basis in the Fock space for the present shell model with con- figuration interaction (SM-CI) approaches. I will stress the importance of the SU(3)-like underlying symmetries of the IPM, discovered by Elliott. Our modern understanding of the effective interac- tions will be highlighted as well. I will review the recent advances that have made of the SM-CI description an unified view of nuclear structure, able to describe deformed and superdeformed states at the same footing than single particle degrees of freedom. A final word will be said about shape coexistence and the occurrence of Islands of Inversion in (neutron) semi-magic, very neutron rich nuclei.

Speaker: Frederic Nowacki (Université de Strasbourg)

9:55 AM The shell model and the nuclear rotation, after seven decades

The shell model as a quantum-many-body approach with various correlations due to nucleon-nucleon interactions has shown the shell evolution phenomena in exotic nuclei. I will survey what consequences the same interactions produce for various collective properties. Type II shell evolution in a long chain of Ni isotopes, and the first-order phase transition in Zr isotopes will be mentioned as examples.
The shapes and rotations of heavy deformed nuclei, such as 154Sm and 166Er, are discussed in the quantum-many-body framework, to which the shell model belongs. Without resorting to the quantization of rotational kinetic energy of a free rigid body, the rotational excitation energies are shown to appear as a consequence of the Hamiltonian for multi-nucleon systems. The prevailing triaxiality is pointed out with two robust mechanisms. A prospect for the Interacting Boson Model is presented as a tool to simulate the afore-mentioned quantum-many-body properties.

The work behind this has been posted to the arXiv as arXiv:2303.11299v4 [nucl-th].

Speaker: Takaharu Otsuka (University of Tokyo)

10:20 AM → 10:50 AM Coffee break

10:50 AM → 12:15 PM Structure of Light Nuclei - Part 1

Convener: Calem Hoffman (Argonne National Laboratory)

10:50 AM Study of oxygen isotopes at neutron-rich extreme

One of the goals of nuclear physics is to understand the properties of
all of the atomic nuclei including ones with large proton-neutron
asymmetry. The nuclear shell structure is a key to understanding the
strongly-interacting many-nucleon system and the doubly magic nucleus is
a cornerstone for that. The shell structure and magic numbers are well
established from the studies of stable nuclei, while the magic numbers
can change when the proton and neutron numbers are much different from
the stable nuclei. Among the stable and unstable nuclei, the candidates
of doubly magic nuclei are very limited. The neutron-rich oxygen isotope
28O was the only one that is experimentally accessible that had yet to
be observed. In the presentation, I will mainly focus on recent
experimental study on 28O and its neighbor 27O.

Speaker: Yosuke Kondo (Tokyo Institute of Technology)

11:15 AM Probing Nuclear Structure of the Edge of the N=20 Region of Deformation through the β-decay of Exotic Ne and Na Isotopes

Neutron-rich nuclei in the N=20 region of deformation have played a key role in our understanding of nuclear structure. In this mass region, so-called intruder states from nucleon occupations in the pf-shell are observed to energetically compete with the expected configurations in the sd-shell. Although nuclei in this mass region can be experimentally challenging to access, β-decay can clearly populate such intruder configurations, which play a critical role in our understanding of nuclear structure by providing clear benchmarks for nuclear theory.
The current presentation will highlight the β-decay of neutron-rich Ne and Na isotopes approaching the dripline. The discussion will focus on data collected from one of the last experiments to run at the NSCL, where neutron-rich isotopes centered on 31Ne were created through the fragmentation of a 48Ca beam and implanted into the β Counting System. Here, β- delayed γ-ray spectroscopy data were collected. Details of the nuclear structure obtained from the implanted 31,30Ne, and 33, 32Na isotopes will be discussed, with a focus on their half-lives and β- branchings. Spin-parity assignments are made from logft and β-branching arguments for observed levels in 31,32,33Mg, and 30,31Na following the β or β-n decay branches.
Notably, the data suggests a novel Jπ = 3/2+ assignment for the ground state of 31Ne—an unnatural parity assignment, which illustrates the presence of significant odd particle-hole configurations in its ground state, rather than the primarily even particle-hole configurations that have been suggested so far. Moreover, this marks the first identification of ground states with odd-particle- odd-hole configurations from a beta-decay measurement in the N=20 region of deformation. A discussion of the confirmation of these states compared to various theoretically predicted states from modern shell-model calculations will be presented. The dominance of these odd particle-hole configurations in the ground-state configuration of 31Ne is a prime example of how this mass region continues to challenge our understanding of nuclear structure.
This work was funded with support provided by the DOE and NSF.

Speaker: Yiyi Zhu (UMass Lowell)

11:35 AM Isomer Discoveries at FRIB with the FDSi Near N=20, 28, and 40

The Facility for Rare Isotope Beams (FRIB) will provide unprecedented access to exotic nuclei; approximately 80% of the isotopes predicted to exist up to uranium (Z = 92) will be produced. The FRIB Decay Station initiator (FDSi), led by the FDSi Coordination Committee and supported by the FDSi Group and Working Groups, is the initial stage of the FRIB Decay Station (FDS).
A brief overview of the FDSi and first two years of operation at FRIB will be given [1,2,3,4]. Emphasis will be placed on new gamma-decaying isomers discovered near N=20, 28, and 40 during the first two years of operation. These isomers provide highly constrained structure possibilities for each region and important landmarks for future exotic beam studies. Implications for 60Ca will be discussed.
This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics.

Speaker: Mitch Allmond (Oak Ridge National Laboratory)

11:55 AM Revealing the nature of near-threshold narrow resonances in 11B with the HELIOS spectrometer

The large branching ratio observed in the β-delayed proton emission of 11Be was explained by the existence of a narrow, near-threshold proton-emitting resonance in 11B. The direct measurement of this process sparked a heated debate surrounding the properties of this resonance and the unusually large β-decay branching ratio that populates it. Since then, several experiments have reported the observation of such an elusive resonance. While there is widespread agreement on the existence of this resonance from both theoretical and experimental standpoints, many open questions remain regarding its nature. One of the main challenges lies in describing the complex structure of 11B and the role of continuum coupling with four different particle emission thresholds in approximately 2 MeV of excitation energy. Moreover, the properties of the states in the vicinity of these thresholds, critical for understanding the structure of 11B, are either unknown or poorly constrained. With the intention of clarifying such an entangled situation, we conducted an experiment to investigate the 11B structure at high excitation energy through the 10B(d,p) reaction in inverse kinematics using the HELIOS spectrometer. The detection of protons in coincidence with heavy recoils in inverse kinematics enabled the determination of low-probability branching ratios of several states around particle emission thresholds and their widths. The much-debated near-proton-threshold resonance at 11.4 MeV was observed thanks to the high-quality recoil identification provided by the experiment. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02- 06CH11357. This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility and used resources of the Facility for Rare Isotope Beams (FRIB) Operations, which is a DOE Office of Science User Facility under Award Number DE-SC0023633. This work has received financial support from Xunta de Galicia (CIGUS Network of Research Centers) and from the Spanish Ministerio de Economia y Competitividad through the Programmes “Ramon y Cajal” with the Grant No. RYC2019-028438-I

Speaker: Yassid Ayyad (Instituto Galego de Fısica de Altas Enerxias, University of Santiago de Compostela)

12:15 PM → 12:30 PM Conference photograph

12:30 PM → 1:30 PM Lunch

1:30 PM → 3:20 PM Facilities, Camapigns, and Devices

Convener: Walter Reviol

1:30 PM Celebrating 60 Years of Nuclear Science at UKAL

The University of Kentucky Accelerator Laboratory (UKAL) celebrates the 60th year of continuous operation of its 7 MV model CN VDG accelerator in 2024. While the laboratory’s capabilities at low energy are quite broad, neutron induced reactions and neutron scattering investigations have dominated the UKAL scientific program for the last 30 years. Nuclear structure investigations using the (n,n'y) reaction focus on the study of nuclei important for 2B and 0v2B decay experiments, with experiments that improve the structure knowledge of the parent [1] and/or daughter nuclei [2]. Other nuclear structure investigations examine shape coexistence and the evolution of few particle and collective excitations across isotopic or isotonic chains. Since its inception the laboratory’s experimental program has also concentrated on the detection of elastically and/or inelastically scattered neutrons [3]. Recently neutron- detection experiments have investigated cross sections on materials important for, e.g., energy production, while simultaneously helping to develop a better basic understanding of how neutrons interact with matter. Scientists from other universities, national laboratories, and industry use the facilities to test detectors, to study background from low-energy VDGs, and to interrogate materials with neutrons. New developments at UKAL will enhance the experimental program going forward. Ongoing developments and experimental investigations related to 0v2B decay, shape coexistence and neutron scattering will be presented. This work is supported by the National Science Foundation (PHY-2209178) and the Department of Energy (DE-SC0021424, DE-SC0021243, DE-SC0021175)

Speaker: Sally Hicks (University of Kentucky)

1:55 PM Nuclear Structure Physics at HIE-ISOLDE

The HIE-ISOLDE facility began operations in 2016 and ran has delivered beams from across the whole chart of nuclides from 7Be up to 228Ra, accelerated by the four superconducting cryomodules to energies ranging from 2 MeV/u up to 9.8 MeV/u for 28Mg.

There are three permanent experimental setups at the end of HIE-ISOLDE's beam lines with the Miniball gamma-ray spectrometer taking most of the beam time until CERN’s second long shutdown (LS2; 2018-2021). The Scattering Experiments Chamber (SEC) is a flexible setup that concentrates mostly on reactions of light nuclei and the ISOLDE Solenoidal Spectrometer (ISS) is a newly commissioned setup designed to perform few-nucleon transfer reactions inside the 4 Tesla magnetic field of a former MRI magnet.

Miniball underwent a total transformation during LS2, returning to action in 2022. There has been a refurbishment of the HPGe detectors, including new cryostats, electronics and preamplifiers, as well as a newly developed data acquisition system.

In this talk I will present preliminary results from the latest experiment campaigns at HIE-ISOLDE, focusing on ISS and Miniball. I will cover exemplar physics cases around the doubly-magic 132Sn, studied using Coulomb excitation and one-neutron transfer reactions, shape coexistence in the neutron deficient Hg isotopes and single-particle structure outside of 68Ni.

Speaker: Liam Gaffney (University of Liverpool)

2:20 PM The AGATA physics campaign at Legnaro National Laboratories

In April 2022, AGATA, the European Ge-array at the forefront of gamma detection technology [1,2] was installed at LNL. Based on the new concept of gamma-ray tracking, it can identify the gamma interaction points (pulse shape analysis) and reconstruct via software the trajectories of the individual photons (gamma-ray tracking). Shortly thereafter a physics campaign has started using stable beams ranging from hydrogen to lead, delivered by the Tandem-ALPI-PIAVE accelerator complex at energies from 20-25 MeV/u (lightest ions) to about 7-8 MeV/u (heaviest ions). In the first phase AGATA has been coupled to the PRISMA heavy-ion magnetic spectrometer to access the study of exotic nuclei produced in multi-nucleon transfer and fusion- fission reactions. Different silicon detector arrays for light charged particles and ions have also been used. The physics cases under study involve shell evolution and configuration mixing in key regions of the nuclear chart, such the N=20 island of inversion and the nuclei around the doubly-magic 78Ni, quadrupole and octupole shapes and collectivity across a wide range of nuclear masses, as well as measurements of astrophysical interest. Several Coulomb-excitation experiments investigated shape coexistence along the Z=40 and Z=50 lines. In this presentation, the current status of the physics campaign and its main results will be discussed.

[1] A. Akkoyun et al., NIM A 668, 26 (2012)
[2] J.J. Valiente-Dobón et al., NIM A 1049, 168040 (2023).

Speaker: Andrea Gottardo (Laboratori Nazionali di Legnaro)

2:40 PM Decay studies of proton-rich nuclei near 100Sn with AIDA at RIKEN

The neutron-deficient region of the nuclear chart in the vicinity of the doubly magic nucleus 100Sn is of great interest in nuclear structure physics. Measurement of the decay properties of the spherical N ≈ Z ≈ 50 nuclei in this region acts as a direct test of the shell model around the major shell closures, and can assist in establishing the location of the proton drip-line. An experiment at the Radioactive Ion Beam Factory (RIBF) facility at RIKEN, Japan was carried out to perform these measurements with the state-of-the-art Advanced Implantation Detector Array (AIDA) silicon detection system, the first use of this system on proton-rich nuclei at this facility. This talk presents a discussion of the experiment and the analysis of data collected with AIDA.

Support for this project from the Science and Technology Facilities Council (STFC), UK Research and Innovation is acknowledged.

Speaker: Corrigan Appleton (Lawrence Berkeley National Laboratory)

3:00 PM Recent results on level structures and lifetimes in 130Cd and 98Cd and their analogies

The search for the mechanisms to drive the possible shell evolution phenomena is ongoing. Originally and up to date, two mechanisms are considered. The first one, is the so called monopole migration [1], which acts for both proton and neutron-rich nuclei, and the second one, the shell quenching due to a softening of the potential shape that results from the presence of an excessive number of neutrons in very neutron-rich nuclei [2]. These mechanisms modify the known magic numbers as a consequence of shifting effective single-particle levels when going towards either the proton or the neutron drip lines. In medium-heavy nuclei the effort to establish shell evolution concentrates around the 100Sn and 132Sn doubly magic nuclei. The Sn isotopes form the longest isotopic chain in the nuclear chart accessible to current experimental study and thus provide a stringent testing ground for nuclear structure models within a large isospin span. A remarkable similarity was found between the decay of 8+ isomers in 98Cd50 [3] and 130Cd82 [4], both of which have a pure g9/2-2 proton-hole configuration. However, the analogue of the known core excited isomer in 98Cd [5] was not observed so far in 130Cd, within experimental sensitivity, thus underlining the differences in the underlying neutron single-particle structure. The understanding of analogies in the structure of both regions of nuclei and the evolution of the N=82 shell gap below 132Sn is of importance in predicting the path of the rapid-neutron capture process which partially drives the production of elements heavier than Fe in nature. A handful of additional information on these two regions and for those two particular nuclei was obtained recently in spectroscopy studies [6,7], and newly, evaluating experimental information collected in various experimental campaigns including EURICA [8], HiCARI [9], and DESPEC [10] in yet unpublished data subsets. The most recent results include identification of new 2-hole level structures, and most importantly, the lifetime information for most of the levels in both nuclei, even if not all with high precision. The results will be discussed and compared with large-scale shell- model calculations using various sets of the realistic residual two-body interaction.

