Nuclear Structure 2022

13 Jun 2022, 08:00 → 17 Jun 2022, 18:00 US/Pacific

Berkeley, CA

Heather Crawford

The Nuclear Structure 2022 conference was held onsite at Lawrence Berkeley National Laboratory in June 2022.

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Nuclear Structure 2022 (NS2022) was the 18th in a biennial series of nuclear structure conferences organized by North American national laboratories. Previous meetings in this series have been held in East Lansing (NSCL), Knoxville (ORNL), Vancouver (TRIUMF), and Argonne (ANL). These conferences are devoted to recent experimental and theoretical developments in the research on nuclei near the limits of isospin, spin and excitation energy.  Nuclear Structure 2020 was canceled due to the COVID-19 pandemic.

Nuclear Structure 2022 was held onsite at LBNL in Berkeley, CA, hosted by Lawrence Berkeley National Laboratory, with an opening reception Sunday, June 12th, and a scientific program running Monday, June 13th through Friday, June 17th, 2022.  We thank the sponsors, organizers and all participants for making this a great conference!

See everyone in 2024 for the next Nuclear Structure conference to be organized by Argonne National Laboratory!

FirstCircular2022.pdf

SecondCircular2022-AbstractOpen.pdf

ThirdCircular2022-RegistrationOpen.pdf

Monday, 13 June

09:00 → 10:30 NS2022 Plenary: Light Nuclei I

Convener: Carl Svensson (Department of Physics, University of Guelph, N1G 2W1 Guelph, Canada)

09:00 Welcome

09:10 A four-neutron system probed through sudden removal of alpha particle from 8He

The search for chargeless nuclei consisting only of neutrons has been a long-lasting challenge in nuclear physics, dating about 50 years back. The tetraneutron, in particular, has attracted a lot of experimental and theoretical attention. Theoretical models agree that nuclear forces do not bind four neutrons together, but struggle to handle the resonance case. On the other hand, no solid experimental information on the tetraneutron ground-state resonance is available as experiments suffer from low statistics and/or large background. The possibility of the tetraneutron forming a resonance state is still an open and fascinating question, which can now be probed theoretically with state-of-the-art ab initio calculations and studied experimentally by employing new techniques in the upgraded, high-intensity, radioactive-ion beam facilities. In this talk I will present results from a novel experiment performed at the SAMURAI setup in RIKEN, Japan, which probed the correlation energy between the four remaining neutrons after the quasi-elastic removal of alpha cluster from ⁸He projectiles.

Speaker: Dr Stefanos Paschalis (University of York)

09:40 Proton inelastic scattering reveals deformation in $^{8}$He

The nucleus $^8$He is the most neutron-rich nucleus known. Its structure, consisting of a $^4$He core surrounded by four neutrons makes it an ideal case to study phenomena in highly neutron-proton asymmetric systems and neutron correlations at the nuclear surface.

An experiment studying proton inelastic scattering of $^8$He has been carried out at the IRIS setup at ISAC-II at TRIUMF. It utilized the novel IRIS solid H$_2$ target in combination with a low pressure ionization chamber for the identification of incoming beam particles and two ΔE-E telescopes to measure the reaction products.

The measurement shows a resonance at 3.54(6) MeV with a width of 0.89(11) MeV. The energy of the state is in good agreement with both coupled cluster and no-core shell model with continuum calculations. The latter describes the measured resonance width as well.

The differential cross section of the resonance has been analyzed with phenomenological collective excitation form factor and microscopic coupled reaction channels framework. Both analyses reveal a large deformation parameter β$_2$=0.40(3), consistent with no-core shell model predictions of a large
neutron deformation.

Speaker: Matthias Holl (Chalmers)

AM1-2-Holl.pdf

10:10 The structure of 7,8,9He in the rotational model*

Inspired by the recent results of Ref. [1] showing strong evidence for a deformed 8He nucleus, we present a study of the structure of the odd-A 7He and 9He isotopes in the rotational model. While the ab initio calculations predict an oblate shape, in this work we consider two cases corresponding to an oblate and a prolate core with deformation |ε2| ≈ 0.38 as inferred in [1].

A comparison of the experimental moment of inertia of 8He, derived from the experimental 2+ energy, is in good agreement with the estimates from the Migdal formula [2], with the proton and neutron radii adjusted to reproduced experimental RMS charge and matter radii. At the adopted deformation, the relevant neutron Nilsson levels arising from the p and sd spherical shells are:

• 7He: [101] 3/2, [110] 1/2 on the prolate and oblate side respectively, and
• 9He: [101] 1/2 , [220] 1/2 on the prolate and [220]1/2 and [202] 5/2 on the oblate side.

Particle plus Rotor Model calculations for both prolate and oblate configurations will be discussed and compared to available experimental data [3,4]. We will present predictions for electromagnetic properties and spectroscopic factors for the 8He(p,d)7He and 8He(d,p)9He reactions, which may stimulate further studies of these exotic nuclei. We also speculate on the structure of 7H, seen as a proton-hole in the 8He deformed core.

The rotational model offers an appealing and intuitive framework that appears to capture the physics at play in the low-lying structure of 7,8,9He and is complementary to shell-model and
ab initio approaches.

• This work is based on the research supported in by the Director, Office of Science, Office of Nuclear Physics, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (LBNL) and Award No. DE-FG02-95ER40934 (Notre Dame).

[1] M. Holl, R. Kanungo, Z.H. Sun, G. Hagen, J.A. Lay, et al., Phys. Lett. B822, 136710(2021).
[2] B. Migdal, Nucl. Phys. 13, 655 (1959).
[3] ENSDF: Evaluated Nuclear Structure Data File. https://www.nndc.bnl.gov/ensdf/
[4] XUNDL: Experimental Unevaluated Nuclear Data List. https://www.nndc.bnl.gov/ensdf/ensdf/xundl.jsp

Speaker: Augusto Macchiavelli (Lawrence Berkeley National Laboratory)

10:30 → 11:00 AM Break

11:00 → 12:30 NS2022 Plenary: Heavy Nuclei I

Convener: Jacklyn Gates (LBL)

11:00 Recent results on nobelium isotopes spectroscopy @ SHELS

The very/super-heavy nuclei area is a unique laboratory in the nuclear chart for the fundamental study of the atomic nucleus since excitation and decay modes are governed by the competition between the short-range strong nuclear interaction, long-range Coulomb repulsion, surface effects and the properties of individual quasiparticle states. For such studies, a wide scientific program has been launched at the FLNR Dubna laboratory with the emergence of new experimental setups such as the SHELS separator [1] and its focal plane detection system, GABRIELA [2] . Thanks to the high α, γ and ICE efficiency detection, some really new results on nobelium isotopes will be presented. Namely, the first γ and ICE spectroscopy of the $^{256}$No nucleus [3] and the revisiting of the $^{254}$No level scheme with an indication of a possible shape coexistence/superdeformed state.

[1] Popeko, A. G. et al. Separator for Heavy ELement Spectroscopy - velocity filter SHELS. Nucl. Instrum. Methods Phys. Res. B 376, 140-143, doi:10.1016/j.nimb.2016.03.045 (2016).
[2] Hauschild, K. et al. GABRIELA: A new detector array for gamma-ray and conversion electron spectroscopy of transfermium elements. Nucl. Instrum. Methods 560, 388-394 (2006).
[3] Kessaci, K. et al. Evidence of high-$K$ isomerism in $_{102}^{256}$No$_{154}$. Phys. Rev. C 104, 044609, doi:10.1103/PhysRevC.104.044609 (2021).

Speaker: Olivier Dorvaux (Universite de Strasbourg, CNRS/IPHC)

11:30 Spectroscopy of Trans-fermium Nuclei Using the Argonne Gas-Filled Analyzer

Spectroscopy of Trans-fermium Nuclei Using the Argonne Gas-Filled Analyzer

D. Seweryniak,$^1$ T. Huang,$^{1,2}$ K. Auranen,$^{1,11}$ A.D. Ayangeakaa,$^{1,12}$ B.B. Back,$^1$ P. Bender,$^3$
M.P. Carpenter,$^1$ P. Chowdhury,$^3$ R.M. Clark,$^4$ P. Copp,$^1$ Z. Favier,$^{10}$ K. Hauschild,$^8$ X.-T. He,$^5$
R.D. Herzberg,$^6$ D. Ho,$^3$ H. Jayatissa,$^1$ T.L. Khoo,$^1$ F.G. Kondev,$^1$ G. Morgan,$^7$ C. Morse,$^4$
A. Korichi,$^8$ T. Lauritsen,$^1$ J. Li,$^1$ C. Mueller-Gatermann,$^1$ D. Potterveld,$^1$ W. Reviol,$^1$ A. Rogers,$^3$ S. Saha,$^3$ G. Savard,$^1$ K. Sharma,$^3$ S. Stolze,$^1$ S. Waniganeththi,$^3$ G. Wilson,$^1$ J. Wu,$^1$ Y.-F. Xu,$^5$,
S. Zhu$^9$

$^1$Argonne National Laboratory, Argonne, IL 60439, USA
$^2$Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
$^3$University of Massachusetts, Lowell, MA 01854, USA
$^4$Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
$^5$College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
$^6$University of Liverpool, Liverpool, UK
$^7$Louisiana State University, Baton Rouge, LA 70803, USA
$^8$CSNSM, IN2P3-CNRS Orsay, France
$^9$Brookhaven National Laboratory, Upton, NY 11973, USA
$^{10}$CEA, Département de Physique Nucléaire, Université Paris-Saclay, 91191, Gif-sur-Yvette, France
$^{11}$Accelerator Laboratory, Department of Physics, University of Jyväskylä, FI-40014 Jyväskylä, Finland
$^{12}$Triangle Universities Nuclear Laboratory, Duke University, Durham, North Carolina 27708, USA

Deformed nuclei near the Z=100, N=152 deformed shell gaps are a stringent testing ground for nuclear models which are used to describe the heaviest known nuclei. Nuclei in this region have been studied using in-beam, K-isomer, $\alpha$-decay and spontaneous fission spectroscopic methods. To extend these studies to odd-A, odd-odd, and to heavier nuclei the Argonne Gas-Filled Analyzer (AGFA) was constructed. During the talk, recent decay and isomer spectroscopy experiments with AGFA in stand-alone mode and in-beam spectroscopy experiments with AGFA coupled to the Gammasphere $\gamma$-ray detector array will be reviewed. Among others, the observation of the ground-state rotational band in the fissile nucleus $^{254}$Rf and the discovery of the new isotope $^{251}$Lr will be discussed. The moment of inertia deduced for $^{254}$Rf shades light on the shape evolution in this region while the observed $\alpha$-decay fine structure in $^{251}$Lr provides information on single-proton orbitals near the Fermi surface. Plans for experimental program with AGFA will be also presented.

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: D. Seweryniak (Physics Division, Argonne National Laboratory, Argonne, IL 60439)

11:50 In-beam and decay spectroscopy of $^{251}$Md

Experiments on the heaviest nuclei are addressing the fundamental issue of the maximum limits of nuclear mass and charge. The difficulty of producing such exotic nuclei, however, creates an inherent challenge for elucidating the detailed structure of these extreme nuclear systems. By contrast, nuclei in the vicinity of $Z=100$ and $N=152$ may be studied in much greater detail, and yet they involve intrinsic excitations of the same single-particle orbits that lie close to the Fermi surface of the spherical superheavy nuclei~[1]. In particular, the structure of the low-lying energy levels in odd-mass transfermium nuclei is dominated by the unpaired particle, which gives direct information about the orbitals involved. The odd-$Z$ nucleus $^{251}$Md has recently been the subject of considerable interest in this regard, with the observation of states based on the $\frac{1}{2}^-$[521] Nilsson level~[2], the $\frac{7}{2}^-$[514] Nilsson level~[3], as well as a high-$K$ isomer~[4]. These experiments are expanding our view of not only the transfermium region, but the superheavy nuclei as well.

We have performed a new experiment to study $^{251}$Md via both in-beam and decay spectroscopy. The $^{205}$Tl($^{48}$Ca,2n) fusion-evaporation reaction was used to populate excited states in $^{251}$Md. The newly commissioned Argonne Gas-Filled Analyzer (AGFA) was used to separate the unreacted beam and the $^{251}$Md recoils, which were implanted into a $160\times160$ pixel silicon double-sided strip detector (DSSD) at the focal plane of AGFA. Prompt $\gamma$ rays were detected with Gammasphere and correlated with $^{251}$Md based on their time-of-flight and implantation energy in the AGFA focal plane. Long-lived isomers were detected by the observation of bursts of conversion electrons in the same DSSD pixel as an implantation event, and delayed $\gamma$ rays emitted after isomer-decay events were detected with the X-array, which was arranged around the DSSD. With this setup, we have observed many new transitions in both the prompt and the delayed $\gamma$ spectra, including transitions above the isomeric state. These results and their implications for the structure of $^{251}$Md will be presented.

