Nuclear Physics Forum

The LBNL Nuclear Physics Forum (NP Forum) is a weekly (more or less) seminar series covering topics in low-energy nuclear science, including nuclear structure, nuclear chemistry and heavy element research, applied physics and much more. The NP Forum series involves the nuclear structure group, heavy element research group and research groups from the UC Berkeley campus. NP Forums are typically scheduled for 11am on Thursdays in the Building 88 2nd floor conference room area, with refreshments (coffee + cookies) available from 10:45am.

2020-2022 Pandemic NP Forum Series

Upcoming Forums...

Wednesday, July 27, 2022, 9:00 AM

Zoom ID: 978 6589 4969 Password: 772614

Dr. Aaron M. Hurst, Department of Nuclear Engineering, University of California, Berkeley

"Neutron-induced gamma rays and radioactive-decay data for basic science and applications"

Patterns of gamma rays produced in neutron-induced reactions provide “fingerprints” characteristic of an absorbing medium that are useful for determining isotopic composition in nondestructive-assay (NDA) applications, e.g., active interrogation. At thermal neutron-induced energies, the distinctive gamma-ray signature usually comes from radiative-capture reactions whereupon high-energy primary gamma rays originating at the capture state, in the region near the neutron-separation energy, are observed. However, neutrons are generally born fast, e.g., in a fission spectrum, and are far more likely to undergo different types of interactions at these higher energies. Indeed, for many materials in the 0.1-10 MeV incident neutron-energy range, inelastic scattering is one of the leading neutron-moderating mechanisms and often provides the dominant contribution to the gamma-ray spectrum, thus, providing an important signature of the isotope being interrogated. In the first part of this talk, I will present select examples of cross-section measurements and data, evaluated by comparison with statistical-model calculations utilizing appropriate photon strength function and nuclear level-density models, that will help augment and improve the neutron-data libraries sourced in NDA applications. Additionally, I will show where the statistical model has proven successful in improving nuclear structure data.

The second part of this presentation concerns a coincidence gamma/gamma and gamma/X-ray database that has recently been developed sourcing radioactive-decay data from the Evaluated Nuclear Structure Data File (ENSDF). This database will be used in support of a robust portable gamma/X-ray coincidence detector system concurrently under development at the Pacific Northwest National Laboratory. Current fieldable spectroscopy techniques often use single detector systems heavily impacted by interferences from intense background radiation fields. These effects may result in low-confidence measurements that could lead to misinterpretation of the collected spectrum. Hitherto, no database exists containing coincident gamma/gamma and gamma/X-ray branching ratios on an absolute scale that, together with the coincident-detector system, will greatly enhance isotopic identification for in-field applications, and help improve interpretation of fission products and short-lived radionuclides produced following a nuclear event. Because the database contains the original ENSDF data for all alpha, beta-minus, and electron-capture/beta-plus radionuclides, translated into a JavaScript Object Notation (JSON) format, it also offers potential as a convenient searchable tool for basic science applications.

Past Forums...

Wednesday, February 20, 2019, 11:00 AM

Prof. Michael Block, GSI Helmholtzzentrum fur Schwerionenforschung

"Precision Measurements in the Region of the Heaviest Elements"

Thursday, February 28, 2019, 11:00 AM

Dr. Boris Pritychenko, NNDC, Brookhaven National Laboratory

"New Approach for Determination of Stellar Nucleosynthesis Abundances"

Thursday, March 7, 2019, 11:00 AM

Prof. Kyle Leach, Colorado School of Mines

"The Current Status of Vud and the Top-Row CKM Unitarity Test: How Did We Get Here?"

Thursday, March 21, 2019, 11:00 AM

Dr. Darren Bleuel, Lawrence Livermore National Laboratory

"Neutrons at the 88-Inch Cyclotron"

Thursday, April 18, 2019, 11:00 AM

Prof. Ritu Kanungo, St. Mary's University

"Reaction spectroscopy unveils new features in rare isotopes"

Thursday, May 2, 2019, 11:00 AM

Dr. Alex Zylstra, Lawrence Livermore National Laboratory

"Studying Nuclear Astrophysics with Laser-Generated High-Energy-Density Plasmas"

Friday, May 17, 2019, 11:00 AM

Prof. Umesh Garg, University of Notre Dame

"Nuclear Incompressibility: How Collective Excitation Modes of a Nucleus Characterize Astrophysical Processes"

