Formkoexistenz in $^{32}$Mg: entdeckung des angeregten 0$^{+}$ zustands

The evolution of shell structure in exotic nuclei as a function of proton (${Z}$) and neutron (${N}$) number is currently at the center of many theoretical and experimental investigations. It has been realized that the interaction of the last valence protons and neutrons, in particular the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the collapse of classic shell closures and the appearance of new shell gaps. The “Island of Inversion” around $^{32}$Mg, which is one of the most studied phenomena in the nuclear chart, is a well known example for such changes in nuclear structure. In this region of neutron-rich nuclei around the magic number${ N}$ = 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed. Due to the reduction of the ${N}$ = 20 shell gap quadrupole correlations can enable low-lying deformed 2${p}$ − 2${h}$ intruder states from the ${fp}$-shell to compete with spherical normal neutron 0${p}$ − 0${h}$ states of the ${sd}$-shell. In this situation the promotion of a neutron pair across the ${N}$ = 20 gap can result in deformed intruder ground states. Consequentially the two competing configurations can lead to the coexistence of spherical and deformed 0$^{+}$ states in the neutron rich nuclei $^{30,32}$Mg. In this work the shape coexistence in $^{32}$Mg was studied by a two neutron transfer reaction at the REX-ISOLDE facility (CERN). The two neutron transfer reaction with a $^{30}$Mg beam involved for the first time the use of a radioactive tritium target in combination with a radioactive heavy ion beam. Light charged particles emitted from the target were detected and identified by the T-REX particle detector while ${\gamma}$-rays were detected by the MINIBALL Germanium detector array. The shape of the angular distribution of the protons allows to unambiguously determine the angular momentum transfer Δ${L}$ of the reaction and thus to identify the 0$^{+}$ states. The analysis of excitation energies and angular distributions led to the first observation of the excited shape coexisting 0$^{+}$ state in $^{32}$Mg . From the cross section the spectroscopic amplitudes can be deduced and compared with shell model calculations. This allows to draw conclusions on the configuration of the populated state.