Session EG: Nuclear Structure II

Probing the quadrupole transition strength of 15C by deuteron inelastic scattering

Valence-neutron decoupling is expected to happen in one-neutron halo nuclei due to the extended wave function of the valence neutron. The degree of coupling can be described by the difference in the neutron and proton contribution to the quadrupole transition.  Core polarization in the transition between the ground state and first excited state in one-neutron halo nucleus ¹⁵C might be weakly induced by the valence neutron. In order to investigate it, deuteron elastic scattering from ¹⁵C and inelastic scattering reactions to the first excited state of ¹⁵C were measured using a radioactive beam of ¹⁵C in inverse kinematics using HELIOS. The matter deformation length δ_d=1.04(10) fm has been extracted from the inelastic scattering differential cross sections. The ratio of neutron and proton matrix element M_n/M_p=3.56(36) has been determined. Neutron and proton effective charges of ¹⁵C will be discussed. Results from ab-initio no-core configuration interaction calculations will also be compared with the experimental observations. This result supports a moderated core decoupling effect of the valence neutron in ¹⁵C, in line with the interpretation of other carbon isotopes. 

    

Neutron-unbound states in 32Na

The N=20 island of inversion is a section of the nuclear landscape centered around ³¹Na where significant structural changes have been observed. Except for a few cases most of the nuclei in this region have only been studied by their bound states. An experiment was performed at the National Superconducting Cyclotron Laboratory using the Modular Neutron Array and Sweeper dipole magnet to investigate neutron-unbound states of nuclei in this island of inversion. An 88 MeV/nucleon ³³Mg beam produced at the coupled-cyclotron facility was delivered to the experimental area and nuclei were populated via nucleon-removal reactions on Be targets. Using a proton removal from ³³Mg, states in ³²Na were populated. Because ³²Na is bound, its unbound excited states are studied using invariant mass spectroscopy by detecting the outgoing neutrons and residual ³¹Na from the decay and using their momenta to reconstruct decay energies. Preliminary results will be presented for this case.

    

Study of the Pygmy Dipole Resonance at the Super-Enge Split-Pole Spectrograph (SE-SPS) at Florida State University

The Pygmy Dipole Resonance (PDR) is expected to develop starting around N = 28. A one-neutron (d,p) transfer study of even-A 48,50Ti was performed to probe excited states belonging to the PDR at the FSU Super-Enge Split-Pole Spectrograph (SE-SPS). Previously, real-photon scattering experiments on 48,50Ti had identified the 1- states in the region of the PDR. In our experiment, several states were observed at energies coinciding with the reported 1- states. Angular distributions were measured up to the neutron-separation energy. These will allow the unambiguous identification of the 1- states populated in (d,p) and the determination of their neutron single-particle structure. First results will be discussed.   

Efficiency comparison between the simulated and experimental results.

Experiment E17011 was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the goal of determining the half-life for the 0₂⁺ state in ⁸⁰Ge by observing the E0 decay of the 0₂⁺→0₁⁺ transition. At the NSCL, the radioactive source was implanted into a position sensitive Cerium Bromide (CeBr3) scintillator, and the beta decay of the source populates the excited states in the daughter nuclei. An array of 16 LaBr3(Ce) detectors and 16 Segmented Germanium Array (SeGA) detectors were used to detect the delayed γ rays. An important component of the experimental analysis is understanding the efficiency of the detector array. A simulation of the detector efficiency has been carried out using GEANT4. The current state of the comparison between the simulated and experimental efficiencies will be discussed.

    

Nuclear Resonance Fluorescence in 74Ge

The germanium isotopes are known to display shape coexistence. Understanding the structure of ⁷⁴Ge is key to tracking the transition from an oblate to a prolate shape along the isotopic chain. We investigate the structure of ⁷⁴Ge using nuclear resonance fluorescence by exiting low-spin levels with a photon beam provided by the High Intensity Gamma-Ray Source (HIGS) at TUNL. This work identified several new levels and decays between 3 and 5.6 MeV and determined their spin, parity, as well as their integrated photon scattering cross sections. We observed decays to the low-lying 0⁺ and 2⁺ states and determined branching ratios and the multipolarity of the electromagnetic transitions, when possible. Comparisons with shell-model calculations will be presented.   