Thursday, May 23, 2019, 10:00 AM

Dr. Jack Henderson, Lawrence Livermore National Laboratory

"Nuclear Collectivity and Deformation"

Thursday, November 14, 2019, 11:00 AM

Prof. Alfredo Poves

"Shape Coexistence and Islands of Inversion"

Tuesday, December 10, 2019, 11:00 AM

Dr. Bethany Goldblum, University of California Berkeley

"Nuclear Science for National Security Applications"

Thursday, December 12, 2019, 10:30 AM

Jonas Karthein, CERN & Universitat Heidelberg, Germany

"High-precision mass measurements for nuclear structure and astrophysics studies"

Monday, December 16, 2019, 11:00 AM

Dr. Fnu Krishichayan, TUNL and Duke University

"Nuclear data activities at TUNL relevant to NNSA interests and isotope production"

Thursday, August 27, 2020

11:00 AM

ZOOM ID: 932 0238 3806; Password: NPForum27

Dr. Michael Skulski

Department of Physics, University of Notre Dame

"Development of 129I AMS at the NSL for Measurements of the Great Lakes Region"

The radionuclide 129I, with a half-life of 15.7 × 106 years, is produced naturally on Earth in the atmosphere through cosmic-ray-induced reactions on xenon isotopes, as well as through spontaneous fission of terrestrial uranium. These contributions to the 129I content on Earth pale in comparison, however, to the amount that has been released by spent nuclear fuel reprocessing centers, having contributed approximately 60 times the natural content. 129I can move very efficiently through the environment, and because of its “point-like” releases from reprocessing centers, 129I has the potential to be a powerful environmental tracer. For this capability to be realized, the 129I distribution throughout the environment needs to be established – previous measurements have primarily focused on nuclear facilities and nuclear disaster sites, leaving a large area of the globe unmeasured. This inspired the Collon group of the Nuclear Science Laboratory (NSL) of the University of Notre Dame to contribute data on 129I levels, specifically in the Great Lakes region. Water samples were collected from Lake Michigan and rivers throughout Michigan and analyzed for their 129I content through the technique of Accelerator Mass Spectrometry (AMS) for separation from the primary interference, 127I, using the time-of-flight technique. The AMS program at the NSL, the development of 129I AMS at the NSL, results of the water survey, and future work on 129I AMS and other AMS projects will be discussed.

Wednesday, May 5, 2021

9:00 AM

ZOOM ID: 948 3122 3509; Password: Nuclei

Mr. Axel Frotscher

Institut fur Kernphysik, TU Darmstadt

"The (p,3p) two-proton removal from neutron-rich nuclei and the development of the STRASSE tracker"

The knockout of nucleons from nuclei is a powerful tool to investigate nuclear structure. It was observed in several occurrences that different final states in a nucleus are populated when produced from one nucleon knockout (p,2p) or from two nucleon knockout (p,3p). The understanding of the latter could provide a new tool for nuclear spectroscopy.

Two experimental campaigns conducted at the RIBF in RIKEN, Japan, were investigated. The scattered proton angular distribution from several neutron-rich medium-mass nuclei were analysed. The radioactive nuclei were impinging onto a 100-mm long liquid hydrogen target. The protons issued from the reaction were measured with the MINOS time-projection chamber surrounding the target, giving access for the first time to angular correlations of the three protons in the final state. The obtained proton distributions were benchmarked against kinematical models assuming three different reaction mechanisms.

To improve the vertex-resolution and enable missing-mass spectroscopy, STRASSE is being developed. The silicon tracker array is configured in a hexagonal shape in two layers around a liquid H2 target. In the future STRASSE with its excellent vertex resolution can be combined with other γ-detector arrays (such as AGATA or GRETA) to perform high resolution in-beam γ-ray spectroscopy using low intensity fast beams.

Wednesday, May 19, 2021

9:00 AM

ZOOM ID: 938 6185 8697; Password: Nucleus

Mr. Anton Samark-Roth

Department of Physics, Lund University

"Spectroscopy along Decay Chains of Element 114, Flerovium"

In the wake of the discovery of superheavy elements, nuclear spectroscopy experiments aim at providing anchor points at the uppermost end of the nuclear chart for nuclear structure theory, which otherwise had to solely rely on extrapolations. In two runs in 2019 and 2020, such a nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z = 114).