[1] T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005).
[2] J. Dobaczewski, I. Hamamoto, W. Nazarewicz and J.A. Sheik, Phys. Rev. Lett. 72, 981 (1994). [3] M. Górska et al., Phys. Rev. Lett. 79, 2415 (1997).
[4] A. Jungclaus, et al., Phys. Rev. Lett. 99, 132501 (2007).
[5] A. Blazhev et al., Phys. Rev. C 69, 064304 (2004).
[6] J. Park et al., Phys. Rev. C 96, 044311 (2017).
[7] S.Y. Jin et al., Phys. Rev. C 104, 024302 (2021).
[8] A. Jungclaus, et al., submitted for publication in Phys. Rev. Lett.
[9] M. Armstrong et al, in preparation.
[10] G. Zhang et al., in preparation.

Speaker: Magdalena Gorska-Ott (Gesellschaft für Schwerionenforschung (GSI))

3:20 PM → 3:50 PM Coffee break

3:50 PM → 5:15 PM Reaction & Structure Related to Nuclear Astrophysics

Convener: Steven Pain (Oak Ridge National Laboratory)

3:50 PM Accessing explosive stellar nucleosynthesis in grounded laboratories

Many elements along the nuclear chart are formed in stellar explosive environments where the reached tem- peratures and densities allow to go beyond stability via capture reactions of light nuclei as p, n, and αAs well as improving our understanding of the origin of elements, signatures of the ongoing nucleosynthesis help us to gain insights in such cosmic outbursts: low-energy γ-ray astronomy, a direct probe of the ongoing nuclear activity, is an excellent example. Within this framework, nuclear reaction rates are key ingredients. Their measurements in accelerator facilities are however challenging due to the involved radioactive nuclei and low cross sections. Two kinds of stellar processes and the associated experimental probes will be discussed here.
Simulations of novae nucleosynthesis predict the production of light elements (up to calcium) via p-captures and β decays. For most p-captures, the rate is dominated at novae peak temperatures (0.2 − 0.5 GK) by narrow resonances which correspond to unbound states located hundreds of keV above the proton threshold in the com- pound nucleus. Measuring the spectroscopic properties of the resonant state allows to determine the resonance strength value. This will be illustrated with the recent work on the Ex=7.785 MeV state in 23Mg associated to the Er=0.204 MeV resonance which dominates the 22Na(p,γ)23Mg reaction that consumes 22Na in novae. This latter radioisotope (τ1/2 = 2.6 yr) is the best γ-ray candidate to bring constrains on ONe novae models. From an experi- ment performed at GANIL facility with the γ-ray tracking spectrometer AGATA, the widths of the resonant state were measured while employing a new approach to femtosecond nuclear lifetimesIn principle, resonance strengths can also be measured by detecting the γ-rays emitted from the resonant states populated in the direct transfer reaction which mimics the astrophysical reaction. With such a method, an experiment was recently performed at FRIB facility with the γ-ray tracking spectrometer GRETINA and the particle spectrometer S800 to determine the resonances in 25Al(p,γ)26Si. This reaction is of keen insterest to constrain the contribution of novae to the 26Al galactic production. Preliminary results of the d(25Al,26Si)γ measurement will be discussed.
Elements heavier than iron are expected to be produced by the rapid and slow n-captures reactions on stable and neutron-rich nuclei. Although recently proven to occur during neutron-star mergers, other sites of the r-process and other mechanisms are still under consideration. For instance, some low metallicity stars present abundances around Z∼40 explained by the weak r-process possibly active in ν-driven winds of core-collapse supernovae: the synthesis toward heavier masses is here driven by (α, n) reactions at astrophysics temperatures of 2 − 5 GK. Due to the lack of experimental data, rates are presently calculated with statistical Hauser-Feshbach models where nuclear inputs like the α-Optical-Model Potential lead to uncertainties of several orders of magnitude. At astrophysics energy (5 − 10 MeV in center-of-mass), the cross sections of these α-induced reactions are high (0.1−100 mb) and, so, can be measured directly with an active (gaseous) target. This relevant approach, in inverse kinematics with an electrically-segmented detector, allows to efficiently measure the excitation function at different energies while the incident beam at few MeV/u slows down in the gas. As an illustration, two experimental works performed at ATLAS facility with the ionisation chamber MUSIC that probed the 88Sr(α,n)91Zr and 87Rb(α,xn)Y weak r-process reactions will be presented.

Speaker: Chloe Fougères (CEA)

4:15 PM Understanding the P-Process Using Gamma-Induced Reactions

The astrophysical source of the p nuclei – a series of rare, proton-rich stable isotopes with abnormally high natural abundances – remains an open question. The p nuclei cannot be produced through the known neutron capture processes, but instead are thought to be synthesized in astrophysical environments via a series of photodisintegration reactions on s-process seeds. A major source of uncertainty in understanding the production of p nuclei is the necessity to use cross sections and reaction rates derived from theoretical models, in particular the statistical Hauser-Feshbach approach. With very little experimental nuclear structure constraint, these cross section predictions can vary by orders of magnitude, preventing a careful assessment of the conditions needed to produce p nuclei in the abundances observed.
To address the need for experimental constraint of the predicted cross sections, the HIgS P- Process Collaboration has undertaken several measurements of gamma-induced charged-particle reactions on p-process nuclei across the range of astrophysical energies using monoenergetic beams of gamma rays at HIgS. Utilizing the SIDAR array of highly-segmented, high resolution silicon detectors coupled to the ORRUBA/GODDESS data acquisition system, the various photodisintegration reaction products can be measured, and the total and partial cross sections derived and compared to predictions. Here, the experimental setup, preliminary analysis, and future plans will be discussed.
This work is supported in part by the U.S. Department of Energy under grant nos. DE-AC05- 00OR22725 and the Romanian Ministry of Research, Innovation and Digitalization under project no. PN-III-P4-PCE-2021-1014 and PN 23 21 01 06.

Speaker: Kelly Chipps (Oak Ridge National Laboratory)

4:35 PM 58Ni(3He,t)58Cu Measurements to Constrain the Astrophysical Rate of 57Ni(p,γ)58Cu

The 57Ni(p,γ)58Cu reaction rate significantly impacts nucleosynthesis in a variety of astrophysical sites. In core-collapse supernovae (CCSNe) this reaction impacts the production of 44Ti, a radioiso- tope whose observed gamma-ray emissions offer an important probe into CCSNe, providing a test of nucleosynthesis models. Furthermore, the 57Ni(p,γ)58Cu reaction rate has been shown to significantly impact vp-process nucleosynthesis in multiple astrophysical environments. Despite the importance of 57Ni(p,γ)58Cu, no experimental rates exist for this reaction. To experimentally constrain this rate, structure properties of 58Cu were measured via the 58Ni(3He,t)58Cu reaction using both GODDESS (GRETINA ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Lab- oratory’s ATLAS facility and the Enge split-pole spectrograph at the University of Notre Dame’s Nuclear Science Laboratory. These measurements provide precise determination of 58Cu level en- ergies. This precision is crucial, as the reaction rate depends exponentially on these level energies. The two measurements also provide additional, complimentary structure information that impacts the reaction rate, such as level spin, proton branching ratios, and gamma branching ratios. Experimental procedures and preliminary analysis will be presented.
Research supported by the following institutions: University of Notre Dame (NSF grant no. PHY2310059 ), Rutgers University (NSF grant number PHY-1812316 and DOE NNSA contract no. DE-NA0003897), Argonne National Laboratory (DOE Office of Science (NP) contract no. DE- AC02-06CH11357), Oak Ridge National Laboratory (DOE Office of Science (NP) contract no. DE- AC05-00OR22725), Lawrence Livermore National Laboratory (DOE Office of Science (NP) contract no. DE-AC52-07NA27344), University of Tennessee (DOE Office of Science (NP) contract no. DE- FG02-96ER40963). This research used resources of Argonne National Laboratory’s ATLAS facility, which is a Department of Energy Office of Science User Facility. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract number DE-SC0014664.

Speaker: Scott Carmichael (University of Notre Dame)

4:55 PM Probing lifetimes of short-lived nuclear resonances of astrophysical interest

Classical novae and Type I X-ray bursts (XRBs) are among the most frequent thermonuclear stellar explosions in the Galaxy. The 30P(p,γ)31S reaction acts as a nucleosynthesis bottleneck in the flow of material to heavier masses, affecting several nova observables. The dominant source of uncertainty in the current recommended reaction rate is the theoretical γ decay width of the 3/2+, 260-keV resonance in 31S. We have observed evidence for γ rays originating from the resonance using the Doppler Shift Lifetimes (DSL) facility at TRIUMF, which was designed for lifetime measurements in the 10^−15 - 10^-12 s range [1]. We have upgraded DSL to DSL2 and successfully commissioned it during the first run of the experiment to determine the lifetime of the key 22Na(p, γ)23Mg resonance in novae [2]. The data analysis is currently in progress, and a second run has been scheduled for October 2024. Our proposal to measure the lifetime of the key 31S resonance using DSL2 has also been approved, aiming to put the 30P(p, γ)31S reaction rate on a fully experimental footing and to eliminate the nuclear uncertainties in simulations of nova observables.

Under XRB conditions, the strength of the NiCu cycle is predicted to have significant impacts on the modeling of X-ray burst light curves and the composition of the burst ashes. Currently, experimental information on the 59Cu(p,γ)60Zn and 59Cu(p,α)56Ni reactions is scarce. We have developed a detection system that utilizes a particle-X-ray coincidence technique (PXCT) to measure lifetimes in the 10^-17 - 10^-15 s range. The performance of the PXCT system has been thoroughly tested and is ready for the 60Ga decay measurement in the stopped-beam area of FRIB. This work will provide the life times and decay branching ratios of discrete 60Zn resonances, thereby constraining the competition between the 59Cu(p,γ)60Zn and 59Cu(p, α)56Ni reactions and the strength of the NiCu cycle [3].

This work is supported by the U.S. National Science Foundation under Grant Nos. PHY-1102511, PHY-1565546, PHY-1913554, PHY-2110365, and PHY-2209429, the U.S. Department of Energy under Award Nos. DE-SC0016052 and DE-SC0024587, the Natural Sciences and Engineering Research Council of Canada, and the National Research Council of Canada.

[1] L. J. Sun, C. Fry, B. Davids et al., Phys. Lett. B 839, 137801 (2023). [2] B. Davids, C. Wrede et al., TRIUMF EEC S2193.
[3] L. J. Sun, J. Dopfer, A. Adams et al., In preparation.

Speaker: Lijie Sun (Facility for Rare Isotope Beams)

Tuesday, July 23

8:30 AM → 9:00 AM Registration, Coffee & Snacks

9:00 AM → 10:25 AM Structure of Medium Mass Nuclei

Convener: Andrew Stuchbery (The Australian National University)

9:00 AM Recent experimental results from the JYFL-ACCLAB along the proton dripline

Since the previous edition of the Nuclear Structure conference in 2022, the experimental results of the Nuclear Structure Group of University of Jyväskylä, Finland, have focused in the proton dripline nuclei through in-beam recoil-decay tagging (RDT), and decay-spectroscopy techniques employing both major research installations hosted by the group, namely the vacuum-mode recoil separator MARA and the gas-filled recoil separator RITU. Some prime examples of the decay-spectroscopy results are the discoveries of many new isotopes. For example, using the MARA vacuum-mode recoil separator we have identified a new proton-emitting isotope 149Lu [1]. The decay Q-value of 1920(20) keV is the highest measured for a ground-state proton decay, and it naturally leads to the shortest directly measured half-life of 450+170 −100 ns for a ground-state proton emitter. Through non-adiabatic quasiparticle calculations we were able to conclude that 149Lu is the most oblate deformed proton emitter observed to date. Additionally, within the same beam time we collected a good number of RDT γ rays feeding the also proton-decaying 147Tm (πh11/2) and 147mTm (πd5/2) states, which we found both to be triaxial [2]. A few more examples of new isotopes discovered in JYFL would be the α-decaying isotopes 190At, discovered by Kokkonen et al [3] using the RITU separator, and 160Os, discovered by Briscoe et al with MARA [4]. In addition to the already mentioned 147Tm, in-beam γ-ray spectroscopy techniques has been used to study the heavy nuclei 211,213Ac by Louko et al [5], who found that the negative-parity low-lying excited states in Ac nuclei tend to follow the 2 +, 4 +, and 6 + states of the even-even Ra core. In other studies at the lead region Ojala et al [6] and Papadakis et al [7] combined two prompt in-beam spectroscopy techniques, namely γ-ray- and conversion-electron spectroscopy, in a RDT study of the triple-shape coexistence nucleus 186Pb and in recoil-tagging study of 188Pb, respectively. In the former the feeding of the 0 + 2 state, the interband 2 + 2 → 2 + 1 transition, and the energies of the electric monopole transitions from the excited 0 + states to the 0 + ground state were observed in 186Pb, based on which the shapes of the excited 0 + states were reassigned. In this talk I will summarize the experimental details, results, and interpretations of some selected results of the work carried out by the Nuclear Spectroscopy Group in JYFL-ACCLAB, together with the future prospects at the MARA-LEB radioactive-beam installation presently under construction in the laboratory.

