Session KP: Nuclear Structure: A < 40

Mirror nucleon removal reactions in p-shell nuclei

Nucleon removal reactions have been shown to be an effective tool for studying the single particle structure of nuclei. However, the theoretical errors associated with the extraction of spectroscopic factors from the measured partial cross sections is still a matter of debate. This work continues efforts to experimentally probe and possibly benchmark the reaction model used to calculate the single particle cross sections. Three different single nucleon knockout reactions were performed from p-shell nuclei with masses A$=$7, 9, and 10 at the National Superconducting Cyclotron Laboratory. The residual fragments from the reactions were detected in coincidence with gamma-rays to allow partial cross sections to individual states to be determined. Variational Monte Carlo radial overlaps and densities were used as inputs into the eikonal reaction model to calculate single particle cross sections in a manner consistent with the nuclear structure model used to calculate the spectroscopic factors. Comparisons between measured and calculated cross sections, as well as between mirror reactions, are performed in an effort to shed light on the limitations of the reaction model in particular the core survival approximation.   

$^8$Li $B(E2; 2^+ \rightarrow 1^+)$ measurement and comparison with ab initio calculations

Measuring electromagnetic transition strengths can provide stringent tests of nuclear $\textit{ab initio}$ calculations in light nuclei. For the A=7 isobars, $^7$Li and $^7$Be, the B(E2) transition strengths of the first excited states have been used to benchmark a variety of $\textit{ab initio}$ calculations (S. L. Henderson $\textit{et al.}$ Phys. Rev. C 99, 064320 (2019) ). We have continued these tests by extending into the A=8 region and performed a Coulomb excitation experiment to measure the $B(E2; 2^+ \rightarrow 1^+)$ of the transition from the first excited state in $^8$Li. This measurement will provide additional constraints to these $\textit{ab initio}$ calculations and these calculations can then give insight into the structural changes from $^7$Li to $^8$Li due to the addition of a neutron.The $^8$Li was produced and separated with TwinSol and the cross sections were measured by observing $\gamma$-ray yields in coincidence with scattered $^8$Li. The newly remeasured B(E2) value will be presented and compared to NCSM calculations for $^8$Li, performed with a variety of nuclear interactions.   

$^{25}$Si $\beta$-decay spectroscopy using the Gaseous Detector with Germanium Tagging (GADGET) system

The protons and $\gamma$ rays emitted in $^{25}$Si $\beta$ decay were measured using the GADGET system. Three $^{24}$Mg $\gamma$-ray lines, eight $^{25}$Al $\gamma$-ray lines, and a 719-keV proton branch were observed for the first time in $^{25}$Si decay. A Monte Carlo method was used to model the Doppler broadening of $^{24}$Mg $\gamma$-ray lines caused by proton emissions. All the proton-bound states of $^{25}$Al are observed to be populated in the $\beta$ decay of $^{25}$Si. We have reported the first measurement of the $^{25}$Si $\beta$-delayed $\gamma$-ray intensities through the $^{25}$Al unbound states. An enhanced decay scheme of $^{25}$Si has been constructed and compared to the mirror decay of $^{25}$Na and the shell-model calculations using two newly-developed $sd$-shell Hamiltonians, USDC and USDI. This work offers insights into the fine nuclear structure of $^{25}$Al.   

Structure of the Transitional Nucleus $^{28}$Mg Studied with the $^{26}$Mg(t,p)$^{28}$Mg Reaction

Past studies of the nuclei surrounding $^{32}$Mg discovered inversions in the usual ordering of shell-model states. Shell-model interactions that incorporate the evolution toward this ``island of inversion" predict low-lying deformed intruder states for nearby nuclei. One such nucleus is $^{28}$Mg, which exists in the transition between stability and the inverted region. The two-neutron transfer reaction $^{26}$Mg(t,p)$^{28}$Mg has been used to study the properties of the ground state and excited 0$^+$ states. This experiment was carried out at Argonne National Laboratory using the HELIcal Orbit Spectrometer (HELIOS). Multi-nucleon transfer is known to be sensitive to the the amplitude and phase of configuration-mixed states, and enhances transfer to those states which are similar to the ground state of the target plus two nucleons in single-particle orbitals. Because multi-nucleon transfers are more complex than single-particle transfers, a shell-model calculation must guide the understanding of which configurations will be strongly populated. A new shell-model calculation using the SDPF-MU interaction provided the structure-related transfer amplitudes, and results comparing the experimental cross sections to those predicted by the transfer amplitudes and DWBA will be presented.   