The experiment conducted at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany, employed an upgraded TASISpec decay station placed behind the gas-filled separator TASCA. The fusion-evaporation reactions 48Ca+242Pu and 48Ca+244Pu provided a total of 32 flerovium-candidate decay chains in effectively 18 days of beam time. Two and eleven decay chains were firmly assigned to even-even 286Fl and 288Fl isotopes, respectively. The – admittedly unexpected – observations include (i) an excited 0+ state at 0.62(4) MeV excitation energy in 282Cn, and (ii) a Qα=9.46(1) MeV decay branch (1 out of 51) from 284Cn into 280Ds [1]. Both observations indicate that there is hardly any shell gap at proton number Z = 114 - at least not at neutron numbers N ≈ 172-174. The remaining decay chains stemming from 289Fl indicate the presence of α-decay fine structure as has been theoretically predicted for odd-A Fl-decay chains. This is the focus of an ongoing analysis [2].

[1] A. Såmark-Roth et al., Phys. Rev. Lett., 126, 032503 (2021).

[2] D.M. Cox et al., to be submitted to Phys. Rev. Lett.

Wednesday, June 9, 2021

8:00 AM

ZOOM ID: 974 0263 1730; Password: Nucleus

Ms. Carlotta Porzio

University of Milan

"Configuration mixing investigation in germanium isotopes through measurement of E0 transition strengths"

Experimental and theoretical studies of the germanium isotopes point increasingly toward the emergence 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. Thus, determining E0 transition strengths is essential for an understanding of the evolution of structures in the Ge isotopes.

Beta-decay experiments populating excited states in the 72,74,76,78Ge isotopes were performed at the TRIUMF-ISAC radioactive beam facility. 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.

Thursday, July 8, 2021

8:00 AM

ZOOM ID: 970 9765 0085; Password: Nucleus

Dr. Özge Aktas

KTH Royal Institute of Technology, Stockholm, Sweden

Gamma-Ray Spectroscopy of Neutron-Rich 111Mo, 85,87Ge and Self-Conjugate 88Ru Far From Stability

Excited states in the neutron-rich nucleus 111Mo were studied following nucleon knockout reactions. 7 γ-ray transitions were identified for the first time using the DALI2 and MINOS detector systems at the BigRIPS and ZeroDegree electromagnetic fragment separator at the RIBF, RIKEN, Japan. Total Routhian surface (TRS) and Particle-Plus-Rotor calculations have been performed to investigate the predicted shape coexistence and its effect on the structure of nuclei in this region of the nuclear chart. Following the results of the calculations, theoretical level schemes are proposed for positive and negative parity states and compared with the experimental findings.

Gamma-ray transitions have been identified for the first time in the extremely neutron-rich (N = Z + 25) nucleus 87Ge following nucleon knockout reactions studied at the RIBF, RIKEN, Japan. Previously unknown γ-ray transitions between excited states in 85Ge were also observed and placed in a tentative level scheme. The results are compared with large-scale shell model (LSSM) calculations and potential energy surface calculations based on the total Routhian surface formalism.

The neutron-deficient self-conjugate (N=Z) nucleus 88Ru was populated via the heavy ion fusion evaporation reaction 54Fe(36Ar, 2n)88Ru in an experiment performed at the GANIL accelerator laboratory in France. Using the AGATA γ-ray spectrometer together with ancillary detectors, prompt γ−γ−2n coincidence and charge particle anticoincidence analysis was performed for the low-lying energy spectrum of 88Ru. The results confirm the previously assigned γ-ray cascade and extend it to the 14+ level. The level scheme is consistent with a deformed rotational system. However, the rotational frequency of the alignment of the valence nucleons has a significantly higher value than what is predicted by theoretical calculations performed without isoscalar neutron-proton pairing. By including isoscalar pairing, an agreement is obtained with the experimentally observed delayed rotational alignment.