[1] K. Auranen et al., PRL 128, 112501 (2022)
[2] K. Auranen et al., PRC 108, L011303 (2023)
[3] H. Kokkonen et al., PRC 107, 064312 (2023)
[4] A. Briscoe et al., PLB 847, 138310 (2023)
[5] J. Louko et al., PRC submitted (2024)
[6] J. Ojala et al., Comm. Phys. 5, 213 (2022)
[7] P. Papadakis et al., PLB. submitted (2024)

Speaker: Kalle Auranen (University of Jyväskylä)

9:25 AM Shape coexistence in Sn isotopes around A=110

A survey of decay properties of excited 0+ states in regions of the nuclear chart well known for shape coexistence phenomena was performed. It was focused on even-even nuclei around the Z=20 (Ca), 28 (Ni), 50 (Sn), 82 (Pb) proton shell closures and along the Z=36 (Kr), Z=38 (Sr) and Z=40 (Zr) isotopic chains [1]. The aim is to identify examples of extreme shape coexistence, namely, coexisting deformed and spherical (or close-to-spherical) nuclear states, the wave functions of which are well separated in the Potential Energy Surface (PES) spanned over deformation space. Due to this separation, the transition between such structures could be substantially hindered. This is in analogy to the 0+ fission shape isomers in the actinides region and to the superdeformed (SD) states at the decay-out spin in medium/heavy mass systems. In the survey, the Hindrance Factor (HF) of the E2 transitions de-exciting 0+ states or SD decay-out states is a primary quantity which is used to differentiate between types of shape coexistence. It is found that a limited number of 0+ excitations (in the Ni, Sr, Zr and Cd regions) exhibit large HF values (>10), few of them being associated with a clear separation of coexisting wave functions, while in most cases the decay is not hindered, due to the mixing between different configurations. More in detail, shape-isomer- like structures, of prolate deformed nature, have been observed at spin zero in the relatively light 64,66Ni nuclei [2,3], by performing gamma-spectroscopy investigation with different types of reaction mechanisms (i.e., proton and neutron transfer, neutron capture and Coulomb excitation). An analogous situation is expected to occur in 112-116Sn isotopes, for which preliminary results will be discussed, from experiments performed at IFIN-HH (Magurele, Romania) with ROSPHERE, and at Legnaro National Laboratory (Padua, Italy) with the AGATA tracking array.
According to theory predictions based on state-of-the-art Monte Carlo Shell Model (MCSM) calculations [4], it is the action of the monopole tensor force which stabilizes and deepens isolated, deformed local minima in the PES. The existence of such minima may thus lead to a significant separation of the wave functions of states residing in these minima and, eventually, to shape isomerism.

[1] S. Leoni, B. Fornal, A. Bracco, Y. Tsunoda, and T. Otsuka, to be published in Prog. Part. Nuc. Phys. [2] S. Leoni, B. Fornal, N. Marginean et al., Phys. Rev. Lett. 118, 162502 (2017)
[3] N. Marginean, et al., Phys. Rev. Lett. 125, 102502 (2020)
[4] Y. Tsunoda et al., Phys. Rev. C 89, 031301 (2014)

Speaker: Giacomo Corbari (University of Milano)

9:45 AM Electromagnetic properties of the Te isotopes: the path to collectivity and the nature of pre-collective nuclei

The tellurium isotopes with 52 protons show a transition from vibrator-like structures near midshell (118Te) to seniority structures near N = 82 (134Te). We report measurements of excited-state g factors and E2-transition strengths following Coulomb excitation, as well as lifetimes from Doppler-broadened line shapes, for the stable isotopes from 124Te to 130Te. These measurements, performed at Australia’s Heavy Ion Accelerator Facility, allow us to map the pathway from the π0g2 seniority structure in 134Te [1] toward 7/2 collective excitations near midshell as successive pairs of neutrons are removed. The experimental results, which give a novel perspective on the nature of pre-collective nuclei, will be presented. It is found that collectivity does not emerge suddenly, with the nucleus becoming collective as a whole, as might be inferred by examining energy patterns such as R4/2 = Ex(4+1 )/Ex(2+1 ) ratios, alone. The E2 transition strengths and g factors show that collectivity develops in subsets of nuclear excitation: the 2+1 state becomes collective first while the 4+ and 6+ states retain a significant π0g2 component. The 11 7/2 4+1 state becomes collective next, while the seniority structure persists in the 6+1 states. For example, it appears that, despite approaching midshell, 124Te retains a seniority structure for the 6+ level, i.e. a significant π0g2 contribution. This persistence of the 1 7/2 shell structure at the 6+1 state is in contrast to the B(E2) values of the lower-excitation 2+1 and 4+1 states in 124Te, and neighboring 120Te and 122Te, for which the collectivity becomes enhanced. Large-basis shell-model calculations can describe the trends, although E2 strengths progressively fall short away from N = 82. It is evident that g-factor ratios such as g(4+1 )/g(2+1 ) and g(6+1 )/g(2+1 ) are an important indicator of emerging nuclear collectivity versus the persistence of seniority structure. This work is supported by Australian Research Council Grants DP170101673 and DP210101201. Support for the Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy is acknowledged.

[1] A.E. Stuchbery, J.M. Allmond et al., Phys. Rev. C 88, 051304 (2013) .

Speaker: Ben Coombes (The Australian National University)

10:05 AM Triaxiality and shape evolution in even-even Ge isotopes

The electromagnetic properties of low-lying states in 70,72,74,76Ge isotopes have been in- vestigated using a series of high-precision multi-step Coulomb excitation measurements carried out at the ATLAS facility of the Argonne National Laboratory. The experimental setup consisted of the GRETINA multidetector array coupled to the charge heavy-ion counter, CHICO2, facilitating precise Doppler correction and clean kinematic separation of the scattered particles. A comprehensive set of transition and static E2 matrix elements were extracted from the measured differential Coulomb cross sections and used to deduce the intrinsic shape parameters - overall deformation and asymmetry - using the model- independent rotational invariant sum rules. The resulting shape parameters are compared with results of configuration-interaction shell-model calculations and computations car- ried out within the framework of the Generalized Triaxial Rotor Model (GTRM). These results will be discussed in the context of the evolution of shape coexistence along the isotopic chain, with emphasis on the role of axial asymmetry in describing the structure of these stable nuclei.
This work was supported by Grants No. DE-FG02-97ER41041 and DE-FG02-97ER41033.

Speaker: Nirupama Sensharma (Argonne National Laboratory)

10:25 AM → 10:55 AM Coffee break

10:55 AM → 12:20 PM Instrumentation and Techniques

Convener: Yassid Ayyad (University of Santiago de Compostela)

10:55 AM Spectroscopy of rare isotopes with the Active Target Time Projection Chamber

The Active Target Time Projection Chamber (AT-TPC) has been used in experiments aimed at the exploration of structural effects in radioactive nuclei using one step reactions such as transfer or elastic and inelastic scattering. When used as a solenoidal spectrometer by placing it inside a magnetic field, the AT-TPC allows to perform this type of measurement in inverse kinematics with much reduced beam intensities, down to 100 particles per second, while preserving a good resolution and a wide range of angular coverage. This presentation will showcase the performance of this detector, based on recent results obtained on nuclei in the beryllium to carbon region using pure proton, deuterium and alpha targets. Highlights will include results on resonances in $^{11}$Be that are indicative of cluster configurations, as well as new results on single particle configuration in unbound states of neutron-rich carbon isotopes.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02- 06CH11357. This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility and used resources of the Facility for Rare Isotope Beams (FRIB) Operations, which is a DOE Office of Science User Facility under Award Number DE-SC0023633.

Speaker: Daniel Bazin (Facility for Rare Isotope Beams)

11:20 AM Nuclear structure studies using cold antiprotons at AEGIS

At CERN's antimatter factory, antiprotons are routinely produced and cooled in bunches utilizing the ELENA/AD decelerators. The low energy antiprotons are distributed to a wide range of trapping experiments primarily aiming at precision tests of fundamental symmetries and interactions [1]. The Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEGIS) at the antimatter factory has achieved remarkable performance in trapping antiprotons for the pulsed creation antimatter bound systems such as antihydrogen for studying the gravitational influence on antimatter [2, 3]. Currently, this technique is being developed for the controlled synthesis of antiprotonic atoms inside the Penning-Malmberg trap, where an antiproton—nearly 2000 times heavier than an electron—replaces the orbiting electron in a conventional atom [4, 5]. The deexcitation of the deeply bound antiproton captured in a selected Rydberg state results in the emission of Auger electrons and X-rays. The X-ray emission from the cascade carries vital information for understanding the strong interaction influence on the deeply bound antiproton orbits and may, in some cases, result in direct resonance effects with the nucleus [7, 8]. These resonance phenomena can provide insight into the nuclear density distribution as well as the spin of short-lived nuclear states, accessible for exploration with gamma spectroscopy. As the bound antiproton approaches the nucleus's surface, it will rapidly annihilate, resulting in the formation of highly charged radioactive nuclear recoil fragments [9]. These highly charged fragments can be trapped and further cooled within a nested trap, opening new avenues for precision studies of nuclear structure and fundamental interactions [10]. The AEGIS collaboration have recently demonstrated the trapping of highly charged ions (HCIs) resulting from antiprotons annihilation. Techniques were developed for manipulating the formed HCIs and identifying the fragments using time-of-flight spectroscopy. The ongoing installation of a negative ion source at AEGIS will enable the first co-trapping of negative ions with antiprotons. These developments facilitate the controlled, laser-triggered formation of antiprotonic atoms and the synthesis of cold radioactive highly charged ions (HCIs) within the trap, paving the way for these novel experimental studies.

[1] Carli, C., et al. Nuclear Physics News 32.3 (2022): 21-27.
[2] Doser, M., et al. Classical and Quantum Gravity 29.18 (2012): 184009.
[3] Amsler, C, et al. Communications Physics 4.1 (2021): 19.
[4] Doser, M. Progress in Particle and Nuclear Physics (2022): 103964.
[5] Rodin, V , et al. JACoW IPAC 2022 (2022): 2126-2129.
[7] Kłos, B., et al. Physical Review C 69.4 (2004): 044311.
[8] Gustafsson, Fredrik P., et al. arXiv (2024) preprint arXiv:2401.06063.
[9] Kornakov, G., et al. Phys. Rev. C 107.3 (2023): 034314.
[10] Blaum, K., et al. Quantum Science and Technology 6.1 (2020): 014002.

Speaker: Fredrik Gustafsson (CERN)

11:40 AM Complete decay spectroscopy of neutron-rich Cl isotopes with FDSi

Nucleon-nucleon correlaRons lead to changes in shell structure, such as the islands of inversion. The N=28 shell gap has been frequently probed in nuclei south of 48Ca, finding that intruder configuraRons, which correspond to neutron excitaRons across the gap, play an important role in those neutron-rich isotopes at low energy. The locaRon of the states is related to the neutron shell structure, and has been studied as a driver for the deformaRon of 44S and 42Si. On the other hand, higher-lying states corresponding to proton excitaRon across the Z=20 shell gap havebeenunchecked. ConvenRonally,innucleiwithN~28andZ<20,theFermisurfacesfor neutrons and protons are located within the pf and sd shells, respecRvely. The selecRvity of beta-decay moRvates decay strength measurements to probe the nuclear shell effects of the parent and daughter nucleus. States populated by the conversion from a pf neutron to a pf proton in the Gamow-Teller transformaRons give good insight into the Z=20 shell gap, while also playing a crucial role in determining fundamental beta-decay properRes.
In this contribuRon, I will present the first complete measurement of the beta-decay strength distribuRon of chlorine isotopes performed at FRIB. The measurements uRlized the two focal plane system of the FRIB Decay StaRon IniRator (FDSi[1]), with a combinaRon of high-resoluRon neutron (NEXTi) and gamma-ray (DEGAi) spectroscopy data alongside total absorpRon spectroscopy data (MTAS). The complete decay strength is extracted for argon isotopes and compared to large-scale shell model calculaRons using the SDPF-MU interacRon. In the decay of 45Cl, this sensiRve approach found that a reduced Z=20 shell gap best reproduced the data. The experimental findings exemplify the ability of Gamow-Teller transiRons to populate states associated with proton excitaRon across major shells, allowing for the first benchmark of the Z=20 shell gap along N=28 below 48Ca.

[1] heps://fds.ornl.gov/iniRator/

This work is supported by: NNSA DOE DE-NA0003899 and DOE DE-FG02-96ER40983

Speaker: Ian Cox (University of Tennessee, Knoxville)

12:00 PM First Identification of the 0^+_4 and 0^+_5 levels in 100Zr at ANL-ATLAS

An abrupt shape change from spherical to deformed ground states has been observed between 98Zr (N = 58) and 100Zr (N = 60) [1]. Monte Carlo shell model (MCSM) cal- culations suggest that the rapid change in the relative excitation energy of spherical and prolate structures fulfills the requirements of a first-order phase transition, and predict an excitation energy of the 0+4 spherical state at ≈ 1.5 MeV in 100Zr [2]. In contrast, interacting boson model calculations with configuration mixing (IBM-CM) predict a con- figuration exchange between 0+1 and 0+2 states from 98Zr to 100Zr, with the spherical 0+2 level located at ≈ 300 keV in 100Zr [3]. Thus, to test the two distinct predictions experi- mentally requires the determination of the excitation energy of the spherical 0+ bandhead in 100Zr and, more generally, more extensive knowledge of 0+ excitations in this nucleus. In this talk, we report on the observation of two new excited 0+ states in 100Zr, where spin and parity quantum numbers were unambiguously determined using the unique an- gular correlation pattern provided by 0+ → 2+ → 0+ cascades, as measured with the high-granularity Gammasphere spectrometer. In addition, the analysis has allowed for the identification of new excited states relative to the previously known level scheme, as well as for the determination of their spins and parities. The new data, recently published in Physical Review C [4], provide a stringent test of the MCSM calculations.
This work is supported by the U.S. Department of Energy, National Nuclear Security Administration, National Science Foundation, and German BMBF

[1] T. A. Khan et al., Z. Phys. A 283, 105 (1977).
[2] T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
[3] N. Gavrielov et al., Phys. Rev. C 105, 014305 (2022).
[4] J. Wu et al., Phys. Rev. C 109, 024314 (2024).

