Study of the neutron-rich region in the vicinity of 208Pb via multinucleon transfer reactions

The multinucleon transfer reaction mechanism was employed to populate isotopes around the doubly- magic 208 Pb nucleus. We used an unstable 94 Rb beam on 208 Pb targets of different thickness. Transfer channels were studied via the fragment-γ and γ-γ coincidences, by using MINIBALL γ spectrometer coupled to a particle detector. Gamma transitions associated to the different Pb isotopes, populated by the neutron transfers, are discussed in terms of excitation energy and spin. Fragment angular distributions were extracted, andcompared with the reaction model

Octupole collectivity in 220Rn and 224Ra

Collective properties of the radioactive nuclei 220Rn and 224Ra have been studied via Coulomb excitation of a 2:8 A.MeV radioactive ion beam (RIB) incident upon 60Ni, 112;114Cd and 120Sn targets. The experiments took place at the REX-ISOLDE RIB facility, CERN. De-excitation g-ray yields following multiple-step Coulomb excitation were detected in coincidence with recoiling target nuclei in the Miniball spectrometer. For the rst time, B(E3; 3+ ! 0+) values have been directly measured with a radioactive ion beam. In the process, 224Ra becomes the heaviest post-accelerated RIB to date at ISOLDE (with the possible exception of the quasi-stable 238U). The measurements presented in this thesis represent a tripling of the number of nuclei around Z ' 88 and N ' 134, for which direct measurements of the octupole collectivity have been performed. The only previous measurements being for the relatively long-lived 226Ra. The g-ray yields, in conjunction with previously measured spectroscopic data, were used to determine electromagnetic matrix elements using the least-squares search code, Gosia. In total, nine E1, E2 and E3 matrix elements were measured in 220Rn (plus six upper limits) and 12 (plus four upper limits) in 224Ra. The measured B(E3; 3+ ! 0+) values in 220Rn and 224Ra are 324 W.u. and 423 W.u., respectively. A new state has been observed at 937.8(8) keV in 220Rn and is proposed to be the 2+ member of the K = 2, g-vibrational band. The results are interpreted in terms of the collectivity and deformation, and are compared with the predictions of self-consistent mean-eld theory and cluster model calculations.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Collectivity and deformation in nuclei . . . . . . . . . . . . . . . 2 1.1.1 Rigid-rotor model . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Reflection-Asymmetric Nuclei . . . . . . . . . . . . . . . . . . . . 6 1.2.1 Energy levels . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2.2 Dipole moments . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.2.3 E3 transition strength . . . . . . . . . . . . . . . . . . . . . . 10 1.2.4 Further evidence . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 Coulomb Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.1 Theoretical description . . . . . . . . . . . . . . . . . . . . . . 13 2.1.1 The Semi-Classical Approximation . . . . . . . . . . . . . . . . . 13 2.1.2 First-order Perturbation Theory . . . . . . . . . . . . . . . . . 15 2.1.3 Multiple-step Coulomb Excitation . . . . . . . . . . . . . . . . . 18 2.2 The Gosia Analysis Code . . . . . . . . . . . . . . . . . . . . . . 19 2.2.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.1 The Radioactive Isotope Facility, ISOLDE . . . . . . . . . . . . . . 22 3.1.1 Isotope production . . . . . . . . . . . . . . . . . . . . . . . . 23 3.1.2 Post-acceleration: REX-ISOLDE . . . . . . . . . . . . . . . . . . 25 3.1.3 Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.2 Miniball . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.2.1 CD Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.2 Eciency determination . . . . . . . . . . . . . . . . . . . . . . 31 3.2.3 Add-back routine . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2.4 Determination of Ge-detector positioning . . . . . . . . . . . . . 35 3.2.5 Doppler correction . . . . . . . . . . . . . . . . . . . . . . . . 37 4 Coulomb excitation of 220Rn and 224Ra . . . . . . . . . . . . . . . . 40 4.1 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.1.1 Time windows . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.1.2 Particle Identication . . . . . . . . . . . . . . . . . . . . . . 44 4.1.3 Particle Multiplicity . . . . . . . . . . . . . . . . . . . . . . 45 4.2 Spectroscopy of 220Rn via Coulomb excitation . . . . . . . . . . . . 49 4.2.1 Vibrational states . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2.2 Data segmentation . . . . . . . . . . . . . . . . . . . . . . . . 53 4.3 Spectroscopy of 224Ra via Coulomb excitation . . . . . . . . . . . . 55 4.3.1 Vibrational states . . . . . . . . . . . . . . . . . . . . . . . . 56 4.3.2 Data segmentation . . . . . . . . . . . . . . . . . . . . . . . . 58 5 Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.1 Gosia Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.2 Fitting Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.2.1 The 220Rn Minimum . . . . . . . . . . . . . . . . . . . . . . . . 64 5.2.2 The 224Ra Minimum . . . . . . . . . . . . . . . . . . . . . . . . 66 5.3 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.3.1 Statistical Errors . . . . . . . . . . . . . . . . . . . . . . . . 67 5.3.2 E1=E3 relative phase . . . . . . . . . . . . . . . . . . . . . . . 68 5.3.3 Beam energy and target thickness . . . . . . . . . . . . . . . . . 69 5.3.4 E4 matrix elements . . . . . . . . . . . . . . . . . . . . . . . . 72 5.3.5 Diagonal E2 matrix elements . . . . . . . . . . . . . . . . . . . 74 5.4 220Rn Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5.5 224Ra Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 6.1 Interpretation of Collectivity . . . . . . . . . . . . . . . . . . . 81 6.1.1 220Rn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 6.1.2 224Ra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 6.2 Theoretical Predictions . . . . . . . . . . . . . . . . . . . . . . 86 6.2.1 Mean-eld approach . . . . . . . . . . . . . . . . . . . . . . . . 86 6.2.2 Cluster models . . . . . . . . . . . . . . . . . . . . . . . . . . 87 6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 6.3.1 Electric-Dipole Moment (EDM) . . . . . . . . . . . . . . . . . . . 90 6.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 A Kinematic approximations . . . . . . . . . . . . . . . . . . . . . . . 93 B Experimental Yields . . . . . . . . . . . . . . . . . . . . . . . . . 95 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . 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