Friday, July 9, 2021

8:00 AM

ZOOM ID: 967 4554 8394; Password: Nucleus

Dr. Xiaoyu Liu

KTH Royal Institute of Technology, Stockholm, Sweden

Experimental Studies of the Neutron Deficient Atomic Nuclei 88Ru and 87Tc, and the Diagonalization of the Pairing Hamiltonian

The low-lying excited states in the N=Z nucleus 88Ru and N=Z+1 nucleus 87Tc were studied via fusion-evaporation reactions induced by 115 MeV 36Ar ions bombarding 6 mg/cm2 thick metallic 54Fe target foils at the Grand Accélérateur National d’Ions Lourds (GANIL), Caen, France. The prompt γγ-neutron and charged-particle coincidences from the de-excitation of the reactions were measured by the AGATA γ-ray spectrometer coupled to the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays. The results for 88Ru confirmed and extended the previous level scheme to a tentative (14+) state. The constructed level structure exhibits a moderately deformed rotational behavior but shows a band crossing at a significantly higher rotational frequency compared with neighboring nuclei with N>Z. The observation of a “delayed” rotation alignment in the deformed N=Z nucleus is consistent with theoretical predictions related to the existence of strong isoscalar neutron-proton pair condensate. The yrast band in 87Tc from the (9/2+) state to the (33/2+) state was established based on six mutually coincident γ-ray transitions. The constructed yrast band exhibits a sharp back bending at ω ≈ 0.50 MeV. In the odd-A isotonic chains around N = 44, approaching the N = Z line, the observed decrease in alignment frequency and increase in alignment sharpness were proposed as an effect of the enhanced isoscalar neutron-proton interactions. In addition to the yrast band in 87Tc, six new mutually coincident γ-ray transitions were identified by comparing the γ-ray intensities in the spectra gated under different reaction channel selection conditions. The constructed level scheme was compared with the shell model and TRS calculations. The results indicate that these low-lying states exhibit spherical behavior different from the previously identified oblate yrast band, and the band might be built on a (7/2+) ground state. In the second part, two OpenMP parallel Fortran programs, PairDiag and PairDiagSph, for the diagonalization of the pairing Hamiltonian were developed. The principles of the method will be introduced.

Thursday, January 20, 2022, 12:00 PM

Zoom ID: 982 0333 4147 Password: 642577

Dr. Mustafa Abdallah, Purdue University, Department of Electrical and Computer Engineering

"Artificial intelligence and machine learning frameworks for securing cyber-physical systems"

Modern cyber-physical systems (CPS) have been facing increasingly sophisticated technical problems due to the growing complexity of these systems and the associated attack surface. Therefore, managers of these systems have to both judiciously allocate security resources efficiently and detect failures (anomalies) in order to reduce their cyber risk and operational cost. In this work, we propose novel artificial intelligence (AI) and machine learning (ML) models for achieving such goals. In particular, we evaluate our frameworks on interdependent CPS with real-world attack scenarios, and with real data collected from deployed sensors in smart agriculture and smart manufacturing systems. Taken together, we show how in these two application domains our frameworks can be leveraged for paving the way for enhancing the future applications of these systems.

Monday, January 24, 2022, 9:00 AM

Zoom ID: 956 7779 4705 Password: 050412

Dr. Behzad Ghazanfari, University of South Carolina

"Machine Learning-Based Decision Making in Autonomous Systems"

Novel perspectives are presented in three separate parts in which lead to better performance and solving some shortcomings.

Abstraction in Reinforcement Learning (RL): We proposed SARM-HSTRL based on sequential association rule mining to extract the hierarchical optimum policy task structure of multiple policies like multi-task RL or Multi-agent RL autonomously in MDPs and FMDPs.

Error Fission: In current machine learning techniques, the error is considered as one scalar value in the literature, we introduce error fission as a novel perspective in which error breaks up into several values. We do error fission as a representation learning perspective of error by breaking up error in form of several features --- highly abstract features of representation of the error.

Quasi-periodic Time Series Classification: The proposed method translates quasi-periodic parts by some identical functions and then uses some functionality on the quasi-periodic parts to magnify the difference and remove similarities.

Monday, April 11, 2022, 9:00 AM

Zoom ID: 916 6707 8390 Password: 183579

Dr. Victor Watson, Commissariat à l’energie Atomique / Université Bretagne Sud

"Statistical tools for extracting information"

Technological advancement generalized access to large amounts of data. The increase in resolutions, the multiplicity of sensors, and also the capacity to store huge volumes of data give the possibility to obtain more accurate information about the phenomena studied in physics. However, this information is not always easy to extract. Indeed, if the information one seeks does not represent the main component of the data, retrieving it is not straightforward.