Speaker: Jin Wu (Brookhaven National Laboratory)

12:20 PM → 1:30 PM Lunch

1:30 PM → 2:55 PM Nuclear Structure Theory

Convener: Anna McCoy (Argonne National Laboratory)

1:30 PM Advancing Nuclear Structure Theory: From the Shell Model to Clustering in Open Quantum Systems

This presentation explores both historical and modern advancements in nuclear structure theory, evolving from the nuclear shell model introduced 75 years ago. It will provide an overview of quantum many-body tools such as the shell model and the configuration interaction (CI) technique, alongside newer methods that incorporate and leverage quantum complexity, entanglement, and the openness of many-body systems. These methods are applied not only to atomic nuclei but also to many other quantum systems.

We will discuss challenges and recent advancements in employing CI techniques to study clustering in light nuclei. This research addresses complex questions about geometry, effective interactions, configuration space limitations, and continuum physics. Recent advances in CI methods, grounded in ab-initio principles, increasingly align theoretical predictions with experimental observations, particularly concerning clustering properties in light nuclei. I will highlight recent cluster asymptotic normalization coefficient (ANC) results for 16O and 20Ne, showcasing the high quality of predictions.

Additionally, significant progress includes studies of alpha clustering near decay thresholds. This presentation will also cover the dynamics of weakly bound or resonance states, discussing the emergence of broad super-radiant states and the decoupling of narrow states from the reaction continuum. Considering the nuclear system as an open quantum system enhances our understanding of clustering dynamics and the emergence of collective degrees of freedom. Experimental validations of these phenomena will also be highlighted.

Speaker: Alexander Volya (Florida State University)

1:55 PM Quasi-SU(3) coupling induced shape evolution in shell-model calculations for heavy nuclei

Shapes and shape evolution have long been a discussion focus in nuclear physics. The study of this topic requires extension of shell model calculation to heavy mass regions. There, one faces two major problems: how to incorporate a large model space in the calculation and how to interpret the results with the vast shell-model output. A promising tool is the Hartree-Fock-Bogolyubov plus generator coordinate method (HFB+GCM), in which the angular-momentum-projected GCM with quadrupole-constrained Hartree-Fock basis states is introduced into shell-model calculations. To discuss physics from shell-model calculations, Zuker et al. [1,2] proposed that the backbone for emergence of large collectivity is the quadrupole correlation in the key partner orbits with  j=2, (1g9/2, 2d5/2) separated by the N = 50 shell gap, for their examples in the mass-60 region. The present talk extends this idea, named quasi-SU(3) coupling [1,2], to heavier mass regions. We have shown that by moving one major shell up, one can expect a similar quasi-SU(3) coupling of (1h11/2, 2f7/2) across the N = 82 shell gap [3], which plays an important role in the sudden increase of quadrupole collectivity found around N = 70 in the Nd isotopes. By moving two major shells up [4], the quasi-SU(3) coupling of (1i13/2, 2g9/2) across the N = 126 shell gap is responsible for the sudden increase of quadrupole collectivity found around N = 104 in the Po isotopes. In Ref. [5], the quasi-SU(3) idea has been extended to an isovector type to describe the triple enhancement in B(E2) around N=Z=40 isotopes, where we interpreted the emerged collectivity as caused coherently by the n-n, p-p, and n-p components of the quasi-SU(3) couplings. Nuclei with A~130 are typically transitional, where different shapes and shape evolution are classified in the Casten triangle [6] based on the algebraic interacting boson model [7]. We found [8] that by considering two quasi-SU(3) couplings, (1g9/2, 2d5/2) and (1h11/2, 2f7/2), one can understand the shape evolution in the extended Casten triangle. The quasi-SU(3) coupling in (1g9/2, 2d5/2) is found to be the main driving force toward the gamma softness of the O(6) limit, while the one in (1h11/2, 2f7/2) is responsible for changing the deformed shapes from oblate to prolate.

[1] A. P. Zuker, J. Retamosa, A. Poves, and E. Caurier, Phys. Rev. C 52, R1741 (1995)
[2] A. P. Zuker, A. Poves, F. Nowacki, and S. M. Lenzi, Phys. Rev. C 92, 024320 (2015)
[3] K. Kaneko, N. Shimizu, T. Mizusaki, Y. Sun, Phys. Rev. C 103, L021301 (2021)
[4] K. Kaneko, N. Shimizu, T. Mizusaki, Y. Sun, to be published.
[5] K. Kaneko, N. Shimizu, T. Mizusaki, Y. Sun, Phys. Lett. B 817, 136286 (2021)
[6] R. F. Casten, Nature Phys. 2, 811 (2006)
[7] F. Iachello and A. Arima, The Interacting Boson Model (Cambridge University Press,
Cambridge, UK, 1987).
[8] K. Kaneko, Y. Sun, N. Shimizu, T. Mizusaki, Phys. Rev. Lett. 130, 052501 (2023)

Speaker: Yang Sun (Shanghai Jiao Tong University)

2:15 PM Unwelcome intruders: Getting a nucleus to come out of its shell (in ab initio calculations)

Intruder states, involving shell model configurations in which nucleons are excited out of the valence shell, feature prominently in the excitation spectra of nuclei across the nuclear chart. In comparison to “normal” states, for which the structure is well described by valence space configurations, “intruder” states have access to a much larger configuration space, allowing them to develop highly collective structure and greater deformation. They thereby attain greater correlation energy, which permits them to “intrude” into the low- lying spectrum. The proximity of weakly deformed normal states and highly deformed intruder states in the spectrum gives rise to shape coexistence, with the concomitant possibility of shape mixing. In so-called “islands of inversion”, an intruder can even take over as the ground state.
Intruder states are notoriously challenging to describe in ab initio calculations, appear- ing too high in the excitation spectrum, if at all. However, with suitably soft interactions and sufficiently large-scale calculations, it becomes possible to welcome these intruders to their rightful place in the low-lying spectrum. In this talk, we will explore low-lying intruder structure in ab initio no-core configuration interaction calculations, for nuclei near the N = 8 shell closure, and examine the consequences for E0 and E2 transitions. We will find that mixing between normal and intruder configurations can take on the form of simple two-state mixing, permitting extraction of a consistent mixing matrix el- ement from the ab initio calculations, even though this mixing is entirely an emergent phenomenon.
This work is supported by the U.S. Department of Energy, Office of Science, under Award No. DE-FG02-95ER40934.

Speaker: Mark Caprio (University of Notre Dame)

2:35 PM Collective rotation of the nuclei near and beyond drip lines: new physical features

Over the years it was established that collective rotation of the nuclei can act as a tool to access new physical features or increase the stability of specific nucleonic configurations. The most well known case is superdeformed (SD) rotational bands: they cannot be formed at low spin in most of the regions of nuclear chart but collective rotation of the nuclei leads to the formation of SD shell gaps at high spin and brings SD rotational bands to the yrast line making their experimental observation feasible.
Cranked relativistic mean field theory without pairing can describe successfully the properties of different types of rotational bands at high spin across the nuclear chart (see Refs. [1, 2]). Using this framework a detailed and systematic investigation of rotational properties of the nuclei near and beyond the proton and neutron drip lines and the impact of collective rotation on their stability has been carried out [3, 4, 5]. It is shown that rotational bands which are particle quasi-bound at zero or low spins can be transformed into particle bound ones at high spin by collective rotation of nuclear systems. This is due to strong Coriolis interaction which acts on intruder high-j orbitals and drives the highest in energy occupied single-particle states of nucleonic configurations into negative energy domain. Particle emission from such particle bound rotational states is suppressed by the disappearance of static pairing correlations at high spins of interest. In addition, a new phenomenon of the formation of giant proton halos in rotating proton-rich nuclei emerges: it is triggered by the occupation of strongly mixed intruder orbitals. These physical mechanisms lead to a substantial extension of the nuclear landscape beyond the spin zero proton and neutron drip lines. The comparison with the results for ground state rotational bands in very neutron-rich 11Be and 39Mg nuclei obtained in Refs. [6] within the particle-plus-core model based on a nonadiabatic coupled-channel formalism and the Berggren single-particle ensemble will also be presented.
This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Award No. DE-SC0013037.

[1] D. Vretenar, A. V. Afanasjev, G. A. Lalazissis and P. Ring, Phys. Rep. 409 (2005) 101.
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[3] A. V. Afanasjev, N. Itagaki and D. Ray, Phys. Lett. B 794 (2019) 7.
[4] A.V. Afanasjev, S. Teeti and A. Taninah, Phys. Scripta, in press.
[5] S. Teeti, A. Taninah, and A. V. Afanasjev, in preparation, to be submitted to Phys.
Rev. C.
[6] K. Fossez, W. Nazarewicz, Y. Jaganathen, N. Michel, M. Ploszajczak, Phys. Rev. C
93, 011305 (2016) and Phys. Rev. C 94, 054302 (2016).

Speaker: Anatoli Afanasjev (Mississippi State University)

2:55 PM → 3:25 PM Coffee break

3:25 PM → 4:50 PM Heavy Nuclei and Super Heavy Elements - Part 1

Convener: Darek Seweryniak (Argonne National Laboratory)

3:25 PM Pursuing New Superheavy Elements: Progress Update from Berkeley Lab

The last time that an element was discovered in the US was seaborgium in 1974. Since that time, all new elements were discovered in other countries, although the US was involved in the discoveries of superheavy (SHE) elements 114 - 118 by providing the targets needed to make those elements. There is growing interest in returning the US to the forefront of element discovery. However, substantial obstacles exist in discovering elements beyond E118. First, the heaviest elements known to exist were all discovered using 48Ca beams impinging on actinide targets. The resulting cross sections were surprisingly high, with production rates on the order of atoms-per-week or higher. However, pushing beyond E118 requires using heavier beams, such as 50Ti, 51V or 54Cr, which could result in cross sections on order of magnitude or lower than those for 48Ca+actinide reactions – resulting in production rates of atoms-per-year or even less. Second, producing these beams at the high intensities required for SHE production is a challenge, as these elements all have high melting points and chemical properties that make them difficult to produce. Third, the predicted half-lives of elements beyond E118 are short, maybe only tens of microseconds or less. Over the last several years, efforts have been made at Berkeley Lab to update our experimental facility from ion source to the target setup to the detector and electronics to demonstrate the ability to produce and detect elements beyond E118. This abstract provides an update on Berkeley Lab's progress towards unveiling new elements, highlighting advancements and prospects in the search for the elusive SHE.

Financial Support was provided by the Office of High Energy and Nuclear Physics, Nuclear Physics Division, and by the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences of the U.S. Department of Energy, under Contract No. DE-AC02-05CH11231

Speaker: Jacklyn Gates (Lwarence Berkeley National Laboratory)

3:50 PM Core-excited configurations of extreme isomers in Pb and Bi isotopes

The nuclear shell model has been quite successful in describing the microscopic structure of nuclei, particularly those which are proximate to magic numbers. While spin isomers at low excitation in nuclei approaching shell closures are well known and appropriately described by the shell model, advances in experimental techniques have enabled the study of metastable states at very high excitation and spin in recent times. In this context, the region of the nuclear chart in the vicinity of the heaviest, doubly-magic nucleus 208Pb is noteworthy. Owing to the presence of a number of high-j orbitals embedded with low-j ones for both protons and neutrons, metastable states at high spin analogous to spin isomers at low excitation are realized. Further, in nuclei with a few valence nucleons, at very high energy (>6-7 MeV), excitations across the Z = 82 and N = 126 shell gaps make it feasible to open up another set of high-j orbitals for occupation. These core-excited configurations have been found to be even more favorable for the realization of long-lived states at very high excitation and spin.
Three of the longest-lived states across the nuclear chart above an excitation energy of 7 MeV were recently discovered by this collaboration in 204Pb, 205Bi and 206Bi, with half-lives of 220(20) μs, 8(2) ms and 27(2) μs, respectively. Data on these and other such isomers will be reported at the conference, with one of them being the newly-identified T1/2 = 1.46(10) μs state at Ex = 8.835 MeV in 207Pb. All of these results have been obtained using the Gammasphere detector array and the ATLAS accelerator at the Ar- gonne National Laboratory. The properties of these isomers (half-life, excitation energy and spin) are at the extremes of what is presently known. It is challenging to obtain a satisfactory description of these isomers and their decay characteristics using large-scale shell-model calculations and the available effective interactions. These experimental re- sults are expected to serve as crucial inputs for improving effective interactions used in large-scale shell-model calculations.
SKT would like to thank the Shiv Nadar Foundation. This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under award numbers DE-FG02-94ER40848 (UML), DE-FG02-97ER41041 (UNC) and DE-FG02-97ER41033 (TUNL), and contract number DE-AC02-06CH11357 (ANL). The research described here utilized resources of the ATLAS facility at ANL, which is a DOE Office of Science user facility.

Speaker: Filip Kondev (Argonne National Laboratory)

4:10 PM Isomer spectroscopy of 251Md

Experiments seeking the heaviest nuclei are exploring the limits at which protons and neutrons can be bound together. Such extreme nuclear systems are difficult to produce and the details of their structure remain scarce. Theory can guide our understanding, but calculations disagree on the shell gaps where the most stable superheavy elements might be found [1, 2]. Alternatively, the nuclides near Z = 100, N = 152 can be produced far more easily and are amenable to greater scrutiny. The orbitals near the Fermi surface in these well-deformed nuclei often arise from the same configurations which define the predicted spherical shell gaps in the superheavy elements. Thus, we can test theory against observations in this region in order to draw inferences about the heaviest systems.
With this in mind, we have undertaken a study of the odd-Z nucleus 251Md. Rotational bands based on the 1/2[521] [3] and 7/2[514] [4] Nilsson configurations, as well as a high-K isomer [5], have recently been reported. Using the Argonne Gas-Filled Analyzer (AGFA) and X-Array, we have studied the γ-ray decay pattern following decay of the isomeric state. The spin, parity, and excitation energy of this three-quasiparticle state are firmly established, and rotational bands are newly observed to be populated in the decay from the isomer. Configurations are suggested and comparison to recent calculations are made, and the implications for the structure of 251Md will be discussed.
This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics.

