Nuclear structure studies of odd-odd and odd-A nuclei in the shape transition region around N = 60

Lifetimes of nuclear excited states are a very important observable in nuclear physics. They deliver information on the structure of excited states, and, in combination with branching ratios and multipole mixing ratios, are necessary to deduce transition matrix elements. These are directly linked to the wave functions of the system and their prediction poses an important test for nuclear structure models. Isomers are a special kind of excited state. Their unusually long lifetime is an embodiment of their unique structure. A study of isomers and their decay often yields a better understanding of evolution of the microscopic structure within a mass region. The measurement of lifetimes in exotic nuclei poses a great experimental challenge. This challenge is met by the recent developments in application of inverse Coulomb excitation and Plunger measurements with radioactive ion beams. Another method which is applicable for the measurement of lifetimes down to 5 ps is the delayed coincidence method with very fast LaBr3(Ce) detectors. This technique was successfully applied in combination with a fission fragment separator for the first time in this work to measure lifetimes in very neutron-rich nuclei. Experiments on neutron-rich nuclei with neutron number N = 59 and N = 60, performed within the scope of this thesis, are discussed and the results are presented. A new microsecond isomer with a 76.5 keV E1 decay transition was unambiguously assigned to the N = 60 nucleus 97Rb. Data on a new decay branch of the K=9/2+[404] deformed isomer in 97Sr are presented. Lifetime measurements of rotational states in 99Y and states with previously unknown structure in 100Nb were performed successfully. Lifetime measurements in the beta background reproduced literature values. These experiments, employing the delayed coincidence technique, convincingly demonstrate the feasibility of precision gamma gamma fast timing measurements of excited states in exotic nuclei, which involve low peak-to-background ratio and an extended beam spot. The data of a large isomer spectroscopy campaign at Lohengrin at the Institute Laue-Langevin, Grenoble, are presented. These measurements yielded more precise microsecond isomer lifetimes in light fission fragments. Furthermore, it was possible to deduce isomer population ratios for many of the observed isomers. These results are important data for comparison with model predictions of mean spin population after fission, and essential information for understanding the very complex fission process. Finally, the experimental results are discussed and compared to nuclear model calculations. For 97Rb a spin assignment of (1/2, 3/2, 5/2)- can be made for the state with the 76.5 keV transition. Comparison to calculations, made by Simpson and Daugas, shows hints at a possible oblate character of the low-lying isomeric state. The new decay branch in 97Sr allows a spin assignment of 5/2+ to the state at 522 keV. Calculations within the IBFM indicate a simple one-quasi-particle structure of this state. The lifetime measurements in 99Y give further evidence for the regular rotational structure of the ground-state band in this nucleus. A calculation applying the quasi particle rotor model, assuming axially symmetric deformation, yields good agreement for the ground-state band. The lifetime measurements in the decay cascade of the microsecond isomer in 100Nb clearly indicate non-collective nature of these excitations

Hybrid polymer sol-gel material for UV-nanoimprint: Microstructure and thermal densification

Hybrid polymer solutions suitable for UV-nanoimprint were synthesized by combination of an alkoxysilane binder mixture with silica nanoparticles. Hydrolysis and condensation reactions were monitored by NMR and viscosity measurements. Thereby long-term stable systems were produced as a prerequisite for industrial application. Dip-coating of glass substrates and subsequent UV-curing yielded thin films. Their thermal densification and microstructural evolution resulted in pure glassy porous coatings, which were in detail characterized by N2-sorption measurements and ellipsometric porosimetry. Results emphasize the importance of the binder-particle interaction within these materials that are destined for the fabrication of microstructured surfaces by cost efficient and industrially feasible UV-based soft lithography. Structured glassy layers with high inorganic content show thermal stability up to >500 C and have a high structure accuracy >85 %

Fast Timing Measurement Using an LaBr3(Ce) Scintillator Detector Array Coupled with Gammasphere