In this presentation, I will address two different cases where getting the in- formation from the data is complex. First, I will present a case involving basic Machine Learning techniques, statistical regressions, used to retrieve information about stellar physical parameters from high resolution spectroscopy. For this specific case, Sliced Inverse Regression (SIR) has been adapted to be used with ill-conditioned covariance matrices and non-linear link between the data and the explanatory variable.

Second, I will describe an approach to detect pollutants from a small set of sensors distributed over a monitored area. To detect weak signals, Generalised likelihood Ratio Test (GLRT) techniques such as Cumulative Sum (CUSUM), are well studied in univariate cases but the extension to multivariate data is still a matter of research. The event can also be transiting over the monitored area such that when the last sensors start monitoring the signal, other sen- sors which monitored it earlier have stopped being exposed. We propose the Temporary-Event-CUSUM to tackle the synchronicity problem caused by the transiting event, without losing the recursive computation, and keeping a low computational cost.

These two examples show how data sciences have become necessary to physics. Statistics allows us to make powerful tools using Machine Learning for estimation and detection.

Friday, April 15, 2022, 9:00 AM

Zoom ID: 961 4839 3619 Password: 714891

Dr. Yonggi Park, Texas A&M University - Corpus Christi

"Recurrent Neural Networks for Dynamical Systems: Applications to Ordinary Differential Equations, Collective Motion, and Hydrological Modeling"

Classical methods of solving spatiotemporal dynamical systems include statistical approaches such as autoregressive integrated moving average, which assume linear and stationary relationships between systems' previous outputs. Development and implementation of linear methods are relatively simple, but they often do not capture non-linear relationships in the data. Thus, artificial neural networks (ANNs) are receiving attention from researchers in analyzing and forecasting dynamical systems. Recurrent neural networks (RNN), derived from feed-forward ANNs, use internal memory to process variable-length sequences of inputs.

This allows RNNs to applicable for finding solutions for a vast variety of problems in spatiotemporal dynamical systems. Thus, in this paper, we utilize RNNs to treat some specific issues associated with dynamical systems.

Specifically, we analyze the performance of RNNs applied to three tasks: reconstruction of correct Lorenz solutions for a system with a formulation error, reconstruction of corrupted collective motion trajectories, and forecasting of streamflow time series possessing spikes, representing three fields, namely, ordinary differential equations, collective motion, and hydrological modeling, respectively. We train and test RNNs uniquely in each task to demonstrate the broad applicability of RNNs in reconstruction and forecasting the dynamics of dynamical systems.

Tuesday, May 31, 2022, 9:00 AM

Zoom ID: 950 6591 1646 Password: 325280

Dr. Jennifer Pore, Lawrence Berkeley National Laboratory

"Groundbreaking Chemistry from Simple-Nuclear Physics: Recent Results on Heavy Elements from FIONA"

The heaviest elements, at the end of the Periodic Table, are still considered to be the “wild west” for fundamental-chemistry measurements. Here, periodic trends are expected to break down as relativistic behaviors of the electron begin to reign. Unfortunately, traditional chemistry techniques cannot be used to study these species. They are radioactive with short half-lives and only trace amounts of material are available. Even though specialized laboratories have been constructed for these studies, there are still huge experimental limitations and very little is known. However, there is now a tremendous opportunity to perform chemical studies of these elements utilizing nuclear-physics techniques. These measurements would not only further our fundamental understanding of chemical behavior and how the Periodic Table should be arranged, but they could also lead to critically important applications in processing nuclear waste, in creating new nuclear medicines, and in nuclear power.

At the LBNL 88-Inch Cyclotron facility, the FIONA spectrometer has already shown itself to be a power-house for nuclear-physics measurements of heavy elements, including the first-ever mass-number measurement for a superheavy element and the identification of several new isotopes. Now, FIONA is being used to perform ground-breaking chemistry. Heavy elements are produced in nuclear reactions and then chemical reactions are performed on these species in FIONA’s Radio-Frequency Quadrupole Paul trap. The reaction products can then be directly identified by their mass-to-charge ratio with the FIONA mass analyzer. Recent results on nobelium and lawrencium will be presented.

