Session HB: Mini-Symposium on Coulomb Excitation of Exotic Nuclei

Coulomb Excitation of Exotic Nuclei

The structure of nuclei far from the stability line is a central theme of research in nuclear physics. Key to this program has been the worldwide development of radioactive beam facilities and novel detector systems, which provide the tools needed to produce and study these exotic nuclei. Coulomb Excitation provides a unique probe to characterize the interplay of collective and single-particle degrees of freedom of the atomic nucleus. In particular, the combination of state-of-the-art charged particle detectors and gamma-ray spectroscopy plays a vital and ubiquitous role in these studies. As an introduction to this Mini-Symposium, I will present a short overview of this powerful technique and selected examples of recent experiments. Future opportunities with a 4 $\pi$ gamma-ray tracking array like GRETA will be discussed.   

Shapes of Exotic Nuclei by Single- and Multi-Step Coulomb Excitation

Coulomb excitation is a powerful technique for probing the shell structure, collectivity, and shapes of atomic nuclei through the measurement of electromagnetic moments. Single- and multi-step Coulomb excitation results from CLARION-BareBall at HRIBF-ORNL (e.g., $^{136}$Te) and GRETINA-CHICO2 at CARIBU-ANL (e.g., $^{106}$Mo and $^{106,110}$Ru) will be presented. A survey of the equipment, techniques, and results will be given. An emphasis will be placed on unique opportunities with $\sim$3-MeV/u beams and future directions at CARIBU and ReA3.   

Triaxiality and shape coexistence in $^{72,76}$Ge: A model independent analysis.

An exploration of the structure of Ge isotopes is important for understanding the microscopic origin of collectivity, the nature of deformation and modifications of shell structure in nuclei of the $N\sim40$ mass region. The present study focuses on the electromagnetic properties of low-lying states in $^{72,76}$Ge obtained via sub-barrier multiple Coulomb excitation with GRETINA and CHICO2. In the case of $^{72}$Ge, the extracted matrix elements agree with a shape coexistence interpretation between the $0_1^+$ and $0_2^+$ states, but require significant mixing between the $0^+$ wavefunctions as well as triaxiality in order to reproduce the data. Similarly, the invariant sum-rule analysis of the $^{76}$Ge data indicates that both the ground state and gamma bands are characterized by the same deformation parameters, with triaxiality $(\gamma \sim 30^\circ)$ being important for a complete description. A summary of these results and data highlighting the nature of gamma deformation in $^{76}$Ge - whether rigid or soft - will be presented.   

Enhanced \textit{E3} Excitations in $^{\mathrm{144,146}}$Ba and the Evolution of Octupole Collectivity

Recent Coulomb excitation studies on $^{\mathrm{144,146}}$Ba using the GRETINA-CHICO2 detection system with post-accelerated CARIBU beams have confirmed the existence of enhanced \textit{E3} transitions in these isotopes which are centered in a region that has long been predicted to exhibit stable octupole-deformed shapes. Furthermore, the widely-varying \textit{E1} strength observed between these isotopes is well-accounted for by models having octupole-deformed potentials, and the variation has been linked to increased occupancies of specific single-particle orbitals in the reflection-asymmetric potential. This talk will summarize the most recent experimental and theoretical results. In addition, data on octupole-related properties in the surrounding isotopes will be discussed in an attempt to better understand the origin and evolution of octupole collectivity in this mass region.   

Measuring the B(E2) of the $\frac{1}{2}^- \rightarrow \frac{3}{2}^-$ transition in $^7Be$

Ab-initio methods have been successful in describing the structure of light nuclei using realistic nucleon-nucleon interactions, but more experimental data is needed for light unstable nuclei. Recent no-core configuration interaction calculations have made predictions for the ratio of E2 transition strengths for the first excited state transition in $^7Be$ and $^7Li$ . Additional calculations that include clustering effects show a significant difference in the $^7Be$ and $^7Li$ B(E2) value. The E2 transition strength of the $^7Be$ first excited state has never been measured, which provides an interesting opportunity to investigate the accuracy of these calculations. To measure this E2 transition strength, a Coulomb Excitation experiment was performed at the University of Notre Dame. $^7Be$ was produced and separated using TwinSol. A beam of $^7Be$ ions were scattered off a gold target and the gamma rays from the inelastically scattered ions were detected using six clover Ge detectors. The most recent results for the E2 transition strength and its comparison to the no-core configuration interaction approach will be shown. In addition, new systematic checks on the experiment will be presented including the first stages of a Geant4 simulation to help account for beam anisotropies.   

Transition strengths in $^{21,22,23}$Mg as tests of ab initio theory

Effective charges compensate for insufficiencies arising from truncations of the nuclear model-space by performing a crude inflation of nucleon charges to account for missing electric-quadrupole ($E2$) transition strength. Recent theoretical developments in ab initio nuclear theory might allow for the calculation of this strength using evolved effective-operators arising from first-principles, removing the need for effective charges and enhancing predictive power. To that end, Coulomb excitation measurements were performed of neutron-deficient $^{21,22,23}$Mg at TRIUMF-ISAC, with the goal of extracting precision $E2$ strengths for comparison with state-of-the-art models. Results will be presented and compared with ab initio in-medium similarity renormalization group (IM-SRG) and symplectic no-core shell-model calculations, as well as phenomenological shell-model results. Results will also be presented in the context of mirror-pairs in the $sd$-shell, providing a systematic evaluation of the reproduction of $E2$ strength with respect to isospin.   

Recoil Distance Method lifetime measurement of the 2$^{+}_{1}$ state in $^{94}$Sr and implications for the structure of neutron rich Sr isotopes

A high precision lifetime measurement of the $2^{+}_{1}$ state in $^{94}$Sr was performed at TRIUMF's ISAC-II facility by coupling the Recoil Distance Method implemented via the TIGRESS Integrated Plunger with unsafe Coulomb excitation in inverse kinematics. Due to limited statistics imposed by the use of a radioactive $^{94}$Sr beam, a likelihood ratio $\chi^2$ method was derived and used to compare experimental data to Geant4-simulated lineshapes. The $B(E2;2^{+}_{1} \rightarrow 0^{+}_{1})$ value extracted from the lifetime measurement of $7.80^{+0.50}_{-0.40}~\mathrm{(stat.)}\pm0.07~\mathrm{(sys.)}$~ps is approximately 25\% larger than previously reported while the relative uncertainty has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with $N \leq 58$ which is a necessary condition for the Quantum Phase Transition interpretation of the onset of deformation in this region. A summary of the experiment, description of the data analysis methods, and a comparison to existing theoretical models will be presented.