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[5] T. Goigoux et al., Eur. Phys. J. A 57, 321 (2021)

Speaker: Chris Morse (Brookhaven National Laboratory)

4:30 PM Octupole correlations in 224U

The neutron-deficient Z = 92 uranium nuclei lie close to the centre of the light-actinide region of enhanced octupole correlations. Despite theoretical predictions of octupole de- formation (e.g. [1, 2]) giving clear motivation for experimental study, there is presently very little existing spectroscopic information concerning the structure of these nuclei. In- deed, at present, the only A < 230 uranium isotope with known excited states is 226U [3, 4] where a classic octupole-band structure has been observed. One of the challenges in the experimental study of these nuclei, is their small production cross sections, meaning that sensitive techniques of channel selection and identification must be used for in-beam spec- troscopy. In the present work, an experiment has been carried out at the Accelerator Lab- oratory at the University of Jyva ̈skyl ̈a in order to study the nucleus 224U using the method of recoil α-decay tagging. The reaction 208Pb(22Ne,4n)224U was used (σ ≃ 500 nb) to- gether with the SAGE eγ spectrometer [5], the RITU recoil separator [6], and the GREAT focal-plane detectors [7]. Excited states have been observed in 224U for the first time. A number of γ-ray and internal-conversion electron transitions have been unambiguously assigned to 224U through correlations with the 224U→220Th→216Ra→212Rn decay chain. The excited states have been arranged into an alternating-parity band, characteristic of a nucleus with enhanced octupole correlations. B(E1)/B(E2) values suggest enhanced electric-dipole moments for several of the states. The new results will be presented and compared to recent theoretical predictions.
This work is supported by the STFC (UK), under grants numbered ST/P005101/1, ST/V001124/1, and ST/L005670/1, and by the Academy of Finland.

[1] L. M. Robledo and G. F. Bertsch, Phys. Rev. C 84, 054302 (2011) [2] K. Nomura et al., Phys. Rev. C 102, 064326 (2020)
[3] P. T. Greenlees et al., J. Phys. G 24, L63 (1998)
[4] R. D. Humphreys et al., Phys. Rev. C 69, 064324 (2004)
[5] J. Pakarinen et al., E. Phys. J. A 50, 53 (2014)
[6] M. Leino et al., Nucl. Instrum. Meth. 99, 653 (1995)
[7] R. D. Page et al., Nucl. Instrum. Meth. 204, 634 (2003)

Speaker: John Smith (University of the West of Scotland)

4:50 PM → 6:00 PM Break

6:00 PM → 8:00 PM Poster Session

Wednesday, July 24

8:30 AM → 9:00 AM Registration, Coffee & Snacks

9:00 AM → 10:05 AM Nuclear Masses

Convener: Guy Savard (Argonne National Laboratory)

9:00 AM High-precision mass measurements for nuclear structure

High-precision atomic mass measurements of radioactive nuclides provide a way to determine nuclear binding energies that can be used to benchmark nuclear models. Important information on nuclear structure, e.g., shell gaps and their evolution, pairing effects and isospin symmetry, can be extracted from the ground-state binding energies. In addition, Penning-trap mass spectrometry, and in particular its phase-imaging ion cyclotron resonance (PI-ICR) technique, has enabled high-precision measurements of isomeric states. The PI-ICR method is very useful for low-lying (< 100 keV) beta-decaying isomers with half-lives longer than around 100 ms. Excitation energies of such isomers are often challenging to unambiguously determine via other techniques. Sometimes the method can also reveal new isomeric states. In this contribution, I will give an overview on high-precision mass measurements with an emphasis on isomeric states and recent results from the JYFLTRAP double Penning trap at the Ion Guide Isotope Separator On-Line (IGISOL) facility.

Speaker: Anu Kankainen (University of Jyväskylä)

9:25 AM Mass measurments of fp-shell nuclei in the vicinity of proton dripline

The two-proton radioactivity (2p decay), where two protons are simultaneously emitted during nuclear decay, was theoretically predicted over 60 years ago[1]. In the early 2000s, 2p decay was discovered in very proton-rich nuclei such as 45Fe and 48Ni [2, 3]. The en- ergy level structure and one- and two-proton separation energies (Sp, S2p) are essential to evaluate the two-proton emission probability of the 2p emitter penetrating through the Coulomb and centrifugal potentials. Since the level structure and mass difference among one- and two-proton removal nuclei are directly related to Sp and S2p, the systematic mea- surement of the masses of nuclei around the 2p emitter leads to a complete understanding of 2p decay.
We performed direct mass measurements of proton-rich Fe isotopes including 45Fe us- ing the TOF-Bρ technique[4] at the SHARAQ beamline of RIBF. Proton-rich isotopes were produced by the fragmentation of the 78Kr primary beam at 345 MeV/nucleon in a 9Be target with a thickness of 2.2 g/cm2. The fragments were separated by the BigRIPS separator and transported to the OEDO beam line followed by the SHARAQ spectrom- eter. OEDO and SHARAQ were operated as a single spectrometer in the dispersion matching mode, which achieved a momentum resolution of 1/15,000. The time of flight (TOF) was measured by diamond detectors installed at the beginning and end of the beamline. Two multiwire drift chamber (MWDC) tracking detectors were also installed to correct the flight-pass length. To measure the Bρ value, a strip-readout parallel- plate avalanche counter (SR-PPAC) newly developed for measuring high-rate heavy-ion beams[5] was used at the intermediate focal plane. Gamma-ray detection systems were placed after the SHARAQ to identify isomers, which could shift the peak in the measured mass spectra.
Proton-rich Ti, Cr, Fe, and Ni isotopes were detected in the vicinity of the proton drip line. Masses of nine isotopes were newly determined for the first time in the present experiment. The separation energies deduced from the mass values exhibit the possible candidates of 2p decay in some proton-rich isotopes beyond the dripline.
This work is supported by JSPS KAKENHI Grant Number JP20H01910 and JP23KJ0609.

[1] V. I. Goldanskii et.al., Nucl. Phys. 19, 484 (1960).
[2] C. Dossat et.al., Phys. Rev. C. 72, 054315 (2005).
[3] K. Miernik et.al., Eur. Phys. J. A 42, 431-439 (2009).
[4] S. Michimasa et al., Phys. Rev. Lett. 121, 022506 (2013).
[5] S. Hanai, et al., Prog. Theo. Exp. Phys. 2023, 123H02 (2023).

Speaker: Shutaro Hanai (CNS, University of Tokyo)

9:45 AM Probing nuclear structure through the mass surface at TITAN

Mass measurement facilities are extremely important in furthering our understanding of nuclear structure away from the valley of stability, including searching for collective be- haviors and probing the appearance and disappearance of shell closures. TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN) is among the world’s premier precision trapping facilities, with the newly added Multiple-Reflection Time-of-Flight Mass Spec- trometer (MR-ToF-MS) expanding its reach. A variety of mass measurement campaigns and results from TITAN’s MR-ToF-MS will be discussed, including investigations of a surfacing island of inversion around N = 40 through neutron-rich Fe masses and tests of the robustness of the N = 32 and N = 34 shell closures in Ca, Ti and V masses.
This work is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Research Council of Canada (NRC).

Speaker: Sam Porter (University of Notre Dame)

10:05 AM → 10:35 AM Coffee break

10:35 AM → 11:40 AM Reactions and Structure Related to Nuclear Astrophysics -- Part 2

Convener: Kelly Chipps (Oak Ridge National Laboratory)

10:35 AM Recent developments in nuclear astrophysics: theory of heavy nuclei

The outcome of astrophysical events depends critically on the structure and reactions of heavy nuclei. These properties often remain unmeasured, leaving theory to fill in the gaps. I will provide a broad overview of recent theoretical progress in the description of heavy nuclei. I will cover progress in structure and reaction models, the behavior of astrophysically relevant nuclear isomers (astromers), the advent of Machine Learning based models, and the prediction of fission properties.

Speaker: Matthew Mumpower (Los Alamos National Laboratory)

11:00 AM β-delayed fission of neutron-rich actinides

Beta-delayed fission (βDF) is a two-step process where a parent nucleus β-decays into a daughter that fissions [1]. βDF plays a role in the termination of the r-process nucleosyn- thesis [2], and so it is of particular interest in the neutron-rich side of the nuclide chart, where only a few studies have been performed. Aiming at expanding the limited infor- mation in this region, an experimental campaign was performed at the ISOLDE facility (CERN, Switzerland) to study the βDF in 230,232,234Ac [3]. A new upper limit for the βDF probability (PβDF ) of 230Ac was deduced to be two orders of magnitude lower than the previously measured PβDF value of 1.19(40)·10−8 [4]. Upper limits for PβDF were also deduced for 232,234Ac and updated values were found for 230,232Fr.
Theoretical calculations of PβDF are ongoing, using the PyNEB code [5] to calculate the fission paths. The aim is to benchmark the models used with reliable experimental values found on the neutron-deficient side of the nuclide chart [1] and then to extend the calculations to the neutron-rich side. The results obtained from the ISOLDE cam- paign will be discussed in this contribution, along with future prospects of a combined experimental and theoretical campaign to study βDF.
This work is supported by Research Foundation Flanders (FWO, Belgium), by BOF KU Leuven (C14/22/104), by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654002 (ENSAR2) and by the FWO and F.R.S.- FNRS under the Excellence of Science (EOS) programme project (40007501).

[1] A. N. Andreyev, et al., Rev. Mod. Phys., Vol.85, No. 4 (2013). [2] S. Goriely, et al., Phys. Rev. Lett. 111, 242502 (2013).
[3] A. N. Andreyev, et al., INT-I-216 (2020).
[4] Y. Shuanggui, et al., Eur. Phy. J. A 10, 1-3 (2001).
[5] E. Flynn, et al., Phys. Rev. C 105, 054302 (2022).

Speaker: Silvia Bara (KU Leuven)

11:20 AM 80Ge(d,pγ) measurements at FRIB to inform (n,γ) reaction rates in weak r-process nucleosynthesis

Individual (n,γ) rates become important in the weak r-process near Z=26-34 and N=50 during freeze out from a hot process. The (n,γ) rates for a handful of specific isotopes exhibit notable impacts on final r-process abundance patterns in sensitivity studies [1]. One such nucleus with enhanced sensitivity is 80Ge and is in reach for reaction studies at FRIB. The 80Ge(d,pγ) reaction will be measured at FRIB in April 2024 using GODDESS (GRETINA ORRUBA: Dual Detectors for Experimental Structure Studies) [2] and the S800 at ~45 MeV/u. This measurement aims to constrain spectroscopic factors for bound states including low-lying ½+-5/2+ doublet. This will be done in combination with a previous measurement at ~4 MeV/u [3], from which direct neutron capture cross sections will be determined. Additionally, the experiment will inform the compound nucleus (n,γ) cross sections via the Surrogate Reaction Method [4]. Experimental set up and preliminary data from the experiment will be discussed.

This work is supported by the National Science Foundation and U.S. Department of Energy National Nuclear Security Administration and Office of Nuclear Physics.

[1] R. Surman et al. AIP Advances 4, 041008 (2014)
[2] S.D. Pain et al. Physics Procedia 90, 455 (2017)
[3] S. Ahn et al. Phys. Rev. C 100, 04613 (2019)
[4] J.E. Escher et al., Rev. Mod. Phys. 84, 353 (2012)

Speaker: Mara Grinder (Rutgers University)

11:40 AM → 11:50 AM Mini-break

11:50 AM → 12:30 PM A ~ 50 Nuclei (teaser)

Convener: Krzysztof Rykaczewski (Oak Ridge National Laboratory)

11:50 AM On the nature of beta-delayed neutron emission near 54Ca

Beta-delayed neutron emission (beta-n) plays an important role in diverse areas of nuclear science, from nuclear reactors to astrophysical nucleosynthesis. Due to its complicated nature, modeling the beta-n process relies on the validity of Bohr’s hypothesis of the compound nucleus (CN) [1]. Here, the decay of the excited nucleus, populated in beta decay, depends only on its spin, parity, and excitation energy and is independent of the formation process [2]. However, recent experimental work suggested evidence of non-statistical beta-n emission near doubly magic 132Sn [3]. Thus, it is essential to verify the foundations of CN in the context of beta decay with a broader spectrum of nuclei, which may result in the revision of beta-n models and will have various consequences for power generation and astrophysical applications.

In this contribution, I will present an experimental work studying the beta-n emission of 51,52,53K at the ISOLDE Decay Station (IDS). In coincidence with the beta decay of 51,52,53K, gamma-ray and neutron-time-of-flight (nTOF) spectra were measured using HPGe clover detectors and VANDLE-like array [4], respectively. We extracted the exclusive neutron-emission branching ratios from the unbound states of 51,52,53Ca, which were populated in beta decay, to the low-lying states in the corresponding residues 50,51,52Ca. The experimental findings were compared with the Hauser-Feshbach statistical model. The results of these comparisons were unexpected and continue to compel us to question existing theories of the process. We propose a model which links the beta-n emission with the nuclear structure.

[1] N. Bohr, Nature 137, 344 (1936).
[2] P. Hansen and B. Jonson, Beta delayed particle emission from neutron-rich nuclei, in Particle Emission from Nuclei, Vol. 3, edited by M. Ivascu and D. Poenaru.
[3] J. Heideman et al., Phys. Rev. C 108, 024311 (2023).
[4] M. Madurga et al, Phys. Rev. Lett 117, 092502 (2016).

Speaker: Zhengyu Xu (1University of Tennessee, Knoxville)

12:10 PM Identification of a new nanosecond isomer in neutron-rich 54Sc

The existence of nuclear isomers offers crucial information for testing nuclear models and understanding shell evolution far from stability. Decay properties including half-lives and transition strengths associated with these isomeric states can be used to characterize them and serve as sensitive probes of their underlying structure.
The second FRIB experiment after it came online involved the production and delivery of a cocktail of neutron-rich nuclei in the region of 54Ca to two distinct high resolution and total absorption focal planes at the FRIB Decay Station initiator (FDSi). This allowed for a thorough decay spectroscopy of the isomeric states investigated in this work. The first is the excited 1+ state, newly identified as a nanosecond isomer and populated following the β decay of 54Ca. The second is a relatively lower-lying state, previously established as a microsecond isomer and populated following the implantation of 54Sc. The presence of these states was inferred from the detection of 247- and 110-keV 𝛾 rays associated with their de-excitation, respectively.
Newly measured and re-evaluated decay properties of these isomers will be presented. Their structure will also be discussed as inferred from interpretations of shell model and ab-initio calculations.