[1] R.-D.~Herzberg \textit{et al.}, Nature 442, 896 (2006)\
[2] A.~Chatillon \textit{et al.}, Phys. Rev. Lett. 98, 132503 (2007)\
[3] R.~Briselet \textit{et al.}, Phys. Rev. C 102, 014307 (2020)\
[4] T.~Goigoux \textit{et al.}, Eur. Phys. J. A 57, 321 (2021)\

Speaker: C. Morse (National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973)

Morse_NS22.pdf

12:10 Laser Spectroscopy of the Heaviest Actinides

The interaction between electrons and the atomic nucleus introduces an avenue for laser spectroscopy measurements to probe changes in the nuclear structure across an isotopic chain, providing a nuclear model-independent method to determine fundamental properties such as the spin, the mean-squared charge radius, and nuclear moments. Laser spectroscopic techniques have been historically successful for wide swaths of the chart of the nuclides, but only recently has the heavy actinide region become accessible through the RAdioactive decay Detected Resonance Ionization Spectroscopy (RADRIS) technique [1,2] used at the Separator for Heavy Ion Products (SHIP) at GSI, Darmstadt, Germany. Experimental access to actinides has opened a window to unusual features in the nuclear structure. Stretching from the $N$ = 126 to the $N = 152$ shell closures, the actinides feature both oblate and prolate nuclei, the presence of $K$-isomers, and rotational bands that are all hallmarks of deformation. Additionally, the large Coulomb repulsion in the heavier actinides results in a central depression in the nuclear density and has been theorized to potentially result in bubble nuclei [3]. The RADRIS technique was used to identify the first atomic transition [4] as well as to determine the nuclear charge radii and nuclear moments of $^{252-254}$No [5]. New technological developments have extended the range of lifetimes accessible by the RADRIS technique and permitted measurement of $^{248,249,250,254}$Fm, crossing the shell closure at $N$ = 152, with additional measurements of $^{255,257}$Fm performed offline with reactor bred samples from Oak Ridge National Laboratory, Oak Ridge, USA, and extended the isotopic chain of nobelium to $^{255}$No. The results of these measurements, which have posed a challenge for existing theoretical models, will be discussed. The development of a new laser spectroscopic technique for use at GSI will be discussed, where nuclear reaction products are measured in a hypersonic gas jet [6,7], retaining the high sensitivity of the RADRIS technique but improving the achievable spectral resolution by an order of magnitude in addition to providing access to short-lived species such as the 275 ms $K$ = 8$^-$ isomer in $^{254}$No.

[1] H. Backe et al., Eur. Phys. J. D 45 (2007) 99-106
[2] F. Lautenschläger et al., Nucl. Instrum. Meth. B 383 (2016) 115-122
[3] J. Dechargé et al., Nucl. Phys. A 716 (2003) 55-86
[4] M. Laatiaoui et al., Nature 538 (2016) 495-498
[5] S. Raeder et al., Phys. Rev. Lett 120 (2018) 232503
[6] R. Ferrer et al., Nat. Commun. 8 (2017) 14520
[7] S. Raeder et al., Nucl. Instrum. Methods. Phys. Res. B 463 (2020) 272-276

Speaker: Jeremy Lantis (Johannes Gutenberg University Mainz)

12:30 → 13:30 Lunch

13:30 → 15:00 NS2022 Plenary: Shapes and Collectivity I

Convener: Marco Siciliano (ANL)

13:30 From shape coexistence to shape isomers in atomic nuclei

Shape coexistence occurs when the potential energy of the nucleus is characterized by local minima for different shapes. Excited states in the secondary minimum may become a isomeric if the potential barrier separating the secondary minimum from the ground-state minimum is sufficiently pronounced. The first examples of such shape isomers were observed in the 1960s, as fission isomers in the actinides. They are located in a secondary minimum at very large deformation and excitation energies of several MeV. Fission is the predominant decay mode due to a strong potential barrier for gamma decay to the ground state.

In even-even nuclei, shape isomers occur as an excited 0$^+$ state residing in a secondary minimum as a false ground state. If located at high excitation energy (above 1-2 MeV) these states will usually not show up as long-lived states, although their reduced decay strength may still be considerably reduced. First excited 0$^+$ states at low energy (below $\sim$1 MeV) are very rare, and only known in a handful of stable nuclei. Research has been going on for several decades to find other examples in exotic nuclei far from nuclear stability.

In my presentation I will concentrate on the research of such low-lying shape isomers, with an emphasis on even-even mass nuclei, where the particularity of 0$^+$ $\rightarrow$ 0$^+$ E0 transitions to the nuclear ground state can be exploited. I will present first results from an experiment at the Experimental Storage Ring (ESR) at GSI to search for the nuclear two-photon decay of such isomers in swift fully-stripped heavy ions.

Speaker: Wolfram KORTEN (CEA Saclay)

NS2022_Korten.pptx

13:50 New developments of shell models for heavy deformed nuclei

Performing shell model calculations for heavy, deformed nuclei has been a challenging problem in nuclear physics. State-of-the-art many-body techniques, such as angular momentum projection (AMP) [1] and generator coordinate method (GCM) [2], are required. In this talk, we present two new developments of shell models with their potential applications. Both developments are benefited by the Pfaffian method proposed by Robledo [3] which helps to solve unambiguously the sign problem in the overlap of two HFB wavefunctions, making the GCM calculation possible for general triaxially-deformed cases.

In a collaboration with Japanese theorists, we have proposed the projected Hartree-Fock-Bogolyubov plus generator coordinate method (HFB+GCM). It starts from the variational calculation to minimize the particle number-projected energy of the HFB wave function with the effective Hamiltonian. The number-projected wave functions for a set of quadrupole moments Q0 and Q2 serve as generator coordinates in the GCM method. Total AMP is then applied, and energies and wavefunctions are finally obtained by solving the Hill-Wheeler equation [4]. Using this model, the sudden emergence of extremely large B(E2) in the N=Z mass-80 region is understood as a triple enhancement of quasi-SU(3) quadrupole collectivity [5].

In the effort to describe complete sets of nuclear spectra and to provide necessary nuclear input for astrophysical models and simulations, we have made progress in the extension of the projected shell model (PSM). It starts from a deformed basis, with the configuration space expanded to include up to 10-qp states for both positive and negative parities [6,7]. AMP technique is then applied to transform the states from the intrinsic to the laboratory one, and many-body wave functions are constructed by superposition of the projected multi-qp states. Thousands of levels including both normal and K-isomeric states with definite spin-parity can be obtained, and electromagnetic transition as well as Gamow-Teller -decay and electron-capture rates [8] can be calculated. Potential applications are for the study of excited nuclear states in astrophysical environments [9] and nuclear level density and chaoticity of nuclear systems [10,11].

Work is collaborated with K. Kaneko, N. Shimizu, T. Mizusaki, G. W. Misch, S. K. Ghorui, S. Garg, S. Dutta, L.-J. Wang, F.-Q. Chen, and supported by NNSF of China under contract No. U1932206.

[1] Y. Sun, Phys. Scr. 91 (2016) 043005.
[2] F.-Q. Chen, Y. Sun, P. Ring, Phys. Rev. C 88 (2013) 014315.
[3] L. M. Robledo, Phys. Rev. C 79 (2009) 021302.
[4] K. Kaneko, N. Shimizu, T. Mizusaki, Y. Sun, Phys. Lett. B 817 (2021) 136286.
[5] K. Kaneko, N. Shimizu, T. Mizusaki, Y. Sun, Phys. Rev. C 103 (2021) L021301.
[6] L.-J. Wang, F.-Q. Chen, T. Mizusaki, M. Oi, Y. Sun, Phys. Rev. C 90 (2014) 011303.
[7] L.-J. Wang, Y. Sun, T. Mizusaki, M. Oi, S. K. Ghorui, Phys. Rev. C 93 (2016) 034322.
[8] L. Tan, Y.-X. Liu, L. J. Wang, Z.- P. Li, Y. Sun, Phys. Lett. B 805 (2020) 135432.
[9] L.-J. Wang, L. Tan, Z.-P. Li, G. W. Misch, Y. Sun, Phys. Rev. Lett. 127 (2021) 172702.
[10] L.-J. Wang, J.-M. Dong, F.-Q. Chen, Y. Sun, J. Phys. G 46 (2019) 105102.
[11] L. J. Wang, F.-Q. Chen, Y. Sun, Phys. Lett. B 808 (2020) 135676.

Speaker: Yang Sun (Shanghai Jiao Tong University)

14:10 Level lifetime measurements in neutron-rich Zr and Mo isotopes around A = 110 through in-beam gamma-ray spectroscopy

Isotopes of zirconium (Zr) with semi-magic atomic number $40$ represent one of the most interesting cases of shape evolution in nuclei. The collective behavior of Zr nuclides is very much suppressed at neutron number $50$, where $^{90}$Zr exhibits properties of a doubly-magic nucleus. On the other hand, a sudden onset of nuclear deformation appears at $N = 40$ and $60$ due to the strong proton-neutron interaction between the overlapping partner $\pi 1g_{9/2}$ and $\nu 1g_{9/2}$ ($\nu 1g_{7/2}$) intruder orbitals. The strong shape transition at $N = 60$ happens in Zr ($Z = 40$) and Sr ($Z = 38$) nuclei which are located at the mid-shell between $N = 50$ to $82$. The abrupt shape transition is limited to the Sr and Zr nuclei, while the neighboring Kr ($Z = 36$) and Mo ($Z = 42$) show a smooth shape evolution pattern in terms of the quadrupole deformation.
Among Zr isotopes, $^{110}$Zr with $Z = 40$ and $N = 70$ shell closures of the harmonic oscillator potential could be another quasi doubly-magic nucleus. However, a previous SEASTAR experiment at the Radioactive Isotope Beam Factory (RIBF) provided evidence for rather well-deformed nature in this isotope by measuring the energy of the first excited state through in-beam gamma-ray spectroscopy [1]. Several questions then remain open, such as the possibility of shape coexistence or triaxial deformation in this $^{110}$Zr isotope as predicted by different theoretical models [2-4].
A high-resolution in-beam gamma-ray spectroscopy study of nuclei around $^{110}$Zr was performed within the HiCARI (High-resolution Cluster Array at RIBF) campaign at the RIBF to measure the level lifetimes [5]. The HiCARI array was comprised of several different types of high-purity germanium detectors, which were six Miniball triple clusters from the Miniball collaboration [6], four 4-fold segmented Clover detectors from the IMP [7], a quad-type 36-fold segmented tracking detector from the RCNP [8], and a triple 36-fold segmented tracking detector P3 from the LBNL [9]. This large array was installed at the F8 focus which is located between the BigRIPS and Zero Degree Spectrometer at the RIBF facility. From this experiment, $^{110}$Zr was populated through proton knockout reactions from $^{111}$Nb and $^{112}$Mo. In addition, states in $^{110}$Mo and $^{112}$Mo could be studied with significantly more statistics
In this talk, preliminary experimental results will be represented. Lifetimes of specific levels in $^{110}$Zr, $^{110}$Mo, and $^{112}$Mo are analyzed based on the line-shape method [10]. These experimental results will allow to distinguish between predictions of different nuclear models concerning the shape of $^{110}$Zr, the key isotope for the evolution of collective properties along the $N = 70$ isotones.

[1] N. Paul et al., Phys. Rev. Lett. 118, 032501 (2017).
[2] M. Borrajo et al., Phys. Lett. B 746, 341 (2015).
[3] J. Libert, Phys. Rev. C 60, 1 (1999).
[4] T. Togashi et al., Phys. Rev. Lett. 117, 172502 (2016).
[5] K. Wimmer et al., RIKEN Accel. Prog. Rep. 54, S27 (2021).
[6] N. Warr et al., Eur. Phys. J. A 49, 40 (2013).
[7] W. Hua et al., Nuclear Structure in China 2012, 98 (2013).
[8] D. Weisshaar et al., Nucl. Instrum. Methods Phys. Res. A 847, 187 (2017).
[9] T. Ross, Neutron damage tests of a highly segmented Germanium detector, Master Thesis, University of Surrey (2009).
[10] P. Doornenbal et al., Nucl. Instrum. Methods Phys. Res. A 613, 218 (2010).

Speaker: Byul Moon (Center for Exotic Nuclear Studies, Institute for Basic Science)

NS2022_BMoon.pdf

14:30 Shape and collectivity in 80Ge studied via Coulomb Excitation

The stable to neutron-rich isotopes of germanium are a critical testing ground for nuclear models due to their complex and rapidly changing nuclear structure. The even-A 72–78Ge isotopes exhibit triaxial deformation, and the presence shape coexistence has also been suggested in 72Ge [1]. A transition from prolate to oblate shapes occurs at 70Ge [2], and another region of triaxiality has been
proposed around the neutron-rich 84,86,88Ge isotopes based on their low-lying level schemes [3]. With two neutrons removed from the singly-magic 82Ge, the rare-isotope 80Ge is important for a systematic understanding of the structural evolution of neutron-rich nuclei in this region of the nuclear chart.
A barrier-energy projectile Coulomb excitation experiment studying 80Ge was performed at the ReA3 facility of the NSCL using the JANUS [4] setup. This technique is sensitive to direct indicators of nuclear shape and deformation, namely E2 transition strengths and quadrupole moments. Electromagnetic matrix elements were extracted from the experimental data via a joint use of the
GOSIA and GOSIA2 codes [5]. Most notably, the quadrupole moment of 80Ge was measured for the first time, and the precision of the B(E2) transition strength was improved. The experimental results indicate a large, prolate deformation for 80Ge.
Two sets of large-scale shell-model calculations, with different effective interactions, were performed for 70–82Ge in order to better understand the current experimental results as well as the structural evolution in this region. The calculations reproduce both the current result for the 80Ge transition strength as well as the trend observed in the heavy Ge isotopes. The quadrupole moments proved more challenging for theory, though both sets of shell-model calculations performed point to a larger prolate deformation in 80Ge compared to its neighboring isotopes. The present measurement is consistent with this picture.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Grant No. DE-SC0020451, the U.S. National Science Foundation (NSF) under Grant No. PHY-1565546, and the DOE National Nuclear Security Administration through the Nuclear Science and Security Consortium, under Award No. DE-NA0003180. B.A.B. acknowledges support
from NSF Grant No. PHY-1811855. Work at LLNL was performed under Contract No. DEAC5207NA27344. Work at the University of Surrey was supported under UKRI Future Leaders Fellowship Grant No. MR/T022264/1.