Quantifying quadrupole collectivity of $^{29}$Ne

The large proton-to-neutron asymmetry as well as low-separation energies found in nuclei located far from stability have been shown to be ingredients leading to unique properties. In particular, isotopes lying in the vicinity of the N=20 Island of Inversion offer an excellent testing ground to investigate the interplay between the shell evolution, deformation, and weakly bound effects. In this talk, a novel technique developed at the National Superconducting Cyclotron Laboratory (NSCL) using inelastic scattering on both light (Be) and heavy (Ta) targets simultaneously in order to extract relevant transition rates from measured cross-sections will be presented. Experimental results of the measurement of E2 transitions rates performed in $^{29}$Ne and in $^{32}$Mg, used as reference, will be discussed in terms of collectivity and deformation.   

The $\beta$-decay of $^{31}$Ne

The neutron-rich Na isotopes approaching N=20 reside along the southern boundary of the “Island of Inversion”. These isotopes are of particular interest to nuclear structure, where observed details in the structure could lead to a deeper understand lying force which causes deformation in this mass region. Recently, the $\Beta$-decay of $^{31}$Ne has been studied, promising to add need structure detail to a region of the chart which had only been accessed using fast in-beam methods. Using the CCF at the NSCL, a $^{48}$Ca beam was fragmented, the fast $^{31}Ne$ isotopes were subsequently selected using the A1900 and implanted in the BCS, allowing event-by-event particle identification to be made. Excited states in the daughter $^{31,30}Na$ isotopes are identified by emitted $\gamma$-rays collected with 16 Clover-style HPGe and 15 LaBr$_3$ detectors surrounding the implant detector in a rhombicuboctahedron geometry and correlated to decay events. The ongoing analysis will be discussed, and preliminary results will be presented.   

Difference of mirror charge radii $^{\mathrm{36}}$Ca-$^{\mathrm{36}}$S and $^{\mathrm{38}}$Ca-$^{\mathrm{38}}$Ar, and implications on the neutron equation of state

The charge radii of mirror nuclei $^{\mathrm{36}}$Ca-$^{\mathrm{36}}$S and $^{\mathrm{38}}$Ca-$^{\mathrm{38}}$Ar were used to deduce he first derivative $L$ of the symmetry energy in the nuclear equation of state (EOS) [1]. Here the linear correlation between the difference of mirror charger radii and $L$ were used [2] to set constraint on $L$. Implications on $L$ will be discussed in terms of correlation with the mean field calculations with Skyrme interactions, the covariant density functional theory (CODF) and the Multi-Reference In-Medium Similarity Renormalization Group (IMSRG) approach. [1] B. A. Brown et al., Phys. Rev. Res. 2, 022035 (R) (2020). [2] B. A. Brown, Phys. Rev. Lett. 119, 122502 (2017).   

Precision Lifetime Measurements of Excited States in $^{38}$Si and $^{36}$Si

Rapid shape transitions are predicted by shell model calculations as a result of the nuclear shell structure significantly evolving in the neutron-rich region at the traditional magic numbers N=20 and 28. The energy ratios between the first 2$^{+}$ and 4$^{+}$ states in the even-even Si isotopes from N=20 to 28 suggest a variety of collectivity evolving from vibrational, to possible triaxial, to rotational modes. The systematic behavior of the level schemes along the Si isotopic chain suggests $^{38}$Si as the turning point in this transition. The lifetime measurements of $^{38}$Si and $^{36}$Si were performed at the National Superconducting Cyclotron Laboratory based on the Recoil-Distance Method using the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA). The data are used to extract the B(E2) ratios which provide useful measurements to assess the nature of collective modes.   

Spectroscopy of $^{38}$S

The $^{38}$S ($Z=16$, $N=22$) excited-state decay scheme has been expanded including a number of new levels with tentative spin-parity assignments. In particular, the yrast sequence has been expanded up to $J^{\pi}=8^+$ with candidates for $J>8$ identified as well. Interpretations of the new data with respect to theoretical calculations and the systematic trends in region have been made and will be discussed. In general, the excited levels of $^{38}$S were well described by shell model calculations invoking the newly developed FSU cross-shell interaction. $^{38}$S nuclei were populated via fusion and subsequent two-proton evaporation of a $^{22}$Ne beam on an $^{18}$O target at $\sim$45~MeV. $^{38}$S recoils were identified using a feed-forward trained neural network including inputs of position, time-of-flight, and energy loss information taken at the focal plane of the FMA. Gretina surrounded the target position and detected prompt $\gamma$-rays in coincidence with $^{38}$S recoils. Coincidences, energy summations, intensities, and angular information were all used in the construction of the final $^{38}$S decay scheme.