Speaker: Timilehin Ogunbeku (Lawrence Livermore National Laboratory)

12:30 PM → 1:30 PM Lunch

1:30 PM → 7:30 PM Excursions, tours, activities, free time

Thursday, July 25

8:30 AM → 9:00 AM Registration, Coffee & Snacks

9:00 AM → 10:25 AM A ~ 50 Nuclei

Convener: Bill Walters (University of Maryland)

9:00 AM Suppressed Electric Quadrupole Collectivity in 49Ti Relative to Semi-Magic 50Ti

Single-step Coulomb excitation of 46,48,49,50Ti is presented. A complete set of E2
matrix elements for the quintuplet of states in 49Ti, centered on the 2+ core excitation,
was measured for the first time, using the CLARION2-TRINITY arrays [1]. A total of
nine E2 matrix elements are reported, four of which were previously unknown. 49Ti shows a 20% quenching in electric quadrupole transition strength as compared to its semi-magic 50Ti neighbour. This 20% quenching is empirically unprecedented, and
contrary to the enhancement recently observed in 129Sb relative to a 128Sn core [2]. Both cases are near double-magic nuclei and have small core B(E2) values. The quenching in 49Ti can be explained with a remarkably simple two-state mixing model, which is also consistent with other ground-state properties such as the magnetic dipole moment and electric quadrupole moment. The simplicity of the 49Ti-50Ti pair (i.e., approximate single-j 0f7/2 valence space and isolation of yrast states from non-yrast states) provides a unique opportunity to disentangle otherwise competing effects in the ground-state properties of atomic nuclei, the emergence of collectivity, and the role of proton-neutron interactions.
This material is based upon work supported in part by the U.S. Department of Energy,
Office of Science, Office of Nuclear Physics under Contract No. DE-AC05-00OR22725
(ORNL). This work was also supported by the U.S. National Science Foundation under
Grant No. PHY-2012522 (FSU).

[1] T.J. Gray, J.M. Allmond et al., Nucl. Instrum. Methods A 1041, 167392 (2022)
[2] T.J. Gray, J.M. Allmond et al., Phys. Rev. Lett. 124, 032502 (2020)

Speaker: Timothy Gray (Oak Ridge National Laboratory and University of Tennessee, Knoxville)

9:25 AM Direct measurement of hexacontatetrapole, E6 γ decay from 53mFe

First-order electromagnetic processes are the primary mechanism by which excited states in atomic nuclei relax, most-often via single γ-ray emission. Since both the initial- and final-state wave functions possess a well-defined spin and parity, conservation laws impose a characteristic multipolarity for each discrete transition. Nature favours pathways that proceed via the lowest- available multipole order. However, situations arise in which the only available decay pathway is hindered by a larger spin-change requirement. The only proposed observation of a discrete, hexacontatetrapole (E6) transition in nature occurs from the isomeric, T1/2 = 2.54-minute decay of 53mFe. However, there are conflicting claims concerning its γ-decay branching ratio, and a rigorous interrogation of γ-ray sum contributions is lacking [1-3]. Experiments performed at the Australian Heavy Ion Accelerator Facility were used to study the decay of 53mFe. For the first time, sum-coincidence contributions to the weak E6, M5 and E4 decay branches have been firmly quantified using complementary experimental and computational methods [4]. Shell-model calculations were also performed in the full pf model space to investigate the nature of these high-multipolarity transitions. This presentation will provide an overview the experiments, results obtained from this work and their interpretation. The authors are grateful for excellent support from technical staff of the Heavy Ion Accelerator Facility. This work is supported by the Australian Research Council Grants No. DP170101673 and DP170101675, the International Technology Centre Pacific (ITC-PAC) under Contract No. FA520919PA138, and NSF Grant PHY-2110365. Support for the ANU Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy program is acknowledged.

[1] J. N. Black, W. C. McHarris, and W. H. Kelly, Phys. Rev. Lett. 26, 451 (1971).
[2] J. N. Black, W. C. McHarris, W. H. Kelly, and B. H. Wildenthal, Phys. Rev. C 11, 939 (1975).
[3] D. Geesaman, Spin gap isomers in 52Fe,
53Fe, and 54Co, Ph.D. thesis, State University of New
York, Stony Brook (USA) (1976).
[4] T. Palazzo et al., Phys. Rev. Lett. 130, 122503 (2023).

Speaker: A. J. Mitchell (The Australian National University)

9:45 AM Isospin symmetry beyond the proton dripline in the fp shell: A=55 T=3/2 mirror nuclei

Using radioactive beams provided by the RIBF facility, Riken, Tokyo, Japan, we explored
the excited states of the exotic proton-rich nucleus 55Cu. We could study such nucleus,
characterized by an isospin T=3/2, using in-beam γ-ray spectroscopy, despite it is un-
bound, lying beyond the proton dripline. The study exploited three distinct reaction
mechanisms: inelastic excitation, single proton, and single neutron knockout. Through
comparative analysis with its isobaric analogue states, the mirror nucleus 55Fe, spins and
parities have been tentatively assigned to the observed excited states, allowing the study
of isospin-non-conserving phenomena at the extreme of the nuclear landscape. We could
understand within the Shell Model the observed excitation energies and mirror energy
differences, demonstrating reasonable agreement with theoretical predictions.

Speaker: Francesco Recchia (Dipartimento di Fisica dell’Universit`a di Padova and INFN, Sezione di Padova)

10:05 AM Nuclear structure of neutron-deficient Se and Kr isotopes

To investigate the microscopic configurations causing the prolate-oblate-triaxial shape
transition near A = 72 and their possible influence on octupole as well as hexadecapole
collectivity, we studied the rare isotopes 74,76Kr and 72Se, as well as stable 74Se via (p, p′) and (p, 2p) reactions in inverse kinematics with GRETINA, the S800, and the NSCL-Ursinus LH2 target [1, 2, 3]. Our work established two regions of distinct electric octupole (E3) transition strengths with an intriguing strength increase at the A = 72
shape-transitional point, which is not yet understood. Additionally, we linked the enhanced electric hexadecapole (E4) transition strength in 74,76Kr to the well deformed
prolate configuration comparing to state-of-the-art nuclear density functional theory calculations [2]. In Ref. [3], we showed that l = 1, 2, 3, and 4 angular momentum transfers are important to understand the population of excited states of 72,74Se in proton removal. A comparison to (d, 3He) data available for stable odd-A nuclei supports that the bulk of the spectroscopic strengths could be found at lower energies in the even-even Se isotopes than in the even-even Ge isotopes around N = 40.

This presentation will discuss these recent results and provide an outlook for further
studies at the Facility for Rare Isotope Beams.

This work was supported by the National Science Foundation under Grant Nos. PHY-
2012522 (WoU-MMA: Studies of Nuclear Structure and Nuclear Astrophysics), PHY-
1565546 (NSCL), PHY-2209429 (Windows on the Universe: Nuclear Astrophysics at
FRIB), by the Department of Energy, Office of Science, Office of Nuclear Physics, Grant Nos. DE-SC0020451 and DE-SC0023633 (MSU), and by the Department of Energy, NNSA, Grant No. DOE-DE-NA0004074 (the Stewardship Science Academic Alliances program). GRETINA was funded by the Department of Energy, Office of Science. The operation of the array at NSCL was supported by the DOE under Grant No. DE-SC0019034.

Speaker: Mark Spieker (Florida State University)

10:25 AM → 10:55 AM Coffee break

10:55 AM → 12:20 PM Structure of Light Nuclei - Part 2

Convener: Ingo Wiedenhoever (Florida State University)

10:55 AM Beyond the proton drip line and onward to the fuzzy proton-rich edge of the chart of nuclides

The landscape beyond the drip lines is filled will resonances containing unbound nucleons which are momentarily constrained by centrifugal and Coulombs barriers. The further one goes past the drip lines, the larger the number of unbound nucleons and the resonances decay by spitting out these unbound nucleons in steps. Each step consisting of the removal of either a single nucleon or pair of nucleons. The structure of these states are profoundly influenced by their coupling to the continuum, a realm of scattering states above the nucleon separation threshold which designate them as open quantum systems. This can be contrasted with closed systems inside the drip lines where the nucleons are tightly bound in a restricted internal space. This talk will show our exploration beyond the proton drip line using invariant-mass spectroscopy with fast beams. Example of resonances that emit multiple protons will be presented including the recent case for 9N, a ground-state five-proton emitter. The location of the diffuse proton-rich border of the chart of nuclides where resonances cease to exist will be discussed.

Speaker: Robert Charity (Washington University in St. Louis)

11:20 AM First β-delayed neutron spectroscopy of 24O

Located at the neutron drip-line, 24O is the heaviest doubly-magic isotope of the oxygen isotopic chain. As the Qβ value increases and the neutron separation energy in the daughter nucleus decreases for the neutron-rich nucleus, beta-delayed neutron emission becomes a dominant decay mode, and neutron energy measurement is vital in studying the beta decay to the neutron unbound states. Also, spectroscopy of such drip-line nuclei may provide important information regarding the effects of nuclear interactions and many-body correlations in determining the limits of nuclear stability [1-3]. The neutron energy spectrum measurement of the beta-delayed neutron precursor 24O was performed for the first time at the National Superconducting Cyclotron Laboratory (NSCL) using a neutron time-of-flight array (VANDLE[2]) accompanied by gamma spectroscopy setup. New half-life and beta decay branching ratios are extracted. Following the decay of 24O, the beta-gamma and beta-delayed neutron measurements provided the excitation energies and the beta decay strength distribution to neutron-bound and unbound states in 24F . The decay of “doubly-magic” 24O is an excellent case to test the quality of the state-of-the-art calculations of the beta-decay strength distribution near the neutron drip line. The present experimental results were compared with shell-model calculations using the standard, empirical USDB interaction, predictions by the shell model embedded in the continuum, and novel ab initio calculations using the valence-space in-medium similarity renormalization group and the coupled-cluster method.

This work was supported by the U.S. Department of Energy, National Nuclear Security
Administration under the Stewardship Science Academic Alliances program through DOE Award No. DENA0002934 and DENA0003899, and NSF Major Research Instrumentation Program Award Number 1919735.

[1] T. L. Tang et al. Phys. Rev. Lett. 124, 212502 (2020).
[2] T. Otsuka et al. Phys. Rev. Lett. 105, 032501 (2010).
[3] G. Hagen et al. Phys. Rev. Lett. 108, 242501 (2012).
[4] W. A. Peters et al., Nucl. Instrum. Methods Phys. Res. A 836, 122 (2016).

Speaker: Shree Neupane (University of Tennessee, Knoxville, Tennessee)

11:40 AM Structure of A = 22 analogue states revealed through mirrored-transfer across the isobaric triplet

The isospin formalism describes protons and neutrons as two projections of the nucleon and provides a powerful tool for identifying and classifying states in the vicinity of the line of N= Z. Under the assumption that isospin is a good quantum number, a number of relations arise to de- scribe isobaric analogue states their properties. This provides access wealth of information, from tests of the isospin-symmetry conserving nature of the nuclear interaction, to applications in nu- clear astrophysics. In truth, however, this assumption is known to be false, broken by the Coulomb interaction and components of the nucleon-nucleon interaction. Here, we employ mirrored transfer reactions using beams of radioactive 21Na and stable 21Ne delivered by the ISAC-II facility at TRIUMF. These are used to populate states in 22Na and 22Ne, respectively, through (d,p), and in 22Mg and 22Na, respectively, through (d,n). Making use of proton-γ and recoil-γ coincidences, we are able to selectively probe the single-particle nature of individual states and investigate the isospin-dependence of the isobaric analogue state population. I will present initial findings, including the reassignment of a number of states in the literature, and assess the single-particle behaviour of isobaric analogue states across the triplet.

Speaker: Jack Henderson (University of Surrey)

12:00 PM Mixing between single particle and intruder states towards the N=20 island of inversion: lifetimes in 37S

The disappearance of the N=20 shell closure in the so-called “island of inversion” around 32Mg is one of the most striking examples of the strength of nucleon-nucleon correlations. In this region, the quadrupole-deformed intruder configuration (based on a multi-particle multi-hole configuration) becomes the ground state, subverting the expected shell ordering predicted by a harmonic oscillator plus spin-orbit term. The odd N=21 isotonic chain provides the possibility to study the single-particle and intruder states as a function of decreasing Z. Available spectroscopic evidence points out the appearance of strong branching ratios among the single-particle and collective intruder configurations in 37S [1], suggesting that they mix significantly, contrary to the notion of 37S being well out the island of inversion. However, a precise quantification of this phenomenon in terms of transition strength is still lacking. The first excited state (3/2− state at 646 keV) is the only one with a measured lifetime [2], but no transition probability has been firmly determined for the intruder states, in particular those decaying to the a priori spherical single-particle states. A combined DSAM+RDDS measurement has been performed to measure such transition probabilities, in particular for the 2p-1h 3/2+ state at 1397 keV and the 3p-2h 7/2− at 2023 keV, exploiting the performance of the AGATA spectrometer in terms of energy and angular resolutions. The 37S nucleus has been produced via the 36S(d,p) reaction in inverse kinematics, detecting the recoiling protons in the silicon array SPIDER to obtain an accurate reconstruction of the excitation energy of 37S. The short lifetimes measured point to large M1 and/or E2 strengths connecting the intruder and spherical states. This would imply a significant mixing between the configurations, arising questions about the determination of the neutron p3/2-p1/2 single-particle strength distribution in 37S.