[1] A. D. Ayangeakaa et al., Phys, Lett. B 754, 254 (2016).
[2] R. Lecomte et al., Phys. Rev. C 22, 1530 (1980).
[3] M. Lettmann et al., Phys. Rev. C 96, 011301 (2017).
[4] E. Lunderberg, et al., Nucl. Instrum. and Meth. in Phys. Res. A 885, 30 (2018).
[5] T. Czosnyka et al., Bull. Am. Phys. Soc. 28, 745 (1983).

Speaker: Daniel Rhodes (MSU/NSCL)

NS2022-Rhodes.pdf

15:00 → 15:30 PM Break

15:30 → 17:00 NS2022 Plenary: Shells/Magicity I

Convener: Mark Caprio (University of Notre Dame)

16:15 Lifetime measurement of first 4+ state in 102Sn

The long chain of Sn isotopes is a formidable testing ground for nuclear models studying the evolution of shell structure and interplay between pairing and quadrupole correlations. A transition from superfluid nuclei at midshell to spherical nuclei is also expected approaching the neutron shell closures at N = 50, where the seniority scheme can be adopted to describe the energy spectra. However, the corresponding B(E2 : 0+→2+) values have shown a presumed deviation from the expected parabolic behavior. From a theoretical point of view, various attempts have been done to explain the experimental results, in particular by including core-breaking excitations in the shell-model calculations by activating protons and neutrons from the g9/2 orbital to the higher ones. From experimental side, limited data are available beyond 104Sn on this very neutron- deficient region, leading to a difficulty in a firmly establishment of core-braking effect.

In this presentation, we will report on the first lifetime measurement for the 4+ state in 102Sn which is sensitive to the balance between the pairing and quadrupole terms in the nuclear interaction. The experiment is performed at GSI based on the use of hybrid AIDA+HPGe+LaBr3(Ce) array, made available by the HIS- PEC/DESPEC collaboration. The nuclei of interest were separated and identified through the FRS separator, following the production via fragmentation reaction of 124Xe beam incident on a 9Be target. The 102Sn ions are stopped by AIDA array and γ rays emitted from the 6+ seniority isomer are collected by FATIMA array which allows a direct lifetime measurement with a precision up to few tens of ps. The obtained experimental data would be compared with theoretical predictions, shedding light on the detailed wave function and the core breaking contribution.

Speaker: guangxin zhang (padova infn)

16:35 New aspects of the physics of charge radii

A systematic global investigation of differential charge radii has been performed within the covariant density functional framework for the first time [1,2,3]. Theoretical results obtained with conventional covariant energy density functionals and separable pairing interaction are compared with experimental differential charge radii in the regions of the nuclear chart in which available experimental data crosses neutron shell closures at $N = 28, 50, 82$ and 126. The analysis of absolute differential radii of different isotopic chains and their relative properties indicate clearly that such properties are reasonably well described in model calculations in the cases when the mean-field approximation is justified. However, while the observed clusterization of differential charge radii of different isotopic chains is well described above the $N=50$ and $N=126$ shell closures, it is more difficult to reproduce it above the $N=28$ and $N=82$ shell closures because of possible deficiencies in underlying single-particle structure [1]. The impact of the latter has been evaluated for spherical shapes and it was shown that the relative energies of the single-particle states and the patterns of their occupation with increasing neutron number have an appreciable impact on the evolution of the $\delta \left < r^2 \right>^{N,N'}$ values. These factors also limit the predictive power of model calculations in the regions of high densities of the single-particle states of different origin. It is usually assumed that pairing is a dominant contributor to odd-even staggering (OES) in charge radii. Our analysis paints a more complicated picture. It suggests a new mechanism in which the fragmentation of the single-particle content of the ground state in odd-mass nuclei due to particle-vibration coupling provides a significant contribution to OES in charge radii [1]. The connections between the physics of nuclear charge radii and atomic physics will also be discussed [4,5].

[1] U. C. Perera, A. V. Afanasjev and P. Ring, Phys. Rev. C. 104, 064313 (2021).
[2] T. Day Goodacre A. V. Afanasjev et al, Phys. Rev. Lett. 126, 032502 (2021).
[3] T. Day Goodacre A. V. Afanasjev et al, Phys. Rev. C 104, 054322 (2021).
[4] S.O. Allehabi et al, Phys. Rev. C 102,024326 (2020).
[5] S.O.Allehabi et al, Phys. Rev. A 103, L030801 (2021).

Speaker: Anatoli Afanasjev

Tuesday, 14 June

09:00 → 10:30 NS2022 Plenary: Heavy Elements II

Convener: Walid Younes (LLNL)

09:00 Gamma-ray spectroscopy of nuclear fission

Gamma-ray spectroscopy is a versatile tool which can be used to study the decay of the excited fragments produced in the complex process of nuclear fission. Gamma ray coincidence and relative time information can give important information on both the nuclear structure of exotic
neutron-rich nuclei and the fission process itself. Recent results from the nu-Ball hybrid gamma-ray spectrometer at the ALTO facility of IJC Lab will be presented. In particular, studies of short-lived states in neutron-rich nuclei will be highlighted [1][2][3] along with recent advances in the understanding on the generation of angular momentum in the fission process [4]. The prospects for new and innovative measurements using gamma spectroscopy of fission will be presented.

[1] Prompt and delayed spectroscopy of the neutron-rich $^{94}$Kr and observation of a new isomer, R-B. Gerst et al. Phys. Rev. C 102, 064323 (2020)

[2] First lifetime investigations of N>82 iodine isotopes: The quest for collectivity, G. Häfner et al. Phys. Rev. C 104, 014316 (2021)

[3] Spectroscopy and Lifetime Measurements in $^{134,136,138}$Te Isotopes and Implications for the Nuclear Structure beyond N = 82, G. Hafner, R. Lozeva, et al. Phys. Rev. C103 034317 (2021)

[4] Angular momentum generation in nuclear fission, J.N. Wilson et al. Nature 590, p566–570 (2021)

Speaker: J. N. Wilson (IJC Laboratory)

09:30 The Broad Impact of Mass-Number Measurements for Heavy Element Studies

Even after decades of research, still very little is known about the fundamental nuclear or chemical properties of the heaviest elements. These elements do not exist naturally on earth, so experimental studies are incredibly challenging. Theoretical work has already predicted where the "island of stability" should be and were the periodic table of the elements should end, but these regimes are not yet accessible experimentally. At Lawrence Berkeley National Laboratory, the recent installation of the FIONA spectrometer has sparked a new era of possible heavy element studies that utilize mass-number identifications.

First measurements have already shown the broad impact of this experimental technique. The production of the new isotopes $^{244}$Md and $^{239}$Es have already been confirmed. It has also been demonstrated that FIONA can be used to perform gas-phase chemistry. In these studies, the products of chemical reactions can be directly-observed via their mass. First measurements have already given insight as to the second-ionization potential of lawrencium. These and more-recent results will be discussed.

Speaker: Jennifer Pore

09:50 Decay Spectroscopy of High-K Isomers in Deformed Neutron-rich 180<A<190 Nuclei via Fragmentation of $^{198}$Pt

Long-lived K-isomers at high angular momenta in very neutron-rich deformed Hf (Z=72) nuclei have been predicted for decades, but their spectroscopy has remained elusive as they are difficult to populate. Not only are these nuclei interesting from a nuclear structure perspective, but they also border the r-process pathway, and thus their $\beta$-decays are of considerable relevance for our understanding of heavy-element nucleosynthesis. Standard reaction mechanisms, such as fusion-evaporation, deep-inelastic and transfer, have led to slow and incremental progress in pushing the spectroscopic frontier by barely two or three neutrons beyond the heaviest stable isotope.

To break this stalemate and make significant inroads into neutron-rich territory, a new reaction mechanism was needed. An experiment was conducted at the NSCL to study neutron-rich nuclides in the Hf-Ta-W region, through the fragmentation of a newly developed $^{198}$Pt primary beam incident at 86 MeV/u on Be and Ni targets. Additional motivation for the experiment was to observe new isotopes, as well as compare angular momenta imparted to the fragments with different primary targets. The products were momentum-analyzed with the A1900 separator and implanted into a stack of Si detectors, consisting of two 140-$\mu$m $\Delta$E detectors, a 500-$\mu$m single-sided strip detector with 16 horizontal strips for momentum sensitivity, a 1000-$\mu$m implant detector for total energy, and backed with a 500-$\mu$m veto detector. This allowed for full event-by-event particle identification ($A, Z, Q$) using measured $\Delta$E-B$\rho$-TKE-ToF parameters. The Si stack was surrounded by the GRETINA $\gamma$-ray tracking array, which provided efficient detection of $\gamma$-ray cascades following isomer decays. Data were acquired in three different central-momentum settings for the very-neutron-rich $^{186}$Hf, $^{189}$Hf and $^{192}$Hf isotopes, with a number of neighboring isotopes populated in the process at each setting. Prior spectroscopic information of $^{190}$W, populated strongly in the $^{186}$Hf (and $^{189}$Hf) settings, was used to benchmark and calibrate the particle identification landscape.

Decay spectroscopy of new and previously-observed multi-quasiparticle isomers in deformed neutron-rich Hf, Ta and W nuclei, with half-live measurements ranging from a few hundred nanoseconds to a few hundred microseconds, will be presented and discussed.

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

Speaker: P. Chowdhury (University of Massachusetts Lowell)

10:10 New evidence for alpha clustering structure in the ground state band of $^{212}$Po

$^{212}$Po has two-protons and neutrons outside the doubly-magic nucleus $^{208}$Pb and it may be assumed that the nuclear structure can be well described within the shell-model. But various experimental properties, such as the short-lived ground state, are better described by an $\alpha$-clustering model. The B(E2) values of the decays of the low-lying yrast states are an important finger print to describe the structure of $^{212}$Po. Especially the missing B(E2; 4$_1^+ \rightarrow$ 2$^+_1$) value is important in this discussion. We have performed an $\alpha$-transfer experiment to investigate excited states of $^{212}$Po and determine the lifetimes using the ROSPHERE $\gamma$-ray detector array at IFIN-HH in Magurele, Romania. This array consisted of 15 HPGe detectors and 10 LaBr$_3$(Ce) scintillator detectors and was supplemented with the SORCERER particle-detector array. The combination of $\gamma$-ray and the particle detectors was an important tool to determine the mean lifetimes of all ground-state band levels up to the 8$^+$ state applying the fast-timing method [Ma. von Tresckow et al., PLB 821, 136624 (2021)]. I will present our lifetime analysis and discuss the results within the shell-model and $\alpha$-clustering model. This work is financially supported by EURONS2, IFA via grant 04FAIR/2020, MCDI via grant PN19060102, UK-STFC via grant ST/P005101/1, Ministry of Science and Higher Education of the Russian Federation under contract No. 075-10-2020-117.

Speaker: Mr Martin von Tresckow (IKP TU Darmstadt)

10:30 → 11:00 AM Break

11:00 → 12:30 NS2022 Plenary: Islands of Inversion

Convener: D. Bazin (NSCL, Michigan State University)

11:00 Investigating the Spin-Orbital splitting in the N=19 isotones using SOLARIS

Spin-Orbital (SO) splitting generates the magic numbers when N>20 but the SO potential itself is not well constrained. Calculations based on Density Function Theory (DFT) predict a “proton bubble” in the $^{34}$Si nucleus, together with a sudden reduction of the SO-splitting between the 0p$_{1/2}$ and 0p$_{3/2}$ orbitals in $^{35}$Si relative to the same orbitals in $^{37}$S. A similar “proton bubble” and a sudden reduction of SO-splitting was also predicted in N=19 isotones. However, a differing interpretation of SO-splitting change has been proposed in terms of the weak-binding effect. In order to investigate the energies of the SO partners in the N=19 isotonic chain, the $^{32}$Si(d,p)$^{33}$Si measurement was carried out using the SOLARIS solenoid spectrometer in the silicon array mode at the ReA Facility at Michigan State University. The goal of the measurement, through the determination of the relative single-particle energies, is to investigate whether any sudden SO reduction occurs $^{33}$Si compared to $^{35}$S and to make a direct comparison with the systematic trends in the N = 21 isotones. The experimentally measured SO-splitting in N=19 will be presented together with calculations based on a Wood-Saxon model, effective interactions via the shell model, and DFT calculations. New insight into the debate about the SO-splitting and the proton central depletion will also be discussed.

Furthermore, as solenoidal spectrometers become mainstay equipment for low-energy direct reactions, inelastic scattering reactions carried out using this type of device can become an important probe of the collective modes in nuclei. Results from a recent measurement of $^{15}$C(d,d’), carried out at the ATLAS Facility at Argonne National Laboratory using the HELIOS spectrometer will be presented. The ratio of neutron and proton quadrupole matrix elements was obtained, which can shed light on the degree of decoupling of the valence neutron in the halo nucleus $^{15}$C. Future opportunities of low-energy direct reactions with radioactive beams will also be discussed.

The authors acknowledge the support by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract Numbers DE-AC02-06CH11357 (ANL), DE-SC0020451(NSCL). SOLARIS is funded by DOE Office of Science under the FRIB Cooperative Agreement DE-SC0000661.

