Session JG: Nuclear Structure IV

Cross-shell Phenomena in Mass 36 Al and Si Isotopes Investigated via β-Decays

The β-decays of ³⁶Mg and ³⁶Al were studied at the National Superconducting Cyclotron Laboratory (NSCL) to extract the half-lives of the parent nuclei and reveal the nuclear structure of the decaying descendants. Neutron-rich ³⁶Mg and ³⁶Al were produced at the NSCL's Coupled Cyclotron Facility via projectile fragmentation of a ⁴⁸Ca beam of energy 140 MeV/u impinged on a 642 mg/cm² thick ⁹Be 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 CeBr₃ scintilator. The β-delayed γ-rays were identified with 16 Ge (SeGA) and 15 LaBr₃ detectors. The half-lives of the two parent nuclei were determined. β-delayed γ-ray transitions were observed in ³⁶Al and ³⁶Si for the first time. Excited energy states of ³⁶Al populated by the β-decay of ³⁶Mg 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 next generation facilities like FRIB.

    

Investigation of 11B and 40Ca levels at 8-9 MeV by nuclear resonance fluorescence

We report on the measurement of ¹¹B and ⁴⁰Ca levels between excitation energies of 8 and 9 MeV using nuclear resonance fluorescence (NRF). The experiment was carried out with nearly-monoenergetic and linearly polarized photon beams provided by the High-Intensity γ-ray Source (HIγS) facility at the Triangle Universities Nuclear Laboratory (TUNL). States in 11B are important for calibrations of NRF measurements, while the properties of 40Ca levels impact potassium nucleosynthesis in globular clusters. For 40Ca, we report on improved excitation energies and an unambiguous 2^- assignment for the state at 8425 keV. For 11B, we obtained improved values for γ-ray multipolarity mixing ratios and branching ratios of the 8920 keV level.   

WalletCraft: A new evaluation of the properties of ground state and long-lived isomers for all known nuclides

The Nuclear Wallet Cards is a collection of selected properties of all known nuclides and their long-lived isomeric states. Introduced in 1971, it has been produced at the National Nuclear Data Center (NNDC) since 1985. The evaluations have been disseminated as a printed booklet, and through an online application hosted on the NNDC website (www.nndc.bnl.gov/wallet). The last edition of the Nuclear Wallet Cards was produced by J.K.Tuli in 2011, and a revision is long overdue.

As part of the Evaluated Nuclear Structure Data File (ENSDF) modernization project, the Wallet Cards have also undergone a full update, resulting in an entirely new database called WalletCraft.

Based on an expansible object-oriented format, WalletCraft was designed to include not only evaluated quantities, but also all the experimental values that were used to calculate them. This will simplify incremental revisions of the recommended values, that will be readily updated whenever new experimental determinations become available.

The first release of WalletCraft, which includes the inclusion of all source measurements, is currently being completed at the NNDC and will be made available as a web-based application in the coming months. Details of the WalletCraft database and of the new evaluation will be presented.   

Beta-delayed neutron emission of 70,71,72Co

Beta-decay strength measurements are required to constrain the nuclear models, which aim to predict nuclear lifetimes and decay branching ratios. Recently, this became an important research topic, especially for neutron-rich nuclei, relevant to the r-process. We studied beta delayed neutron emission of neutron-rich cobalt isotopes to (Z=28) nickel nuclei to complete the strength distribution measurements which previously used gamma-ray spectroscopy methods. The neutron emission was investigated with the VANDLE neutron time of flight detector array at NSCL. Neutron energy spectra and emission probabilities were obtained, and beta-n-gamma coincidences where they could be determined. The obtained beta-delayed neutron emission for the decays of the nuclei Co-70,71,72 are interpreted using a newly developed framework that combines shell-model and statistical-model calculations.   

Withdrawn

Beta-decays of neutron-rich Cu isotopes with N≥50 carry information on the major neutron-orbit spin partners expected in the beta-decay of doubly-magic ⁷⁸Ni [1]. The decay of Cu nuclei with N>50 is also impacted by the N=50 shell gap. These nuclei are strong beta-delayed neutron emitters and the energy distribution of the neutrons becomes vital to studying the beta-decay to neutron-unbound states.

An experiment was performed at RIBF, Japan, aiming to measure the energy of beta-delayed neutrons from the nuclei in the ⁷⁸Ni region using time-of-flight spectroscopy with VANDLE [2] and YSO implantation detector [3]. In this contribution, the first measurement of the beta-decay strength to neutron-unbound states in the beta decay of ⁷⁹Cu is reported. Furthermore, the results are compared to the prediction of the shell-model calculation using NuShellX [4]. Finally, the model in agreement with the experiment is used to predict the beta decay of ⁷⁸Ni.

