Reiner Kruecken

The neutron-rich cadmium isotopes around the well-known magic numbers at Z=50 and N=82 are prime candidates to study the evolving shell structure observed in exotic nuclei. Additionally, the extra binding energy observed around the nearby doubly-magic ¹³²Sn has direct implications for in astrophysical models, leading to the second r-process abundance peak at A≈130 and the corresponding waiting-point nuclei around N=82. The β-decay of the N=82 isotope ¹³⁰Cd into ¹³⁰In was investigated in 2002, but the information for states of the lighter indium isotope ¹²⁸In is still limited. Detailed beta-gamma-spectroscopy of ¹²⁸,¹³¹,¹³²Cd was accomplished using the GRIFFIN facility at TRIUMF. In ¹²⁸In, 32 new transitions and 11 new states have been observed in addition to the four previously observed excited states. The ¹²⁸Cd half-life has also been remeasured via the time distribution of the strongest gamma-rays in the decay scheme with a higher precision. For the decay of ¹³¹,¹³²Cd, results are compared with the recent EURICA data. These new results are compared with recent shell model and IMSRG calculations, highlighting the necessity to re-investigate even "well-known" decay schemes for missing transitions.

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The sudden onset of ground state deformation and the emergence of shape-coexisting states in the vicinity of N~60 and Z~40 has been a subject of substantial interest for many years. It has been shown that the emergence of deformed low-energy configurations can be explained in the shell model by the evolution of single particle structure and the interaction between protons and neutrons in certain valence orbitals. However, the numerous theoretical models that have been developed for this transitional region are limited by the experimental data that is available. In particular, a description of the underlying single particle configurations of low energy states is essential for a detailed description of this region. In this work, the single particle structure of states in ⁹⁵Sr and ⁹⁶Sr has been investigated through the one-neutron transfer reactions ⁹⁵ ⁹⁶Sr(d,p) in inverse kinematics at TRIUMF. In each of these experiments, a 5.5 MeV/u Sr beam was impinged on a 5.0 mg/cm² CD₂ target, and emitted particles and γ-rays were detected using the SHARC and TIGRESS detector arrays, respectively. Using an angular distribution analysis, firm spin assignments have been made for the first time of the low-lying 352 keV, 556 keV and 681 keV excited states in ⁹⁵Sr from ⁹⁴Sr(d,p), and a constraint has been made on the spin of the higher-lying 1666 keV excited state in ⁹⁵Sr. Similarly, angular distributions have been extracted for 14 states in 96Sr from ⁹⁵Sr(d,p), and new experimental constraints have been assigned to the spins and parities of 8 states in ⁹⁶Sr. Additionally, two new states in ⁹⁶Sr have been identified in this work. A measurement of the mixing strength between the 1229 keV and 1465 keV shape-coexisting states in ⁹⁶Sr was also made, which was found to be a²=0.48(17).

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Electromagnetic probes represent a fundamental tool to study nuclear structure and dynamics. The perturbative nature of the electromagnetic interaction allows for a clean connection between calculated nuclear structure properties and measured cross sections. Ab initio methods have long represented the gold standard for calculations of nuclear structure observables in light nuclei. Thanks to recent developments in the scientific community, ab initio calculations have finally reached the medium- and heavy-mass region of the nuclear chart. However, the challenges modern nuclear structure calculations face are multiple, ranging from the construction of nuclear forces from chiral effective field theory (χEFT) and the solution of the highly correlated quantum many-body problem, to a quantitative description of observables with solid treatment of uncertainties.The work presented in this thesis aims to contribute addressing some of these challenges, using the ab initio coupled-cluster (CC) theory formulation of the Lorentz integral transform (LIT) method. We combine the CC and LIT methods for the computation of electromagnetic inelastic reactions into the continuum. We show that the bound-state-like equation characterizing the LIT method can be reformulated based on extensions of the coupled-cluster equation-of-motion (EOM) method, and we discuss strategies for viable numerical solutions. We then focus on the calculation of the electric dipole polarizability (α_D), which quantifies the low-energy behaviour of the dipole strength and is related to critical observables such as the radii of the proton and neutron distributions. Using a variety of chiral interactions, and singles and doubles excitations, we study ⁴He, ¹⁶ ²²O and ⁴⁰ ⁴⁸Ca. Exploiting correlations between α_D and the charge radius, we predict the neutron-skin radius and the polarizability for the double-magic ⁴⁸Ca, the latter recently measured by the Osaka-Darmstadt collaboration. Finally, we study the impact of triples excitations on the dipole strength in ⁴He and ¹⁶O.

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The nuclear shell model (SM) has been very successful in describing the properties and the structure of near-stable and stable isotopes near the magic nuclei. Today, the advent of powerful facilities capable of producing radioactive isotopes far from stability has enabled the test of the SM on very proton-rich or neutron-rich magic nuclei. 100/50Sn50 is a proton-rich doubly-magic nucleus, but is nearly unstable against proton emission. Key topics of nuclear structure in this region include the location of the proton dripline, the effect of proton-neutron interactions in N ~ Z nuclei, single-particle energies of orbitals above and below the N = Z = 50 shell gaps, and the properties of the superallowed Gamow-Teller decay of ¹⁰⁰Sn. A decay spectroscopy experiment was performed on ¹⁰⁰Sn and nuclei in its vicinity at the RIKEN Nishina Center in June 2013. The isotopes of interest were produced from fragmentation reactions of 124/54Xe on a 9/4Be target, and were separated and identified on an event-by-event basis. Decay spectroscopy was performed by implanting the radioactive isotopes in the Si detector array (WAS3ABi) and observing their subsequent decay radiations. β⁺ particles and protons were detected by WAS3ABi, and γ rays were detected by a Ge detector array (EURICA). Of the proton-rich isotopes produced in this experiment, over 20 isotopes as light as ⁸⁸Zr and as heavy as ¹⁰¹Sn were individually studied. New and improved measurements of isotope/isomer half-lives, β-decay endpoint energies, β-delayed proton emission branching ratios, and γ-ray transitions were analyzed. In general the new results were well reproduced by the SM, highlighting a relatively robust ¹⁰⁰Sn core. However, the level scheme of ¹⁰⁰Sn's β-decay daughter nucleus ¹⁰⁰In was not conclusively determined because of several missing observations which were expected from various SM predictions. Significantly higher β-decay and γ-ray statistics are required on several nuclei, including ¹⁰⁰Sn, to evaluate the limit of the current understanding of their structure.

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