Lifetime measurements in the lead region and the emergence of collectivity in 98Zr

The evolution from the single-particle, seniority-like structure to the emergence of collectivity has been investigated in the lead region through lifetime measurements. The fast-timing, the recoil-distance Doppler shift (RDDS), and the Doppler-shift attenuation (DSA) methods have been employed in the scope of this thesis. Lifetimes of low-lying states in 211At were measured in the 208Pb( 6Li,3n) fusion-evaporation reaction via the fast-timing method and in the 209Bi( 16O, 14C) two-proton transfer reaction via the RDDS and DSA methods. The deduced transition probabilities have been interpreted by two shell-model calculations. One was a multi-j shell-model calculation done using the Kuo-Herling residual interaction. The other one was done using a semi-empirical interaction for protons confined to the single-j 0h9/2 orbital. The Kuo-Herling calculations overestimate some of the ground-state transitions, which has been interpreted as a presence of particle-hole excitation in the ground-state wave function. However, the discrepancy between the calculated values and the measured ones is smaller than in the case of 210Po. This shows that the effects of the particle-hole excitations are reduced when moving away from the 208Pb core. The results from the single-j calculations describe the reduced transition probabilities well, which shows that seniority could be regarded as a good quantum number. The 209Po nucleus was studied in the electron-capture decay of 209At via the fast-timing technique. The deduced transition probabilities were compared to shell-model calculations done using the V low−k approach based on the Bonn nucleon-nucleon potential. The results show that some of the states in 209Po could be interpreted as single-particle neutron-hole states coupled to a 210Po core. However, some additional contributions in the wave functions are needed to describe the experimental data better. The onset of collectivity in the region was studied via fast-timing measurements of the lifetimes of the 4+1 states in 204Po and 206 Po. The nuclei were studied via the 197Au( 11B,3n) and the 198Pt(12C,4n)206Po reactions, respectively. The results indicate that the transition to a collective behavior of the 4+1 states happens below N=122. Additionally, the rapid onset of collectivity in the A ≈ 100 region was investigated in the scope of this thesis. The low-spin structure of the 98Zr nucleus was studied in the 96Zr( 18O, 16O) two-neutron transfer reaction. Lifetimes were deduced using the RDDS and DSA techniques. The results were interpreted by a Monte Carlo shell model and an inter-acting boson model with configuration mixing calculations. Both calculations reproduce most of the data but there are still some open questions regarding the low-spin structure of 98Zr

A study on the transition between seniority-type and collective excitations in 204Po and 206Po

Low-lying yrast states in 204Po and 206Po have been investigated by the γ-γ fast timing technique with LaBr3(Ce) detectors. Excited states of these nuclei were populated in the 197Au(11B,4n) and the 198Pt(12C,4n) fusion-evaporation reactions, respectively, at the FN-Tandem Facility at the University of Cologne. The lifetimes of the 4+1 states in both nuclei were measured, along with an upper limit for the 2+1 state in 204Po. The preliminary results are discussed in the scope of the systematic behavior of the transition strengths between yrast states in polonium isotopes

Coulomb Excitation of Proton-rich N = 80 Isotones at HIE-ISOLDE

A projectile Coulomb-excitation experiment was performed at the radioactive ion beam facility HIE-ISOLDE at CERN. The radioactive ¹⁴⁰Nd and ¹⁴²Sm ions were post accelerated to the energy of 4.62 MeV/A and impinged on a 1.45 mg/cm²-thin ²⁰⁸Pb target. The γ rays depopulating the Coulomb-excited states were recorded by the HPGe-array MINIBALL. The scattered charged particles were detected by a double-sided silicon strip detector in forward direction. Experimental γ-ray intensities were used for the determination of electromagnetic transition matrix elements. Preliminary results for the reduced transition strength of the B(M1;23+→21+)=0.35(19)μN2 of ¹⁴⁰Nd and a first estimation for ¹⁴²Sm have been deduced using the Coulomb-excitation calculation software GOSIA. The 2³₊ states of ¹⁴⁰Nd and ¹⁴²Sm show indications of being the main fragment of the proton-neutron mixed-symmetry 2⁺₁,ms state