Thursday, June 2, 2022, 9:00 AM

Zoom ID: 959 4421 8502 Password: 137775

Dr. Erich Leistenschneider, CERN-ISOLDE

"Negative Radioactive Ions: a novel spectroscopy tool for rare isotope science"

The electron affinity (EA) of a chemical element is defined as the energy released as an electron is attached to a neutral atom. The binding of such an “extra” electron does not arise from the net charge of the atomic system but is a result of complex electron-electron correlations. Hence, precise measurements of EAs are powerful benchmarks of atomic theories reliant on many-body quantum methods. The EA is also an important parameter for understanding the chemical behavior of an element, since it is strongly related to how much such element is prone to form chemical bonds by sharing electrons. However, the EAs of several rare and radioactive elements are still unknown, especially among the superheavies.

The standard technique for precision determination of EAs is the laser photodetachment threshold (LPT) method, in which a photon with sufficient energy is used to detach an electron from a negative ion. However, given the low photodetachment probabilities, this technique has been restricted to mostly stable, abundant species. At CERN-ISOLDE, we are currently exploring the use of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) technique to enhance the sensitivity of LPT and enable the study of rare and radioactive species. The method is based on an electrostatic ion trap to greatly extend the ion's exposure time to lasers. We have demonstrated the technique offline using stable chlorine anions. By confining a few thousands of Cl- ions for a few hundreds of ms in the trap, neutralized atoms have been experimentally observed following the laser photodetachment. For photon energies only 5 meV above the threshold, a continuous laser power as low as 0.4 mW has been demonstrated to be sufficient to observe a photodetachment signal.

Our novel measurement scheme paves the way for unprecedented new spectroscopy studies of rare isotopes. For example, the measurement of isotope shifts in EAs can provide crucial support for atomic and nuclear structure investigations that need precise knowledge of the so-called Specific Mass Shift, a quantity required to accurately extract nuclear charge radii from conventional laser spectroscopy data. The technique will also confer LPT an atom-at-a-time sensitivity required to measure electron affinities of superheavy elements.

In this talk, I will provide an introduction to negative ion spectroscopy, present our novel technique and its first experimental achievements, and provide an outlook on the scientific cases it will enable.

Wednesday, June 22, 2022, 9:00 AM

Zoom ID: 962 8484 6666 Password: 949346

Dr. Taiki Tanaka, Australian National University

"Reaction Dynamics of Heavy and Superheavy Element Synthesis - Towards Expanding New Frontiers of Nuclear Physics and Chemistry"

Discovery of new chemical elements has been of fundamental importance in progressing our society - in particular, artificial elements have contributed much to our society in nuclear medicine (atomic number Z = 43; Technetium for computed tomography), nuclear energy (Z = 94 Plutonium), fire alarm detectors (Z = 95 Americium), and neutron sources for non-destructive inspection (Z = 98 Californium). Heavy and superheavy elements have unusual nuclear and chemical properties due to their large number of nucleons and electrons. These may be useful for our future society. Nuclei located around the next doubly magic nuclide beyond 208Pb are expected to have longer half-lives than the known superheavy nuclei, forming “the island of stability”. The long-lived superheavy nuclei would also open new types of chemistry study, which require a longer time to complete chemical procedures. A few research groups [1-8] have attempted (or will attempt) to synthesize new superheavy isotopes around the island. Understanding the reaction dynamics of heavy and superheavy element synthesis to optimize the reaction is vital for the success of these attempts. In this talk, I will discuss my contributions towards solving the two vital components; (1) study of the capture process utilizing quasielastic (QE) barrier distributions and (2) study of the quasifission process utilizing mass and angle distributions (MADs).

