Search

The quantum tunneling and emission of a single constituent nucleon provide a beautifully simple and unique window into the complex properties of atomic nuclei at the extreme edge of nuclear existence. In particular, for odd-odd proton emitting nuclides, the associated decay energy and partial half-life can be used to probe the correlations between the valence neutrons and protons which have been theoretically predicted to favour a new type of nuclear superfluidity, isoscalar neutron-proton pairing, for which the experimental “smoking gun" remains elusive. In the present work, proton emission from the lanthanum isotope 11657La59, 23 neutrons away from the only stable isotope 13957La82, is reported. 116La nuclei were synthesised in the fusion-evaporation reaction 58Ni(64Zn, p5n)116La and identified via their proton radioactivity using the mass spectrometer MARA (Mass Analysing Recoil Apparatus) and the silicon detectors placed at its focal plane. Comparisons of the measured proton energy (Ep = 718 ± 9 keV) and half-life (T1/2 = 50 ± 22 ms) with values calculated using the Universal Decay Law approach indicate that the proton is emitted with an orbital angular momentum l = 2 and that its emission probability is enhanced relative to its closest, less exotic, odd-even lanthanum isotope (11757La60) while the proton-emission Q-value is lower. We propose this to be a possible signature for the presence of strong neutron-proton pair correlations in this exotic, neutron deficient system. The observations of γ decays from isomeric states in 116La and 117La are also reported. peerReviewed

For the first time, excited states in 185Pb have been observed in in-beam gamma-ray spectroscopic measurements using the recoil-decay tagging method. The resulting level scheme reveals a strongly coupled yrast band structure that originates from coupling of the i_13/2 quasineutron to a prolate deformed core. The band is also observed to de-excite via the spherical alpha-decaying 13/2+ isomeric state. ispartof: Physical Review C, Nuclear Physics vol:80 issue:3 status: published

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 nrpages: 112 status: published