Deutsche Übersetzung des Titels: Der neue Beschleuniger für radioaktive Atomkerne REX/ISOLDE-MINIBALL : niederenergetische Coulombanregung von 30,32 Mg und die Insel der Inversion

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Abstract

The understanding of the structure of nuclei far from stability is one of the key problems in nuclear structure physics today. The questions to be addressed range from understanding of the limits of nuclear stability to the origin of the elements. While in nature there are only about 300 stable isotopes, there are more than 3000 unstable nuclides that have already been observed and an equal number that is expected to exist. Beams at most accelerator laboratories are restricted to the few stable isotopes. Therefore, only stable nuclei or nuclei close to stability are well studied (with some exceptions). On the contrary, little is known about the properties of especially neutron-rich exotic nuclei, characterized by an extreme ratio of neutron to proton number; for some of them only their existence is known. With the commissioning of the REX-ISOLDE facility and the MINIBALL gamma-ray spectrometer, in which our group had a leading role, it is now possible to study nuclides far from stability with standard nuclear physics techniques such as safe Coulomb excitation, allowing to make systematic studies of key properties of exotic nuclei over wide regions in the nuclear chart, yielding a significant contribution to the understanding of nuclear structure. In this work the main aspects of the REX-ISOLDE accelerator and of the MINIBALL array are presented. After the commissioning phase two experimental campaigns were carried out with the aim of shedding new light on the interesting region of the Island of Inversion. In this first study the reduced transition probabilities B(E2) of the neutron-rich isotopes 30,32Mg were measured using the model-independent technique of safe Coulomb excitation. While the B(E2) of 30Mg can be understood in terms of pure sd shell model configurations, the large value of 32Mg is consistent with a pure 2p-2h configuration with a breakdown of the N=20 shell closure at Z=12. The reasons for this surprisingly abrupt transition into the Island of Inversion are discussed.