Understanding the distribution of Coulomb barrier heights [9] is of fundamental importance to optimize heavy-ion induced fusion reactions for synthesis of superheavy elements [10–13]. To this end, the excitation functions of QE scattering [14] for 10 reactions, 48Ca+208Pb, 50Ti+208Pb, 48Ca+238U, 22Ne+248Cm, 26Mg+248Cm, 30Si+248Cm, 34S+248Cm, 40Ar +248Cm, 48Ca+248Cm and 50Ti+248Cm, were measured using the RIKEN Heavy-ion Linear Accelerator and the gas-filled recoil ion separator GARIS. We developed a new experimental method to measure QE scattering under low-background conditions by analyzing rigidities of recoiling particles at zero degrees with GARIS. This method, for the first time, enabled us to derive the barrier distribution for angular momentum l ~ 0, which has been expected to decisively influence the fusion cross sections of superheavy nuclei synthesis and their excitation functions [12,13]. The extracted QE barrier distributions were compared to coupled-channels calculations [15]. The calculations indicated that the deformation of actinoid target nuclei, the vibrational and rotational excitations of the colliding nuclei, as well as neutron transfers before contact, affect the structure of the barrier distribution. The peak of the 2n evaporation residue (ER) cross section of the cold fusion reactions 48Ca+208Pb and 50Ti+208Pb – relevant to the synthesis of No (Z = 102), Rf (Z = 104) – emerged at the same energy as the peak of the barrier distributions. The ER cross section of the hot fusion reactions 22Ne+248Cm, 26Mg+248Cm, 48Ca+238U and 48Ca+248Cm – relevant to the synthesis of Sg (Z = 106), Hs (Z = 108), Cn (Z = 112) and Lv (Z = 116), which are the frontier of the known superheavy nuclei – peak at an energy between the experimentally determined average Coulomb barrier height and the Coulomb barrier height for a side collision, where the projectile approaches along the short axis of a prolately deformed nucleus. This suggests that the hot fusion reactions benefit from a compact collision geometry with the projectile impacting on the side of the deformed target nucleus [16,17]. From the systematic study above, we proposed a new method to estimate the optimum incident energy to synthesize unknown superheavy nuclei from the experimental barrier distribution [13].

MAD measurements have illuminated many aspects of the physical variables controlling quasifission [18-20]. This tool has been exploited to probe the dynamics of the nuclear fusion reactions used to synthesize heavy and superheavy elements. A fundamental understanding of quasifission, and how it can be minimized, is sought to optimize synthesis of new superheavy isotopes. MADs for the 52Cr+198Pt and 54Cr+196Pt reactions (both forming 250No) were measured by the ANU CUBE fission spectrometer. A new experimental method [21], involving subtraction of two measured MADs, has enabled the first direct determination of the dependence of the fast quasifission sticking time on angular momentum Lℏ, as well as new information on fast quasifission mass evolution. The results are consistent with a transition from slow quasifission (and fusion) at the lowest L, through fast quasifission at intermediate L to deep-inelastic collision at the highest L. Time-dependent Hartree-Fock theoretical calculations [22] show good agreement with the experimental relationship between sticking time and L. The mass evolution observed for fast quasifission is inconsistent with the standard picture [18,19] of the peak mass yield drifting towards symmetric splits. Surprisingly, the mass distributions can extend to symmetric splits even though the peak yield remains close to the entrance channel masses. This result opens a window into the transition from fast energy dissipative deep-inelastic collisions to quasifission, and implies a very strong role for fluctuations in determining the fission-like mass partition during the first 10 zs after contact.

[1] Y. T. Oganessian et al., Phys. Rev. C 69, 021601(R) (2004).

[2] Y. T. Oganessian et al., Phys. Rev. C 69, 054607 (2004).

[3] Y. T. Oganessian et al., Phys. Rev. C 70, 064609 (2004).

[4] Y. T. Oganessian et al., Phys. Rev. C 74, 044602 (2006).

[5] S. Hofmann et al., Eur. Phys. J. A 52, 180 (2016).

[6] S. Dmitriev et al., EPJ Web Conf. 131, 08001 (2016).

[7] H. Haba. Nat. Chem. 11, 10 (2019).

[8] J. Khuyagbaatar et al., Phys. Rev. C 102, 064602 (2020).

[9] M. Dasgupta et al., Annu. Rev. Nucl. Sci. 48, 401 (1998).

[10] S. S. Ntshangase et al., Phys. Lett. B 651, 27 (2007).

[11] S. Mitsuoka et al., Phys. Rev. Lett. 99, 182701 (2007).

[12] T. Tanaka et al., J. Phys. Soc. Jpn. 87, 014201 (2018).

[13] T. Tanaka et al., Phys. Rev. Lett. 124, 052502 (2020).

[14] H. Timmers et al., Nucl. Phys. A 584, 190 (1995).

[15] K. Hagino et al., Comput. Phys. Commun. 123, 143 (1999).

[16] D. J. Hinde et al., Phys. Rev. Lett. 74, 295 (1995).

[17] K. Nishio et al., Phys. Rev. C 62, 014602 (2000).

[18] J. Toke et al., Nucl. Phys. A 440, 327 (1985).

[19] W. Q. Shen et al,. Phys. Rev. C 36, 115 (1987).

[20] D. J. Hinde et al., Phys. Rev. Lett. 101, 092701 (2008).

[21] T. Tanaka et al., Phys. Rev. Lett. 127, 222501 (2021).

