Nuclear Structure Near The Drip Line Using Large γ-ray Detector Arrays

Abstract

Big Ge‐arrays like Euroball or Gasp have been built with the main purpose of studying high angular momentum phenomena in nuclei. When coupled with ancillary detectors they became also excellent instruments to explore the properties of very exotic nuclei far from β‐stability. A large fraction of the experiments has been therefore devoted to study both proton‐rich and neutron‐rich nuclei populated using stable beams provided by the Legnaro and Strasbourg accelerators. Nuclei lying close to the N=Z line are of particular interest being a laboratory where collective excitations as well as fundamental properties of the nuclear force can be tested, like isospin symmetry and isospin breaking terms, proton neutron pairing, dripline effects. Due to their special symmetry they allow to test nuclear models. An example is the impact of nuclear physics calculations on the predictions of the standard model of the electro‐weak interaction which requires unitarity for the Cabibbo‐Kobayashi‐Maskawa (CKM) matrix. Available data suggest that the CKM matrix fails the unitarity test, pointing to physics beyond the standard model. Since such result depends on corrections for nuclear isospin mixing which must be calculated for heavy N=Z nuclei it is very important to test experimentally the model predictions. The isospin mixing probability can be determined using isospin forbidden γ transitions. If the charge symmetry of the nuclear force is exact, in the limit of long wavelengths, the E1 transition operator is purely isovector and therefore E1 transitions are forbidden in N=Z nuclei between states of equal isospin and have equal strength in mirror nuclei. Failure of this symmetry rule due to the isospin non conserving Coulomb interaction can be experimentally observed as an apparent “induced isoscalar term” through the presence of forbidden E1 transitions. An alternative method to test isospin symmetry is based on the investigation of the electromagnetic decay properties in mirror pairs. In such a case the determination of the isovector and isoscalar components of E1 transitions allow higher sensitivity compared to the determination of the transition probability of forbidden transitions in N=Z nuclei as a consequence of the interference between forbidden and allowed strengths.