Session DG: Nuclear Structure I

Neutron-unbound states in 34,35Al* and 34Mg*

The interplay between deformation, the degeneracy of the fp shells and the coupling with the continuum plays an important role in the structure of neutron-rich nuclei between N=20 and N=28. However, information on neutron-unbound states is scarce and yet essential to benchmark the theoretical models in this region. An experiment at the National Superconducting Cyclotron Laboratory (NSCL) was performed to investigate the neutron-unbound states of 34,35Al* and 34Mg* using a 100~MeV/u 36Si secondary beam. The beam-velocity reaction products were detected in the forward direction in coincidence and a kinematically complete measurement was made using the MoNA-Sweeper setup: the neutrons emitted from the decay were detected using 272 plastic scintillator bars of the MoNA-LISA array. The charged fragments were deflected by the Sweeper dipole magnet, and their tracking information and identification were measured by a suite of charged particle detectors. Spectroscopy of the unbound excited states of 34,35Al* and 34Mg* is performed using the invariant-mass technique. The decay energy is calculated using the reconstructed momenta of the coincident fragment+n events. Preliminary results from the ongoing analysis including the decay energy spectra for these three cases will be presented.   

Observation of an elusive Near-Threshold Proton Resonance in 11B

The study of near-threshold resonances in weakly bound systems is an exciting topic in experimental and theoretical nuclear physics since they provide important information on the interaction of discrete states with the continuum. Recent theoretical developments aim to describe the continuum interaction. However, questions on the behavior, structure, and properties of the many-body systems close to the particle emission threshold remain open, making the experimental studies of near-threshold states and its characteristics crucial for constraining the theoretical efforts. In particular, a near-threshold proton resonance in 11B has long been sought since it would provide a less exotic explanation for the controversial observation of the unexpectedly large β−p+ branching ratio in 11Be. In this talk, I will discuss a recent experiment carried out at the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University where the near-threshold proton state was observed at resonance energy of 211(40) keV in 11B via the 10Be(d,n)11B reaction. The results and implications will be presented.   

Studying structure of 10Li by 9Li(p,p)9Li of T=2 isobaric analogue states in 10Be

The structure of the neutron unbound system ¹⁰Li is not yet fully determined. The low-lying level structure of ¹⁰Li is important for describing the ⁹Li core plus neutron interaction which plays a role in 2n-halo systems ¹¹Li. In the ground state, the neutron is unbound by about 25 keV which has been understood as an s-wave scattering state in neutron decay experiments. The ¹⁰Li system can be explored by investigating T=2 isobaric analogue states in ¹⁰Be. Here we present preliminary experimental results with the TexAT-TPC to measure the excitation function of those T=2 states in ¹⁰Be via ⁹Li(p,p)⁹Li.   

A Search for the 12Be Isomeric State

Spectroscopy of neutron unbound ¹³Be can provide insights into the unexpected structural changes of neutron-rich nuclei around N=8. Invariant mass spectra from past experiments do not agree on the location of the 1/2^- p-wave that is expected to strongly decay to a long-lived 0⁺₂ state in ¹²Be with a mean lifetime of 331(12) ns. Its decay scheme has a 20% branch through the ¹²Be(2⁺ ) to the ground state, giving a 2.1 MeV gamma ray, and 80% for an E₀ transition giving two 511 keV gamma rays from positron annihilation. An experiment was performed by the MoNA Collaboration at the National Superconducting Cyclotron Laboratory of Michigan State University to study the one neutron decay of ¹³Be to this isomeric state in ¹²Be. The experiment featured a new telescope to identify the ¹²Be fragment, and utilized 96 modules of the MoNA-LISA neutron array as well as the gamma ray detector CAESAR. The latter was placed around the telescope due to the long lifetime of the expected isomer. An overview and preliminary results of the experiment will be presented and discussed.   

Spectroscopy of Beryllium isotopes using TexAT-TPC

The detailed spectroscopic data that can be extracted from single nucleon transfer reactions provide a useful experimental probe of nuclear structure. This work aims to complement existing data for the 12Be system by measuring the spectroscopic factors to low-lying states in 12Be with the 13B(d,3He)12Be reaction in inverse kinematics. Using the Momentum Achromat Recoil Spectrometer (MARS) at Texas A&M University Cyclotron Institute (TAMU-CI), a 13B beam was delivered to the TEXas Active Target (TexAT) Time Projection Chamber (TPC). By reconstructing reaction vertices and tracks in TexAT, differential cross sections and thus spectroscopic data can be extracted. Ongoing analysis and preliminary results will be discussed.   

