Abstract
This thesis presents measurements of the reduced electromagnetic transition probabilities, B(E2 : 2+ -> 0+), in the nuclei 164Dy, 166Dy and 178W, from experimental determination of the yrast 2+ excited state lifetimes. Experiments were performed using the accelerator at the ALTO facility at the IPN Orsay. Gamma-ray spectroscopy studies were performed with the HPGe-LaBr3(Ce) hybrid spectrometer, ν-Ball. In the first experiment of the campaign, excited states were populated using reactions between an 18O beam, at three separate primary beam energies of 71, 76 and 80 MeV on the same gold-backed, isotopically-enriched 164Dy target of thickness 6.3 mg/cm2. Coulomb excitation/inelastic nuclear scattering, two-neutron transfer reactions, and fusion-evaporation reactions produced excited states in 164Dy, 166Dy and 178;9W nuclei, respectively. A measurement of the yrast I{pi} = 2+ excited state lifetime in 166Dy was used to determine a first measurement for the quadrupole deformation of this nucleus. This work makes 166Dy the nucleus with the largest valence product, NpNn, with a measured electromagnetic transition rate from the yrast 2+ to the 0+ ground state, below A=208. The experimental methodology was validated through demonstrating consistency between previously determined excited state lifetimes in 164Dy and 178W. The methodology and effectiveness of multiplicity/sum-energy gating, for channel selection between fusion evaporation events and lower multiplicity/energy events from inelastic nuclear scattering and Coulomb excitation of the target, and from two-neutron transfer reactions to 166Dy, is also demonstrated. Following the selection of gamma rays emitted by 164Dy or 178W using multiplicity/sum-energy constraints, lifetime measurements of the yrast 2+, 4+ and 6+ excited states in these nuclei were made. Excited state lifetimes were deduced using the fast-timing coincidence method using LaBr3-LaBr3 coincident events. The yrast 2+ lifetimes in 164Dy, 166Dy and 178W were determined to be 2.35(6), 2.3(2) and 1.08(4) ns, respectively, which correspond to B(E2 : 2+ ! 0+) values of 214(6), 199(19) and 161(6) W.u., respectively. These can be used within the context of the rotational model to correspond to values of the quadrupole deformations for these nuclei of β2 = 0.352(5), 0.339(16) and 0.272(5), respectively. The dysprosium values are compared with theoretical predictions of the evolution of quadrupole deformations through the dysprosium isotopic chain as the N=104 valence maximum system, 170Dy is approached. The final part of this thesis reports first results on a separate study of the production yields of primary fission products produced following fast-neutron induced fission of 238U, performed at the LICORNE facility at the IPN-Orsay, also using the nu-Ball gamma-ray spectrometer.
These data are compared with predictions of the GEF code.