[22] C. Simenel et al., Phys. Rev. Lett. 124, 212504 (2020).

Friday, July 15, 2022, 9:00 AM

Zoom ID: 923 3861 8618 Password: 324793

Dr. Daniel Hoff, Lawrence Livermore National Laboratory

"From Isomers to Isospin"

The nuclear landscape provides a wide expanse of nuclear systems to study, from light to heavy nuclei and across the valley of stability. Each region of the nuclear landscape provides a diverse set of nuclear phenomena to explore, and I will share some of my research from around the chart of nuclides. Starting in medium-mass proton-rich nuclei, I will show results on the observation of isospin symmetry breaking in the mirror pair 73Sr/73Br. From this data the proton separation energy of 73Rb was also extracted, which is relevant to the astrophysical rapid-proton capture process. Journeying across the valley of stability, we will then look at the astrophysically-relevant isomer, or “Astromer”, of 128Sb. Recently, in collaboration with the Canadian Penning Trap group at Argonne National Laboratory, we measured the mass of the ground state and isomer of 128Sb, providing the first key step to understand its role in the rapid-neutron capture process.

Wednesday, July 20, 2022, 9:00 AM

Zoom ID: 922 5261 4665 Password: 603881

Dr. Mathis Wiedeking, iThemba LABS, Cape Town, South Africa, University of the Witwatersrand, Johannesburg, South Africa

"Electromagnetic Dipole Response of Nuclei: Exploring Nuclear Structures and Constraining Nucleosynthesis Processes"

The gamma-ray decay of nuclear states in the quasi-continuum provides important insights into nuclear structure effects and constraints to nucleosynthesis processes. In particular, measurements of Nuclear Level Densities (NLDs) and Photon Strength Functions (PSFs) have and will continue to play a central role as we are entering an era of incredible potential for novel measurements. This is due to many institutes across the world having established programs to provide enhanced, state-of-the-art research infrastructure. These range from significant increases in efficiencies for particle and gamma-ray detectors, to new or upgraded radioactive ion beam facilities. In parallel, several new experimental and analytical techniques were developed which allow for more reliable PSF and NLD studies, even on nuclei away from stability. All this progress will undoubtedly lead to unprecedented insight into the structure of nuclei and provide reaction rates of relevance to nucleosynthesis processes.

In this talk, I will provide an overview of the most significant advances made and how these have laid the foundation for novel and ambitious measurements of PSFs and NLDs at radioactive and stable ion beam facilities. I will further discuss recent progress in exploring the underlying nuclear structure of resonances from PSF measurements, focusing on the scissor’s mode and the low-energy enhancement, whose mechanisms are still not fully understood. Our understanding of observed isotopic abundances can be improved greatly through the measurement of PSFs and NLDs, as will be demonstrated for the p-nuclei 138 La and 180 Ta. I will conclude with an outlook on future physics opportunities, envisaged measurements and plans for evaluating and disseminating PSF and NLD data.

This work is supported by the National Research Foundation of South Africa under grant number 118840.

Friday, July 22, 2022, 9:00 AM

Zoom ID: 975 2772 8157 Password: 341694

Dr. Anastasia Georgiadou, Department of Nuclear Engineering, U. C. Berkeley, and Nuclear Science Division, Lawrence Berkeley National Laboratory

"Nuclear Data for Science and Applications"

Nuclear reaction data on structural materials are critical for nuclear energy, space exploration, nuclear security, and nuclear medicine applications. However, in many cases, nuclear data evaluations of the reaction cross-sections of interest are unavailable or unreliable due to the complexity of performing consistent, high-quality measurements over a broad neutron energy range. In this talk, I present double differential 54Fe(n, Z), cross-sections, and angular distributions measured at the WNR facility at LANSCE, covering energies up to 30 MeV. The results enabled new evaluations [1] on discrete level cross-sections and angular distributions, which play an essential role in a future updated ENDF library. Additionally, my work unravels inadequacies in the inelastic channel evaluation of 54Fe, which reflects the status of this channel’s evaluation of multiple other structural materials. Current work on NaCl data obtained with GENESIS at LBNL is discussed, confirming the promising avenue of neutron-γ coincidences to shed light on neutron inelastic cross-section measurements.

[1] H. I. Kim, et al. Nuclear Instruments and Methods in Physics Research Section A, 964, 163699 (2020).

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