Speaker: Jie Chen (Physics Division, Argonne National Laboratory)

11:30 Single-nucleon transfer reactions probed in inverse kinematics with the ISOLDE Solenoidal Spectrometer - recent highlights

The ISOLDE Solenoidal Spectrometer (ISS) is a new magnetic spectrometer that has been developed to study direct reactions with exotic beams produced at the ISOLDE facility, CERN. ISS was fully commissioned during 2021 and the first physics campaign took place using a new on-axis position-sensitive silicon array, constructed by the University of Liverpool, mounted inside the 4T former-MRI magnet.
A number of preliminary highlights from the physics campaign will be presented here. These include a measurement to probe changing shell structure in to the $N=20$ island of inversion via a measurement of the $d$($^{30}$Mg,$p$)$^{31}$Mg reaction. These data, combined with previous measurements of the $d$($^{28}$Mg,$p$)$^{29}$Mg reaction [1], provide details on the evolution of single-particle structure across the boundary of the island of inversion with which to assess modern shell-model calculations.
Additionally, at the other end of the nuclear chart, a measurement of the $d$($^{212}$Rn,$p$)$^{213}$Rn reaction identified excited states outside $N=127$. This measurement provides the first spectroscopy of low-lying, single-particle levels in $^{213}$Rn. These data will contribute to our understanding of the evolution of single-particle structure outside the $N=126$ closed neutron shell and the role of the nucleon-nucleon interaction in driving observed trends. Preliminary results and comparisons to calculations will be presented.

[1] P.T.MacGregor et al. Phys. Rev. C. 104 L051301 (2021).

This work was supported by the U.K. Science and Technology Facilities Council [Grants No. ST/V001027/1, No. ST/P004598/1, No. ST/N002563/1, No. ST/M00161X/1 (Liverpool), No. ST/P004423/1 (Manchester), the ISOL-SRS Grant (Daresbury), No. ST/R004056/1 (Gaffney), and No. ST/T004797/1 (Sharp)], the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357 (ANL), the Research Foundation Flanders (FWO, Belgium), the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 617156.

Speaker: David Sharp (University of Manchester)

2022-NS-Sharp.pdf

11:50 Investigation of the 46Ar proton wave function: the 46Ar(3He, d)47K direct reaction

Direct reactions represent a unique mechanism to investigate the nuclear-structure properties and the nature of single-particle states of nuclei along shell closures. In this contribution, the $^{46}$Ar($^3$He, d)$^{47}$K proton-transfer reaction is proposed for the study of properties of the ground state of the radioactive neutron-rich $^{46}$Ar isotope. The interest behind this isotope stems from the observed discrepancies between the well-established shell model with SDPF-U interaction and measurements of transition probability B(E2) between the ground state and the first excited state. The proton component of the wave function has been pointed out as the source of this discrepancy [1,2].

The measurement, performed at the SPIRAL 1 facility in GANIL, France with a post-accelerated radioactive 9 MeV/u $^{46}$Ar beam impinging on a high-density cryogenic $^3$He target [3], aimed at quantifying the transfer cross section to the 3/2$^+$ level relative to the 1/2$^+$ ground state in $^{47}$K. The experiment relied on a state-of-the-art experimental setup for a precise reconstruction of the kinematics of the reaction. The heavy reaction fragment was identified by the high acceptance magnetic spectrometer, VAMOS [4], while the high-granularity silicon DSSD detector, MUGAST [5], allowed the measurement of the angular distribution of the light ejectile while also performing particle identification. The AGATA gamma-ray tracking germanium array [6] measured the photons produced by the decay of the $^{47}$K excited states.

The experimental evidence indicates a substantially suppressed L=2 transfer to the first excited state of $^{47}$K, at odds with shell-model calculations that predict the s$_{1/2}$ and d$_{3/2}$ orbitals as almost degenerate and not entirely occupied. The results will be discussed in the framework of ab initio and mean-field calculations. In these theoretical results, the low occupancy of the s$_{1/2}$ orbital, in agreement with the high relative spectroscopic factor measured, implies a central depletion of the proton wavefunction. This work presents the first experimental evidence of this phenomenon in $^{46}$Ar.

[1] A. Gade et al., Phys. Rev. C 74, 034322 (2006)
[2] S. Calinescu et al., Phys. Rev. C 93, 044333 (2016)
[3] F. Galtarossa et al., Nucl. Inst. Meth. A 1018, 165830 (2021)
[4] M.Rejmund et al., Nucl. Inst. Meth. A 646, 184-191 (2012)
[5] M. Assié et al., Nucl. Inst. Meth. A 1014, 165743 (2021)
[6] S. Akkoyun et al., Nucl. Inst. Meth. A 668, 26-58 (2012)

Speaker: Daniele Brugnara (Laboratori Nazionali di Legnaro, INFN, Italy)

12:10 Nuclear Structure of $^{36}$Al and $^{36}$Si via $\beta$-decays of $^{36}$Mg and $^{36}$Al

The $\beta$-decays of $^{36}$Mg and $^{36}$Al have been studied at The National Superconducting Cyclotron Laboratory (NSCL) in order to extract the half-lives of the parent nuclei and reveal the nuclear structure of the decaying descendants. Neutron-rich $^{36}$Mg and $^{36}$Al were produced at the NSCL's Coupled Cyclotron Facility via projectile fragmentation of a $^{48}$Ca beam of energy 140 $\text{MeV/u}$ impinged on a 642 $\text{mg/cm}^2$ thick $^{9}$Be target. The fragmented beam was delivered to the decay station after being resolved by the A1900 separator. Two Si p-i-n detectors were used for the particle identification whereas the ions were implanted on a 3-mm thick $\text{CeBr}_3$ scintilator coupled to a position-sensitive photo multiplier tube (PSPMT). The $\beta$-delayed $\gamma$-rays were identified with 16 segmented Ge detector array (SeGA) and 15 $\text{LaBr}_3$ detectors. The half-lives of the two parent nuclei were determined and were compared to the previous measurements. $\beta$-delayed $\gamma$-ray transitions were observed in $^{36}$Al and $^{36}$Si for the first time and their level schemes were built from the correlated $\beta$ decays of $^{36}$Mg and $^{36}$Al. Excited energy states of $^{36}$Al populated by the $\beta$-decay of $^{36}$Mg are proposed, whereas only the ground state information was available prior to this work. The experimental results were interpreted by using the nuclear configuration interaction studies with the FSU shell-model Hamiltonian. The results will shed light on our understanding of the structure of more exotic neutron-rich nuclei to be produced with the FRIB.

Speaker: Rebeka Sultana Lubna (Facility of Rare Isotope Beams, Michigan State University)

12:30 → 13:30 Lunch

13:30 → 15:00 NS2022 Plenary: Shapes and Collectivity

Convener: Peter Bender (University of Massachusetts Lowell)

13:30 Lifetime measurements in fission fragments: Evolution of nuclear shapes and triaxiality

Neutron-rich nuclei with mass around A=100 exhibit a rapid change of nuclear deformation. A sudden onset of deformation is observed for the Sr and Zr isotopes at neutron number N=60. For Mo and Ru isotopes the transition is more gradual and the shape becomes increasingly triaxial. Nuclear shapes in this wider mass region were studied by measuring lifetimes of excited states in an experiment at GANIL. Neutron-rich fission fragments were produced in fusion-fission reactions between a 238U beam at 6.2 MeV/u and a 9Be target. The fission fragments were identified in mass, charge, and atomic number in the magnetic spectrometer VAMOS on an event-by-event basis. The velocity of nuclei exiting the target foil was slowed down in a degrader that was mounted in a plunger device at variable distances from the target. The AGATA gamma-ray tracking array was used to measure picosecond lifetimes with the recoil-distance Doppler shift (RDDS) method. The setup furthermore comprised 24 LaBr3(Ce) detectors from the FATIMA array, which were employed to measure longer lifetimes using fast-timing techniques. The experiment produced a wide range of neutron-rich fission fragments, for which lifetimes could be measured under identical experimental conditions, and a large number of new lifetimes was determined. The VAMOS spectrometer not only provided identification of the fission fragments, but also yielded a precise measurement of their velocity. This was used to develop a new variant of the RDDS method, where gating on the velocity provides several data points for a single target-degrader distance.

An overview of the results will be presented, with emphasis on the chain of Ru isotopes. New lifetimes were measured for both odd and even-mass Ru isotopes. In case of 110Ru it was possible to measure lifetimes in both the ground-state band and the gamma band. The comparison of experimental results with the triaxial rotor model and with beyond-mean field calculations provides quantitative information on the degree of triaxiality.

Speaker: Andreas Görgen (University of Oslo)

13:50 First Evidence of Axial Shape Asymmetry and Shape Coexistence in 74Zn: Suggestion for a Northern Extension of the N=40 Island of Inversion

Results from recent experiments studying nuclei in the $^{78}$Ni region suggest that the $N=50$ shell closure persists, in agreement with state-of-the-art shell-model calculations. However, how collectivity manifests and evolves in this region of the Segrè chart is still an open question, particularly concerning phenomena such as vibrational modes, axial shape asymmetry (triaxiality) and shape coexistence. This is especially true in the Zn isotopic chain in the neutron-rich region, in which even definitive spin assignments are unavailable except for the very low-lying states.
In this talk, I will present the results of a recent experiment performed at the TRIUMF laboratory (Vancouver, Canada) using the GRIFFIN $\gamma$-ray spectrometer. The excited states of $^{74}$Zn were investigated via $\gamma$-ray spectroscopy following $^{74}$Cu $\beta$-decay. By exploiting $\gamma$-$\gamma$ angular correlation analysis, the $2_2^+$, $3_1^+$, $0_2^+$ and $2_3^+$ states in $^{74}$Zn were firmly identified. The $\gamma$-ray branching and $E2/M1$ mixing ratios for transitions de-exciting the $2_2^+$, $3_1^+$ and $2_3^+$ states were measured, allowing for the extraction of relative $B(E2)$ values. In particular, the $2_3^+ \to 0_2^+$ and $2_3^+ \to 4_1^+$ transitions were observed for the first time. The levels observed were organized into rotational-like bands and the results were compared with large-scale shell-model calculations from which the shapes of individual states were determined. Enhanced triaxiality is suggested to characterize $^{74}$Zn in its ground state. Furthermore, an excited $K=0$ band with a different shape is identified. A shore of the $N=40$ island of inversion appears to manifest above $Z=28$, previously thought as its northern limit in the nuclide chart.

Speaker: Marco Rocchini (Department of Physics, University of Guelph, N1G 2W1 Guelph, Canada)

74Zn-Rocchini.pdf

14:10 Octupole collectivity in 74,76Kr studied with inelastic proton scattering in inverse kinematics

Atomic nuclei close to ⁷²Kr are expected to feature enhanced octupole correlations since both proton and neutron single-particle levels are close to the Fermi surface, which differ by Δj = Δl = 3. Previous QRPA calculations predicted only small electric octupole strength in this mass region, which is at odds with the experimental systematics gathered in the stable Kr isotopes and other isotopic chains. At the same time, these calculations underlined that the strength fragmentation is intimately connected to the type of quadrupole ground-state deformation. During the last two decades, the latter has been accessed in various experimental studies which revealed a delicate interplay between prolate and oblate configurations at low excitation energies challenging theoretical models.
Inelastic proton scattering experiments in inverse kinematics were performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University using GRETINA, the S800 spectrograph and the URSINUS/NSCL Liquid Hydrogen Target to study octupole collectivity in a region of developing deformation and shape coexistence. Besides low-spin positive-parity states, the first and second 3^- states of ^74,76Kr were populated and their previously unknown B(E3) strengths determined. Results will be presented.

M.Sp. acknowledges support from the National Science Foundation (NSF) under Grant No. PHY-2012522 (WoU-MMA:Studies of Nuclear Structure and Nuclear Astrophysics) and through the FRIB Visiting Scholar Program for Experimental Science 2020. This work was also supported by the NSF under Grant No. PHY-1565546 (NSCL). 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)

14:30 Collectivity evolution of proton-rich Mo isotopes

The self-conjugate N=Z nuclei have been an intriguing subject for a long time due to their peculiar characteristics for proton-neutron correlations and quadrupole-quadrupole interactions. In particular, a significant shape change has been anticipated among the medium-mass nuclides. The structure of N=Z molybdenum (Z=42) isotope, 84Mo, attracts attention in this viewpoint since the theoretical results represented by the ground-state shape are sensitive to shell model calculation with different model spaces and a choice of the interaction. For example, a shell model calculation based on the Nilsson SU(3) scheme revealed that 84Mo is a transitional nuclide that prolate-oblate shape competition emerges. Thus, a detailed study of the Mo isotope provides valuable results to feedback the nuclear theories. We aimed at investigating the collectivity and shape of 84Mo and its neighbors through a first 2+ state lifetime measurement. The experiment was performed at NSCL/MSU with a 140-MeV/u 92Mo primary beam impinging on a 235-mg/cm2 9Be target. The TRIPLEX plunger setup coupled to the GRETINA was employed to populate the low-lying states and measure the lifetime. The results of the new lifetime measurement for 84Mo and 86Mo are presented. Furthermore, the change of the collectivity around A=70-80 is discussed with the shell model calculation with the DNP-ZBM3 effective interaction.

