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 γ-ray yields following multiple-step Coulomb excitation were detected in coincidence with recoiling target nuclei in the Miniball spectrometer. For the first 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 γ-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 32±4 W.u. and 42±3 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, γ-vibrational band. The results are interpreted in terms of the collectivity and deformation, and are compared with the predictions of self-consistent mean-field theory and cluster model calculations