Lifetime measurements of isomeric states following decay of neutron-rich 31, 32, 33Na isotopes

Nuclear transition rates are sensitive probes for identifying differences in the underlying configurations of nuclei. One method to measure half-lives is through the direct measurement of the time delay between two emitted radiations following β decay. A β-decay experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) to study the β decay and β-delayed neutron decay daughters of ^{31, 32, 33}Na isotopes found at the center of the neutron-rich N=20 “island of inversion.” Radioactive nuclei were implanted within a thin, position-sensitive CeBr₃ detector and β-delayed radiation was correlated to the implanted nuclei. Ancillary arrays surrounding the CeBr₃ detector were used for γ-ray detection, including 15 LaBr₃ (Ce) detectors for fast-timing measurements. Preliminary results of half-lives of isomeric states populated following the β decay and β-delayed neutron decay of implanted sodium isotopes will be presented.   

Spectroscopic Factor Investigation in the N=40 Island of Inversion

The focus of this work is on the Fe and Mn neutron-rich isotopes with N~40, which lie within one of the so-called Islands of Inversion. Here, a quenching of the N=40 shell gap allows deformation to develop in the ground-state configurations. Limited spectroscopic information is available in the region of N~40 below the Ni isotopes. For the even-even nuclei, this consists of systematics of 2⁺₁ and 4⁺₁ state energies and, for the Fe and Cr isotopes, of B(E2; 2⁺₁→0⁺₁) values up to ⁶⁸Fe and ⁶⁴Cr. Large-scale shell model calculations well reproduce the energy systematics of the observed low-lying states of the even-even Fe and Cr isotopes around N=40. A good agreement is found also within the rotational Nilsson model. Such two descriptions, however, provide different predictions for proton spectroscopic factors. A measurement of such quantity would thus allow us to probe the validity of the considered models.

In this context, proton knockout reactions on the neutron-rich N=38 and N=40 isotopes ^64,66Fe and ^63,65Mn have been performed to investigate the proton spectroscopic factors of the parent nuclei. The experiment took place at the NSCL laboratory in the US and exploited the ??-ray tracking array GRETINA coupled to the S800 spectrograph to perform an in-beam ??-ray spectroscopic study. Preliminary results of the data analysis will be presented.   

Beta-delayed neutron spectroscopy of $^{24}O$.

Neutron spectroscopy is an important tool to probe the nuclear structure effects of neutron-rich nuclei, where beta-delayed neutron emission is a dominant decay mode. Beta-decay of parent nuclei populates neutron unbound states in the daughter nuclei which de-excite via neutron emission. Measuring neutron energy provides information about the excited states in the emitter which is otherwise not accessible through normal beta-decay measurement. The neutron energy spectrum measurement of the beta-delayed neutron precursor $^{24}O$ was performed for the first time at National Superconducting Cyclotron Laboratory (NSCL) using two different neutron detector arrays: NEXT and VANDLE. NEXT is a newly developed high-resolution segmented neutron time-of-flight array with neutron interaction position localization and neutron-gamma discrimination capabilities that enable precise energy measurement. The result from beta-delayed neutron emission measurement for $^{24}O$ will be presented.   

The distribution of proton strength in 11B studied via the 10Be(d,n)11B reaction

Recent experiments have provided some evidence for a 1/2+ level in ¹¹B located just above the proton threshold, with a large l_p=0 spectroscopic factor. Such a level could have important implications for ¹¹Be β decay and ¹⁰Be+p reaction rates. We have measured angular distributions of the proton transfer reaction ¹⁰Be(d,n)¹¹B in normal kinematics for incident deuteron energies of 4 and 7 MeV. Neutrons were detected using the time-of-flight technique, with experimental conditions optimized to provide an excitation energy resolution of 40 keV in the proton threshold region of ∼11 MeV. The angular distributions to specific final states allow the orbital angular momentum of the transferred proton l_p to be identified, while the absolute cross sections provide information about the spectroscopic factors. We will present spectroscopic factors and l_p values for all states in ¹¹B observed, up to an excitation energy of 14 MeV.