Speaker: Jeongsu Ha (IKS, KU Leuven)

15:00 → 15:30 PM Break

15:30 → 17:00 NS2022 Plenary: Statistical Structure and Astrophysics

Convener: Dorthea Gjestvang

15:30 Photon Strength Function Studies: Progress and Outlook

Significant progress has been made in the study of photon strength functions (PSF) over the last few years. The nature of the so-called low-energy enhancement of the PSF is being unraveled. In addition, PSF and nuclear level density (NLD) measurements have provided unprecedented constraints on nucleosynthesis processes through much improved neutron capture cross sections. The successful development of novel experimental and analytical techniques now allows for the investigations of previously inaccessible nuclei and improved reliability of results.
In this presentation, I will review recent experimental and analytical developments to study PSFs and NLDs at radioactive and stable ion beam facilities with a particular focus on the inverse-Oslo [1] and Shape methods [2]. The latter determines the functional form of the PSF and the slope of the NLD, which can be obtained simultaneously, even in the absence of neutron resonance spacing data.
I will further discuss the current understanding of the underlying nuclear structure responsible for the low-energy enhancement and will demonstrate the power of PSF and NLD measurements to constrain nucleosynthesis processes and astrophysical environments for the production of 180Ta [3] and 138La [4] p-nuclei. In light of the many new and improved experimental facilities and capabilities now available across the world, I will conclude by exploring prospects for future PSF measurements.

[1] V.W. Ingeberg et al., Eur. Phys. J. A 56, 68 (2020).
[2] M. Wiedeking et al., Phys. Rev. C 104, 014311 (2021).
[3] K.L. Malatji et al., Phys. Lett. B 791, 403 (2019).
[4] B.V. Kheswa et al., Phys. Lett. B 744, 268 (2015).

This work is supported by the National Research Foundation of South Africa under Grant Number 118840.

Speaker: Prof. Mathis Wiedeking (iThemba LABS and University of the Witwatersrand)

15:50 β-decay strength distributions of neutron-rich isotopes for r-process nucleosynthesis

Approximately half of the elements heavier than iron are thought to be produced in the r process. Recent insights into the astrophysical site of this critical process highlight the need for experimental data on short-lived neutron-rich nuclei. R-process nucleosynthesis sensitivity studies show that the final abundance distributions of r-process nuclei are greatly impacted by β-decay properties, such as half-lives and β-delayed neutron-emission probabilities [1]. To inform global models used to calculate these properties, the β-decay strengths for a series of neutron-rich Co isotopes have been measured using the technique of total absorption spectroscopy with the Summing NaI (SuN) detector. The resultant β-decay intensities and deduced Gamow-Teller strengths are compared to QRPA calculations, which are typically used in r-process models. Results from several different experiments will be presented along with determined trends which may inform future theoretical calculations.

This work was supported by the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory operated by Battelle for the US Department of Energy.

[1] M. R. Mumpower, R. Surman, G.C. McLaughlin, A. Aprahamian. Progress in Particle and Nuclear Physics 86 (2016), 86-126.

Speaker: Stephanie Lyons (Pacific Northwest National Laboratory)

16:10 Measurements of the E1-strength in Fe and Ni nuclei around the threshold

The γ -ray emission from the nuclei 62,64 Fe following Coulomb excitation at bombarding energy of 400-
440 AMeV was measured with special focus on E1 transitions in the energy region 4-8 MeV. The
unstable neutron-rich nuclei 62,64 Fe were produced at the FAIR-GSI laboratories and selected with the
FRS spectrometer. The γ decay was detected with AGATA HPGE tracking array. From the measured γ -ray spectra the summed
E1 strength is extracted and compared to microscopic quasi-particle phonon model calculations. The
trend of the E1 strength with increasing neutron number is found to be fairly well reproduced with
calculations that assume a rather complex structure of the 1− states (three-phonon states) inducing a
strong fragmentation of the E1 nuclear response below the neutron binding energy.

Speaker: Oliver Wieland (INFN seziona di Milano)

16:30 Gamma-Particle Coincidence Studies of 93Sr(d,p)94Sr via the Surrogate Reaction Method

Neutron-capture cross sections play a vital role in our understanding of heavy element nucleosynthesis. In astrophysical processes such as the intermediate neutron-capture process and rapid neutron-capture process, element formation occurs in neutron-rich environments and involves short-lived isotopes for which capture cross sections cannot be measured via direct techniques. Instead reaction rates in these regions rely on calculations that have uncertainties up to a few orders of magnitude. Recent measurements of the $\beta$-decay of $^{94}$Rb [1], which compared the neutron-to gamma-ray-branching ratio of state decays above the neutron separation energy in $^{94}$Sr, suggest an enhanced $\gamma$-ray branch which would in turn lead to an unexpectedly large $^{93}$Sr(n,$\gamma$) cross section. If confirmed, such an enhancement could have a strong impact on our understanding of nucleosynthesis processes involving nuclei in this region. In order to investigate this potential enhancement of the $^{93}$Sr(n,$\gamma$) cross section, an experiment was performed at TRIUMF using an 8~MeV/u $^{93}$Sr beam impinging on a CD$_2$ target. The (d,p$\gamma$) coincidence data was measured using the SHARC and TIGRESS arrays. Experimental details from the measurement of $^{93}$Sr(d,p)$^{94}$Sr will be presented along with preliminary gamma-particle coincidence analysis using the Surrogate Reaction Method [2,3].

*This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.

[1]. J. L. Tain, et al. Phys. Rev. Lett. 115, 062502 (2015).
[2]. J. Escher, et al., Phys. Rev. Lett. 121, 052501 (2018).
[3]. A. Ratkiewicz, et al., Phys. Rev. Lett. 122, 052503 (2019).

Speaker: Andrea Richard (Lawrence Livermore National Laboratory, Facility for Rare Isotope Beams)

ARichard_NS2022.pdf

17:30 → 19:30 Poster Session

Wednesday, 15 June

09:00 → 10:30 NS2022 Plenary: Mirrors and Proton-Rich Nuclei

Convener: A. Rogers (University of Massachusetts Lowell, USA)

09:00 Symmetry and collectivity of mirror nuclei

Unlike any other physical system the nucleus represents a unique dual quantum many-body system. Its constituents, protons and neutrons, are assumed to be identical, except for their electric charge. They can be seen as two representations of the nucleon, with isospin components t$_z$ = ±1/2 for neutrons and protons, respectively. Under the assumption of charge independence of the strong interaction, hence invariance under rotation in the isospin space, the excitation energy spectra of mirror nuclei should be identical.

Isospin breaking effects, besides the dominating electromagnetic force, are usually studied through mirror energy differences, testing the charge symmetry and triplet energy differences, testing the charge independence of the nuclear force.
However, a more rigorous way to test isospin symmetry are electromagnetic matrix elements.

In this talk, I will present the results of our studies of isospin symmetry performed at the Radioactive Isotope Beam Factory at the RIKEN Nishina Center in Japan. I will present our study of the A = 70, T = 1 triplet where the unusually large excitation cross section of the 2$^+_1$ state of $^{70}$Kr can be interpreted in terms of a shape change in the mirror nuclei. Furthermore, I will present new results on the A=62 triplet and mirror symmetry of the A=61 quartet.

Speaker: Dr Kathrin Wimmer (GSI)

09:30 Determination of the neutron-deficient $^{54}$Ni charge radius and symmetry energy constraints using the difference in mirror pair charge radii

Different parameterizations of Skyrme energy density functionals show large variations in the stiffness of the neutron equation of state (EOS), making extrapolations to higher densities uncertain [bro00]. It has been shown that the difference in mirror pair charge radii is correlated with the L parameter, which is the slope of the symmetry energy in the nuclear EOS [bro17]. By placing constraints on L, the neutron equation of state can thereby be constrained. In the present study, the charge radius of neutron-deficient 54Ni was determined for the first time using collinear laser spectroscopy at the BEam COoling and LAser spectroscopy (BECOLA) facility [min13, ros14] at NSCL/MSU. Using the difference in mirror pair charge radii between 54Ni and 54Fe, a constraint of 20 ≤ L ≤ 70 MeV has been placed [pin21], consistent with results from the gravitational wave event of the GW170817 neutron star merger [rai19] and barely consistent with those from PREX-II [ree21]. Constraints on the neutron skin for 48Ca from this experiment are in agreement with the preliminary CREX results released at DNP 2021, implying a “soft” EOS and contradictory to the PREX results. In addition to the experimental results, a new trend analysis will be discussed, which evaluates the reliability of the difference in mirror charge radii as a good isovector indicator [rei22].

*This work was supported in part by grants NSF No. PHY-15-65546, PHY-21-10365 and PHY-21-11185; German Research Foundation SFB1245.

References
[min13] K. Minamisono et al., Nucl. Instrum. Methods Phys. Res., Sect. A 709, 85 (2013).
[ros14] D. M. Rossi et al. Rev. Sci. Instrum. 85, 093503 (2014)
[bro17] B. A. Brown, Phys. Rev. Lett. 119, 122502 (2017).
[bro00] B. A. Brown, Phys. Rev. Lett. 85, 5296 (2000).
[pin21] S. V. Pineda et al. Phys. Rev. Lett. 127, 182503 (2021).
[ree21] B. T. Reed et al., Phys. Rev. Lett. 126, 172503 (2021).
[rai19] C. Raithel and F. Özel, Ap. J. 885:121 (2019).
[rei22] P.-G. Reinhard and W. Nazarewicz, Phys. Rev. C 105, L021301 (2022).

Speaker: Skyy Pineda (Department of Chemistry, Michigan State University)

09:50 Discovery of the proton emitter 116La

The discovery of the new proton emitter 116La, 23 neutrons away from stable 139La, will be reported. 116La nuclei were synthesised in the fusion-evaporation reactions at the University of Jyväskylä Accelerator Center and identified via their proton radioactivity using the MARA recoil mass spectrometer. Comparisons of the measured proton energy (E= 718 keV) and half-life (T1/2 = 50 +-22ms) with values calculated using microscopic nuclear barrier penetration theory indicate that the proton is emitted with an orbital angular momentum l=2 and that its emission probability is enhanced relative to its closest, less exotic, odd-even lanthanum isotope (117La) while the proton-emission Q-value is lower. We propose this unusual feature to be a signature for the presence of strong isovector neutron-proton pair correlations in this exotic, neutron deficient system. The observations of gamma decays from isomeric states in 116La and 117La will also be discussed.

Speaker: Bo Cederwall (KTH Royal Institute of Technology)

10:10 Advances in experimental methods to investigate the isospin breaking effects in isobaric triplets

Atomic nuclei having (almost) the same number of neutrons (N) and protons (Z) attract a great deal of attention due to the various interesting physics phenomena associated with these systems. During the past few decades the focus has been on the isospin symmetry properties of the nuclei located around the N = Z line. More recently, owing to the development of radioactive ion beam techniques, collective properties and shape evolution around the N = Z line have also been investigated.

Arising from the charge-symmetry and charge-independence characteristics of the strong nuclear force, isobaric analog nuclei with the same mass A = N + Z, but the neutron and proton numbers differing as N = Z - 2, N = Z and N = Z + 2, should contain the same set of excited states at similar excitation energies. This manifestation of isospin symmetry is broken by the Coulomb interaction and leads to Coulomb energy differences (CED) in the level energies. The CED can give information on the microscopic structure of nuclei and allows to investigate if the isospin symmetry is further broken in addition to the Coulomb interaction. Actually, several shell-model analyses made for the isobaric triplets in the A=50-70 mass region have indicated a need for a schematic isospin-breaking two-body interaction beyond the Coulomb force in order to reproduce the experimental CED data.

Currently, information on the T = 1 states in N = Z - 2 and N = Z nuclei with mass A = 62-78 are scarce due to the experimental challenges to study these systems. For 62Ge there are no firm data on the excited states and for 62Ga the available information on the T = 1, 2+ state is contradictory. In 66As and 66Se the T = 1 bands are known up to the 6+ state, 70Kr and 74Sr are both known up to the 4+ state and in 78Zr excited states are not known at all. This is clearly a major hindrance to advance the isospin symmetry breaking investigations at the heavier end of the N = Z line.

During the past two years, the isobaric triplets in the A = 62-78 mass region have been systemically studied at the Accelerator Laboratory of the University of Jyväskylä (JYFL-ACCLAB). These studies, using fusion evaporation reactions, employed the vacuum-mode recoil separator MARA and JUROGAM 3 germanium array together with the JYtube charged-particle veto detector and new position sensitive scintillation detector for the detection of fast and high-energy beta particles. The well-established method of recoil-beta tagging, where the reaction product of interest is identified based on its characteristic beta-decay properties, has been the primary tool to find the weak gamma-ray transitions originating from the exotic N ≈ Z nuclei. However, this method does not allow for the unambigous discrimination between the gamma rays from the N = Z - 2 and N = Z members of the triplet, which both are Fermi super-allowed beta emitters with nearly identical beta decay properties. Therefore, to get access to the neutron-deficient N = Z - 2 member, recoil-beta-beta tagging method was recently successfully demonstrated to unambiguously identify gamma-ray transitions from 62Ge for the first time.

In this presentation, the recent experimental advances in the instrumentation and methodologies achieved at JYFL-ACCLAB to increase the detection sensitivity for the studies of heavy N ≈ Z nuclei will be discussed together with the new spectroscopic data on these nuclei obtained during the past two years.

Speaker: Panu Ruotsalainen (University of Jyväskylä)

10:30 → 11:00 AM Break

11:00 → 12:30 NS2022 Plenary: New Facilities

Convener: Andrew Boston

12:30 → 13:30 Lunch

Thursday, 16 June

09:00 → 10:30 NS2022 Plenary: Lasers

Convener: Dr Kyle Leach (Colorado School of Mines)

09:00 Recent highlights and new developments in (fluorescence based) collinear laser spectroscopy at ISOLDE

Electromagnetic properties of short-lived radionuclides represent sensitive probes for the structural evolution of atomic nuclei far away from stability. Experimentally, these can be accessed, for instance, by laser spectroscopy in which measurements of the atomic hyperfine structure reveal electromagnetic moments and charge radii of nuclear ground states and long-lived isomers.