A fast-timing experiment was performed at the Argonne National Laboratory in December 2015 and January 2016, measuring decay radiation of fission products from a 252Cf fission source. Details of the set-up, integration with Digital Gammasphere, and the data acquisition system are presented. The timing performance of the set-up, capable of measuring lifetimes from the nanosecond region down to tens of picoseconds, is discussed. First preliminary results from the fast-timing analysis of the fission fragment data are presente

γ -ray spectroscopy of low-lying excited states and shape competition in Os 194

The properties of excited states in the neutron-rich nucleus 194Os have been investigated using the 192Os(18O,16O)194Os reaction with an 80 MeV beam provided by the IFIN-HH Laboratory, Bucharest. Discrete γ -ray decays from excited states have been measured using the hybrid HPGe-LaBr3(Ce) array RoSPHERE. The current work identifies a number of previously unreported low-lying nonyrast states in 194Os as well as the first measurement of the half-life of the yrast 2+ state of 302(50) ps. This is equivalent to a B(E2 : 2+ → 0+) = 45(16) W.u. and intrinsic quadrupole deformation of βeff = 0.14(1). The experimental results are compared with Hartree-Fock-Bogoliubov–interacting-boson-model calculations and are consistent with a reduction in a quadrupole collectivity in Os isotopes with increasing neutron number

γ -ray spectroscopy of low-lying excited states and shape competition in Os 194

The properties of excited states in the neutron-rich nucleus 194Os have been investigated using the 192Os(18O,16O)194Os reaction with an 80 MeV beam provided by the IFIN-HH Laboratory, Bucharest. Discrete γ -ray decays from excited states have been measured using the hybrid HPGe-LaBr3(Ce) array RoSPHERE. The current work identifies a number of previously unreported low-lying nonyrast states in 194Os as well as the first measurement of the half-life of the yrast 2+ state of 302(50) ps. This is equivalent to a B(E2 : 2+ → 0+) = 45(16) W.u. and intrinsic quadrupole deformation of βeff = 0.14(1). The experimental results are compared with Hartree-Fock-Bogoliubov–interacting-boson-model calculations and are consistent with a reduction in a quadrupole collectivity in Os isotopes with increasing neutron number

Multi-quasiparticle sub-nanosecond isomers in 178W

We report on the first measurement of the half-lives of K(pi) = 11- and 12+ four-quasiparticle states in the even-even nucleus W-178. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3 scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array as a whole. The measured half-lives are 476(44)ps and 275(65)ps for the I(pi) = 11- and 12+, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Half-life measurements in 164,166Dy using γ-γ fast-timing spectroscopy with the ν-Ball spectrometer

We report on the measurement of lifetimes of excited states in the near-mid-shell nuclei 164,166Dy using the gamma-ray coincidence fast-timing method. The nuclei of interest were populated using reactions between an 18O beam and a gold-backed isotopically enriched 164Dy target of thickness 6.3 mg/cm2 at primary beam energies of 71, 76, and 80 MeV from the IPN-Orsay laboratory, France. Excited states were populated in 164Dy, 166Dy, and 178,179Wfollowing Coulomb excitation, inelastic nuclear scattering, two-neutron transfer, and fusion-evaporation reaction channels respectively. Gamma rays from excited states were measured using the ν-Ball high-purity germanium (HPGe)-LaBr3 hybrid γ -ray spectrometer with the excited state lifetimes extracted using the fast-timing coincidence method using HPGe-gated LaBr3-LaBr3 triple coincident events. The lifetime of the first Iπ = 2+ excited state in 166Dy was used to determine the transition quadrupole deformation of this neutron-rich nucleus for the first time. The experimental methodology was validated by showing consistency with previously determined excited state lifetimes in 164Dy. The half-lives of the yrast 2+ states in 164Dy and 166Dy were 2.35(6) and 2.3(2) ns, respectively, corresponding to transition quadrupole moment values of Q0 = 7.58(9) and 7.5(4) eb, respectively. The lifetime of the yrast 2+ state in 166Dy is consistent with a quenching of nuclear quadrupole deformation at β ≈ 0.35 as the N = 104 mid-shell is approached.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard