Nuclear shell effects in neutron-rich nuclei around N=20 and N=32,34

Nuclear shell effects in neutron-rich nuclei around N=20 and N=32,34 were studied by means of reduced transition probabilities, i.e. B(E2) and B(M1) values. To this end a series of Coulomb-excitation experiments, employing radioactive 31Mg and 29,30Na beams, as well as a precise lifetime experiment of excited states in 56Cr were performed. The collective properties of excited states of 31Mg were the subject of a Coulomb-excitation experiment at REX-ISOLDE, CERN, employing a radioactive 31Mg beam at a beam energy of 3.0 MeV/u. The beam intensity amounted to 3000 ions/s on average. The highly efficient MINIBALL setup was employed, consisting of eight HPGe cluster detectors for gamma-ray detection and a segmented Si-detector for coincident particle detection. The level scheme of 31Mg was extended. Spin and parity assignment of the observed 945 keV state yielded 5/2+ and its de-excitation is dominated by a strong collective M1 transition. Comparison of the transition probabilities of 30,31,32Mg establishes that for the N=19 magnesium isotope not only the ground state but also excited states are largely dominated by a deformed pf intruder configuration. This implies that 31Mg is part of the so-called "island of inversion". Coulomb-excitation experiments of radioactive 29,30Na were carried out at REX-ISOLDE, CERN, at a final beam energy of 2.85 MeV/u. De-excitation gamma rays were detected by the MINIBALL gamma spectrometer in coincidence with scattered particles in a segmented Si-detector. Despite rather low beam intensities transition probabilities to the first excited states were deduced. Results of very recently published experiments at MSU and TRIUMF could be largely confirmed and extended. The measured B(E2) values agree well with shell-model predictions, supporting the idea that in the sodium isotopic chain the ground-state wave function contains a significant intruder admixture already at N=18, with N=19 having an almost pure 2p2h deformed ground-state configuration. Excited states in 56Cr have been populated after 48Ca(11B,p2n) reactions at a beam energy of 32 MeV, provided by the Cologne FN tandem accelerator. The Cologne coincidence plunger device surrounded by a gamma-ray detector array of one EUROBALL cluster detector and five Ge detectors was employed to determine lifetimes with the recoil distance Doppler-shift method. gamma-gamma-coincidence data were analyzed using the differential decay curve method and precise lifetimes for the first 2+ and 4+ states were extracted. The corresponding B(E2,2+ -> 0+) value quantifies with a high accuracy the puzzling discrepancy between experimental B(E2) values in N=32 isotones and theoretical results from large-scale shell-model calculations employing modern effective nucleon-nucleon interactions.