Due to the high experimental precision and accuracy, these observables represent critical benchmarks for modern nuclear-structure theory. Today, ab initio calculations can be extended well into the medium- to heavy-mass region where they connect with nuclear density functional theory.The predictive power of these theoretical methods has recently been benchmarked by collinear laser spectroscopy (CLS) performed on short-lived nickel (Ni) isotopes [2] at the COLLAPS beamline at ISOLDE/CERN. This work establishes a theoretical accuracy of ∼1% for the description of nuclear charge radii in the Ni region.

Conventional CLS is based on fluorescence detection from laser-excited ions or atoms. This limits its application to radionuclides with yields of >100-10,000 ions/s, depending on the specific case and spectroscopic transition. Thus, the study of the ‘most exotic’ nuclides synthesised at today’s radioactive ion beam facilities with very low production yields demands for more sensitive experimental methods.
To this end, we have developed the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) [3]. This novel approach exploits a Multi-Reflection Time-of-Fight (MR-ToF) device in which ions bounce back and forth between two electrostatic mirrors. Hence, the trapped ions are probed by the laser beam during each revolution inside the MRToF device compared to a single passage through a laser-ion integration region in conventional CLS. Thus, the MIRACLS approach enhances the sensitivity of CLS by a factor of 30-700.
In addition to the discussion of the Ni results at COLLAPS, this talk will present the MIRACLS
concept as well as recent experimental work on a proof-of-principle experiment which demonstrates the strength of the MIRACLS approach.

[1] K. Blaum, et al., Phys. Scr. T152, 014017 (2013); P. Campbell et al., Prog. Part. and Nucl. Phys. 86, 127-180 (2016); R. Neugart et al., J. Phys. G: Nucl. Part. Phys. 44, 064002 (2017).
[2] S. Malbrunot-E/enauer et al., Phys. Rev. Le/. 128, 022502 (2022).
[3] S. Sels et al., Nucl. Instr. Meth. B 463, 310 (2020); V. Lagagki et al., Nucl. Instr. Meth. A 1014, 165663 (2021).

Speaker: Stephan Malbrunot-Ettenauer (CERN, ISOLDE)

09:30 COLLINEAR LASER SPECTROSCOPY ON THE PALLADIUM ISOTOPIC CHAIN

The use of high resolution optical measurements of the atomic structure is at the forefront of modern subatomic physics. Laser spectroscopy provides model-independent nuclear data of nuclear spins, moments and charge radii across long chains of isotopes~\cite{Campbell}. This allows the study of the evolution of nuclear observables versus particle number to probe shape deformation, configuration mixing and structural evolution effects.

The collinear laser spectroscopy setup~\cite{de Groote} at the IGISOL facility~\cite{Moore2013} in the Accelerator Laboratory of the University of Jyväskylä, has been used to perform measurements on palladium isotopes (Z=46) in the mass range A=98-118. Thanks to the chemically insensitive IGISOL ion-guide production method~\cite{Moore2014}, it has been possible to reduce the existing gap in optical spectroscopy data below Z=50, created by the refractory character of these elements.

This contribution will present the latest results on laser spectroscopy measurements on the Pd isotopic chain. Special attention will be paid to the magnetic dipole and electric quadrupole moments. The main results of trends in the mean-square charge radii have recently been accepted for publication~\cite{Geldhof}, nevertheless, new complementary results regarding the odd isotopes will be presented. These observables will be compared to state of the art theoretical calculations. Three different models for the interpretation of our data will be confronted, Large Scale Shell Model (LSSM), Fayans Energy Density Functionals (EDF) and Beyond-mean field calculations.

References:

P. Campbell et al. / Progress in Particle and Nuclear Physics 86 (2016) 127–180

R. P. de Groote et al. / Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 463 (2020): 437-440.

I.D. Moore et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 208–213

I.D. Moore et al. / Hyperfine Interactions volume 223, pages 17–62 (2014)

S. Geldhof et al. / Physical Review Letters 128.15 (2022)

Speaker: Alejandro Ortiz Cortes (GANIL)

09:50 Nuclear moments of indium isotopes reveal abrupt change at magic number 82

In this contribution, we present measurements of the nuclear magnetic dipole moments and nuclear electric quadrupole moments of the 113-131In isotope chain, performed using the Collinear Resonance Laser Spectroscopy experiment at ISOLDE, CERN.

We show that the electromagnetic properties of the neutron-rich indium isotopes significantly differ at N = 82 compared to N < 82, despite the single unpaired proton dominating the behaviour of this complex many-body system. This challenges our previous understanding of these isotopes, which were considered a textbook example for the dominance of single-particle properties in nuclei [1, 2].

To investigate the microscopic origin of our experimental results, we performed a combined effort with developments in two complementary nuclear many-body methods: ab-initio valence space in-medium similarity normalization group [3,4] and density functional theory [5].

When compared with our experimental results, contributions from previously poorly constrained time-odd channels [6,7], and many-body currents [8] are found to be important, demonstrating electromagnetic properties of ‘proton-hole’ isotopes around magic shell closures at extreme proton-to-neutron ratios can give us crucial insights.

[1] - K. Heyde. The Nuclear Shell Model. Springer Series in Nuclear and Particle Physics. Springer Berlin Heidelberg, Berlin, Heidelberg, 1990.
[2] - J. Eberz et al. Nuclear Physics A, 464(1):9–28, 1987
[3] - R. Stroberg et al. Annual Review of Nuclear and Particle Science, 69(1), 2019.
[4] - P. Gysbers et al. Nature Physics, 15(5):428–431, 2019.
[5] - J. Dobaczewski et al. J. Phys. G: Nucl. Part. Phys., 48(10):102001, 2021.
[6] – J.Engel and J. Menéndez. Reports on Progress in Physics, 80(4):046301, 2017
[7] – J. Dobaczewski et al. Phys. Rev. Lett., 121:232501, 2018.
[8] - S. Pastore et al. Phys. Rev. C, 87:035503, 2013.

Speaker: Adam Vernon (Massachusetts Institute of Technology)

NS2022_ARV2.odp

NS2022_ARV_pdf.pdf

10:10 Beta-delayed neutron emission near $^{54}$Ca

Beta-delayed neutron emission ($\beta$-n) plays a vital role in shaping the elemental abundance distribution in the $r$-process via modifying the decay path back to stability and by contributing significantly to the neutron flux after freeze-out [1]. Modeling the $\beta$-n process requires a good model of the beta-strength functions and of the neutron emission mechanism. Statistical neutron-emission models assume gamma and neutron decay from a compound nucleus following beta decay and have successfully predicted gross properties of $\beta$-n probabilities ($P_{1n}$, $P_{2n}$, etc) in some medium- and heavy-mass nuclei. However, recent experimental work found evidence of non-statistical neutron emission after the beta decay in the vicinity of doubly magic $^{132}$Sn [2]. Therefore, it is of great importance to study $\beta$-n spectroscopy in a broader area of the nuclear chart to provide stringent experimental constraints to the theories, which in astrophysical applications predict those properties for many more $r$-process nuclei currently out of experimental reach.

We expanded our study onto the nuclei near $^{54}$Ca, which is thought to be doubly magic. An experiment studying $\beta$-n spectroscopy of $^{52,53}$K was carried out at the ISOLDE Decay Station (IDS). These isotopes have large Q$_{\beta}$ values (energy window for $\beta$ decay) and low neutron-separation energies in their daughters ($^{52,53}$Ca respectively), making them ideal for the studies of the $\beta$-n process. In coincidence with the beta decay of $^{52,53}$K, gamma-ray and neutron-time-of-flight (TOF) spectra were measured using HPGe clover detectors and VANDLE [3]. In this contribution, I will present the latest results from the experiment, including the reconstructed excitation energies (Εx) and apparent beta feeding (I$_{\beta}$) of the neutron unbound states in $^{52,53}$Ca, together with their exclusive neutron-emission branching ratios to the states in $^{51,52}$Ca, respectively. The experimental findings were compared with the shell-model calculations and Hauser-Feshbach statistical model. These comparisons provide valuable insights into the $\beta$-n process and its
connection with the nuclear structure far from the stability line.

[1] R. Yokoyama et al, Phys. Rev. C 100, 031302(R) (2019).
[2] J. Heideman et al, being reviewed by Phys. Rev. Lett.
[3] M. Madurga et al, Phys. Rev. Lett 117, 092502 (2016).

Speaker: Zhengyu Xu (University of Tennessee, USA)

Xu_NS2022.pdf

10:30 → 11:00 AM Break

11:00 → 12:30 NS2022 Plenary: Light Nuclei II

Convener: Prof. D.G. Jenkins (Dept of Physics, Univ of York)

11:00 Ab initio description of nuclear reactions with applications to astrophysics

Thermonuclear reactions between light nuclei play an important role in explaining the origin and evolution of our universe, but are generally very difficult or even impossible to measure at the astrophysically relevant energies of tens to hundreds of keVs due to the hindering effect of Coulomb repulsion. As a result, they are almost always estimated by extrapolation from higher-energy measurements, a process that leads to significant uncertainty. Along with high-precision experimental data, first-principle (or ab initio) calculations with quantified uncertainties based on validated chiral nucleon-nucleon and three-nucleon forces can set a new standard for the evaluation of thermonuclear reaction rates that will lead to more robust predictive capabilities for astrophysical models. In this talk, I will present ab initio predictions of thermonuclear reactions obtained within the framework of the no-core shell model with continuum, including the recent ab initio informed evaluation of the $^7$Be(p,$\gamma$)$^8$B solar fusion reaction and discuss initial results and prospects on bridging ab initio calculations and three-body reaction models.

Prepared by LLNL under Contract No. DE-AC52-07NA27344.

Speaker: Sofia Quaglioni (Lawrence Livermore National Laboratory)

11:30 Short-range correlations in the mirror nuclei 3H and 3He

Short-range correlations (SRC) in nuclei - nucleon pairs with large relative momentum, but small total momentum - arise from the strong, short-distance NN interaction. These are important components of the nuclear ground state, but it is difficult to study their contributions in low-energy reactions. GeV-scale electron scattering measurements have been used to study scattering from SRCs in nuclei and to map out their strength and isospin structure in nuclei for a range of light and heavy nuclei. Measurements using two-nucleon knockout have shown that np-SRCs dominate over pp-SRCs, although these measurements involve measuring a three-particle final state and are limited by statistics and corrections for nucleon rescattering in the final state.

We present a new measurement providing an extraction of the isospin dependence of SRCs with dramatically improved precision over previous results by comparing inclusive scattering from the mirror nuclei 3H and 3He. In this experiment, the target provides the sensitivity to isospin structure using clean and well-understood inclusive scattering. We find that np-SRCs are enhanced relative to pp-SRCs in 3He, but this enhancement is significantly smaller than observed in heavier nuclei. Understanding these unexpected and, as yet, unexplained result will help illuminate the structure of these light nuclei and may allow us to better constrain the short-range part of the N-N interaction.

Speaker: John Arrington (Lawrence Berkeley National Laboratory)

11:50 Study the isomeric state of 16N via the $^{16}$N$^{g,m}$(d,$^{3}$He) reaction

We report the result of a study of the isomeric state in $^{16}$N via the $^{16}$N^{g,m}($d$,$^3$He)$^{15}$C reaction in inverse kinematics at 11.8 MeV/u using the HELIOS spectrometer. The radioactive 16N beam, with a 24(2)% isomer component, was produced using the ATLAS RAISOR in-flight separator at ANL. The simultaneous measurements of the reactions provided the most direct and reliable comparison of the spectroscopic factors due to the cancellation of most of the systematic uncertainties from the experiment and DWBA theory. The ratio of the spectroscopic factors between the $^{16}$N$^{g}$→$^{15}$C(0.74) and $^{16}$N$^m$→$^{15}$C(0.00) reactions was found to be 0.82±0.22. The result indicates that the s-orbital between 16Nm and 15C ground state is similar and the isomer state of 16N is an excited neutron halo state.

This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract Number DE-AC02-06CH11357 (ANL), DE-AC05-00OR22725 (ORNL), DE-SC0020451 (NSCL), and DE FG02-96ER40978 (LSU). This work was supported by the National Science Foundation, No. PHY-2012522 (FSU). This work was also supported in part by the UK Science and Technology Facilities Council Grants No. ST/P004423/1 (Manchester). This work also supported by the Hirose International Scholarship Foundation from Japan. This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility.

Speaker: Tsz Leung Tang (Florida State University)

12:10 Symmetries as a framework for understanding the emergence of nuclear collective behavior

Ab initio nuclear theory, for which the only input is the inter-nucleon interaction, can provide a deeper understanding of the origins of nuclear collective behavior. In no-core shell model (NCSM) calculations of p-shell nuclei, rotational bands with vastly different structure and deformation appear within the same nucleus (shape coexistence). To gain insight into the underlying symmetries and correlations which give rise to this emergent collective behavior, we decompose the calculated wave functions by symmetry content. In particular, we consider the symmetries associated with Elliott's SU(3) rotational model and its multishell extension Sp(3,R), which further incorporates giant monopole and quadrupole resonance degrees of freedom. Through the decompositions, we demonstrate that these symmetries provide a natural framework for understanding the emergence of rotational behvior in the $p$-shell and discuss how these symmetries can also be used to guide investigation of collective behavior in heavier nuclei.

