Low collectivity of the 2(1)(+) state of Po-212

International audienceThe lifetime of the $2^+_1$ state of $^{212}$Po was measured in the $^{208}$Pb($^{12}$C,$^{8}$Be)$^{212}$Po transfer reaction by γ -ray spectroscopy employing the recoil distance Doppler shift (RDDS) method. The derived absolute B(E2) value of 2.6(3)W.u. indicates a low collectivity and contradicts previous claims of α-cluster components in the structure of the $2^+_1$ state. It is demonstrated that a consistent description of the properties of the $2^+_1$−$4^+_1$−$6^+_1$−$8^+_1$ sequence in $^{212}$Po cannot be achieved in the framework of a single-j shell-model calculation, either. This puzzle is traced to the properties of the seniority-2 configurations in $^{210}$Pb and $^{210}$Po

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

Lifetime measurement in the mass region A~100 with the PreSPEC-AGATA setup

Mid-shell nuclei in the mass region A~100 show collective behavior. For instance, neutron rich even-even molybdenum isotopes are associated with a shape-phase transition. A spherical vibrator nucleus is observed for N=48 neutrons, while measurements indicate a deformed rotor nucleus for N=64 neutrons. In order to characterize the shape of molybdenum nuclei better, thus the shape-phase transition in molybdenum isotopes, a PreSPEC-AGATA experiment was performed at GSI. In particular, the experiment aimed at measuring the lifetime of the 4+ excited state of 108Mo to deduce the decay transition rate of the 4+ state. The production of 108Mo was performed in two steps. First, the GSI Fragment-Separator (FRS) selected 109Tc ions produced via fission of a 238U beam. Then, 109Tc ions underwent a fragmentation reaction on a beryllium secondary target. Half-speed of light fragments produced in this second reaction were identified by the Lund-York-Cologne CAlorimeter (LYCCA). gamma-rays were detected by the AGATA array positioned around the secondary target at forward angles. A complex electronics and data-acquisition system was set up to record the coincidences between a flying ion and the gamma-rays it emitted. The interaction position of gamma-rays inside an AGATA crystal is determined with good resolution. Therefore, combined with the large Doppler shift of the gamma-ray, it provides a unique possibility to measure lifetime with the Geometrical-Doppler-Shift-Attenuation method. From the data analysis which included several new techniques, the half-life of T1/2 = 11 (+16;-6) ps was extracted for the first 4+ state of 108Mo. The statistics recorded in this experiment is relatively low which leads to a large uncertainty on this measurement. With this large uncertainty, it is not possible to conclude on the exact behavior of the 108Mo nucleus, even though the comparison of our measurement with geometrical models would favor a transitional nucleus with approximate X(5) solution of the Bohr-Hamiltonian. However, this measurement proves the possibility of lifetime determination coupling relativistic ion beams with the AGATA array. Therefore, it represents a crucial step towards more precise measurements that will be performed at HISPEC with the increased beam intensity provided by the Super-FRS at FAIR

APCAD - Analysis Program for the Continuous-Angle DSAM

We present a novel method for the analysis of Doppler-broadened γ-ray lineshapes observed by position-sensitive detector systems with the purpose to determine nuclear level lifetimes. This advance of the well-known Doppler-Shift Attenuation Method (DSAM) is specifically tailored to experiments employing γ-ray tracking detectors and takes advantage of the position resolution of these detector systems. The novel continuous-angle Doppler Shift Attenuation Method (caDSAM) is implemented in the comprehensive computer program APCAD for the calculation of Doppler-broadened γ-ray lineshapes, their fit to experimental data and extraction of level lifetimes. Extensions of the method qualifying it for measurements with radioactive and relativistic ion beams as well as for the measurement of “long” level lifetimes of the order of ∼200 ps are presented