Nuclear Physics Division

The study of so-called 'deformed' atomic nuclei with a non-spherical charge distribution is essential for testing nuclear interactions and structure models. Almost all nuclei have an intrinsic prolate (elongated) shape and very few are oblate (flattened). A very small number of nuclei exhibit coexistence of shapes (e.g. prolate-oblate), a phenomenon allowed by the quantum nature of the atomic nucleus. One of the research themes of the nucleus structure group of the DPhN (Departement de Physique Nucléaire) is to search for these nuclei within the Segrè map in order to study them and characterise their shape.



Experiments will be performed with the European germanium detector array AGATA. The unprecedented efficiency and resolution of this new detector will permit spectroscopic studies further away from the valley of stability than previously possible. The Nuclear Structure Group is strongly involved in the exploitation of AGATA with a particular focus on the study of shape coexistence.

The nuclear two-photon, or double-gamma decay is a rare decay mode in atomic nuclei whereby a nucleus in an excited state emits two gamma rays simultaneously. Even-even nuclei with a first excited 0+ state are favorable cases to search for a double-gamma decay branch, since the emission of a single gamma ray is strictly forbidden for 0+ ? 0+ transitions by angular momentum conservation. The double-gamma decay still remains a very small decay branch (<1E-4) competing with the dominant (first-order) decay modes of atomic internal-conversion electrons (ICE) or internal positron-electron (e+-e-) pair creation (IPC). Therefore we will make use of a new technique to search for the double-gamma decay in bare (fully-stripped) ions, which are available at the GSI facility in Darmstadt, Germany. The basic idea of our experiment is to produce, select and store exotic nuclei in their excited 0+ state in the GSI storage ring (ESR). For neutral atoms the excited 0+ state is a rather short-lived isomeric state with a lifetime of the order of a few tens to hundreds of nanoseconds. At relativistic energies available at GSI, however, all ions are fully stripped of their atomic electrons and decay by ICE emission is hence not possible. If the state of interest is located below the pair creation threshold the IPC process is not possible either. Consequently, bare nuclei are trapped in a long-lived isomeric state, which can only decay by double-gamma emission to the ground state. The decay of the isomers would be identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state state by their (very slightly) different revolution time in the ESR, and to observe the disappearance of the isomer peak in the mass spectrum with a characteristic decay time. After a first successful experiment establishing the double-gamma decay in 72Ge a new experiment has been accepted by the GSI Programme Committee and its realization is planned for early 2024.