    

Spectroscopic studies on the beta-delayed neutron emission of 53K

An experimental study on the β-n spectroscopy of ⁵³K was carried out at the ISOLDE Decay Station (IDS). This neutron-rich isotope has a large Q_β value (energy window available for the β decay) and low neutron-separation energy (S_n) in the daughter (⁵³Ca), making it an ideal case for detailed studies of β-n process, similar to the ¹³⁴In decay [1]. In this work, we measured the β decay of ⁵³K in the coincidence of γ-ray and neutron-time-of-flight (nTOF) spectroscopy using HPGe and INDIe [2] arrays respectively at IDS. In this contribution, I will present the latest results from the experiment, mainly focusing on the apparent beta feeding (I_β) of the ⁵³K decay. In addition, exclusive neutron-emission branching ratios following β decay will be discussed. The experimental findings are compared with the shell-model calculations and Hauser-Feshbach statistical models, providing valuable insights into the β-decay and β-n process far from the stability line and into their connections to the neutron shell gap at N=34.

[1] J. Heideman et al, being reviewed by Phys. Rev. Lett.

[2] M. Madurga et al, Phys. Rev. Lett 117, 092502 (2016).   

Coulomb Excitation of 74Ge

The electromagnetic properties of low-lying states in ⁷⁴Ge have been investigated using a multi-step coulomb excitation measurement carried out at the ATLAS facility of the Argonne National Laboratory. The experimental setup consisted of the GRETINA multidetector array coupled to the heavy-ion counter, CHICO2. This coupling allowed for precise Doppler correction and clean kinematic separation of the scattered particles. The ground-state and quasi-gamma bands were coulomb excited to spins 8⁺ and 4⁺, respectively. Preliminary results from the analysis will be presented in the context of similar studies performed for the neighboring ^72,76Ge [1, 2] nuclei.

[1] A. D. Ayangeakaa, et. al. PLB 754, 254-259 (2016)

[2] A. D. Ayangeakaa, et. al. PRL 123, 102501 (2019)   

“Piston” mechanism in a time-dependent two-level model

The “piston” mechanism concept [1] offers a unique motivation to use (p,2p) reactions to systematically study the evolution of neutron-neutron correlations in exotic neutron-rich nuclei. It seems clear that such a program will feature prominently in current and future rare-isotopes facilities worldwide.

In this work [2] we analyze the specific case of the high-energy one-proton knockout reaction on ¹⁹N leading to unbound states in ¹⁸C [1] in the framework of a time-dependent two-level mixing model.  This simple formalism allows us to assess the validity of the piston mechanism and to suggest some indicators for its applicability.  Differences in the time scales of the reaction, the lifetime and the oscillation period of the decaying state should be considered in relating the measured n-n correlations to the pairing properties of the ground state in the final nucleus.

While an extension of the two-level system to include more mixing states will be more realistic, our scenario captures the main physical ingredients of the problem.

[1] A. Revel, F. M. Marques, O. Sorlin, et al. (R3B-LAND Collaboration), Phys. Rev. Lett. 120, 152504 (2018).

[2] A. O. Macchiavelli, O. Civitarese, S. M. Lenzi,  R. J. Liotta, and D. R. Bès,  Phys. Rev.  C 104, L061304 (2021).

    

Commissioning of the LSU-Argonne Conversion Electron Spectrometer

A collaboration between Louisiana State University (LSU) and Argonne National Laboratory (ANL) has developed a program of internal conversion electron (CE) spectroscopy to complement the strong decay spectroscopy program at the Argonne Tandem Linac Accelerator System (ATLAS)/Californium Rare Ion Breeder Upgrade (CARIBU). Internal Conversion Electron (ICE) spectroscopy is an established experimental method that has been used to determine transition multipolarities, the spin and parity of the energy levels, and uniquely able to observe electric monopole (E0) transitions between 0⁺ states.

The LSU-Argonne Conversion Electron Spectrometer (LACES) is a 200 mm² by 5-mm thick LN₂-cooled Si(Li) detector system designed to be coupled to the X-Array HPGe detectors and the Scintillator and Tape Using Radioactive Nuclei (SATURN) moving tape collector system at ATLAS at ANL. LACES will be used to study the low-spin K^π = 1⁺ isomeric state at 150.3-keV of the deformed, odd-odd nucleus ¹⁵⁶Pm via ¹⁵⁶Nd beta decay from CARIBU beams.  Also, the low-energy efficiency will be measured from the beta decay of ¹⁴¹Cs and ¹²⁸Sn. The LACES design, upgrades to the X-Array/SATURN system, performance and preliminary commissioning data will be presented.

*Supported by Louisiana State Board of Regents RCS LEQSF(2016-19)-RD-A-09, the U.S. Department of Energy, Office of Nuclear Physics, under Award No. DE-AC02-06CH11357 (ANL) and DE-SC0021315 (LSU).