Speaker: Anna McCoy (Institute for Nuclear Theory)

12:30 → 13:30 Lunch

13:30 → 15:00 NS2022 Plenary: Shapes and Collectivity III

Convener: Gordon Ball (TRIUMF)

13:30 Study of cross-shell excitations near the 'island of inversion' using fusion-evaporation and Doppler shift methods

The 'island of inversion' centered on $^{32}$Mg is characterized by ground state configurations with an inverted ordering of $sd$ and $pf$ (intruder) neutron orbitals due to nuclear deformation and nucleon-nucleon interactions. For neutron rich $sd$ shell nuclei outside of the 'island of inversion', similar configurations incorporating the neutron $pf$ shell occur in levels with high excitation energy and spin. Several recent studies have used fusion-evaporation reactions to preferentially populate and study these intruder states, including a recent experiment at the ISAC-II facility at TRIUMF in which the nuclides $^{25}$Na and $^{28}$Mg were produced following $^{12}$C + $^{18}$O fusion [1, 2].

In this experiment, fusion-evaporation exit channels were separated via time coincident identification of charged particles and gamma rays. Gamma-ray spectroscopy utilized the TIGRESS array at ISAC-II. Charged particles were detected and identified using a recently completed CsI(Tl) `ball' scintillator array, developed at Simon Fraser University and commissioned at TRIUMF [3]. Lifetime measurements of excited states populated in the channels of interest were performed using Doppler shift methods.

Six new excited states in $^{25}$Na and $^{28}$Mg were identified, including candidates for the $I^{\pi}=5^+_1,6^+_1$ levels in $^{28}$Mg. Evidence for negative parity states was also observed, including a candidate for the $I_{\pi} = 13/2^-_1$ level in $^{25}$Na and an unusually long-lived state in $^{28}$Mg thought to decay by an M2 transition ($I^{\pi}=(0,4)^{-}$). The energies of these levels are consistent with predicted intruder states arising from single neutron excitation to the $pf$ shell, using the SDPF-MU and FSU shell model interactions. This data and its interpretation with respect to the `island of inversion' will be discussed, along with future plans to extend this work towards $N=20$ by studying $^{32}$Si and other nearby nuclides populated following $^{12}$C + $^{22}$Ne fusion.

[1] J. Williams et al., PRC 100 014322 (2019).
[2] J. Williams et al., PRC 102 064302 (2020).
[3] J. Williams et al., NIM A 939 1-9 (2019).

Speaker: Jonathan Williams (TRIUMF)

JW_NS2022.pdf

13:50 Mapping the N = 40 Island of Inversion with neutron-rich iron isotopes at TITAN

Mass measurement facilities are extremely important in furthering our understanding of nuclear structure away from the valley of stability, including aiding in the search for collective behaviors in exotic nuclei. 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 Spectrometer (MR-ToF-MS) expanding its reach. The TITAN MR-ToF-MS was used in the measurement of neutron-rich iron isotopes around $N = 40$. These masses are critical in investigating a potential Island of Inversion at $N = 40$, which has been supported previously in literature by increased collectivity seen in this region. In total, the masses of $^{67-70}$Fe were measured, with $^{69}$Fe and $^{70}$Fe constituting first time measurements and $^{67}$Fe and $^{68}$Fe resulting in improvements over current literature uncertainties. The impact of these mass measurements on the presence of a surfacing Island of Inversion will be discussed.

Speaker: William Porter (University of Notre Dame)

Nuclear Structure 2022 Presentation.pdf

14:10 K=4+ Band-heads in 160Gd

Detailed spectroscopy of neutron-rich, deformed rare-earth nuclei is of broad interest for nuclear structure. The structure of nuclei midshell in both proton and neutron number helps to understand the evolution of deformed subshell gaps and the interplay of single-particle and collective degrees of freedom in these nuclei. High-statistics decay spectroscopy of $^{160}$Gd resulting from the $\beta$-decay of $^{160}$Eu was performed using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. An updated half-life for the $\beta$-decaying isomer in $^{160}$Eu was measured, and a number of newly observed excited states, transitions, lifetimes, $\gamma$-$\gamma$ angular correlations, and mixing ratios were measured for the first time in $^{160}$Gd. In particular, the lifetimes and angular correlations relating to the two $K^\pi=4^+$ band-heads in $^{160}$Gd were measured and shed light on the possible structure of these bands. Additionally, lifetime and mixing ratio measurements were performed for the proposed $(5^-)$ state at 1999 keV that is heavily populated by the ground-state $\beta$-decay of $^{160}$Eu. The measurements raise questions about the assigned spin and parity as well as underlying quasi-particle configurations for both the 1999 keV state in $^{160}$Gd and the ground-state of the parent $^{160}$Eu.

In this talk, I will discuss the new measurements and implications on the $K^\pi=4^+$ bands in regards to the interpretation of these bands as being strongly-mixed quasi-particle configurations versus hexadecapole vibrational bands, as well as an updated proposal for the configuration of the ground-state of the parent $^{160}$Eu.

Speaker: Daniel Yates (TRIUMF)

DYates_NS22.pdf

14:30 Configuration mixing investigation in Ge isotopes through E0 strength measurements

Experimental and theoretical studies of the germanium isotopes point increasingly toward exotic combinations of nuclear-structure effects, with indications of triaxiality, configuration mixing, and shape coexistence. Studies of the $E0$ strengths, which can provide a direct measure of the amount of configuration mixing, are lacking along the Ge chain. Thus, an experimental determination of $E0$ transition strengths is essential for an understanding of the evolution of structures in the germanium isotopes.

Beta-decay experiments populating excited states in the $^{72,74,76,78}$Ge isotopes were performed at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. The GRIFFIN spectrometer combined with the PACES silicon array enabled us to perform both gamma-ray and electron spectroscopic investigations, to measure $E0$ strengths between states of $J>0$. Preliminary results from this study will be discussed.

Speaker: Carlotta Porzio (LBNL)

CPorzio_NS2022.pdf

15:00 → 15:30 PM Break

15:30 → 17:00 NS2022 Plenary: Shells and Magicity II

Convener: Corina Andreoiu (Simon Fraser University)

15:30 Investigating the Nuclear Shell Evolution in Neutron-Rich Calcium

Nuclei away from the line of stability have been found to demonstrate behavior that is inconsistent with the traditional magic numbers of the spherical shell model. This has led to the concept of the evolution of nuclear shell structure in exotic nuclei, and the neutron-rich Ca isotopes are a key testing ground of these theories; there have been conflicting results from various experiments as to the true nature of a sub-shell closure for neutron-rich nuclei around $^{52}$Ca. In November of 2019, an experiment was performed at the ISAC facility of TRIUMF; $^{52}$K, $^{53}$K, and $^{54}$K were delivered to the GRIFFIN gamma-ray spectrometer paired with the SCEPTAR and the ZDS ancillary detectors for beta-tagging, as well as DESCANT for neutron-tagging. Using this powerful combination of detectors we combine the results to construct level schemes for the isotopes populated in the beta-decay. Preliminary results from the analysis of the gamma, beta, and neutron spectra will be presented and discussed in the context of an N=32 shell closure in neutron-rich nuclei.

Speaker: Robin Coleman (University of Guelph)

Coleman_Structure2022.pptx

15:50 New experimental decay information of $\beta$-delayed neutron emitters for very exotic neutron-rich nuclei in the north-east of double magic $^{78}$Ni region

New experimental $\beta$-decay half-lives, $\beta$-delayed neutron emission probabilities (P$_{xn}$), and $\gamma$ ray for neutron-rich nuclei in the $A\approx$90-100 region are of interest for nuclear structure, industrial applications, and astrophysics. The scarce experimental data in very the neutron-rich regions involved in the $r$-process nucleosynthesis is the main source of uncertainty of the astrophysical models to better understand the observed abundance distribution, since they rely heavily on theoretical data for yet unmeasured nuclei. The neutrons released during the decays of $\beta$-delayed neutron emitters in these regions play an important role in late phases of the $r$-process during the freeze-out by shifting decaying material to different mass chains and providing additional neutrons for further capture reactions.

Since 2016, the BRIKEN collaboration has investigated several neutron-rich regions between $A$=70 and $A$=200 with the aim of determining properties of hundreds of isotopes with unknown or incomplete decay information that were not accessible before. The experimental setup at the fragmentation RIB facility of RIKEN in Japan consisted of a DSSDs array to register the implanted ions and $\beta$-decays, and the BRIKEN neutron counter, which also included two HPGe clover-type $\gamma$-ray detectors for high resolution spectroscopy.

In this contribution, I will report the $\beta$-decay half-lives, P$_xn$ values, and preliminary $\gamma$ information of neutron-rich Ge, As, Se, and Br nuclei around mass A$\approx$90, north-east of the doubly-magic $^{78}$Ni.The implications of the results in theoretical models and astrophysical simulations will also be discussed.

Speaker: Dr Roger Caballero-Folch (TRIUMF)

16:10 Beta decay strength of $^{78}$Ni to neutron-unbound states revealed by $^{79}$Cu

$^{78}$Ni is one of the few radioactive doubly-magic nuclei on the nuclear landscape [1]. It has a large neutron-to-proton ratio and engages in beta-delayed neutron and gamma emissions. Decays of Cu isotopes with significant neutron excess provide a laboratory to study such doubly-magic nuclei. The nuclear shell structure of $^{79}$Cu (N=50; Z=29) is highly similar to the one of $^{78}$Ni (N=50; Z=28), except an extra proton above the Z=28 shell gap in $1f_{5/2}$ orbital [2]. The similarities in the nuclear shell structure are also reflected in the beta decay properties. $^{79}$Cu is also a strong beta-delayed neutron emitter, which allows us to infer beta-strength function $(S_{\beta}(E))$ to neutron-unbound states in the beta decay of $^{78}$Ni.

The determination of $(S_{\beta}(E))$ requires measurement of the intensity of neutron resonances post beta decay. Neutron-gamma coincidences are also vital to arranging strength with excitation energy. An experiment for measuring the strength distribution in the $^{78}$Ni region (27$\geq$Z$\geq$33) was performed at the Radioactive Ion Beam Factory (RIBF) facility at RIKEN Nishina Center, JAPAN using a YSO-based implantation detector [3] and VANDLE [4] array for Time-of-Flight-based spectroscopy of the beta-delayed neutrons. For $\gamma$-ray detection, two HPGe clovers and 10 $LaBr_{3}$ detectors were used.

This work reports the first direct measurements of beta-decay strength to neutron-unbound states in the decay of $^{81, 80, 79}$Cu crossing over the N=50 shell closure. The results from the experiment are compared to NuShellX [5] calculations with various sets of single-particle energies and residual interactions. Finally, $^{78}$Ni beta decay is predicted based on the model best describing $^{79}$Cu. The results reveal neutron and proton spin-orbit partners providing significant contributions to $(S_{\beta}(E))$ in the beta decay of $^{78}$Ni. The results also reveal the impact of the N=50 on the shell gap in the decay of $^{80, 81}$Cu.

$\textbf{References}$
1. R. Taniuchi $\textit{et al.}$, Nature 569, 53–58 (2019).
2. L. Olivier $\textit{et al.}$, Phys. Rev. Lett. 119, 192501 (2017).
2. M. Singh $\textit{et al.}$, (drafted).
4. W.A. Peters $\textit{et al.}$, Nucl. Instrum. Methods Phys. A 836, 122 (2016).
5. NuShellX, W.D.M. Rae, http://www.garsington.eclipse.co.uk.

Speaker: Maninder Singh (University of Tennessee, Knoxville)

Nuclear_structure_Maninder_Singh.pdf

16:30 First β-decay spectroscopy of 135In and new β-decay branches of 134In

The $\beta$ decay of the neutron-rich $^{134}$In and $^{135}$In was investigated experimentally with the aim of providing new insights into the nuclear structure of the tin isotopes above $N=82$. Better understanding of exotic nuclides from the $^{132}$Sn region is required for accurate modeling of the rapid neutron capture nucleosynthesis process ($r$ process), due to the $A \approx 130$ peak in the $r$-process abundance pattern being linked to the $N=82$ shell closure [1, 2]. Because a vast number of nuclei involved in the r process are $\beta$-delayed neutron ($\beta n$) emitters, new experimental data that can verify and guide theoretical models describing $\beta n$ emission are of particular interest. Neutron-rich isotopes $^{134}$In and $^{135}$In – being rare instances of experimentally accessible nuclides for which the $\beta 3n$ decay is energetically allowed [3] – constitute representative nuclei to investigate the competition between $\beta n$ and multiple-neutron emission as well as the $\gamma$-ray contribution to the decay of neutron-unbound states.

The $\beta$-delayed $\gamma$-ray spectroscopy measurement was performed at the ISOLDE Decay Station. Three $\beta$-decay branches of $^{134}$In were established, two of which were observed for the first time [4]. Population of neutron-unbound states decaying via $\gamma$ rays was identified in the two daughter nuclei of $^{134}$In, $^{134}$Sn and $^{133}$Sn, at excitation energies exceeding the neutron separation energy by 1 MeV. The $\beta n$- and $\beta 2n$-emission branching ratios of $^{134}$In were determined and compared with theoretical calculations. The $\beta n$ decay was observed to be dominant $\beta$-decay branch of $^{134}$In even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of $^{134}$Sn. Transitions following the $\beta$ decay of $^{135}$In are reported for the first time, including $\gamma$ rays tentatively attributed to $^{135}$Sn [4]. A transition that might be a candidate for deexciting the missing neutron single-particle $\nu 1i_{13/2}$ state in $^{133}$Sn was observed in both $\beta$ decays and its assignment is discussed. Experimental level schemes of $^{134}$Sn and $^{135}$Sn are compared with shell-model predictions, including calculations considering particle-hole excitations across the $N=82$ shell gap [5].

[1] B. Pfeiffer, K. L. Kratz, F. K. Thielemann, and W. B. Walters, Nucl. Phys. A 693, 282 (2001).
[2] M.R. Mumpower, R. Surman, G.C. McLaughlin, and A. Aprahamian, Prog. Part. Nucl. Phys. 86, 86 (2016).
[3] M. Wang, W. J. Huang, F. G. Kondev, G. Audi, and S. Naimi, Chin. Phys. C 45, 030003 (2021).
[4] M. Piersa-Siłkowska et al. (IDS Collaboration), Phys. Rev. C 104, 044328 (2021).
[5] H. Jin, M. Hasegawa, S. Tazaki, K. Kaneko, and Y. Sun, Phys. Rev. C 84, 044324 (2011).

Speaker: Monika Piersa-Silkowska (CERN)

Friday, 17 June

09:00 → 10:30 NS2022 Plenary: Fundamental Symmetries

Convener: M. Wiedeking (iThemba LABS)

09:00 Sub-keV Decay-Recoil Spectroscopy with Superconducting Quantum Sensors

The recoiling daughter nucleus in weak nuclear decay processes ($\beta$ or EC decay) is a unique probe for a wide range of nuclear structure properties and BSM physics scenarios. In particular, precision measurements of the recoil can be used to access model-independent information on the neutrino mass, the $\beta$-$\nu$ angular correlation, precise excited- and ground-state energies, and decay branching fractions. Despite the unique information contained in these recoiling nuclei, few cases have been studied due to the technical challenges associated with measuring heavy ions with eV-scale kinetic energies with high resolution. To address this, we have developed a program to perform these low-energy measurements using implanted rare isotopes in thin-film superconducting tunnel junctions (STJs). STJs are a cryogenic-charge sensitive quantum sensor technology that allow for count rate of up to 10 kHz/pixel, provide energy resolutions of 1-5 eV, and have low-energy thresholds of a few eV [1]. In this talk I will discuss this novel experimental concept and present data on the eV-scale measurements we have performed with STJs using $^7$Be (T$_{1/2}$ = 53 days) EC decay relevant to structure and astrophysics [2] and BSM neutrino physics [3,4]. I will also present plans on extending this concept to perform measurements on short-lived species on-line at RIB facilities, and how our community can better leverage these technologies for high-impact structure measurements.
[1] S. Friedrich et al., J. of Low Temp. Phys. 200, 200 (2020)
[2] S. Fretwell et al., Phys. Rev. Lett. 125, 032701 (2020)
[3] K.G. Leach and S. Friedrich, arXiv: 2112.02029 (2021)
[4] S. Friedrich et al., Phys Rev. Lett. 126, 021803 (2021)

Speaker: Dr Kyle Leach (Colorado School of Mines)

09:30 Precision spectroscopy studies of radioactive molecules for fundamental physics

Precision molecular experiments provide a unique tool for the measurement of electroweak nuclear properties and searches for physics beyond the Standard Model (SM). Compared to atoms, certain molecules can offer more than eleven orders of magnitude enhanced sensitivity to violations of fundamental symmetries, enabling precision tests of the SM and the possibility to probe energy scales beyond hundreds of TeV. Containing octupole-deformed nuclei, radium monofluoride (RaF) molecules are expected to be highly sensitive to the parity-violating nuclear anapole moment as well as to the parity- and time-reversal violating nuclear Schiff and magnetic quadrupole moments [Phys. Rev. A 82, 052521 (2010); J. Chem. Phys. 152, 044101 (2020)]. In this talk, I will present the latest results obtained from a series of laser spectroscopy experiments performed on short-lived RaF molecules at the ISOLDE facility at CERN. Using a collinear resonant ionization setup, the rotational and hyperfine structure of $^{226}$RaF and $^{225}$RaF were measured with high precision. This allowed us to establish a laser cooling scheme for these molecules, and to extract the value of the nuclear magnetic moment of the $^{225}$Ra nucleus. Our new results exhibit high sensitivity to the distribution of the nuclear magnetization in the $^{225}$Ra nucleus and represent an increase in precision of at least 3 orders of magnitude compared to our previous studies [Nature 581, 396 (2020); Phys. Rev. Lett. 127, 033001 (2021)]. They are the first of their kind performed on radioactive, short-lived molecules, opening the way for future precision studies of electroweak nuclear properties and new physics searches in these systems.

Speaker: Silviu-Marian Udrescu (Massachusetts Institute of Technology)

Nuclear_structure_2022_v2.pptx

09:50 Searching for Tensor Currents in the Weak Interaction in the $\beta$-decay of $^{8}$Li and $^{8}$B

The weak interaction of the Standard Model is well-described by a vector-axial vector, or ‘V-A’, structure, which reproduces both maximal parity violation and left-handed neutrinos in beta- decay. However, there is no first-principles reason that other the interactions, such as scalar (S) or tensor (T), may not be present. To search for possible tensor contributions to the weak interaction we measure the beta-neutrino correlation coefficient a$_{\beta\nu}$ in the Gamow-Teller beta-decays of $^{8}$Li and $^{8}$B. The $A = 8$ decays are ideal to probe a$_{\beta\nu}$ due to the large Q-value and delayed alpha emission from the excited state $^{8}$Be. These measurements are completed with the Beta decay Paul Trap (BPT) at Argonne National Lab. The BPT is surrounded on 4 sides with double-sided silicon strip detectors backed by plastic scintillator detectors, which allow the kinematics of decay products from the trapped mass-8 ions to be over-constrained. We will present results from a high statistics measurement from the decay of $^{8}$Li, the first results from the decay of $^{8}$B, and the first R-Matrix fit of the $^{8}$B $\beta$-decay final state distribution.

We acknowledge NSERC, Canada, App. No. 216974, the U.S. DOE Contract No. DE-AC02- 06CH11357 [ANL] and DE-AC52-07NA27344 [LLNL], NSF grant no. 1144082 and the ANL ATLAS facility.

Speaker: Aaron Gallant (LLNL)

10:10 Evidence of seniority conservation via lifetimes measurements in the N=50 isotones towards 100Sn

The experimental evidence of the seniority conservation is a direct evidence of the validity of the short-range pairing interaction, with far-reaching implications for nuclear structure in the validity of the BCS theory and therefore of the quasiparticle representation of the atomic nucleus [1]. In theory, this symmetry is preserved up to $j<=7/2$ and, contradictory experimental results exist for orbitals with larger angular momenta. In order to shed light on the open question of the seniority conservation in the proton $g_{9/2}$ orbital in the $N=50$ isotones, reduced transition probabilities in $^{90}$Zr, $^{92}$Mo and $^{94}$Ru nuclei, have been determined experimentally for the first time via lifetime measurements at the GANIL laboratory. The unconventional use of multi-nucleon transfer reaction [2] with a differential plunger device [3] allowed to measure lifetimes of the yrast low-spin states despite the presence of isomers in the proton-rich isotones. The required sensitivity to the lifetimes could only be achieved with the AGATA+VAMOS++ detection system [4,5].
The reduced transition probabilities for the $4^+\rightarrow 2^+$ and $2^+\rightarrow 0^+$ yrast transitions in $^{92}$Mo and $^{94}$Ru and for the $4^+\rightarrow 2^+$ and $6^+\rightarrow 4^+$ yrast transitions in $^{90}$Zr determined in this experiment will be discussed in this contribution and, the results, will be interpreted on the basis of realistic shell-model calculations in the $f_{5/2}$, $_{p3/2}$, $_{p1/2}$, $g_{9/2}$ proton valence space, where it emerges that seniority is conserved in the first $\pi g_{9/2}$ orbital [6]. The results are relevant as well in the understanding of the evolution of the nuclear effective interaction in the $Z=28$ isotopes towards $^{78}$Ni, located much further away from the stability line than the $N=50$ isotones.

[1] S. Cohen et al., Phys. Lett. 10, 195 (1964)
[2] R. Broda et al., Phys. Lett. B 251, 245 (1990)
[3] A. Dewald et al., Prog. Part. Nucl. Phys. 67, 786 (2012)
[4] S. Akkoyun, et al., Nucl. Instr. Meth. Phys. Res. A 668, 26 (2012)
[5] M. Rejmund, et al., Nucl. Instr. Meth. Phys. Res. A 646,184 (2011)
[6] R.M. Pérez-Vidal, et al. Submitted to Phys. Rev. Lett.

Speaker: Rosa María Pérez Vidal

10:30 → 11:00 AM Break

11:00 → 12:30 NS2022 Plenary: Shapes and Collectivity IV

Convener: Yang Sun

11:00 Nuclear structure of the neutron-deficient cadmium isotopes close to $^{100}$Sn

The heaviest $T_z = 0$ doubly-magic nucleus, $^{100}$Sn, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction. From Cd to Te, many common features and phenomena have been observed experimentally along the isotopic chains, leading to theoretical studies devoted to a more general and comprehensive study of the region.

Having only two proton holes in the $Z = 50$ shell, the Cd isotopes are expected to present properties similar to those found in the Sn isotopic chain. Therefore, one may expect that the experimental information on the $Z = 48$ nuclei may not only be important in itself, but it may also provide an insight into the structure of the corresponding $Z = 50$ isotones.

While Sn isotopes have been considered the paradigms of pairing dominance for decades, the cadmium isotopes have been seen as a textbook example of harmonic quadrupole-vibrational nuclei. On the other hand, the electromagnetic properties of the Cd isotopes, i.e., quadrupole moments and transition strengths, put their vibrational character in doubt. However, the lack of precise experimental information makes it difficult to assess whether the vibrational picture still holds for the neutron-deficient species.

In order to obtain a precise information on the structure of neutron-deficient cadmium isotopes and on the nuclear interaction in the region close to $^{100}$Sn, a multi-nucleon transfer reaction was used together with the Recoil Distance Doppler-Shift method. This allowed to directly measure the lifetime of low-lying states in $^{102-108}$Cd. The transition strengths corresponding to the measured lifetimes were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach.

Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the $Z = 48$ and $Z = 50$ isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with $\beta_2 \approx 0.2$. At this deformation $Z = 48$ becomes a closed-shell configuration, which is favored with respect to the spherical one.

Speaker: Marco Siciliano (Argonne National Laboratory)

Siciliano_LifetimeCd.pdf

11:20 First results from CLARION2-TRINITY: Coulomb Excitation of 49Ti and the proton-neutron interaction

The first results from CLARION2-TRINITY, a new charged-particle and HPGe array are presented: Coulomb excitation of $^{49}$Ti. Ti-49 can be treated as a neutron hole plus semimagic $^{50}$Ti core within the particle-core coupling scheme. Reduced electric quadrupole transition probabilities, or $B(E2)$ strengths, for the $2^{+} \otimes f_{7/2}$ multiplet members and candidate $p_{3/2}$ state were measured. The total electric quadrupole strength of $^{49}$Ti is compared to the $B(E2; 0^+ \rightarrow 2^+)$ of the $^{50}$Ti core in search of enhanced quadrupole collectivity, similar to that recently observed in $^{129}$Sb relative to a $^{128}$Sn core [1]. Both cases are near double-magic nuclei and have small core $B(E2)$ values. The results are compared to shell-model calculations with state-of-the art nucleon-nucleon interactions. Any enhancement is in stark contrast to the expectations of the successful particle-core coupling scheme and it is thought to arise when the long-range part of the proton-neutron (PN) residual interaction rapidly develops compared to the short-range pairing interaction (PP or NN). Enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

[1] T.J. Gray, et al., Phys. Rev. Lett. 124, 032502 (2020)

*This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics.

Speaker: Dr Timothy Gray (Oak Ridge National Laboratory)

11:40 Prediction of nuclear masses with an artificial neural network

The many-body Hamiltonian that describes atomic nuclei is exceedingly complex and remains unknown. Thus, the lowest energy state cannot be calculated directly from first principles for heavy nuclei. This has lead to many theoretical endeavors to predict the ground-state properties based on the mean-field concept. In a novel and alternative approach, we show that it is possible to predict nuclear masses directly from a probabilistic artificial nuclear network. We use approximately ~25% of the Atomic Mass Evaluation for training and predict the remaining ~75%. We achieve extraordinary accuracy on the order of ~250 keV for the predictions. In contrast to contemporary mass models, each mass prediction is associated with a corresponding uncertainty. We discuss the interpretability of our model and impactful trends observed in our mass and uncertainty predictions.

Speaker: Matthew Mumpower (Los Alamos National Laboratory)

12:00 Microscopic calculations of neutron-rich isotopes near 40Mg

Nuclides near the neutron drip line are the frontier in nuclear structure experiment and theory. But full configuration-interaction (FCI) calculations of such nuclides, which provide detailed spectra, can be computationally prohibitive. Instead, we approximate FCI calculations by embedding so-called ``beyond mean-field'' methods in a shell-model framework. Previous work showed that angular-momentum projected Hartree-Fock calculations, especially when exploiting multiple shape minima, provide surprisingly good excitation spectra (as well as systematics of binding energies such as odd-even staggering), in a variety of nuclides, including odd-A and odd-odd nuclides. Here we generalize by adding additional reference states, specifically occupation-representation of deformed Slater determinants, which can reproduce features of complex spectra such as multiple J^pi = 0+ states, and apply these methods to nuclides around 40Mg. This hybrid approach provides a flexible and computationally tenable framework for modeling nuclides as experiment continues to probe towards the driplines.

This work was supported by the U.S. Department of Energy, Office of Science, under award number DE-FG02-03ER41272.

Speaker: Stephanie Lauber (San Diego State University)

12:30 → 13:30 Lunch

13:30 → 14:30 NS2022 Plenary: Closing Session

Convener: Heather Crawford

13:30 Poster Session Winner I

13:45 Poster Session Winner II

14:00 Closing Remarks