Beam Transport to RC4 using MAD-X

The development and construction of a ISOLDE decay station (IDS) have been proposed. The setup will allow ddifferent type of decay studies throughout the whole Nuclear Chart: beta-delayed fission, alpha-delayed gamma decay, beta-delayed gamma decay, charge particle, neutron detection, etc. The development of the setup has already started by the collaboration which includes universities and institutions from several European countries. The goal of the summer project is to describe a beam transport up to the position of the IDS. This has been done by using the program MAD-X for optics simulations

Measurement of the

The r-process has been shown to be robust in reproducing the abundance distributions of heavy elements, such as europium, seen in ultra-metal poor stars. In contrast, observations of elements 26 < Z < 47 display overabundances relative to r-process model predictions. A proposed additional source of early nucleosynthesis is the weak r-process in neutrino-driven winds of core-collapse supernovae. It has been shown that in this site (α,n) reactions are both crucial to nucleosynthesis and the main source of uncertainty in model-based abundance predictions. Aiming to improve the certainty of nucleosynthesis predictions, the cross section of the important reaction 86Kr(α,n)89Sr has been measured at an energy relevant to the weak r-process. This experiment was conducted in inverse kinematics at TRIUMF with the EMMA recoil mass spectrometer and the TIGRESS gamma-ray spectrometer. A novel type of solid helium target was used

Search for Isovector Valence-Shell Excitations in 140Nd and 142Sm via Coulomb excitation reactions of radioactive ion beams

Projectile Coulomb excitation experiments were performed at HIE-ISOLDE at CERN with the radioactive ion beams of 140Nd and 142Sm. Ions with an energy of 4:62 MeV/A were impinging on a 1.45 mg/cm2 thick 208Pb target. The γ-rays depopulating the Coulomb-excited states were recorded by the HPGe-array MINIBALL and scattered particles were detected by a double-sided silicon strip detector. Experimental intensities were used for the determination of electromagnetic transition matrix elements. A preliminary result of the B(M1; 2+3 → 2+1) of 140Nd and an upper limit for the case of 142Sm are revealing the main fragments of the proton-neutron mixed-symmetry 2+1;ms states

Search for Isovector Valence-Shell Excitations in Nd-140 and Sm-142 via Coulomb excitation reactions of radioactive ion beams

Projectile Coulomb excitation experiments were performed at HIE-ISOLDE at CERN with the radioactive ion beams of 140Nd and 142Sm. Ions with an energy of 4.62 MeV/A were impinging on a 1.45 mg/cm2 thick 208Pb target. The γ-rays depopulating the Coulomb-excited states were recorded by the HPGe-array MINIBALL and scattered particles were detected by a double-sided silicon strip detector. Experimental intensities were used for the determination of electromagnetic transition matrix elements. A preliminary result of the B(M1; 2+ 3→2+ 1 ) of 140Nd and an upper limit for the case of 142Sm are revealing the main fragments of the proton-neutron mixed-symmetry 2+ 1,ms states.peerReviewe

Coulomb Excitation of Proton-rich N = 80 Isotones at HIE-ISOLDE

A projectile Coulomb-excitation experiment was performed at the radioactive ion beam facility HIE-ISOLDE at CERN. The radioactive 140Nd and 142Sm ions were post accelerated to the energy of 4.62 MeV/A and impinged on a 1.45 mg/cm2-thin 208Pb target. The γ rays depopulating the Coulomb-excited states were recorded by the HPGe-array MINIBALL. The scattered charged particles were detected by a double-sided silicon strip detector in forward direction. Experimental γ-ray intensities were used for the determination of electromagnetic transition matrix elements. Preliminary results for the reduced transition strength of the B(M1 23+ to 21+)=0.35(19) μN2 of 140Nd and a first estimation for 142Sm have been deduced using the Coulomb-excitation calculation software GOSIA. The 23+ states of 140Nd and 142Sm show indications of being the main fragment of the proton-neutron mixed-symmetry 21, ms+ state.peerReviewe

Isomeric Lifetime Measurement in the Neutron-rich 189^{189}Ta

International audienceIsomeric states of the neutron-rich isotope 18973Ta116 were populated via fragmentation of a primary beam of 208Pb ions at 1 GeV/u impinging on a 9Be target at GSI, Darmstadt, Germany. The isotopes of interest were separated, identified and delivered to the DESPEC setup. Two isomers were deduced in 189Ta116 and their lifetimes were measured based on the γ-ray time distributions