Speaker: Luca Zago (Laboratori Nazionali di Legnaro)

12:20 PM → 1:30 PM Lunch

1:30 PM → 2:55 PM A Mixed Session: Theory & Reactions & Structure

Convener: Alexander Volya (Florida State University)

1:30 PM Exploring the Realm Beyond Exponential Decay in Open Quantum Systems

Recent advancements in quantum mechanics have challenged the classical understanding of decay processes, traditionally encapsulated by the exponential decay law. This presentation, based on our previous work [1], delves into the nonexponential decay regimes in open quantum systems, a domain governed by the continuum. The study illuminates the theoretical predictions and experimental opportunities surrounding deviations from exponential decay, particularly in the context of atomic nuclei, yet extends its relevance to a broad array of many-body open quantum systems including hadrons, atoms, molecules, and nanostructures.

The research introduces novel observables for experimental exploration of the post-exponential decay regime, focusing on the decay of threshold resonances, particle correlations in three-body decays, and interference between near-lying resonances. Through detailed methodological advancements, we shed light on the quantum interference in nonexponential decay and propose promising candidates and scenarios for further experimental verification in the uncharted territory of quantum decay dynamics.

This work is supported by the National Key Research and Development Program (MOST 2022YFA1602303 and 2023YFA1606404); the National Natural Science Foundation of China under Contract No. 12347106 and No. 12147101; the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under award numbers DE-SC0013365 (Michigan State University), DE-SC0009883 (Florida State University), and DE-SC0023175 (NUCLEI SciDAC-5 collaboration).

[1] S. M. Wang, W. Nazarewicz, A. Volya, and Y. G. Ma, Phys. Rev. Research 5, 023183 (2023).

Speaker: Simin Wang (Fudan University)

1:55 PM Deuteron quasi-free scattering reactons: a tool to probe nucleon-nucleon short-range correlatons in atomic nuclei

The experimental evidence points to the existence, at short distances, of strongly correlated neutron-proton pairs much like they are in the deuteron or in free sca0ering processes. As it moves through the nuclear medium, a “bare” nucleon in the presence of the nucleon- nucleon interacton becomes “dressed” in a quasi-deuteron cloud, about 20% of the time [1]. A phenomenological analysis [2] of the quenching of spectroscopic factors and recent data from JLAB [3] points to an isospin dependence of the independent-particle model content in a dressed nucleon. It is expected that this dependence should also be reflected in the dressed amplitude and thus, in the virtual quasi-deuteron content in the ground state. Following from the qualitatve arguments above, quasi-free sca0ering (QFS) of deuterons for which the fast reacton time tR becomes comparable to the time scale of the virtual excitatons, 𝑡!~ℏ/Δ𝐸, could offer a sensitve probe to examine these concepts. In this contributon, we will discuss these ideas within a single-j approximaton and put forward an experimental case that can serve as a template to test the above conjecture, i.e., measuring the (p,pd) QFS cross-secton for knocking out a deuteron in 10,14,16C rela2ve to 12C as an additonal tool to probe short-range correlatons and their isospin dependency.

This work was supported by the Royal Society, UK STFC and the Laboratory Directed Research and Development Program of Oak Ridge Natonal Laboratory, managed by UT-BaTelle, LLC, for the US Department of Energy.

[1] K. Brueckner, in Proceedings of the Rutherford Jubilee Int. Conf. Manchester 1961 (Heywood &Company LTD, London, 1961)
[2] S. Paschalis, M. Petri, A.O. Macchiavelli, O. Hen, and E. Piasetzky, Physics LeTers B 800 (2020) 135110
[3] M. Duer, et al., Nature 560 (2018) 617

Speaker: Augusto Macchiavelli (Oak Ridge National Laboratory)

2:15 PM Evolution of collectivity in even-even platinum nuclei from fast-timing measurements with LaBr3 detectors

The even-even platinum isotopes from A = 190 − 198 are stable and have well-established B(E2,2+1 → 0+1 ) and derived τ(2+1 ) values measured via Coulomb excitation. These tran- sitional isotopes are gamma soft and show some evidence of triaxiality, exhibiting low-lying γ bands built on 2+2 states. In contrast, the mid-shell isotopes exhibit shape-coexistence, having level structures consistent with theoretical models that predict prolate-deformed 0+1 ground states and oblate-deformed excited 0+2 states. The transition from triaxiality to the shape-coexisting picture is complex and measurements of 2+1 state lifetimes would pro- vide valuable insight into the evolution of collectivity. Unfortunately, there is considerable disagreement between the most recent 2+1 lifetime evaluations for the A = 180−186 nuclei from ENSDF (using both e−γ coincidence and recoil-distance methods) and more recent measurements based on a mixture of recoil-distance and γγ-fast timing measurements.
We will report the results of τ(2+1 ) measurements for even-even platinum nuclei that were undertaken at the Heavy Ion Accelerator Facility at the Australian National Uni- versity using LaBr3 detectors and γγ-fast timing methods. We confirm the validity of our methodology by comparison of the new result for τ(2+1 ) in 190Pt with the well- established value from Coulomb excitation. Further measurements under similar con- ditions for 180−190Pt have been made using a range of reactions involving 28,30Si beams on a 156Gd target and 16,18O beams on 172,174,176Yb targets.
A key result is an ≈ 30% increase of τ(2+1 ) for 188Pt as compared to the ENSDF evaluation, while new results for lighter isotopes now resolve existing discrepancies, es- tablish smooth trends with mass number, and are in agreement with other properties such as changes in the average nuclear charge radii determined from laser spectroscopy. The new measurements establish that triaxial behaviour extends down to 188Pt, with a change to nominally prolate ground states (albeit mixed due to prolate-oblate shape co- existence) occurring at 186Pt. We will present new General Collective Model calculations that result from fits to experimental data with 8 free parameters in the Hamiltonian, as well as calculations with the General Bohr Hamiltonian where the potential energy and mass parameters were obtained from the microscopic mean-field produced with both Skyrme SIII/SLy4 interactions and UNEDF0/UNEDF1 density functionals. The energy surfaces from the GCM and GBH approaches show features consistent with experimental observations and the new interpretation of the shape evolution.
This work is supported by Australian Research Council Grant DP210101201 and the In- ternational Technology Center Pacific (ITC-PAC) under Contract No. FA520919PA138. Support for the Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy is acknowledged.

Speaker: Gregory Lane (The Australian National University)

2:35 PM The missing proton emitter – the case of 125Pm

The phenomenon of proton radioactivity, where the atomic nucleus is energetically unstable to the spontaneous emission of a proton is a crucial source of nuclear structure and mass- landscape information at, and beyond, the proton drip line. In addition, in contrast to alpha decay where the formation of an alpha particle close to the nuclear surface must be considered, the pre-existence of protons within the nucleus allows for a cleaner theoretical treatment. This phenomenon has been observed in the most neutron-deficient isotopes of all odd-Z nuclides with atomic numbers between 53 and 83, with promethium (Z=61) being the only exception. Previous searches for proton emission from 125Pm have been unsuccessful likely due to either a smaller than expected cross section or a short half-life for decay. However, recent theoretical calculations of proton emission, utilizing a non-adiabatic quasiparticle model, have indicated that observing the decay of 125Pm should be within the limits of state-of-the- art experimental setups utilizing digital electronics that allow for the analysis of pile-up waveforms with sophisticated analysis techniques. The observation of proton emission from 125Pm will be an important benchmark for both theories of proton decay, given the large, predicted deformation and the important role of the Coriolis interaction, and of mass models. Here, we report on a recent experimental search using the Fragment Mass Analyzer (FMA) at Argonne National Laboratory’s ATLAS facility. Evidence for possible proton emission from 125Pm will be presented and the important role of fine structure in the decay discussed. In addition, perspectives will be presented for future observation of even more exotic nuclear decays.

Speaker: Dan Doherty (University of Surrey)

2:55 PM → 3:25 PM Coffee break

3:25 PM → 4:50 PM Fundamental Symmetries & Precision Measurements

Convener: Jeremy Lantis (Argonne National Laboratory)

3:25 PM Probing the Electroweak Properties of Nuclei with Radioactive Atoms and Molecules

Atoms and molecules containing nuclei with extreme proton-to-neutron ratios can be artificially created to enhance sensitivity to particular nuclear phenomena. Precision laser spectroscopy measurements of atomic species provide access to the ground-state electromagnetic properties of nuclei, which play a critical role in our understanding of nuclear structure.

On the other hand, the electronic structure of certain molecules can be used to isolate the effects of the nuclear electroweak structure, enabling the possibility of measuring yet-to-be-discovered parity and time-reversal violating nuclear properties. In this talk, I will present recent highlights and perspectives from laser spectroscopy experiments on these radioactive species. I will also discuss the relevance of these experiments in addressing open problems in nuclear and particle physics.

Speaker: Ronald Garcia Ruiz (Massachussetts Institute of Tachnology)

3:50 PM Nuclear structure observables as a way to determine ββ decay nuclear matrix elements

Neutrinoless ββ decay is a special nuclear process where a nucleus decays into its isobar with two more protons by only emitting two electrons. This beyond-standard-model decay can establish the nature of neutrinos and shed light into the matter-antimatter asymmetry of the universe [1]. The process, if exists, is sensitive to the structure of the initial, intermediate and final nuclei of the ββ decay [2].
In this talk I will present different ways in which nuclear structure information can be used to learn about the unknown nuclear matrix elements of the neutrinoless ββ decay. First, recent spectroscopy studies of the relevant nuclei, such as 136Cs, allows one to test different shell-model Hamiltonians which predict different values for the nuclear matrix elements [3]. These measurements complement the nucleon removal and addition experiments carried out at Argonne National Laboratory in the past two decades [4].
Second, I will propose nuclear structure observables which in shell-model and other many-body calculations appear to be correlated with ββ decay nuclear matrix elements, such as double Gamow-Teller [5] and double magnetic-dipole transitions [6, 7]. Measure- ments of these nuclear structure observables, which are being pursued by different groups, would provide very valuable insights on neutrinoless ββ decay nuclear matrix elements in nuclear structure experiments.
This work is supported by by MCIN/AEI/10.13039/5011 00011033 from the following grants: PID2020-118758GB-I00, RYC-2017-22781 through the Ram ́on y Cajal program funded by FSE El FSE invierte en tu futuro, CNS2022-135716 funded by the European Union NextGenerationEU/PRTR, and CEX2019-000918-M to the Unit of Excellence Mara de Maeztu 2020-2023 award to the Institute of Cosmos Sciences.

[1] M. Agostini, G. Benato, J. A. Detwiler, J. Men ́endez, and F. Vissani, Rev. Mod. Phys. 95, 025002 (2023).
[2] J. Engel and J. Men ́endez, Rep. Prog. Phys. 80, 046301 (2017).
[3] B. Rebeiro et al., Phys. Rev. Lett. 131, 052501 (2023).
[4] S.J. Freeman, J.P. Schiffer, J. Phys. G 39 124004 (2012).
[5] N. Shimizu, J. Men ́endez, and K. Yako, Phys. Rev. Lett. 120, 142502 (2018). [6] B. Romeo, J. Men ́endez, and C. Pen ̃a Garay, Phys. Lett. B 827, 136965 (2022). [7] L. Jokiniemi and J. Men ́endez, Phys. Rev. C 107, 044316 (2023).

Speaker: Javier Menendez (University of Barcelona)

4:10 PM Charge Radius of 32Si and Symmetry Energy in Nuclear EOS from Difference of Mirror Charge Radii 32Si- 32Ar

The nuclear charge radius of 32Si was first determined [1] from the isotope-shift of hyperfine structure measured at the BECOLA facility at the Facility for Rare Isotope Beams, Michigan State University. A SiO+ molecular beam was produced in the batch mode ion source (BMIS), transported at 30 keV and broken up at BECOLA to produce Si+ for laser spectroscopy. The extracted charge radius provides ideal ground to benchmark ab initio calculations, and are compared to lattice and VS-IMSRG calculations as well as mean-field calculations.

The obtained charge radius of 32Si completes the radii of the mirror pair 32Ar - 32Si, whose difference is correlated [2] to and used to constrain the slope parameter 𝐿𝐿 of the symmetry energy in the nuclear equation of state (EOS) [1]. The present result of the constraint on L is consistent with our previous measurements [3, 4], the lattice ab inito calculation and the analysis of gravitational wave form from the binary neutron star merger. However, it shows systematic shift from the PREX result. The details of experiment and results will be discussed.

This work was supported in part by the National Science Foundation under Grant No. PHY-21- 11185 and PHY-21-10365, and the U.S. Department of Energy under the grants DE-SC0021176, DE-SC0021152, DE-SC0013365, DE-SC0023658, SciDAC-5 NUCLEI Collaboration.

Speaker: Kei Minamisono (Facility for Rare Isotope Beams)

4:30 PM Precision studies of radioactive molecules for nuclear science

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 electroweak structure 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.

This contribution will present recent highlights from studying radioactive molecules of particular interest for fundamental symmetries studies at ISOLDE-CERN [Udr24]. A highlight of which includes the first observation of the distribution of nuclear magnetization in the structure of a molecule [Wil24]. An outlook will be given on the range of opportunities for nuclear structure studies available studying radioactive molecules at the Facility for Rare Isotope Beams.

[Arr23] Arrowsmith-Kron, G. et al., Rep. Prog. Phys Accepted (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. 20 202-207 (2024)
[Wil24] Wilkins, S. et al., arXiv 2311 04121 (2024) [Yan23] Yang, X. et al., PPNP 129, 104005 (2023)

Speaker: Shane Wilkins (Massachusetts Institute of Technology)

6:00 PM → 10:00 PM Conference Dinner

Friday, July 26

8:30 AM → 9:00 AM Registration, Coffee & Snacks

9:00 AM → 10:25 AM Heavy Nuclei and Super Heavy Elements - Part 2

Convener: Rodney Orford (Lawrence Berkeley National Laboratory)

9:00 AM Exploring the structure of the heaviest nuclei through laser spectroscopy and mass spectrometry

Exploring the limits of nuclear existence is at the forefront of contemporary nuclear physics. At the GSI in Darmstadt, we have explored the limit in the region of superheavy nuclei for more than 50 years resulting in the discovery of six new elements, for example. Recently, the program has been expanded towards a comprehensive investigation of the atomic, nuclear, and chemical properties of the heaviest elements. Pioneering experiments in Penning-trap mass spectrometry and resonance ionization laser spectroscopy have provided a wealth of new data for the elements from fermium to dubnium. Accurate mass measurements allowed us to investigate the nuclear shell structure evolution in the region of the N=152 deformed shell gap. The high mass resolving power of SHIPTRAP also enabled studies on longer-lived nuclear isomers with low excitation energy in nobelium, lawrencium, and rutherfordium isotopes. Furthermore, changes in nuclear charge radii in a long chain of fermium isotopes were inferred from isotope-shift measurements by laser spectroscopy, while hyperfine laser spectroscopy provided information on electromagnetic moments in nobelium isotopes. I will present select highlights of the recent measurement campaigns at the GSI in Darmstadt, Germany, using the SHIPTRAP, RADRIS, and JetRIS setups and discuss future perspectives.

Speaker: Michael Block (GSI Helmholtzzentrum für Schwerionenforschung)

9:25 AM Recent results obtained with the Argonne Gas-Filled Analyzer: observation of the ground-state rotational band in the highly fissile nucleus 250No

The island of deformed nuclei around the Z=100, N=152 deformed shell gaps serves a stringent testing ground for nuclear models aiming to describe super-heavy nuclei. In fact, without the presence of shell corrections, these nuclei would undergo instantaneous fission. Nuclei in this region have been extensively studied using decay and in-beam spectroscopic methods. During the presentation, recent experiments conducted with the Argonne Gas-Filled Analyzer (AGFA) both in stand-alone mode and coupled to the Gammasphere -ray detector array will be reviewed. Notably, the talk will cover the first observation of the ground-state rotational band in the highly fissile nucleus 250No, which fissions rapidly with a half-life of only 4 s. This nucleus presents a unique opportunity to investigate the competition between -ray decay and fission processes. Additionally, the results of a search for short-lived K-isomers in proton-rich Lr isotopes will be presented. Finally, plans for experimental program with AGFA will be outlined.

This material is based upon work supported by the U.S Department of Energy, Office of Science, Office of Nuclear Physics, under contract number DE-AC02-06CH11357. This research used resources of ANL's ATLAS facility, which is a DOE Office of Science User Facility.

Speaker: Darek Seweryniak (Argonne National Laboratory)

9:45 AM New half-lives measurements for r-process in A∼225 Po-Fr nuclei

The astrophysical rapid-neutron capture process (r-process) of explosive nucleosynthesis is re- sponsible for the formation of half of the heavy nuclei above Fe [1]. Actinides are produced towards the end of this process, when the neutron flux is expected to be minimal, and it is supported also by fission processes. Given that the r-process path runs far away from the accessible species, in this heavy region of the chart of nuclides, experimental inputs on β decay for nuclei beyond N=126 are particularly useful to test predictions of global nuclear models.
In this paper results from a recent experiment performed at GSI-FAIR (Darmstadt, Germany) within the HISPEC-DESPEC experimental campaign, as part of the FAIR Phase-0 program, will be discussed. The experiment populated 220<A<230 Po-Fr nuclei in a relativistic fragmentation reaction induced by a 1 GeV 238U beam. The species were selected and identified using the FRagment Separator (FRS) and implanted in the DEcay SPECtroscopy (DESPEC) station [2] to study their subsequent β decay. The DESPEC station is composed of a stack of Double Sided Silicon-Strip Detectors (DSSD) sandwiched between two plastic scintillator detectors, surrounded by a hybrid γ-detection array consisting of high-resolution HPGe and fast timing LaBr3(Ce).
The extracted β-decay half-lives are discussed with the help of recent theoretical models to assess the impact of the measured values in the predictions of the r-process. Perspectives of future measurements in the region will be provided.

[1] M.R. Mumpower et al., Prog. Part. Nucl. Phys. 86 (2016) 86;
[2] A. K. Mistry et al., Nucl. Instrum. Methods Phys. Res. A 1033 (2022) 166662

Speaker: Marta Polettini (University of Padova and INFN Padova)

10:05 AM Nuclear resonance fluorescence of 242Pu

The electromagnetic dipole response of 242Pu was studied for the first time using the nuclear resonance fluorescence (NRF) method, hence with real photons. The experiment was performed at TU Darmstadt, where monoenergetic electrons are provided by the superconducting Darmstadt linear electron accelerator S-DALINAC to produce bremsstrahlung by impinging on a gold radiator target. A sample of PuO2 with a total mass of about 1 g was irradiated by a bremsstrahlung beam, having a continuous energy distribution up to 3.7 MeV. Resonantly scattered photons were detected with two high-purity Germanium detectors at angles of 90◦ and 130◦ relative to the direction of the incident photon beam, which allows us to distinguish between dipole and quadrupole transitions based on their different angular distributions. The highly-enriched 242Pu target was placed in a special con- tainer taking into account the sample’s total radioactivity of about 370 MBq. To identify the NRF signals originating from the target, NRF spectra of an empty target container, γ-ray spectra of the sample’s radioactivity and background measurements were compared. Evidence for decays of photo- excited states of 242Pu was found – making 242Pu the heaviest nuclide for which NRF data is available for the moment. The experiment was the first step towards characterizing dipole excitation modes of transuranium actinides by means of real photons. Details of the experiment, γ-ray spectra and preliminary results will be presented.
We thank the Institute of Resource Ecology of HZDR for providing the 242Pu sample. This work was supported by the State of Hesse within the LOEWE program and by the Deutsche Forschungsge- meinschaft (DFG, German Research Foundation) under project-ID 499256822 – GRK 2891 ”Nuclear Photonics”.

Speaker: Maike Beuschlein (Technische Universita ̈t Darmstadt)

10:25 AM → 10:55 AM Coffee break

10:55 AM → 12:40 PM Decay Spectroscopy

Convener: Gordon Ball (TRIUMF)

10:55 AM Recent highlights from the GRIFFIN spectrometer

Large arrays of gamma-ray detectors coupled with auxiliary detection systems provide a powerful and versatile tool for studying exotic nuclei through nuclear spectroscopy at radioactive ion beam facilities. GRIFFIN (Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei) is a state-of-the-art facility dedicated to beta decay spectroscopy with rare-isotope beams, situated at the TRIUMF-ISAC-I facility in Vancouver, Canada [1]. GRIFFIN is composed of 16 Compton-Suppressed HPGe clover detectors complemented by a powerful suite of ancillary detector sub-systems that includes plastic-scintillators for beta tagging, LN2-cooled Si(Li) detectors for conversion electron measurements and an array of eight LaBr3(Ce) scintillators for fast-timing measurements. The spectrometer supports a variety of research in the areas of nuclear structure, nuclear astrophysics, and fundamental symmetries. Recent experiments using GRIFFIN will be discussed including: transition strengths in 50Sc from lifetime measurements, using the half life of 34Ar to guide the choice of theoretical radiative and isospin symmetry breaking corrections for Fermi super allowed beta emitters, probing cross-shell excitations on the border of the island of inversion using gamma-gamma angular correlations in 34Si, and the nature of quasiparticle configurations in 160Gd are explored using the high-statistics beta decay of 160Eu.

[1] A.B. Garnsworthy et al. ,NIM. A 918, 9 (2019)

Speaker: Victoria Vedia (TRIUMF)

11:20 AM Shape transition in neutron-rich A~190 nuclei via isomer decays in fragmentation reactions

The 180<A<190 Hf-Ta-W region near the valley of stability display robust axially symmetric prolate deformation and associated high-K isomerism. Mapping the evolution of shapes in approaching the Z=82 and N=126 shell closure from the very deformed rare-earth mid-shell region is of great interest for honing nuclear structure models, with a loss in axial symmetry and a transition from prolate to oblate shape expected. The very neutron-rich nuclei in this region are far less explored experimentally as they cannot be accessed via fusion-evaporation or transfer reactions, and long-standing predictions of shape transitions remain untested. Different theoretical models predict different shape evolution characteristics. Firm experimental evidence is needed to refine and tune the predictions on these exotic systems.

A fragmentation reaction was utilized for the first time to access these nuclei, using a newly developed 198Pt primary beam at NSCL incident on Ni and Be targets, populating isotopes in their isomeric states in the region of interest. These isotopes were implanted in a stack of Silicon detectors surrounded by the GRETINA array to detect delayed gamma rays correlated to their respective isomeric decay. Isotope identification was achieved with ΔE-B𝜌-TKE-ToF information recorded on an event-by event basis for each implant. A range of isomers were populated in 72<Z<77 nuclei, with half-lives ranging from a few hundred ns to few hundred μs and many of them observed for the first time. These provide first spectroscopic data on high-spin excitations in this previously inaccessible region of the nuclear chart, with detailed level schemes deduced in some cases with the available 𝛾-𝛾 statistics. The results of this experiment and analysis will be presented, with special emphasis on the detailed level structure of a N=116 nucleus, which provide a critical experimental test to model predictions in this very neutron-rich region of the nuclear chart.

This work is supported by the U.S. Department of Energy and the National Science Foundation.

Speaker: Partha Chowdhury (University of Massachusetts Lowell)

11:40 AM Measurements of β-delayed neutron emission probabilities around the medium-mass mid to the doubly-closed shell region

Beta-delayed neutron emission (βn) is the prevalent decay mode in very neutron-rich nu- clei that are involved in the nucleosynthesis via rapid (r-) neutron capture process [1]. Characterized as a two-step decay mechanism, it involves β-decay feeding into excited states beyond the neutron separation energy in the daughter nucleus, succeeded by neu- tron emission, resulting in a lower-mass nucleus. The probabilities of βn (Pxnvalues) are intricately tied to factors such as Qβn windows, decay strength distribution, de-excitation, and neutron emission models [2], emphasizing the critical need for precise experimental data on βn emitters. Such data not only deepen our understanding and modeling of βn processes but also serve as vital inputs for nucleosynthesis models. Yet, a dearth of reliable experimental information persists, particularly regarding multi-neutron emission channels process [3]. In this contribution, we will present recent experimental results on the Pxnvalues, with a focus on the neutron-rich mid-shell region around mass A=110 and around doubly-closed shell region at 132Sn [4]. The experiments were conducted within the BRIKEN project at RIKEN RIBF, an extensive survey program of the Pxn values. The systematic of the results shows discernible odd-even and shell effects, underscoring the sensitivity of Pxn values to nuclear structure information and their important role in benchmarking theoretical models. Additionally, we will introduce a new experimen- tal program at RIKEN’s RIBF, employing neutron time-of-flight and γ-ray spectroscopy setups in tandem with MRTOF mass measurements, aimed at further elucidating the βn.
This work is supported by the U.S. Department of Energy, National Science Foundation, JSPS, Spanish Ministerio de Economia y Competitividad, UK Science and Technology Facilities Council, National Research Foundation (NRF) in South Korea, Polish National Science Center, Natural Sciences and Engineering Research Council of Canada (NSERC).

Speaker: Vi Ho Phong (RIKEN Nishina Center)

12:00 PM Beta-decay study of the shape coexistence in 98Zr

Anomalies in the systematics of nuclear properties challenge our understanding of the underlying nuclear structure. One such anomaly emerges in the Zr isotopic chain as a dramatic ground- state shape change, abruptly shifting from spherical into a deformed one at N=60. Only a few state-of-the-art theoretical models have successfully reproduced this deformation onset in 100Zr and helped to establish the shape coexistence in lighter Zr isotopes [1, 2]. Of particular interest is 98Zr, a transitional nucleus lying on the interface between spherical and deformed phases. Exten- sive experimental and theoretical research efforts have been made to study the shape coexistence phenomena in this isotope [3,4,5]. Although they provide an over-all understanding of 98Zr’s nu- clear structure, uncertainties remain in interpreting its higher-lying bands. Specifically, two recent studies utilizing Monte Carlo Shell Model (MCSM) [6] and Interacting Boson Model with config- uration mixing (IBM-CM) [7] calculations have presented conflicting interpretations. The MCSM predicts multiple shape coexistence with deformed band structures, whereas the IBM-CM favours a multiphonon-like structures with configuration mixing.
To address these uncertainties, a β-decay experiment was conducted at TRIUMF-ISAC facility utilizing the 8π spectrometer with β-particle detectors. The high-quality and high-statistics data obtained enabled the determination of branching ratios for weak transitions, which are crucial for assigning band structures. In particular, the key 155-keV 2+2 → 0+3 transition was observed, and its branching ratio measured, permitting the B(E2) value to be determined. Additionally, γ-γ angular correlation measurements enabled the determination of both spin assignments and mixing ratios. As a result, the 0+, 2+, and I = 1 natures for multiple newly observed and previously known (but not firmly assigned) states have been established. The new results revealed the collective character of certain key transitions, supporting the multiple shape coexistence interpretation provided by the MCSM framework. These results will be presented and discussed in relation to both MCSM and IBM-CM calculations.

[1] T. Togashi, Y. Tsunoda, T. Otsuka, and N. Shimizu, Phys. Rev. Lett. 117, 172502 (2016). [2] N. Gavrielov, A. Leviatan and F. Iachello, Phys. Rev. C 105, 014305 (2022).
[3] K. Heyde and John L. Wood, Rev. Mod. Phys. 83, 1467 (2011).
[4] T. Kibedi, A.B. Garnsworthy, J.L. Wood, Prog. Part. Nucl. Phys. 123, 103930 (2022).
[5] P. E. Garrett, M. Zielinska, E. ClĂŠment, Prog. Part. Nucl. Phys. 124, 103931 (2022).
[6] P. Singh, W. Korten et al., Phys. Rev. Lett. 121, 192501 (2018).
[7] V. Karayonchev, J. Jolie et al., Phys. Rev. C 102, 064314 (2020).

Speaker: Desislava Kalaydjieva (University of Guelph)

12:20 PM Hand off to hosts of Nuclear Structure 2026