Modernizing the Evaluated Nuclear Structure Data File

The Evaluated Nuclear Structure Data File (ENSDF) is the authoritative source for nuclear structure and decay data. As such, it is a critical resource both for basic research and applications. However, at present, ENSDF data is stored in an 80-column text format designed for FORTRAN punch card readers over 70 years ago. As a result, interacting with ENSDF data directly is challenging both for humans and computer systems. In addition, a large amount of data has accumulated in unformatted comments in the database, which is difficult to retrieve in a standardized way. To remedy this situation, ENSDF is in the process of being migrated to a new object-oriented database based on Java Script Object Notation (JSON). This talk will present the new format and the current status of the ENSDF modernization project, as well as future opportunities which the new format is anticipated to enable.   

Electromagnetic Transition Rates in 28Mg

Neutron rich Mg isotopes far from stability belong to the island of inversion, a region where the single particle energy state description of the shell model breaks down and the predicted configuration of the nuclear states becomes inverted. Nuclei in this region also exhibit collective behaviour in which multiple particle interactions play a significant role in nuclear wavefunctions and transitions. This can be observed through electromagnetic transition strength measurements.

Using TIGRESS in conjunction with the TIGRESS Integrated Plunger for charged particle transitions at TRIUMF, Canada's particle accelerator centre, an experiment was performed to measure the lifetime of the first excited state in ²⁸Mg. This experiment used both the Doppler Shift Attenuation Method and the Recoil Distance Method, which exploit the Doppler shift of gamma rays emitted in flight in order to be sensitive to lifetimes of both short- and long-lived excited states in the nucleus. The current state of data analysis and the impacts on nuclear physics will be discussed.   

Lifetime measurement of the 39Ca 11/2- state and E2 transition strengths of mirror nuclei near N=Z

Assuming perfect isospin symmetry, the E2 transition matrix elements of isobaric multiplets are expected to follow a linear trend as a function of isospin projection. However, measurements of the 2+→0+ transition in the A=38 triplet of Ca, K, and Ar show a deviation from this trend with an enhanced transition in ³⁸Ca with respect to its mirror ³⁸Ar. By studying an analogue transition in ³⁹Ca we can ascertain if this enhancement persists in neighboring nuclei or if ³⁸Ca is anomalous. A recoil-distance method (RDM) lifetime measurement of the ³⁹Ca (11/2-) state was conducted at the NSCL utilizing a ⁴²Sc secondary beam, the TRIPLEX plunger, GRETINA, and the S800 spectrograph. Our data on the ³⁹Ca (11/2-)→7/2- transition provides a useful reference for the 2+→0+ transition in ³⁸Ca, assuming that the ³⁹Ca states are formed by the f7/2 neutron coupled to the ³⁸Ca core. A preliminary analysis of the lifetime measurement will be presented and a possible enhancement in the ³⁹Ca E2 strength will be discussed via a comparison to a simple core shell calculation with various effective charges.   

Measuring cross-sections of unbound mirror states: a systematic study of one-proton and one-neutron reactions adding on N=Z targets

One-neutron adding reactions are often used to infer proton-capture rates in mirror systems. While isospin is approximately symmetric, the degree to which it's still symmetric in systems with weak binding is unknown. Isobaric analog states should exhibit the same behavior, but weakly bound states experience Thomas-Erhman shifts and it is unknown how this affects the cross section. A series of experiments at Florida State University has been completed with the aim to be able to quantify asymmetries in spectroscopic factors due to weakly bound or unbound protons for calculating astrophysical reaction rates.

Targets of N=Z nuclei ²⁴Mg and ²⁸Si were used to populated mirror states via one-neutron and one-proton adding reactions. Beams of deuterium and ³He were impinged onto these targets, and four reactions were measured in the Super-Enge Split-Pole Spectrograph: ²⁴Mg(d,p)²⁵Mg, ²⁴Mg(³He,d)²⁵Al, ²⁸Si(d,p)²⁹Si and ²⁸Si(³He,d)²⁹P. Using the same targets, experimental setup and beam energy per nucleon, these systematic measurements populated states up to and beyond the respective neutron and proton separation energies (from 0 to at least 8 MeV in all cases), which in each mirror pair differs by ~5 MeV. Preliminary cross sections will be presented for these bound and unbound mirror states.